Introduction to General Packet Radio Service (GPRS)GPRS stands for General Packet Radio Service, and is a protocol for passing data over a mobile phone network. GPRS offers what has become known as "always on" data connection for GSM mobile phones, allowing for faster browsing of Internet content, and faster access to online services such as WAP and email. GPRS is a packet-based wireless communication service that promises data rates from 56 up to 114 Kbp (kilobits per second) and continuous connection to the Internet for mobile phone and computer users. The higher data rates allow users to take part in video conferences and interact with multimedia Web sites and similar applications using mobile handheld devices as well as notebook computers. GPRS is based on Global System for Mobile (GSM) communication and complements existing services such circuit-switched cellular phone connections and the Short Message Service (SMS).
GPRS packet-based services cost users less than circuit-switched services since communication channels are being used on a shared-use, as-packets-are-needed basis rather than dedicated to only one user at a time. It is also easier to make applications available to mobile users because the faster data rate means that middleware currently needed to adapt applications to the slower speed of wireless systems are no longer be needed. As GPRS has become more widely available, along with other 2.5G and 3G services, mobile users of virtual private networks (VPNs) have been able to access the private network continuously over wireless rather than through a rooted dial-up connection. GPRS also complements Bluetooth, a standard for replacing wired connections between devices with wireless radio connections. In addition to the Internet Protocol (IP), GPRS supports X.25, a packet-based protocol that is used mainly in Europe. GPRS is an evolutionary step toward Enhanced Data GSM Environment (EDGE) and Universal Mobile Telephone Service (UMTS).
GPRS customers enjoy advanced, feature-rich data services such as colour Internet browsing, e-mail on the move, powerful visual communications such as video streaming, multimedia messages and location-based services. For operators, the adoption of GPRS is a fast and cost-effective strategy that not only supports the real first wave of mobile Internet services, but also represents a big step towards 3GSM (or wideband-CDMA) networks and services. GPRS communication is designed to compliment but not replace current circuit-switched networks, being used solely as an extra means of data communication. In practice, connection speeds will be significantly lower than the theoretical maximum, depending upon the amount of traffic on the network and upon the number of simultaneous channels supported by the handsets. In practice, GPRS is an evolutionary step towards enhanced data for global evolution (EDGE) and IMT-2000 systems. GPRS is a packet-based wireless data communication service designed to replace the current circuit-switched services available on the second-generation global system for mobile communications (GSM) and time division multiple access (TDMA) IS-136 networks. GSM and TDMA networks were designed for voice communication, dividing the available bandwidth into multiple channels, each of which is constantly allocated to an individual call (circuit-switched). These channels can be used for the purpose of data transmission, but they only provide a maximum transmission speed of around 9.6Kbps.Key User Features of GPRS
Theoretical maximum speeds of up to 171.2 kilobits per second (kbps) are achievable with GPRS using all eight timeslots at the same time. This is about three times as fast as the data transmission speeds possible over today's fixed telecommunications networks and ten times as fast as current Circuit Switched Data services on GSM networks. By allowing information to be transmitted more quickly, immediately and efficiently across the mobile network, GPRS may well be a relatively less costly mobile data service compared to SMS and Circuit Switched Data.GPRS facilitates instant connections whereby information can be sent or received immediately as the need arises, subject to radio coverage. No dial-up modem connection is necessary. This is why GPRS users are sometimes referred to be as being "always connected". Immediacy is one of the advantages of GPRS (and SMS) when compared to Circuit Switched Data. High immediacy is a very important feature for time critical applications such as remote credit card authorization where it would be unacceptable to keep the customer waiting for even thirty extra seconds.GPRS facilitates several new applications that have not previously been available over GSM networks due to the limitations in speed of Circuit Switched Data (9.6 kbps) and message length of the Short Message Service (160 characters). GPRS will fully enable the Internet applications you are used to on your desktop from web browsing to chat over the mobile network. Other new applications for GPRS, profiled later, include file transfer and home automation - the ability to remotely access and control in-house appliances and machines.
GPRS involves overlaying a packet based air interface on the existing circuit switched GSM network. This gives the user an option to use a packet-based data service. To supplement circuit switched network architecture with packet switching is quite a major upgrade. GPRS standard is delivered in a very elegant manner - with network operators needing only to add a couple of new infrastructure nodes and making a softwareupgrade to some existing network elements.PRESENT STATE OF NATURE
GPRS, is a new non-voice, value added, for GSM (Global System for Mobile Communications) networks. It makes sending and receiving small bursts of data, such as email and web browsing, as well as large volumes of data over a mobile telephone network possible. A simple way to understand packet switching is to relate it to a jigsaw puzzle. Image how you buy a complete image or picture that has been divided up into many pieces and then placed in a box. You purchase the puzzle and reassemble it to form the original image. Before the information is Switch in network.
Circuit Switched vs. Packet Switched
There are two types of wireless data transmission - Circuit Switched and Packet Switched. Circuit Switched employs a dedicated voice channel to transmit and receive data, essentially like keeping a single phone line open during your entire conversation. A cellular modem uses Circuit Switched transmission, which allows you to dial up a computer over your wireless phone just like you would using a landline connection. Packet switched data transmission compresses the data and sends short data bursts between or during gaps in conversations on the voice channels. Packet data digital transmission is ideal for using your phone to send short messages, including E-mail, or access news headlines, and stock quotes from the Internet.
Packet switching means that GPRS radio resources are used only when users are actually sending or receiving data. Rather than dedicating a radio channel to a mobile data user for a fixed period of time, the available radio resource can be concurrently shared between several users. This efficient use of scarce radio resources means that large numbers of GPRS users can potentially share the same bandwidth and be served from a single cell. The actual number of users supported depends on the application being used and how much data is being transferred. Because of the spectrum efficiency of GPRS, there is less need to build in idle capacity that is only used in peak hours. GPRS therefore lets network operators maximize the use of their network resources in a dynamic and flexible way, along with user access to resources and revenues.
GPRS should improve the peak time capacity of a GSM network since it simultaneously:
Allocates scarce radio resources more efficiently by supporting virtual connectivity.
migrates traffic that was previously sent using Circuit Switched Data to GPRS instead.
Reduces SMS Center and signaling channel loading by migrating some traffic that previously was sent using SMS to GPRS instead using the GPRS/SMS interconnect that is supported by the GPRS standards.
For the first time, GPRS fully enables Mobile Internet functionality by allowing interworking between the existing Internet and the new GPRS network. Any service that is used over the fixed Internet today - File Transfer Protocol (FTP), web browsing, chat, email, telnet - will be as available over the mobile network because of GPRS. In fact, many network operators are considering the opportunity to use GPRS to help become wireless Internet Service Providers in their own right.The World Wide Web is becoming the primary communications interface - people access the Internet for entertainment and information collection, the intranet for accessing company information and connecting with colleagues and the extranet for accessing customers and suppliers. These are all derivatives of the World Wide Web aimed at connecting different communities of interest. There is a trend away from storing information locally in specific software packages on PCs to remotely on the Internet. When you want to check your schedule or contacts, instead of using something like "Act!", you go onto the Internet site such as a portal. Hence, web browsing is a very important application for GPRS.Because it uses the same protocols, the GPRS network can be viewed as a sub-network of the Internet with GPRS capable mobile phones being viewed as mobile hosts. This means that each GPRS terminal can potentially have its own IP address and will be addressable as such.
It should be noted right that the General Packet Radio Service is not only a service designed to be deployed on mobile networks that are based on the GSM digital mobile phone standard. The IS-136 Time Division Multiple Access (TDMA) standard, popular in North and South America, will also support GPRS. This follows an agreement to follow the same evolution path towards third generation mobile phone networks concluded in early 1999 by the industry associations that support these two network types.
Limitations of GPRS
It should already be clear that GPRS is an important new enabling mobile data service which offers a major improvement in spectrum efficiency, capability and functionality compared with today's nonvoice mobile servicesGPRS does impact a network's existing cell capacity. There are only limited radio resources that can be deployed for different uses - use for one purpose precludes simultaneous use for another. For example, voice and GPRS calls both use the same network resources. The extent of the impact depends upon the number of timeslots, if any, that are reserved for exclusive use of GPRS. However, GPRS does dynamically manage channel allocation and allow a reduction in peak time signaling channel loading by sending short messages over GPRS channels instead.
Achieving the theoretical maximum GPRS data transmission speed of 172.2 kbps would require a single user taking over all eight timeslots without any error protection. Clearly, it is unlikely that a network operator will allow all timeslots to be used by a single GPRS user. Additionally, the initial GPRS terminals are expected be severely limited - supporting only one, two or three timeslots. The bandwidth available to a GPRS user will therefore be severely limited. As such, the theoretical maximum GPRS speeds should be checked against the reality of constraints in the networks and terminals. The reality is that mobile networks are always likely to have lower data transmission speeds than fixed networks.At the time of writing, there has been no confirmation from any handset vendors that mobile terminated GPRS calls (i.e. receipt of GPRS calls on the mobile phone) will be supported by the initial GPRS terminals. Availability or not of GPRS MT is a central question with critical impact on the GPRS business case such as application migration from other nonvoice bearers.
By originating the GPRS session, users confirm their agreement to pay for the delivery of content from that service. This origination may well be performed using a Wireless Application Protocol (WAP) session using the WAP micro browser that will be built into GHPRS terminals. However, mobile terminated IP traffic might allow unsolicited information to reach the terminal. Internet sources originating such unsolicited content may not be chargeable. A possible worse case scenario would be that mobile users would have to pay for receiving unsolicited junk content. This is a potential reason for a mobile vendor NOT to support GPRS mobile Terminate in their GPRS terminals.
However, there is always the possibility of unsolicited or unwanted information being communicated through any media, but that does not mean that we would wish to preclude the possibility of any kind of communication through that means altogether. A network side solution such as GGSN or charging platform policing would be preferable rather than a non-flexible limitation built into all the GPRS handsets.When we asked Nokia about this issue, it commented: "Details of the Nokia GPRS terminals are not available at this time. It is too early to confirm whether MT will be supported in the first Nokia GPRS terminals". The company's policy is not to make details available about products before they are announced. Readers should contact the GSM Association, Mobile Streams Limited and/or the vendors directly to encourage them to incorporate support for GPRS MT in their initial terminals.
