GPRS Tutorial

GPRS Technical Review By Natalia V. Rivera and Dr. Belka Kraimeche

(Pending for publication) Today the digital systems such as the Global System for Mobile Communication (GSM), Code Division Multiple Access (CDMA based on IS-95), and United States Time Division Multiple Access (US-TDMA based on IS-136) conformed to what is known as second-generation systems. All these three systems have made voice communications to go one step ahead in wireless and multimedia networking resulting in a great demand for services such as text messaging and access of data networks through the Internet. Hence, Internet have been utilized as the fundamental access for these types of services as well as for more advanced services inciting leading markets and customers with increasingly finding value for services such as streaming audio, http, video, remote banking, and e-commerce. Advanced Second generation systems and Third generation systems such General Packet Radio Service (GPRS) and Universal Mobile Telecommunication Services (UMTS) respectively, are based on the concept presented above, enabling enhanced and high quality multimedia communications between end-to-end users at the cost of higher data rates and innovative communications capabilities resulting in the wireless domain a persistent interest in IP technology. Such that it is growing intensively since most of the communication capabilities to support ground-breaking communications can be obtained at a low cost with appropriate network architecture and highly efficient and secure protocols. The arrival of imminent handset devices packed-based systems such as GPRS or UMTS based will make possible to access the Internet not only by using a fixed connection, but also using a mobile terminal. Nevertheless, because of all different technologies and frequency allocation throughout the United States and the world, global roaming will continue to require specific arrangements between operators, such as multi-mode and multi-band handset and roaming gateways between the different core networks. Furthermore, in spite of cost for new technology, it is believed that the demand for such technology will be higher and hence a further growth for 3G communication systems will take place especially with the perception of all IP-based core networks, which are currently under standardization. As mentioned before, there are currently three Second-generation technologies, however two are the most promising to succeed and evolve in Third-generation systems: First, is GSM, which with an improved core network (circuit and packed based integrated) and advanced new features that are not included in GSM phase I or II+ will offer high data rate, flexibility and efficient utilization of scarce bandwidth across the air interface. Second, is CDMA, which for Third generation adopts two new concepts concerning multi-carrier transmission for CDMA2000 and wideband CDMA for UMTS, both resulting in high data rates and spectrum efficiency, but differs mainly in physical characteristics. Furthermore, an emerging alternative, which lately has been growing, is GPRS with an all IP-based core network. As it is

know, GPRS is the data network for GSM technology however, its architecture and protocols are slightly different. Figure 1 illustrates an example of standardized concept as regards to connections between typical core networks and air interface that are currently considered for 2.5+G and 3G networks.

Figure 1. Core network relation to the 3G air interface alternatives The growth of all these technologies is based of the evolution and high technology advances on crucial protocols, one of the main widely used is known as Transmission Control Protocol (TCP/IP). The architecture and application of TCP/IP protocol will not be covered in this paper, however a small explanation is given in order to facilitate understanding of packet networks. TCP is a connection-oriented transport protocol of the datagram network layer (IP). TCP applies a Selective Repeat Protocol (SRP) with positive acknowledgements and timeouts to provide reliable end-to-end byte transport. However, if reliable transmission is not required a different protocol can be applied. Such protocol is connectionless transport and is known as User Datagram Protocol (UDP). Note that UDP is used for application requiring a fast transport with low delay, where occasional packet loss is less important. An example of such application is stream video. History and Evolution of GPRS GSM (Global System Mobile communications) standardized by ETSI (European Telecommunication Institute) is one of today’s largest second generation cellular system. GSM functions under the 900 MHz cellular frequency band as well as under the 1800 MHz and 1900 MHz personal communication frequency bands. Technology for GMS has evolved rapidly since the introduction of its data services, however in order to support such services GSM adapted a new solution called GPRS (General Packet Radio

Service), which is intended to provide actual packet radio for GSM and TDMA users with corresponding enhanced architecture adjustments, new response times and implementation of innovative features. The main set of GPRS specification was approved by SMG#25 (Special Mobile Group) in 1997, and was completed by 1999. The purpose of GPRS is to accommodate efficiently data sources that are bursty in nature as well as to obtain high data rate, flexibility and efficient utilization of bandwidth across the air interface that is why GPRS reuses the same infrastructure in order to provided end-to-end packet switched services. Thus, to comply with these concepts new radio channels are defined, in where the allocation of these channels is flexible such that one to eight time slots can be allocated to a user or several active users at a giving time. This implies that one or more users could share the same time slot resulting different allocation formats for uplink and downlink transmissions. Hence, radio resources can be shared dynamically between speech and data services as a function of traffic load or operator preference, which can be accomplished through various radio channel code schemes that have been implemented successively in order to allow bit rates from 9kbps to more than 171.2 kbps per user. Furthermore, since GPRS and GSM share the same frequency bands with telephone and circuit switched data traffic and both make use of many physical properties at the physical layer such its TDMA frame structure, modulation technique and structure of GSM time slots, the migration to data services is moving rapidly and optimistically. To facilitate a better understating of the evolution of GPRS, a standard GSM architecture is provided as well as its data network implementation i.e. GPRS system.