GPRS modulation
GPRS is based on a modulation technique known as Gaussian minimum-shift keying (GMSK). EDGE is based on a new modulation scheme that allows a much higher bit rate across the air interface - this is called eight-phase-shift keying (8 PSK) modulation. Since 8 PSK will also be used for UMTS, network operators will need to incorporate it at some stage to make the transition to third generation mobile phone systems.
GPRS packets are sent in all different directions to reach the same destination. This opens up the potential for one or some of those packets to be lost or corrupted during the data transmission over the radio link. The GPRS standards recognize this inherent feature of New wireless packet technologies and incorporate data integrity and retransmission strategies. However, the result is that potential transit delays can occur.Because of this, applications requiring broadcast quality video may well be implemented using High Speed Circuit Switched Data (HSCSD). HSCSD is simply a Circuit Switched Data call in which a single user can take over up to four separate channels at the same time. Because of its characteristic of end to end connection
between sender and recipient, transmission delays are less likely.
Whereas the Store and Forward Engine in the Short Message Service is the heart of the SMS Center and key feature of the SMS service, there is no storage mechanism incorporated into the GPRS standard, apart from the incorporation of interconnection links between SMS and GPRS.
Applications for GPRS
A wide range of corporate and consumer applications are enabled by nonvoice mobile services such as SMS and GPRS. This section will introduce those that are
particularly suited to GPRS.Chat can be distinguished from general information services because the source of the information is a person with chat whereas it tends to be from an Internet site for information services. The "information intensity" - the amount of information transferred per message tends to be lower with chat, where people are more likely to state opinions than factual data. In the same way as Internet chat groups have proven a very popular application of the Internet, groups of like-minded people - so called communities of interest - have begun to use nonvoice mobile services as a means to chat and communicate and discuss.Because of its synergy with the Internet, GPRS would allow mobile users to participate fully in existing Internet chat groups rather than needing to set up their own groups that are dedicated to mobile users. Since the number of participants is an important factor determining the value of participation in the newsgroup, the use of GPRS here would be advantageous. GPRS will not however support point to multipoint services in its first phase, hindering the distribution of a single message to a group of people. As such, given the installed base of SMS capable devices, we would expect SMS to remain the primary bearer for chat applications in the foreseeable future.
GPRS services
A wide range of content can be delivered to mobile phone users ranging from share prices, sports scores, weather, flight information, news headlines, prayer reminders, lottery results, jokes, horoscopes, traffic, location sensitive services and so on. This information need not necessarily be textual- it may be maps or graphs or
other types of visual information.
The length of a short message of 160 characters suffices for delivering information when it is quantitative - such as a share price or a sports score or temperature. When the information is of a qualitative nature however, such as a horoscope or news story, 160 characters is too short other than to tantalize or annoy the information recipient since they receive the headline or forecast but little else of substance. As such, GPRS will likely be used for qualitative information services when end users have GPRS capable devices, but SMS will continue to be used for delivering most quantitative information services. Interestingly, chat applications are a form of qualitative information that may remain delivered using SMS, in order to limit people to brevity and reduce the incidence of spurious and irrelevant posts to the
mailing list that are a common occurrence on Internet chat groups.
Still images such as photographs, pictures, postcards, greeting cards and presentations, static web pages can be sent and received over the mobile network as they are across fixed telephone networks. It will be possible with GPRS to post images from a digital camera connected to a GPRS radio device directly to an
Internet site, allowing near real-time desktop publishing.
Over time, the nature and form of mobile communication is getting less textual and more visual. The wireless industry is moving from text messages to icons and picture messages to photographs and blueprints to video messages and movie previews being downloaded and on to full blown movie watching via data streaming on a mobile device.
Sending moving images in a mobile environment has several vertical market applications including monitoring parking lots or building sites for intruders or thieves, and sending images of patients from an ambulance to a hospital. Videoconferencing applications, in which teams of distributed sales people can have a regular sales meeting without having to go to a particular physical location, is
another application for moving images.
Using Circuit Switched Data for web browsing has never been an enduring application for mobile users. Because of the slow speed of Circuit Switched Data, it takes a long time for data to arrive from the Internet server to the browser. Alternatively, users switch off the images and just access the text on the web, and end up with difficult to read text layouts on screens that are difficult to read from.
As such, mobile Internet browsing is better suited to GPRS.
Mobile data facilitates document sharing and remote collaborative working. This lets different people in different places work on the same document at the same time. Multimedia applications combining voice, text, pictures and images can even be envisaged. These kinds of applications could be useful in any problem solving exercise such as fire fighting, combat to plan the route of attack, medical treatment, advertising copy setting, architecture, journalism and so on. Even comments on which resort to book a holiday at could benefit from document sharing to save everyone having to visit the travel agent to make a decision. Anywhere somebody can benefit from having and being able to comment on a visual depiction of a situation or matter, such collaborative working can be useful. By providing sufficient bandwidth, GPRS facilitates multimedia applications such as document sharing.Despite many improvements in the quality of voice calls on mobile networks such as Enhanced Full Rate (EFR), they are still not broadcast quality. There are scenarios where journalists or undercover police officers with portable professional broadcast quality microphones and amplifiers capture interviews with people or radio reports dictated by themselves and need to send this information back to their radio or police station. Leaving a mobile phone on, or dictating to a mobile phone, would simply not give sufficient voice quality to allow that transmission to be broadcast or analyzed for the purposes of background noise analysis or voice printing, where the speech autograph is taken and matched against those in police storage. Since even short voice clips occupy large file sizes, GPRS or other high
speed mobile data services are needed.
With up to half of employees typically away from their desks at any one time, it is important for them to keep in touch with the office by extending the use of corporate email systems beyond an employee's office PC. Corporate email systems run on Local Area computer Networks (LAN) and include Microsoft Mail, Outlook,
Outlook Express, Microsoft Exchange.
Since GPRS capable devices will be more widespread in corporations than amongst the general mobile phone user community, there are likely to be more corporate email applications using GPRS than Internet email ones whose target market is
more general.
Internet email services come in the form of a gateway service where the messages are not stored, or mailbox services in which messages are stored. In the case of gateway services, the wireless email platform simply translates the message from SMTP, the Internet email protocol, into SMS and sends to the SMS Center. In the case of mailbox email services, the emails are actually stored and the user gets a notification on their mobile phone and can then retrieve the full email by dialing in
to collect it, forward it and so on.
Upon receiving a new email, most Internet email users do not currently get notified of this fact on their mobile phone. When they are out of the office, they have to dial in speculatively and periodically to check their mailbox contents. However, by linking Internet email with an alert mechanism such as SMS or GPRS, users can be
notified when a new email is received.
When mobile workers are away from their desks, they clearly need to connect to the Local Area Network in their office. Remote LAN applications encompasses access to any applications that an employee would use when sitting at their desk, such as access to the intranet, their corporate email services such as Microsoft Exchange or Lotus Notes and to database applications running on Oracle or Sybase or whatever. The mobile terminal such as handheld or laptop computer has the same software programs as the desktop on it, or cut down client versions of the applications accessible through the corporate LAN. This application area is therefore likely to be a conglomeration of remote access to several different information types - email, intranet, databases. This information may all be accessible through web browsing tools, or require proprietary software applications on the mobile device. The ideal bearer for Remote LAN Access depends on the amount of data being transmitted,
but the speed and latency of GPRS make it ideal.
As this generic term suggests, file transfer applications encompass any form of downloading sizeable data across the mobile network. This data could be a presentation document for a traveling salesperson, an appliance manual for a service engineer or a software application such as Adobe Acrobat Reader to read documents. The source of this information could be one of the Internet communication methods such as FTP (File Transfer Protocol), telnet, http or Java - or from a proprietary database or legacy platform. Irrespective of source and type of file being transferred, this kind of application tends to be bandwidth intensive. It therefore requires a high speed mobile data service such as GPRS, EDGE or UMTS to run satisfactorily across a mobile network
TECHNOLOGY
EDGE (Enhanced Data GSM Environment) to be effective it should be installed along with the packet-switching upgrades used for GPRS. This entails the addition of two types of nodes to the network: the gateway GPRS service node (GGSN) and the serving GPRS service node (SGSN). The GGSN connects to packet-switched networks such as internet protocol (IP) and X.25, along with other GPRS networks, while the SGSN provides the packet-switched link to mobile stations.
The additional implementation of EDGE systems requires just one EDGE transceiver unit to be added to each cell, with the base stations receiving remote software upgrades. EDGE can co-exist with the existing GSM traffic, switching to EDGE mode automatically.
GPRS is based on a modulation technique called Gaussian minimum-shift keying (GMSK). This modulation technique does not allow as high a bit rate across the air interfaces as 8 PSK modulations if introduced into EDGE systems. 8 PSK modulations automatically adapts to local radio conditions, offering the fastest transfer rates near to the base stations, in good conditions. It offers up to 48Kbps per channel, compared to 14Kbps per channel with GPRS and 9.6Kbps per channel for GSM. By also allowing the simultaneous use of multiple channels, the technology allows rates of up to 384Kbps, using all eight GSM channels. Because the basic infrastructure interfaces with the existing GPRS, GSM or TDMA infrastructure, the major vendors are the incumbent GPRS and GSM suppliers such as Ericsson, Nokia, Motorola and Alcatel.
To use GPRS, users specifically need:
• A terminal that supports GPRS (and with a MCL Client available)
• A subscription to a mobile telephone network that offers GPRS
• Use of GPRS must be enabled for that user. Automatic access to the GPRS may be allowed by some mobile network operators, others will require a specific opt-in
• IP Name and if required login/password
• Knowledge of how to send and/or receive GPRS information using their specific terminal
• A destination (remote server) to send or receive information through GPRS this is likely an Internet address.
• A destination that must be permanently connected to the Internet.
The GPRS technology provides the following benefits:• Enables the use of a packet-based air interface over the existing circuit-switched GSM network, which allows greater efficiency in the radio spectrum because the radio bandwidth is used only when packets are sent or received.• Supports minimal upgrades to the existing GSM network infrastructure for those network service providers who want to add GPRS services on top of GSM, which is currently widely deployed.• Supports data rates of about 115 Kbps, which is greater than the traditional 9.6 Kbps rate available in a circuit-switched connection.• Supports larger message lengths than Short Message Services (SMS).• Supports virtual private network (VPN)/Internet service provider (ISP) corporate site access.
To use GPRS, users specifically need:
Mobile phone or terminal that supports GPRS (existing GSM phones do NOT support GPRS)
A subscription to a mobile telephone network that supports GPRS;
Use of GPRS must be enabled for that user. Automatic access to the GPRS may be allowed by some mobile network operators, others will require a specific opt-in;
knowledge of how to send and/or receive GPRS information using their specific model of mobile phone, including software and hardware configuration (this creates a customer service requirement);
A destination to send or receive information through GPRS. Whereas with SMS this was often another mobile phone, in the case of GPRS, it is likely to be an Internet address, since GPRS is designed to make the Internet fully available to mobile users for the first time. From day one, GPRS users can access any web page or other Internet applications- providing an immediate critical mass of uses;
Having looked at the key user features of GPRS, lets look at the key features from a network operator perspective.
SIMPLE GPRS TECHNICAL OVERVIEW
As mentioned earlier GPRS is not a completely separate network to GSM. Many of the devices such as the base transceiver stations and base transceiver station controllers are still used. Often devices need to be upgraded be it software, hardware or both. When deploying GPRS many of the software changes can be made remotely.
There are however two new functional elements which play a major role in how GPRS works.
The Serving GPRS Support Node (SGSN) and the Gateway GPRS support node (GGSN).GPRS system architecture
GPRS as a new data service uses a packet-mode technique to transfer high-speed and low speed data and signaling in an efficient manner. GPRS optimizes the use of network and radio resources. Strict separation between the radio subsystem and network subsystem is maintained, allowing the network subsystem to be reused with other radio access technologies.
Network ArchitectureThe GSM Environment Today
Existing GSM networks (Phase 1or Phase 2) consist of a radio access network called a Base Station Subsystem (BSS), a core network solution referred to as a Network Switching Subsystem (NSS), and an Operation Subsystem (OSS). The BSS consists of Base Station Controllers (BSC) which are responsible for the radio resource control, Base Transceiver Stations (BTS) which handle ciphering, encoding, burst generation, radio frequency generation, etc. The Tran coder and Rate Adaptor Unit (TRAU) compresses 64 kpbs voice data to 13 kbps (Full Rate), 12.2 kbps (Enhanced Full Rate), and 5.6 kbps (Half Rate) and performs rate adaptation for data applications.
The NSS is made up of Mobile Services Switching Centers (MSC), which perform classical exchange tasks including traffic switching, flow control, and signaling data analysis. In cooperation with other network elements, the NSS handles mobile-specific tasks such as mobility management and authentication. Logically, MSCs may be either Visited MSCs (VMSC), which are responsible for all the mobile devices in its supply area, or Gateway MSCs (GMSC), which are the interworking nodes to the external public telephone networks. The Visitor Location Register (VLR) associated with the VMSC holds relevant subscriber data for all subscribers currently within the range of the VMSC - including international mobile subscriber identity (IMSI) and a record of subscribed services. The Home Location Register (HLR) supplies the VLRs with this data and supports the Mobile Terminating Calls (MTC). The
Authentication Center (AC or AuC) generates the Triplets (RAND, SRES, kc) necessary for the authentication of the subscriber. Finally, the optional Equipment Identity Register (EIR) is used to check the validity of the subscribers’ handheld. GSM networks are circuit switched and normally use SS7 for signaling and control information.GPRS Enhancements to the GSM Network
With the introduction of GPRS, both the BSS and the NSS must be enhanced to support the key features outlined above .Several new logical network elements enable the following high-level GPRS functions.
Network Access Control:
A set of procedures are defined in GPRS to control access to the network’s services and facilities. The subscriber may access the network via the air interface or an external packet data network. The operator can offer support for several protocols (X.25, IPv4, etc.) for access to external PDNs. The operator determines the extent to which services and access are restricted; six network access control functionalities are defined within the GPRS recommendations.
Registration:
The user and the services to which he or she has subscribed must be known at the HLR. This includes the packet data protocols (PDP) subscribed for, the external PDNs (so-called access points) he or she is allowed to use, and the addresses (X.25, IPv4, etc.) of the mobile device.
Authentication and Authorization:
These processes verify the subscriber’s right to access the network and to use a specific service. The accompanying procedures correspond to those used in GSM.
Admission Control:
When a subscriber requests a certain minimum amount of resources (quality of service) with a service, admission control checks whether they can be made available.
Message Screening:
This functionality is used to filter unsolicited and unauthorized messages/data to and from the subscriber. In GPRS Phase 1, this is only network controlled.
Packet Terminal Adaptation:
The maximum size of packets which can be transmitted via the GPRS network is limited to 1500 octets. Larger packets have to be segmented.
Billing Data Collection
Packet Routing and Transfer:
Routing is the process of determining the paths available for transmitting data packets from their source to their destination, selecting the most appropriate path and adapting datagram formats to fit the underlying transmission technology. If a connectionless network service is applied, datagram’s can take different routes between the same source and destination. Several functions are closely related to packet routing and transfer: Relay, Address Translation and Mapping, Encapsulation, Tunneling, Compression, Encryption, and Domain Name Server. This last function is used to translate logical names into the corresponding network element addresses. The logical name “Internet” can be translated so that the subscriber is connected to the closest network element providing Internet access.
Mobility Management:
Keeping track of subscribers’ locations is a crucial task in mobile networks. Instead of administrative sets of cells organized into
Location Areas, Routing Areas are introduced in GPRS. Each Routing Area is assigned to an SGSN.
Logical Link Management:
When running bursts applications, subscribers only require physical resources when sending or receiving data. While there is no transmission, these physical resources can be released and allocated to other subscribers. By doing so, higher resource efficiency can be realized on both the air interface and the transmission lines. But as long as the subscriber has not terminated the session, a logical link must continue to exist, so that downloads can be continued after a break, etc.
Radio Resource Management:
This function comprises three functional groups:
• Um Interface: realizes the “capacity on demand” concept.
• Um Tranx: Medium access, the packet multiplexing, error detection and correction of the packet switched traffic via the air interface must be controlled and combined with a flow control.
• Path Management: The operator can determine the maximum amount of packet switched traffic realized by a set of cells. Hence, only a certain capacity of transmission resources is necessary between the BSS and the packet switched part of the NSS. The data rate over an established link for an individual subscriber can fluctuate over time. Therefore, the subscribers’ traffic should be multiplexed on the transmission line between the BSS and NSS.
Network Management:
Existing Operation and Maintenance Systems must be enhanced to supply the operator with all the necessary information to guarantee smooth running of the GSM-GPRS network. This includes alarms, remote control and statistical data collection.
GPRS Network Element Overview
To prepare existing GSM networks for GPRS, six new network elements are introduced.
GPRS Mobile Station: On the subscriber side, new handhelds are necessary to handle packet switched traffic over the air interface. Three different classes are defined:
• Class A
mobiles can handle both GSM circuit services and GPRS packet switched services simultaneously. GSM and GPRS signaling and control are also carried out simultaneously.
• Class B
mobiles can handle both GSM and GPRS signaling, but only GSM or GPRS traffic can be transmitted at any one time. If for example, a subscriber accepts a circuit switched call while downloading information from the Internet, GPRS data transmission is interrupted. As soon as the voice call is terminated, the download continues since the logical link between the mobile and the GPRS network still exists.
• Class C
mobiles can handle either GSM or GPRS. If a mobile station is connected to a GSM call, it is not available for GPRS traffic (and vice versa).
Two of the new logical network elements are introduced to upgrade the BSS:
Packet Control Unit (PCU): The PCU is responsible for the capacity on demand feature. It decides which radio resources are dynamically allocated to packet switched and circuit switched use.
The Base Station Controller (BSC) then manages the radio resources allocated for circuit switched use, while the PCU manages radio resources for the GPRS traffic itself. This includes channel access control, channel bundling, and data packet segmentation and re-assembly. The Um-Management Function and the Um- Tranx-Function are implemented by the GPRS mobile station and the PCU.
The location of the PCU can be next to the SGSN (as stand alone unit), it can be next to or within the BSC cabinet, or at the BTS site.
Channel Codec Unit (CCU): The CCU implements the new coding schemes, power control, and timing advance procedures. In the beginning, most operators will only introduce the CS-1 and CS-2 codec’s, because they can normally be implemented with a BTS software upgrade. The CS-3 and CS-4 codec’s, however, require modifications to the BTS and may not be implemented as rapidly .In the NSS, a packet switched network is implemented parallel to the circuit switched domain. Three of the new logical network elements are introduced here.
GPRS introduces the following two new major network elements:• SGSN—Sends data to and receives data from mobile stations, and maintains information about the location of a mobile station (MS). The SGSN communicates between the MS and the GGSN. SGSN support is available from Cisco partners or other vendors.• GGSN—A wireless gateway that allows mobile cell phone users to access the public data network (PDN) or specified private IP networks. The GGSN function is implemented on the Cisco Systems' router.
Connectivity between the SGSN & GGSN
The connection between the two GPRS Support Nodes is made with a protocol called GPRS
Tunneling Protocol (GTP). GTP sits on top of TCP/IP and is also responsible for the collection of mediation and billing information. GPRS is billed on per megabyte basis unlike GSM. In practice the two GSN devices may be a single unit.
Serving GPRS Support Node (SGSN):
The SGSN is located on the same hierarchical level as the VMSC/VLR and performs similar tasks as outlined below. An SGSN is connected to the BSS, to neighboring SGSNs and GGSNs.
During the Network Access Control process, the SGSN is involved in the authentication and authorization procedures. The admission control procedure includes determination of QoS availability.
• Mobility Management is realized based on the same principles as in the MSC/VLR. Note: There is a database in the SGSN which realizes the same tasks as the VLR, but is not a logical network element of its own.
• The SGSN is responsible for switching traffic to the BSS and the network elements which establish the interconnection to external PDNs. An SGSN thus also performs the tasks of an ordinary (packet) router.
• As several subscribers can be dynamically multiplexed onto a single timeslot, ciphering can no longer be performed by the BTS (as in circuit switched transmission) and is thus outsourced from the BTS to the SGSN. User data must be compressed before it is encrypted, so compression is also performed in the SGSN. On the other end, the same functions are performed by the GPRS mobile station.
• The domain name server is logically associated with the SGSN.
• Logical link management is realized between the SGSN and the mobile station, independent of the radio access system. A logical link between the SGSN and the handheld can be maintained even if there are no physical resources in use. Logical link management includes establishment, maintenance, and release.
• Physical resources are managed between the SGSN and BSS (PCU) as part of the path management.
• Billing information and statistical data are collected at the SGSN.
• Interfaces to the BSS (PCU), the GGSNs, neighboring SGSNs, HLRs, EIRs, SMS-Centers, other PLMNs and the MSC/VLR are specified.