Figure 2. GSM System Architecture As seen from Figure 2, GSM system architecture includes three standard interfaces; the air interface (Um), the Abis interface, and the A interface. The GSM functionalities are divided into the Mobiles Stations (MS), the Base Station Subsystem (BSS), which is composed of two main systems (1) Base Transceiver Station (BTS), which handles the radio interface towards the MS and (2) Base Station Controller (BSC), which manages the radio resources and control handovers. The GSM core network contains the (Mobile Switching Center (MSC), which permits access to public networks such as Public Switched Telephone Network (PSTN), Integrated Service Digital (ISDN), Circuit Switched Public Data Network (CSPN), and Packet Switched Public Data network (PSPDN). In addition, three databases, the Home Location Register (HLR), the Visitor Location Register (VLR), and the Authentication Center (AUC) support the MSC. All these networks components will be modified at their different level of architecture so that a migration step toward 3G will be accomplished. A standardized concept is presented in the Figure 3.

Figure 3. GPRS System Architecture What is GPRS? General Packet Radio Services is a new and interesting service to value added in the field of the services that it concurs with the customers to send information using the mobile telephony. GPRS is a packet-switched service that provides lots of advantages, but also requires hardware and software changes in the GSM networks. GPRS is known for its defined characteristics that can be a continuation of new technological advances, which implies speed, immediacy, and new data features. GPRS facilitates the instantaneous logon between various finished networks since it is always connected, thus that as the information reached its destination instantaneously at all times. GPRS corresponds to Internet, e-commerce applications amongst others. The basic idea of GPRS is to provide a packet-switched bearer in a GSM network in where resources are use more efficiently through bursty data applications and general flexibility. In brief from Figure 3, it can be deduced that GPRS can be described as a service providing optimized access to the Internet, while reusing to a large degree existing

GSM infrastructure. The GPRS network is created to support theoretically a maximum speed of 171.2 kbps; hence the concept of packet-switched network is adopted throughout the infrastructure and implementation of a GPRS network. It is foreseeable that its architecture will not contain any MSC, VLR or AUC applications, but instead will enclose two new network GSN (GPRS Support Node) elements: a Gateway GPRS Support Node (SGSN) and a Serving GPRS Support Node (GGSN). The gateway GPRS Support Node (GGSN) as the name implies it serves as the gateway between the GPRS network and other packet networks. GGSN is also responsible for routing data to the mobile stations at their current points of attachments to the network. In other words, GGSN acts as a logical interface to external packet data networks. On the other hand, the Serving GPRS Support Node (SGSN) is responsible for the delivery of packets to the Mobile Systems (MSs) within its particular geographical service area. Within the GPRS network, Protocol Data Units (PDUs) are encapsulated at the originating GSN and decapsulated at the destination GSN. In between the GSNs, the Internet protocol (IP) is used as the backbone to transfer PDUs. This process is known as GPRS tunneling (GTP). The GSNs that make up a GPRS PLMN are interconnected via an IP backbone, in where standards IP routers are employed to help the SGSN to perform the routing and data transfer functionality that is needed concerning the user related data, which is constantly stored within the network Home Location Register (HLR). The GPRS Public Land Mobile Network (PLMN) is made up of a number of network elements and communications links which connections are based on the Internet Protocol (IP) standard stack. On the interface the resources are assigned mobile-to-mobile stations only temporarily on a per-packet basis thus radio resources are only assigned for the duration of one or few IP packets. Generally, it is believed that the idea if introducing GPRS will enable the following features: • Circuit and packet switched services in one mobile radio network • Efficient use of the scarce radio resources • Fast setup/access times • Connectivity to other external packet data network, based on IP and X.25 A visual example of an ideal GPRS network is giving in Figure 4.

Figure 4. GPRS Network Additionally, the GPRS mobile stations are known to operate in four modes in order to provide access:

1. Class A: Simultaneous use of GPRS/EDGE other GSM services 2. Class B: Alternate use of GPRS/GPRS and other GSM services 3. Class B136: Alternate use of EDGE and there is-136 services 4. Class C: GPRS/EDGE only (data service only)

Thus to use GPRS services, users will need the following: ! A mobile phone or terminal that supports GPRS since existing GSM mobile

phones do not support GPRS. ! A subscription to a mobile phone network that support GPRS ! Use of GPRS must be always enabled for the user. Some mobile networks operator

may allow automatic access to GPRS, others will require a specific opt-in. ! Knowledge of how to send /received GPRS information using their specific

mobile phone, including software and hardware configuration resulting in a customer service requirement

BSC

BTS BSC

BTS

GGSIntra PLMN IP

GGSN

SGSN

SGSN

GGSN

HL

GPRS PLMN X.25

IP

SS7

! A destination to send/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 from the first time. This implies that it provides an immediate critical mass of uses to all its subscribers.