Gateway GPRS Support Node (GGSN): The GGSN is the interworking node between the external packet data networks and the packet switched part of the Network Switching Substation. It is located on the same hierarchical level as the GMSC in GSM networks and performs comparable tasks.
• The GGSN is responsible for the packet routing and transfer procedures.
The SGSNs and GGSNs are connected via an IP backbone. IPv4 can be implemented initially, but on the long run IPv6 shall be put into action.
• The GGSN is involved in the Mobility Management process: When a call is placed to a mobile handheld, the GGSN sends a request to the Home Location Register to determine the SGSN currently serving the subscriber.
• In GPRS Phase 1 it is responsible for the network orientated screening.
• Billing information and statistical data are collected at the GGSN.
• Interfaces to the SGSNs, external PDNs, and HLRs are specified.
HLR Extension: The HLR must be extended to store new subscriber data associated with packet switching. While the GGSNs and SGSNs are new hardware elements, the HLR Extension normally is just a software upgrade. The HLR (Extension) is involved in
• Registration, authentication and authorization processes,
• ciphering (cipher key) and
• Mobility management.
GPRS Interface Overview
In addition to the GSM interfaces, nine new interfaces are specified in GPRS, all of them beginning with the letter “G”. The GPRS reference model of the European Telecommunications Standards Institute (ETSI) in Figure shows the basic GSM and GPRS network elements and the interfaces specified between them.
As shown in the figure, both the GSM circuit switched network and the GPRS packet switched network are connected to the BSS. Additional external networks can also be seen: The GPRS NSS part is connected to external packet data networks, while the GSM NSS part is attached to external PSTNs. Additionally the GPRS network can be directly connected to GGSNs in other operators’ mobile networks. The solid lines represent all GPRS interfaces which are used both as transmission and signaling planes, while the dotted lines indicate interfaces used only as signaling planes.
The central task of GPRS is the transmission of packet data from external networks to the mobile and vice versa. When a user data packet arrives at the GGSN, it must be routed to the SGSN in the supply area where the subscriber is currently located. The packet is then delivered to the BSS for transmission via the air interface. Finally, the packet arrives at the mobile station, where higher level applications process the user data packet.
Hence following transmission and signaling planes are mandatory for GPRS:
The Gi interface is the reference point between the external PDNs and the GPRS network. The network operator and the external ISP must agree on the transmission technology (layer 1 and 2) used to connect their packet data networks. Interworking with the X.25 and IPv4 protocols is specified; extensibility to future protocols is given.
Once the GGSN has accepted the packet, it determines the SGSN where the subscriber is currently located and transmits the packet to the SGSN via the Gn interface. IP routing is used between GPRS Support Nodes. The user data must be transparently transferred between the external packet data network and the GPRS mobile station. The transmission is split into two segments between the GGSN and SGSN, and the SGSN and GPRS mobile station. Methods known as encapsulation and tunneling are applied. The user data packet is equipped with Gn-specific protocol information which reduces the amount of interpretation of the user data packet by the GPRS network and enables an easy introduction of future interworking protocols.
Note: the Gn interface is also defined between different SGSN of the same GPRS network.
When the user data packet arrives at the SGSN, it must be transmitted to the GPRS mobile station. The logical link to the GPRS mobile station is managed via the Gb interface. In order
The perform the physical transmission; the SGSN has to be connected to the BSS (PCU). The PCU receives instructions as to the quality of service with which the user data packet is to be transmitted via the air interface. This information is also sent over the Gb interface from the SGSN to the BSS (PCU).
There are two additional interfaces over which both transmission and signaling planes are implemented, though both are optional and often unnecessary for a basic GPRS network:
The Gp interface is used between the SGSN and the GGSN in another operator’s network. In its functionality it is quite similar to the Gn interface.
• The Gd interface is specified between the SGSN and an SMS gateway (SMS-GMSC). This interface is based on the SS7 protocol stack and enables the GPRS network to transmit long SMS messages.
In addition to the aforementioned interfaces, four pure signaling interfaces were defined in the ETSI GSM recommendations. The first three are connections to the registers - their protocol stacks are an enhancement of the GSM interfaces to the databases.
• The Gr interface between the SGSN and the HLR is the only mandatory interface of these 4 interfaces. It is based on the SS7 MTP, SCCP, TCAP, and MAP signaling stacks. If a subscriber appears in the supply area of an SGSN, the SGSN can request subscriber information from the HLR via the Gr interface.
• The Gc interface between the GGSN and the HLR is optional. If the first user data packet arrives at the GGSN and the subscriber has a fixed address, the subscriber’s location must be retrieved from the HLR. The Gc interface offers a direct path for this query. If this interface does not exist, the request can be sent via the Gn interface to a home SGSN,which then forwards the request to the HLR via the Gr interface. The routing information is then delivered by the HLR to the SGSN, which passes it on to the GGSN.
• The Gf interface from the SGSN to the EIR is not mandatory, as the EIR is optional in both GSM and GPRS networks.
The fourth optional signaling interface is a connection between the MSC/VLR and the SGSN.
• The Gs interface between the SGSN and the MSC/VLR can be used for common procedures like location updates. If, for example, a subscriber moves from one Location Area to another
Location Area, then both location and routing area must be updated. If the Gs interface does not exist, both update procedures must be performed separately over the air interface. If the interface is present, a GPRS routing update can be initiated, and the SGSN informs the
MSC/VLR that a location updates must also be initiated. The use of the Gs interface thus conserves valuable resources on the air interface. The Gs interface is a strongly reduced version of the A interface protocol stack.
Protocol Layers
Overview
The layered protocol structure realized over the GPRS interfaces distinguishes between transmission and signaling planes. Transmission planes transfer user information, associated with transfer control information such as error correction, error recovery, flow control, multiplexing and de-multiplexing, and segmentation and re-assembly. The NSS platform is based on a packet switched IP backbone, and is kept independent of the BSS and the radio interface using the Gb interface.
Operators interested in migrating their networks to UMTS in the future can reuse investments in the SGSNs, GGSNs, and the transmission network in between.
A logical connection between the GPRS mobile station and the SGSN is maintained using the Logical Link Control Layer (LLC). Above this layer,
Sub network Dependent Convergence Protocol packets can be transmitted. LLC packets are transparently transmitted between the GPRS mobile station and SGSN.
HLR
The HLR or Home Location Register is a database that contains subscriber information, when a device connects to the network their MSISDN number is associated with services, account status information, preferences and sometimes IP addresses.
Classes of GPRS services
Mobile devices can request different types of traffic to be prioritised in an attempt to give the user their desired level of connectivity. There are 4 types of class:
Precedence Class
An application can be assigned a Precedence Class 1, 2 or 3. If an application has a
Higher precedence (1) than another (3) then its traffic will be given a higher priority.
Delay Class
Applications can request predictive delay classes which guarantee an average and 95-percentile delay. There are 4 classes, 1 being the fastest.
Reliability class
Applications can request differing levels of reliability for its data depending on its tolerance to data loss.
Throughput Class
Applications can choose different profiles for throughput. There are 2 distinctions in class, peak and mean. Peak throughput class is used mainly for burst transmissions with a variable in octets per second describing the throughput required for burst of specified size. Mean is the average data transfer rate over a period of time measured in octets per hour.
GPRS QoS
Just because GPRS uses many of the components of a standard GSM network it would be foolhardy to assume that the same standards should apply. Things to be taken into account include provider general network architecture, radio interface and throughput. Here are some of the key elements briefly explained.
Network Architecture
Provider networks have to be upgraded. As mentioned earlier the GSN’s are new to the standard GSM network. If GPRS is to stand-up to customer expectations network performance will be vital.
Radio Interface
The ETSI (European Telecommunications Standard Institute) has defined 3 new coding schemes for Radio Interface. When the GPRS device talks to the base station they can use 1 of the 4 schemes. The schemes are CS – 1 through CS – 3 where CS – 1 is the same as standard GSM.
In simple terms CS – 1 is highly redundant but because of this is slow, 2 and 3 have less redundancy, whilst 4 has the least - removing all forward error control - but is capable of maximum throughput. If radio quality is bad then coding scheme 1 is used, as the quality improves less error control is needed.
GPRS with GSM
GPRS is a new service designed for Global System for Mobile Communications (GSM) networks. GSM is a digital cellular technology that is used worldwide, predominantly in Europe and Asia, with current estimates of 400 million subscribers and growing. GSM is the world's leading standard in digital wireless communications
GSM system architecture
A GSM system has two major components: the fixed installed infrastructure (network) and the mobile stations (MS). The mobile subscribers use the services of the network and communicate over the radio interface. GSM together with other technologies is part of an evolution of wireless mobile telecommunication that includes High-Speed Circuit-Switched Data (HSCSD), General Packet Radio System (GPRS), Enhanced Data GSM Environment (EDGE), and Universal Mobile Telecommunications Service (UMTS). Fig below illustrates the GSM system architecture
Mobile station (MS)
The MS consists of two major components: the Mobile Equipment (ME) and SIM module.
Fixed Network
The fixed installed GSM network can be subdivided into three subsystems. the BSS (Base Station Subsystem), the SMSS (Switching and Management Subsystem), and the OMSS (Operation and Maintenance Subsystem).
The BTS (Base Transceiver Station) and the BSC (Base Station Controller) together form the BSS. A cell is formed by the radio coverage of a BTS. The BTS provides the radio channels for signaling and user data traffic in a cell. Several BTS can be controlled by one BSC.
The SMSS consists of the mobile switching centers (MSC) and the databases which store the data required for routing and service provision. The MSC performs all the switching functions of a fixed-network switching node. A GSM PLMN has several databases. The SMSS consists of two databases, i.e. the HLR (Home Location Register) and the VLR (Visitor Location Register). The HLR stores all permanent subscriber data and the relevant temporary data of all subscribers permanently registered in the HLR. The IMSI (International Mobile Subscriber Identity) and authentication data are stored in it. The VLR stores the data of all MSs which are currently staying in the administrative area of the associated MSC.
The ongoing network operation is controlled and maintained by the OMSS. Network control functions are monitored and initiated from an OMC (Operation and Maintenance Center) . Two more databases are defined in this subsystem. They are responsible for various aspects of system security. System security of GSM networks is based primarily on the verification of equipment and subscriber identity. These databases serve for subscriber identification and authentication and for equipment registration. Confidential data and keys are stored and generated in the AUC (Authentication Center). The EIR (Equipment Identity Register) stores the serials numbers (supplied by the manufacturer) o the terminals (IMEI).
SIM
A personal chip card SIM can be a fixed installed chip (plug-in SIM) or an exchangeable SIM module. The SIM is a secure microprocessor-based environment implemented on a credit-card-sized platform with on-board non-volatile memory. Two types of SIM cards are used in GSM,
i.e. ID-1 and plug-in card .
There are three types of memory, i.e., ROM, RAM, and EEPROM. The ROM contains the operating system, the applications, and security algorithms A3 and A8, which implements important functions for the authentication and user data encryption based on the subscriber identity IMSI and secretes RAM is used to buffering transmission data and executing. The EEPROM consists of subscriber identification (IMSI, PIN), call number (IMSI and MSISDN), network-related information (TMSI, LAI), and the equipment identifier IMEI.
The security features supported by the SIM are authentication of the subscriber identity to the network, data confidentiality over the air interface, and file access conditions. The first two features are presented in section 4. The SIM can support five access conditions. One of the access conditions is PIN which is used to control user access to the SIM. If the subscriber typed three incorrect PIN code, the SIM will be blocked.
Therefore, use of the SIM, the whole of MS can be protected together with PIN again unauthorized access.
Identities
In this section, some identities related to GSM security are introduced. The associationof the most important identifiers and their storage locations are summarized as follows.
Subscriber is identified by IMSI, MSISDN, TMSI, MSRN.
Mobile Equipment is identified by IMEI; IMSI, MSISDN, and MSRN are stored in HLR. The LMSI, MSRN, IMSI, TMSI, MSISDN, and LAI are stored in VLR; The IMSI, RAND, SRES, Ki, Kc are stored in AUC.IMEI is stored in EIR.
When registering for service with a mobile network operator, each subscriber receives a unique identifier, the IMSI (International Mobile Subscriber Identity). This IMSI is stored in the SIM. A mobile station can only be operated, if a SIM with a valid IMSI is inserted into equipment with a valid IMSI, since this is the only way to correctly bill the appropriate subscriber. The IMSI consists of several parts: mobile country code (MCC).3 decimal digits, internationally standardized; mobil Network code (MNC).2 decimal digits, for unique identification of mobile networks within a country; Mobile Subscriber Identification Number (MSIN). Maximum 10 decimal digits, identification number of the subscriber in his mobile home network.
MSISDN: The real telephone number of MS is MSISDN (Mobile Subscriber ISDN Number).
The VLR responsible for the current location of a subscriber can assign a TMSI (Temporary
Mobile Subscriber Identity) which has only local significance in the area handled by the VLR. It is used in place of the IMSI for definite identification and addressing of the
MS. This way nobody can determine the identity of the subscriber by listening to the radio channel, since this TMSI is only assigned during the mobile stations presence in the area by the network operator who stores it in EIR.
GPRS different to GSM
GPRS is different to GSM because it offers the following key features:
Higher bandwidth and, therefore, data speeds
Seamless, immediate and continuous connection to the Internet - 'always on-line'
New text and visual data and content services (due to data speeds and the Internet), such as email, chat, still and moving images, information services (stock prices, weather reports, train times), video conferencing, e-commerce transactions (buying flight and cinema tickets) and Internet-based remote access to corporate intranets and public networks (rather than dial-up remote access which incurs long distance phone calls)
Packet-switching rather than circuit-switching, which means that there is higher radio spectrum efficiency because network resources and bandwidth are only used when data is actually transmitted even though it is always connected
Different mediation, rating and billing requirements such as collecting records from GPRS and IP networks, charging for volumes of data transferred rather than connection time and new and multiple members of the billing value chain
Support for leading Internet communications protocols - Internet protocol (IP) and X. 25
Additional components and protocols to the GSM network - the key elements are SGSN (serving GPRS support node), GGSN (gateway GPRS support node) and a charging gateway
Different devices (not GSM phones) - GPRS will be available from laptops or handheld computers that are either connected to GPRS-capable cellular phones, external modems or that have PC card modems, smart phones that have full screen capability and cellular phones that have WAP micro browsers. All of these devices have user interfaces that will allow users to utilize GPRS service the first important step on the path to 3G.
GSM (Global System for Mobile) - is known as a 2G (second generation) digital. GSM has maximum data speeds of 9.6 kbit/s and is based on circuit switching technology.
HSCSD (High Speed Circuit Switched Data) -the first step towards faster data speeds on GSM circuit switched networks. HSCSD concentrates up to four GSM timeslots and allows data.
GSM and GPRS Security Functions
This section gives a detailed description of the three GSM and GPRS security functions.
Subscriber Identity Confidentiality
The purpose of this function is to avoid an intruder to identity a subscriber on the radio path (e.g. Traffic Channel or signaling resources) by listening to the signaling exchanges. This function can be achieved by protecting the subscriber’s IMSI and any signaling information elements. Therefore, a protected identifying method should be used to identify a mobile subscriber instead of the IMSI on the radio path. The signaling information elements that convey information about the mobile subscriber identity must be transmitted in ciphered form. And also a ciphering method is used.
Identifying method.
The TMSI is used in the method. It’s a local number and only valid in a given location area. The TMSI must be used together with the LAI to avoid ambiguities.
The network manages the databases (e.g. VLR) to keep the relation between TMSIs and IMSIs
Location updating in the same MSC area.
In this case, the original and new location area are controlled by the same MSC. The TMSI is issued by the VLR.
Location updating in a new VLR, old VLR reachable.
This case happens when the original location area and the new one depend on different VLRs. The MS is still registered in VLR old and requests registration in VLR new.
Subscriber Identity Authentication
This function can be triggered by the network whenever one of the following events happens
Subscriber applies for a change of subscriber-related information element in the VLR or HLR . The subscriber-related information element includes location updating involving change of VLR), registration, or erasure of a supplementary services).
Subscriber accesses to a service. The service may be setting up mobile originated or terminated calls, activation or deactivation of a supplementary service.
Subscriber accesses to the network for the first time after restarting of MSC/VLR.
The cipher key sequence number mismatch.
Confidentiality of Signaling information elements, connectionless user data, and user information on Physical Connections
GSM confidentiality
The signaling information elements related to the user, such as IMEI, IMSI, and Calling
Subscriber directory number (mobile terminated or originated calls) need to be protected after connection establishment.
GPRS confidentiality
GPRS network still needs this security feature. However the ciphering scope is different. The scope of GSM is between BTS and MS. The scope of GPRS is from the SGSN to the MS. A new ciphering algorithm GPRS-A5 is used because of the nature of GPRS traffic. The ciphering is done in the Logical Link Control (LLC) layer.
ROAMING
Roaming is a general term in wireless telecommunications that refers to the extending of connectivity service in a location that is different from the home location where the service was registered. The term "roaming" originates from the GSM (Global System for Mobile Communications Association) sphere. Traditional GSM Roaming is defined (cf. GSM Association Permanent Reference Document) as the ability for a cellular customer to automatically make and receive voice calls, send and receive data, or access other services, including home data services, when travelling outside the geographical coverage area of the home network, by means of using a visited network. This can be done by using a communication terminal or else just by using the subscriber identity in the visited network. Roaming is technically supported by mobility management, authentication, authorization and billing procedures.
In the GSM system the mobile station consists of two parts, the Mobile Equipment (ME) and Subscriber Identity Module (SIM). With the current standardized SIM-ME interface, the user is able to roam between the Public Land Mobile Networks (PLMNs), which have different air interfaces. This is called SIM-roaming. It enables the user to have one subscription and be reached trough the same directory number regardless of the network she is roaming into. Instead of carrying her mobile station, the user can take only her subscriber card (SIM card) with her and use it in different mobile equipment to access to a desired network The Figure 1 shows the difference between MS-roaming and SIM-roaming. The European Telecommunications Standards Institute (ETSI) has standardized the interfaces to allow roaming. The interfaces are marked with two-headed arrow lines.
SIM and MS roaming and the standardized interfaces.
Roaming can be divided also into intra-PLMN roaming and inter-PLMN-roaming according to who are the roaming partners. Inter-PLMN roaming means roaming between two different mobile operators and intra-PLMN roaming stands for roaming inside one operator. The roaming between operators can further be split into national and international roaming. When roaming is discussed in this paper, it refers to the international inter-PLMN roaming unless otherwise mentioned.
General Packet Radio Service
The GPRS architecture overview
The existing GSM network does not provide adequate functionality to support packet data routing. Therefore GPRS introduces two new logical network entities in the GSM PLMN: GPRS Support Nodes (GSNs) called Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN). The functionality of GGSN and SGSN is quite closely related to functionality of the home agent and the foreign agent in IETF Mobile IP, respectively .
The logical architecture of the GPRS network
GPRS backbone networks
There are two kinds of GPRS backbone networks as the Figure 3 shows. They are called intra-PLMN backbone network and inter-PLMN backbone network.
Intra and Inter PLMN backbones
The intra-PLMN backbone network is the IP network interconnecting GSNs within the same PLMN. The inter-PLMN backbone network is interconnects GSNs and intra-PLMN backbone networks in different PLMNs. Every intra-PLMN backbone networks is a private IP network intended for GPRS data and signaling only and there must be some access control mechanism in order to achieve a required level of security. Two intra-PLMN backbone networks are connected via the Gp interface using the Border Gateways (BGs) and an inter-PLMN backbone networks.
The BG functionality is not defined in the GPRS standards but it should include security and access control mechanisms. The inter-PLMNn backbone can be e.g. the public Internet or a leased line.
GPRS Roaming
Packet transmission
When a subscriber is roaming into another PLMN, the Visited PLMN (VPLMN), she has to first attach to the network. In GPRS attach MS informs SGSN its willingness to connect with the network. The MS reports information about its identity, capability and location to the SGSN. SGSN then checks the MS’s identity and does the authentication procedures to be able to secure the transmission path. The attach is completed after SGSN has received the roaming subscriber data from HLR of the subscriber’s Home PLMN (HPLMN) and finished the location update procedure.
After GPRS attach the MS sends an Activate PDP Context Request to SGSN. GPRS subscriber data contains one or more Packet Data Protocol (PDP) contexts per International Mobile Subscriber Identity (IMSI), which explicitly identifies the subscriber. The PDP context contains the type and address of the PDP (e.g. IP address), QoS profile subscribed, a record which specifies whether the MS is allowed to use the Access Point Name (APN) in the domain of the HPLMN only, or additionally the APN in the domain of the VPLMN. The APN is a reference to the GGSN to be used in GPRS backbone or it defines the access point to the external network. The PDP context is part of the subscriber data that is fetched from HLR to SGSN during GPRS attach. The SGSN sends the context data to GGSN in charge. The GGSN then handles the packet
When a GGSN gets Mobile Terminated (MT) packets, it has to activate the PDP context if there is no active context available. The GGSN fetches the routing information from HLR and sends a request to SGSN. SGSN then informs the MS and after that the attach procedure is same as previously explained
Transmission in roaming situation
Basically the same procedure is done when subscriber is roaming in VPLM. The difference is whether the GGSN of VPLM is used or do the packets travel through the Home PLMN (HPLMN). Figure shows the general model for GPRS networkinterworking
General interworking between GPRS networks to support roaming subscribers
New roaming services key to wireless
The wireless world is moving toward 3G high-speed networking, but several obstacles must be overcome before users can transmit data at 384K bit/sec using their wireless handsets. Just as wireless service providers need to deploy new hardware and software to support data transmissions that are 20 times as fast as the speediest mobile service today, they will also have to map out roaming and interconnection agreements.
Network operators are scrambling to make connections that will let these next-generation wireless networks deliver the same level of domestic and international roaming service as standard wireless voice networks such as GSM or Code Division Multiple Access (CDMA). While CDMA networks are more popular in the U.S., there are several GSM wireless service providers in North America. And the GSM numbers in the U.S. are set to grow as AT&T Wireless upgrades its dated Time Division Multiple Access network to GSM-based 3G technology called general packet radio services (GPRS).
GPRS is an interim technology that is typically called 2.5G because it supports up to 144K bit/sec instead of the 384K bit/sec support, which by definition is a requirement for any 3G network. Service providers say they are ready for GPRS, but have yet to address how they can support regional or international roaming that will let users make and receive calls as well as transmit data.
The three main applications of the GPRS roaming are as follows
WAP Access
WEB Access (Internet Access, Browsing & E-mail over GPRS)
MMS (Peer to Peer while Roaming)
Wireless Roaming
Ideal for applications where vehicles must move beyond the range of a single hub radio.
All the hubs are typically connected, through a backbone network, back to a central site. This backbone network can be wired or wireless. The overall system supports the following features:
Mobile nodes automatically attach to the strongest hub.
As a mobile unit moves and the link to its hub fades, the mobile radio changes autonomously to attach to a stronger hub.
Connectivity to a central site, through a backbone network, is maintained when a mobile changes hubs. Packet routing is switched over autonomously throughout the network so that packets are correctly routed immediately after the mobile radio changes hubs.
Hub radios can be used to support the roaming operation and also be part of the backbone network.HSCSD AND EDGE ARE USED IN GPRS
Ideal for applications where vehicles must move beyond the range of a single hub radio.
All the hubs are typically connected, through a backbone network, back to a central site. This backbone network can be wired or wireless. The overall system supports the following features:
Mobile nodes automatically attach to the strongest hub.
As a mobile unit moves and the link to its hub fades, the mobile radio changes autonomously to attach to a stronger hub.
Connectivity to a central site, through a backbone network, is maintained when a mobile changes hubs. Packet routing is switched over autonomously throughout the network so that packets are correctly routed immediately after the mobile radio changes hubs.
Hub radios can be used to support the roaming operation and also be part of the backbone network.HSCSD AND EDGE ARE USED IN GPRS
High Speed Circuit Switched Data (HSCSD)
GSM Circuit Switched Data supports one user per channel per time slot. High Speed Circuit Switched Data (HSCSD) gives a single user simultaneous access to multiple channels (up to four) at the same time. As such, there is a direct trade-off between greater speed and the associated cost from using more radio resources- it is expensive for end users to pay for multiple simultaneous calls.
Assuming a standard Circuit Switched Data transmission rate of 14.4 kilobits per second (kbps), using four timeslots with High Speed Circuit Switched Data (HSCSD) allows theoretical speeds of up to 57.6 kbps. This is broadly equivalent to providing the same transmission rate as that available over one ISDN B-Channel. Some Mobile Switching Centre’s (MSCs) are limited to 64 kbps maximum throughput- this restriction is removed with GPRS.
In networks where HSCSD is deployed, GPRS may only be assigned third priority, after voice as number one priority and HSCSD as number two. In theory, HSCSD can be preempted by voice calls- such that HSCSD calls can be reduced to one channel if voice calls are seeking to occupy these channels. HSCSD does not disrupt voice service availability, but it does affect GPRS. Even given preemption, it is difficult to see how HSCSD can be deployed in busy networks and still confer an agreeable user experience- i.e. continuously high data rate. HSCSD is therefore more likely to be deployed in start up networks or those with plenty of spare capacity- since it is relatively inexpensive to deploy and can turn some spare channels into revenue streams. High Speed Circuit Switched Data (HSCSD) is however easier to implement in mobile networks than General Packet Radio Service (GPRS) because some GSM vendor solutions require only a software upgrade of base stations and no new hardware. This is not the case with D-AMPS networks and some GSM vendor solutions.
There are a couple of reasons why HSCSD may be the preferred bearer for certain applications when compared to GPRS. The fact that associated packets can be sent in different directions to arrive at the same destination should in theory make the transmission more robust since there are many different ways of achieving the end result. However, this nature of packet transmission means that packets are subject to variable delay and some could be lost. Whilst packet retransmission is incorporated into the GPRS standards, naturally this process does take time and in the case of applications such as video transmission can cause poor quality images.
Another preferred application for HSCSD could be the fact that whilst GPRS is complementary for communicating with other packet-based networks such as the Internet, HSCSD could be the best way of communicating with other circuit switched communications media such as the PSTN and ISDN. HSCSD is mainly supported by Nokia with little success.
EDGE, Enhanced Data GSM Environment
Enhanced Data rates for Global Evolution (EDGE) is a radio based high-speed mobile data standard. It allows data transmission speeds of 384 kbps to be achieved when all eight timeslots are used. In fact, EDGE was formerly called GSM384. This means a maximum bit rate of 48 kbps per timeslot. Even higher speeds may be available in good radio conditions.EDGE was initially developed for mobile network operators who fail to win Universal Mobile Telephone System (UMTS) spectrum. EDGE gives incumbent GSM operators the opportunity to offer data services at speeds that are near to those available on UMTS networks.
EDGE can also provide an evolutionary migration path from GPRS to UMTS by implementing now the changes in modulation that will be necessary for implementing UMTS later. The idea behind EDGE is to eke out even higher data rates on the current 200 kHz GSM radio carrier by changing the type of modulation used, whilst still working with current circuit (and packet) switches.
Implementation of EDGE by network operators has been designed to be simple. Only one EDGE transceiver unit will need to be added to each cell. With most vendors, it is envisaged that software upgrades to the BSCs and Base Stations can be carried out remotely. The new EDGE capable transceiver can also handle standard GSM traffic and will automatically switch to EDGE mode when needed.
EDGE capable terminals will also be needed- existing GSM terminals do not support the new modulation techniques and will need to be upgraded to use EDGE network functionality. Some EDGE capable terminals are expected to support high data rates in the downlink receiver only (i.e. high dates rates can be received but not sent), whilst others will access EDGE in both uplink and downlinks (i.e. high data rates can be received and sent). The later device types will therefore need greater terminal modifications to both the receiver and the transmitter parts.
In addition, the TDMA industry association, the Universal Wireless Communications Corporation, has introduced what it calls EDGE Compact. This a spectrum efficient version of EDGE that will support the 384 kbits mandated packet data rates but will require only minimum spectral clearing and therefore could work for network operators with limited spectrum allocations. In fact, as a result of this, EDGE has been renamed Enhanced Data Rates for GSM and TDMA Evolution.
EDGE provides speed enhancements by changing the type of modulation used and making a better use of the carrier currently used, for example the 200kHz carrier in GSM systems. EDGE also provides an evolutionary path to third-generation IMT-2000-compliant systems, such as universal mobile telephone systems (UMTS), by implementing some of the changes expected in the later implementation in third-generation systems.
EDGE builds upon enhancements provided by general packet radio service (GPRS) and high-speed circuit switched data (HSCSD) technologies that are currently being tested and deployed. It enables a greater data-transmission speed to be achieved in good conditions, especially near the base stations, by implementing an eight-phase-shift keying (8 PSK) modulation instead of Gaussian minimum-shift keying (GMSK).
SPECTRUM EFFICIENCY
Packet switching means that GPRS radio resources are used only when users are actually sending or receiving data. Rather than dedicating a radio channel to a mobile data user for a fixed period of time, the available radio resource can be concurrently shared between several users. This efficient use of scarce radio resources means that large numbers of GPRS users can potentially share the same bandwidth and be served from a single cell. The actual number of users supported depends on the application being used and how much data is being transferred. Because of the spectrum efficiency of GPRS, there is less need to build in idle capacity that is only used in peak hours. GPRS therefore lets network operators maximize the use of their network resources in a dynamic and flexible way, along with user access to resources and revenues.
GPRS should improve the peak time capacity of a GSM network since it simultaneously:
Allocates scarce radio resources more efficiently by supporting virtual connectivity.
Immigrates traffic that was previously sent using Circuit Switched Data to GPRS instead, and reduces SMS Center and signaling channel loading by migrating some traffic that previously was sent using SMS to GPRS instead using the GPRS/ SMS interconnect that is supported by the GPRS standards.
Relatively high mobile data speeds may not be available to individual mobile users until Enhanced Data rates for GSM Evolution (EDGE) or Universal Mobile Telephone System (UMTS) are introduced.
GPRS and EDGE Technologies
GPRS simply is an extension of the GSM standard to provide packet data services. GSM uses TDMA technology that allows eight users on one 200-kHz RF channel. These users are assigned to slots, with each slot’s duration in time being 577 µs. Together, all eight slots become a frame with a corresponding duration of 4.615 ms. We also know from some previous calculations on the corresponding channel capacity that the number of bits per time slot or burst is equal to 147 with GMSK modulation.
In a voice mode, the user always would be assigned one slot for transmission of voice data. The same is true for a form of wireless data service called circuit-switched data. With circuit-switched voice or data, we take advantage of the frame system used in a number of telephone networks to transmit the data. You are assigned a time slot, and that time slot always is yours to use whether there is data present or not.
With a GSM system, voice or data is sent during each slot every frame. This means that the phone is ramping up and sending data whether data is present or not and then ramping down. Circuit-switched operation is inherently inefficient because a slot always is assigned whether or not the mobile has any information to send.
Recently, designers of these mobiles have recognized this, and to save battery life, they have developed a means to shorten the burst using a method of detecting whether information is present and then discontinuing the burst. This is known as DTX. This is only a battery-saving function since the mobile still is always assigned to that particular slot.
Things change dramatically with packet data services such as GPRS or EDGE. These systems allow the mobile to transmit any number of slots either consecutively or within a frame as directed by the network. This permits the system operator to take advantage of dead air time associated with circuit-switched networks to increase capacity and data rates. GPRS and EDGE data sessions are not circuit switched and, for that reason, require a new package routing network to interface to the IP.
GPRS and EDGE Modulation Formats
GPRS is an evolution of the existing GSM modulation and channel formats. The modulation scheme is the same as for GSM and called GMSK. The GMSK digital modulation format relies on gently shifting the carrier 180° in phase to produce a binary modulation scheme capable of delivering 1 bit/symbol.
GPRS Channel Coding
Channel coding in a GPRS or EDGE system protects the data that is being transported across the air interface and is implemented to correct errors in the bit stream caused by the RF environment. GPRS and EDGE use different convolutional encoding and decoding processes, bit-puncturing schemes, and interleaving processes to account for different RF channel conditions and QOS requirements.
This is required because the air interface can be highly destructive to the RF channel. If we are to realize the benefits of higher data rates, then we want to provide as little encoding as possible to preserve the data being transferred. But we also need to ensure that we do not drop the call when conditions suddenly change due to fading.
Below the Figure shows an example of a channel-coding scheme for EDGE and indicates the relationship among the various processes. Bits of the data stream are removed or punctured to reduce the size of the bit stream using a predetermined mask for that particular channel-coding scheme. The data sent between the MS and the network is not a 1:1 burst-to-block conversion.
The data in each burst is broken into subsections and dispersed into the RLC blocks using a predetermined mapping scheme across multiple bursts in a process called interleaving. Finally, convolutional coding on the bit stream uses redundancy bits so that a decoder can detect errors in the bit stream and correct them.
The scheme shown in Figure is MCS-7, which uses the 3/8 8PSK modulation scheme. Channel-coding schemes used in GPRS and EDGE must be bidirectional, meaning that they are encoded and decoded the same way. If we look at this example from a decoding point of view, you can see that we are taking four consecutive bursts and deinterleaving the information or symbols.
After deinterleaving, the bits are depunctured and sent to a 1/3 rate convolutional decoder. Further parsing these bits provides us with the RLC blocks that are the basis for all of our signaling or protocol needed to set up an EDGE call.
While GPRS uses the same modulation scheme as GSM, there are four different channel-coding schemes to provide varying levels of protection to the packets either going or coming from the mobile. Table 1 (see below) shows the impact that different channel-coding schemes and the number of slots can have on effective data rates.
Table 1. GPRS Data Rates for Channel-Coding Schemes 1 through 4
Channel Coding Scheme
Slot Combinations
1 Slot
4 Slots
8 Slots
CS1
9.2 kb/s
36.8 kb/s
73.6 kb/s
CS2
13.55 kb/s
54.2 kb/s
108.4 kb/s
CS3
15.75 kb/s
63 kb/s
126 kb/s
CS4
21.55 kb/s
86.2 kb/s
172.4 kb/s
First-generation GPRS systems will use four slots in the downlink (base station to mobile) and one or two slots in the uplink (mobile to base station) direction to transfer data. This is due to the fact that during a data session, typically more information is received than transmitted.
EDGE uses nine channel-coding schemes separate from GPRS schemes CS1 through CS4. In Table 2 (see below), we see a sample of a few of the corresponding data rates for channel-coding schemes MCS1-4 that use GMSK and MCS5-9 using 3/8 8PSK modulation. There are dramatic increases in the data rates from 1 bit/symbol GMSK modulation to 3 bits/symbol data rates of 3/8 8PSK and the difference that channel coding (protection) can make in effective overall data rates.Table 2. EDGE Data Rates Using 3/8 8PSK Modulation vs. Different Channel-Coding Schemes and Slot Combinations
Channel Coding Scheme
Modulation
Slot Combinations
1 Slot
4 Slots
8 Slots
MCS1
GMSK
8.8 kb/s
35.2 kb/s
70.4 kb/s
MCS4
GMSK
17.6 kb/s
70.4 kb/s
140.8 kb/s
MCS5
8PSK
22.4 kb/s
89.6 kb/s
179.2 kb/s
MCS9
8PSK
59.2 kb/s
236.8 kb/s
473.6 kb/s
GPRS and EDGE have much in common with conventional GSM systems. They share the same slot structure and timing and the same 200-kHz channel bandwidth. However, significant differences exist between a GSM voice system that is circuit switched and a GPRS or EDGE system that is packet routed.
GPRS 8 PSK
A method is disclosed for operating a wireless mobile station to receive a time slot sent through a radio channel from a transmitter. In a GPRS embodiment the time slot has a Header portion and a Data portion, wherein the Header portion contains information for specifying whether the Data portion is modulated using 8-PSK modulation or π/4-shifted DQPSK modulation. When modulated using 8-PSK modulation, the Data portion includes a plurality of Pilot symbol sequences at predetermined locations.
The method has steps of
(a) Estimating the quality of the radio channel using symbol sequences found at the predetermined locations, the symbol sequences being assumed to be Pilot symbol sequences.
(b) Determining a magnitude of a detection error between assumed transmitted Pilot symbol sequences and the detected symbols found at the predetermined locations.
(c) Calculating a mean of the detection error powers.
(d) Comparing the calculated mean of the detection error powers with a threshold value.
(e) Selecting the modulation type of the Data portion as being one of 8-PSK or π/4-shifted DQPSK based on the result of the step of comparing.GPRS phones
GPRS phones are a step up in terms of technology from any mobile phones. As well as the basic voice capability; the phones will offer a range of high-speed e-mail and mobile internet services. GPRS, which stands for general packet radio service, is also known as 2.5G. This is because it is seen as being half-way between the second-generation (2G) phones UK users have today and the coming third-generation (3G) technology. It is 3G that has caused the telecoms operators to build up such big debts, with the requirement to buy new licences and build new networks.
GPRS different from Wap
GPRS refers to the way data is downloaded while Wap is the browser technology that allows users to view downloaded information. The key difference between GPRS phones and the Wap phones that have been available until now is that GPRS provides an "always-on" connection to the internet, so users won't have to sign on to check if they've received e-mail or dial up to view internet pages.
Time spent attempting, and sometimes failing, to connect to the internet has been a major complaint of those using the Wap (wireless application protocol) phones that have been available until now. Essentially, GPRS users will be able to download the same data faster than standard Wap phone users. But the data will still look the same when it arrives on the screen - GPRS phones have Wap browsers, with all the limitations that entails. Operators say GPRS phones will download data about three times faster than standard Wap phones. Time spent reading or replying to e-mails is free - users only pay for information sent or received.
GSM Circuit Switched Data supports one user per channel per time slot. High Speed Circuit Switched Data (HSCSD) gives a single user simultaneous access to multiple channels (up to four) at the same time. As such, there is a direct trade-off between greater speed and the associated cost from using more radio resources- it is expensive for end users to pay for multiple simultaneous calls.
Assuming a standard Circuit Switched Data transmission rate of 14.4 kilobits per second (kbps), using four timeslots with High Speed Circuit Switched Data (HSCSD) allows theoretical speeds of up to 57.6 kbps. This is broadly equivalent to providing the same transmission rate as that available over one ISDN B-Channel. Some Mobile Switching Centre’s (MSCs) are limited to 64 kbps maximum throughput- this restriction is removed with GPRS.
In networks where HSCSD is deployed, GPRS may only be assigned third priority, after voice as number one priority and HSCSD as number two. In theory, HSCSD can be preempted by voice calls- such that HSCSD calls can be reduced to one channel if voice calls are seeking to occupy these channels. HSCSD does not disrupt voice service availability, but it does affect GPRS. Even given preemption, it is difficult to see how HSCSD can be deployed in busy networks and still confer an agreeable user experience- i.e. continuously high data rate. HSCSD is therefore more likely to be deployed in start up networks or those with plenty of spare capacity- since it is relatively inexpensive to deploy and can turn some spare channels into revenue streams. High Speed Circuit Switched Data (HSCSD) is however easier to implement in mobile networks than General Packet Radio Service (GPRS) because some GSM vendor solutions require only a software upgrade of base stations and no new hardware. This is not the case with D-AMPS networks and some GSM vendor solutions.
There are a couple of reasons why HSCSD may be the preferred bearer for certain applications when compared to GPRS. The fact that associated packets can be sent in different directions to arrive at the same destination should in theory make the transmission more robust since there are many different ways of achieving the end result. However, this nature of packet transmission means that packets are subject to variable delay and some could be lost. Whilst packet retransmission is incorporated into the GPRS standards, naturally this process does take time and in the case of applications such as video transmission can cause poor quality images.
Another preferred application for HSCSD could be the fact that whilst GPRS is complementary for communicating with other packet-based networks such as the Internet, HSCSD could be the best way of communicating with other circuit switched communications media such as the PSTN and ISDN. HSCSD is mainly supported by Nokia with little success.
EDGE, Enhanced Data GSM Environment
Enhanced Data rates for Global Evolution (EDGE) is a radio based high-speed mobile data standard. It allows data transmission speeds of 384 kbps to be achieved when all eight timeslots are used. In fact, EDGE was formerly called GSM384. This means a maximum bit rate of 48 kbps per timeslot. Even higher speeds may be available in good radio conditions.EDGE was initially developed for mobile network operators who fail to win Universal Mobile Telephone System (UMTS) spectrum. EDGE gives incumbent GSM operators the opportunity to offer data services at speeds that are near to those available on UMTS networks.
EDGE can also provide an evolutionary migration path from GPRS to UMTS by implementing now the changes in modulation that will be necessary for implementing UMTS later. The idea behind EDGE is to eke out even higher data rates on the current 200 kHz GSM radio carrier by changing the type of modulation used, whilst still working with current circuit (and packet) switches.
Implementation of EDGE by network operators has been designed to be simple. Only one EDGE transceiver unit will need to be added to each cell. With most vendors, it is envisaged that software upgrades to the BSCs and Base Stations can be carried out remotely. The new EDGE capable transceiver can also handle standard GSM traffic and will automatically switch to EDGE mode when needed.
EDGE capable terminals will also be needed- existing GSM terminals do not support the new modulation techniques and will need to be upgraded to use EDGE network functionality. Some EDGE capable terminals are expected to support high data rates in the downlink receiver only (i.e. high dates rates can be received but not sent), whilst others will access EDGE in both uplink and downlinks (i.e. high data rates can be received and sent). The later device types will therefore need greater terminal modifications to both the receiver and the transmitter parts.
In addition, the TDMA industry association, the Universal Wireless Communications Corporation, has introduced what it calls EDGE Compact. This a spectrum efficient version of EDGE that will support the 384 kbits mandated packet data rates but will require only minimum spectral clearing and therefore could work for network operators with limited spectrum allocations. In fact, as a result of this, EDGE has been renamed Enhanced Data Rates for GSM and TDMA Evolution.
EDGE provides speed enhancements by changing the type of modulation used and making a better use of the carrier currently used, for example the 200kHz carrier in GSM systems. EDGE also provides an evolutionary path to third-generation IMT-2000-compliant systems, such as universal mobile telephone systems (UMTS), by implementing some of the changes expected in the later implementation in third-generation systems.
EDGE builds upon enhancements provided by general packet radio service (GPRS) and high-speed circuit switched data (HSCSD) technologies that are currently being tested and deployed. It enables a greater data-transmission speed to be achieved in good conditions, especially near the base stations, by implementing an eight-phase-shift keying (8 PSK) modulation instead of Gaussian minimum-shift keying (GMSK).
SPECTRUM EFFICIENCY
Packet switching means that GPRS radio resources are used only when users are actually sending or receiving data. Rather than dedicating a radio channel to a mobile data user for a fixed period of time, the available radio resource can be concurrently shared between several users. This efficient use of scarce radio resources means that large numbers of GPRS users can potentially share the same bandwidth and be served from a single cell. The actual number of users supported depends on the application being used and how much data is being transferred. Because of the spectrum efficiency of GPRS, there is less need to build in idle capacity that is only used in peak hours. GPRS therefore lets network operators maximize the use of their network resources in a dynamic and flexible way, along with user access to resources and revenues.
GPRS should improve the peak time capacity of a GSM network since it simultaneously:
Allocates scarce radio resources more efficiently by supporting virtual connectivity.
Immigrates traffic that was previously sent using Circuit Switched Data to GPRS instead, and reduces SMS Center and signaling channel loading by migrating some traffic that previously was sent using SMS to GPRS instead using the GPRS/ SMS interconnect that is supported by the GPRS standards.
Relatively high mobile data speeds may not be available to individual mobile users until Enhanced Data rates for GSM Evolution (EDGE) or Universal Mobile Telephone System (UMTS) are introduced.
GPRS and EDGE Technologies
GPRS simply is an extension of the GSM standard to provide packet data services. GSM uses TDMA technology that allows eight users on one 200-kHz RF channel. These users are assigned to slots, with each slot’s duration in time being 577 µs. Together, all eight slots become a frame with a corresponding duration of 4.615 ms. We also know from some previous calculations on the corresponding channel capacity that the number of bits per time slot or burst is equal to 147 with GMSK modulation.
In a voice mode, the user always would be assigned one slot for transmission of voice data. The same is true for a form of wireless data service called circuit-switched data. With circuit-switched voice or data, we take advantage of the frame system used in a number of telephone networks to transmit the data. You are assigned a time slot, and that time slot always is yours to use whether there is data present or not.
With a GSM system, voice or data is sent during each slot every frame. This means that the phone is ramping up and sending data whether data is present or not and then ramping down. Circuit-switched operation is inherently inefficient because a slot always is assigned whether or not the mobile has any information to send.
Recently, designers of these mobiles have recognized this, and to save battery life, they have developed a means to shorten the burst using a method of detecting whether information is present and then discontinuing the burst. This is known as DTX. This is only a battery-saving function since the mobile still is always assigned to that particular slot.
Things change dramatically with packet data services such as GPRS or EDGE. These systems allow the mobile to transmit any number of slots either consecutively or within a frame as directed by the network. This permits the system operator to take advantage of dead air time associated with circuit-switched networks to increase capacity and data rates. GPRS and EDGE data sessions are not circuit switched and, for that reason, require a new package routing network to interface to the IP.
GPRS and EDGE Modulation Formats
GPRS is an evolution of the existing GSM modulation and channel formats. The modulation scheme is the same as for GSM and called GMSK. The GMSK digital modulation format relies on gently shifting the carrier 180° in phase to produce a binary modulation scheme capable of delivering 1 bit/symbol.
GPRS Channel Coding
Channel coding in a GPRS or EDGE system protects the data that is being transported across the air interface and is implemented to correct errors in the bit stream caused by the RF environment. GPRS and EDGE use different convolutional encoding and decoding processes, bit-puncturing schemes, and interleaving processes to account for different RF channel conditions and QOS requirements.
This is required because the air interface can be highly destructive to the RF channel. If we are to realize the benefits of higher data rates, then we want to provide as little encoding as possible to preserve the data being transferred. But we also need to ensure that we do not drop the call when conditions suddenly change due to fading.
Below the Figure shows an example of a channel-coding scheme for EDGE and indicates the relationship among the various processes. Bits of the data stream are removed or punctured to reduce the size of the bit stream using a predetermined mask for that particular channel-coding scheme. The data sent between the MS and the network is not a 1:1 burst-to-block conversion.
The data in each burst is broken into subsections and dispersed into the RLC blocks using a predetermined mapping scheme across multiple bursts in a process called interleaving. Finally, convolutional coding on the bit stream uses redundancy bits so that a decoder can detect errors in the bit stream and correct them.
The scheme shown in Figure is MCS-7, which uses the 3/8 8PSK modulation scheme. Channel-coding schemes used in GPRS and EDGE must be bidirectional, meaning that they are encoded and decoded the same way. If we look at this example from a decoding point of view, you can see that we are taking four consecutive bursts and deinterleaving the information or symbols.
After deinterleaving, the bits are depunctured and sent to a 1/3 rate convolutional decoder. Further parsing these bits provides us with the RLC blocks that are the basis for all of our signaling or protocol needed to set up an EDGE call.
While GPRS uses the same modulation scheme as GSM, there are four different channel-coding schemes to provide varying levels of protection to the packets either going or coming from the mobile. Table 1 (see below) shows the impact that different channel-coding schemes and the number of slots can have on effective data rates.
Table 1. GPRS Data Rates for Channel-Coding Schemes 1 through 4
Channel Coding Scheme
Slot Combinations
1 Slot
4 Slots
8 Slots
CS1
9.2 kb/s
36.8 kb/s
73.6 kb/s
CS2
13.55 kb/s
54.2 kb/s
108.4 kb/s
CS3
15.75 kb/s
63 kb/s
126 kb/s
CS4
21.55 kb/s
86.2 kb/s
172.4 kb/s
First-generation GPRS systems will use four slots in the downlink (base station to mobile) and one or two slots in the uplink (mobile to base station) direction to transfer data. This is due to the fact that during a data session, typically more information is received than transmitted.
EDGE uses nine channel-coding schemes separate from GPRS schemes CS1 through CS4. In Table 2 (see below), we see a sample of a few of the corresponding data rates for channel-coding schemes MCS1-4 that use GMSK and MCS5-9 using 3/8 8PSK modulation. There are dramatic increases in the data rates from 1 bit/symbol GMSK modulation to 3 bits/symbol data rates of 3/8 8PSK and the difference that channel coding (protection) can make in effective overall data rates.Table 2. EDGE Data Rates Using 3/8 8PSK Modulation vs. Different Channel-Coding Schemes and Slot Combinations
Channel Coding Scheme
Modulation
Slot Combinations
1 Slot
4 Slots
8 Slots
MCS1
GMSK
8.8 kb/s
35.2 kb/s
70.4 kb/s
MCS4
GMSK
17.6 kb/s
70.4 kb/s
140.8 kb/s
MCS5
8PSK
22.4 kb/s
89.6 kb/s
179.2 kb/s
MCS9
8PSK
59.2 kb/s
236.8 kb/s
473.6 kb/s
GPRS and EDGE have much in common with conventional GSM systems. They share the same slot structure and timing and the same 200-kHz channel bandwidth. However, significant differences exist between a GSM voice system that is circuit switched and a GPRS or EDGE system that is packet routed.
GPRS 8 PSK
A method is disclosed for operating a wireless mobile station to receive a time slot sent through a radio channel from a transmitter. In a GPRS embodiment the time slot has a Header portion and a Data portion, wherein the Header portion contains information for specifying whether the Data portion is modulated using 8-PSK modulation or π/4-shifted DQPSK modulation. When modulated using 8-PSK modulation, the Data portion includes a plurality of Pilot symbol sequences at predetermined locations.
The method has steps of
(a) Estimating the quality of the radio channel using symbol sequences found at the predetermined locations, the symbol sequences being assumed to be Pilot symbol sequences.
(b) Determining a magnitude of a detection error between assumed transmitted Pilot symbol sequences and the detected symbols found at the predetermined locations.
(c) Calculating a mean of the detection error powers.
(d) Comparing the calculated mean of the detection error powers with a threshold value.
(e) Selecting the modulation type of the Data portion as being one of 8-PSK or π/4-shifted DQPSK based on the result of the step of comparing.GPRS phones
GPRS phones are a step up in terms of technology from any mobile phones. As well as the basic voice capability; the phones will offer a range of high-speed e-mail and mobile internet services. GPRS, which stands for general packet radio service, is also known as 2.5G. This is because it is seen as being half-way between the second-generation (2G) phones UK users have today and the coming third-generation (3G) technology. It is 3G that has caused the telecoms operators to build up such big debts, with the requirement to buy new licences and build new networks.
GPRS different from Wap
GPRS refers to the way data is downloaded while Wap is the browser technology that allows users to view downloaded information. The key difference between GPRS phones and the Wap phones that have been available until now is that GPRS provides an "always-on" connection to the internet, so users won't have to sign on to check if they've received e-mail or dial up to view internet pages.
Time spent attempting, and sometimes failing, to connect to the internet has been a major complaint of those using the Wap (wireless application protocol) phones that have been available until now. Essentially, GPRS users will be able to download the same data faster than standard Wap phone users. But the data will still look the same when it arrives on the screen - GPRS phones have Wap browsers, with all the limitations that entails. Operators say GPRS phones will download data about three times faster than standard Wap phones. Time spent reading or replying to e-mails is free - users only pay for information sent or received.