From the points mentioned above, it is embedded that GSM MS design must be enhanced in several ways so as to support GPRS. First, new protocol layers must be added. These include MAC, RLC, LLC, and SNDCP. It must also be modified to operate on shared traffic channels with the adoption of new modulation schemes such Gausssian Minimum Shift Keying (GMSK) applicable to GPRS, and Eight level Phase Shift Keying (8-PSK) adopted to the Enhanced GPRS (EGPRS), both occupying a channel width of 200 kHz. An additional feature is that GSM BSS have been modified such that GPRS software capability was added to BTS and BSC with minima BSC hardware changes in order to support packet data services, thus that the BSS must interface with the SGSN via frame relay and also several new protocols which will be discusses further in detailed. Applications of GPRS GPRS can act as a mobile access network to the Internet. Its purpose is to enable a variety of new and unique services to the mobile wireless subscriber such offered services contained several unique characteristics that enhance the values to the customer. First among them is mobility, which is the ability to maintain constant voice and data communications while roaming. Second, is immediacy, which allows subscribers to obtain connectivity when needed, regardless of location and without a lengthy login session. This is why GPRS users 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 critical time applications. Finally, third is localization, which allows subscribers to obtain information relevant to their current location. As a consequence, the combination of all these three major characteristics integrates possible applications that can be offered to mobile user in two different categories: corporate and consumer. In addition, due to its efficient support of bursty traffic, GPRS is expected for application areas such as: ! Communications, i.e. email, fax, unified messaging (SMS), intranet and Internet

access ! Values added Services (VAS), i.e. Information services, games ! E-Commerce i.e. retail, ticket purchasing, financial trading ! Location based application i.e. Navigation, traffic condition, airline. Rail

schedules, location finder ! Vertical application i.e. Freight delivery, flee management, sale force automation ! Advertising A graphical example of GPRS is given below. Figure 5

Figure 5. GPRS common applications

Conceptually, GPRS achieves theoretical maximum speeds of up to 171.2 kbps using all eight time-slots as described earlier. This is about three times as fast as the data transmission speed possible over today’s network fixed communications networks and ten times as fast as current circuit switched data devices on GSM networks. Likewise, GPRS provides a point-to point and point-to- multipont connections to facilitate wide range of applications. As a result GPRS uses the scarce radio resources more efficiently and supports a number of applications with different requirements, which are accomplished by a set of several preferences. Such preferences are commonly known as Quality of Service (Qos) classes. This concept allows network operators to change data transmission schemes for different operations according to its applicable fee charges. QoS classes of GPRS phase 1 are provided by its standard specification, which can be found in [2]. In phase 2, GPRS’ objective is to create the QoS applications comparable to third generations systems such Universal Mobile Telecommunications Service (UMTS) and adaptable to further generations. Hence, today many GPRS operators are either planning to deploy or are investigating 3G while GPRS is shaping under its standards. Effectively, GPRS can be seen as a migration step toward several 3G technologies as the innovations of GPRS terminals and infrastructures are driving factors. A standard QoS classes are specified in Table 1.

Email

Multimedia

File Transfer

Telemetry

FAX

Web Applications

Credit cardTransactions

Text messaging

Parameter

Values

Precedence High, Normal, Low Reliability Packet loss probability, e.g. 249 10,10,10 −−−

Class 1 2 3 4 Mean(s) <0.5 <5 <50 Best effort

Delay for packets of 128 octets

95% (s) <1.5 <25 <250 Best effort Maximum Bit Rate 8 kbps – 2 Mbps

Mean Bit rate 0.22 bps – 111 kbps Current GPRS limit 160 kbps (practical)

Table 1. QoS classes

GPRS Network Architecture As it has already been explained, two main components were integrated into a GPRS system, GGSN and SGSN. The functional grouping defined in GPRS include Network access, packet routing and transfer, mobility management, logical link management, radio resources and network management. As mentioned earlier, GPRS network access support the standard point-to-point data transfer and anonymous access. These function include: ! Registration ! Authentication and authorization ! Admission controls ! Message screening ! Packet terminal adaptation ! Charging information collection for packets transmission across the GPRS

network. Packet routing and transfer functions route the data between an MS and the destination through the serving and gateway GSNs and their functions include: ! Relay functions that is used by the BSS to forward packets between an MS and a

serving GSN; it is also used by the serving GSN to forward packets between a BSS and a serving or gateway GSN

! Routing ! Address translations and mapping that converts a GPRS network address to an

external data network address and vice versa ! Encapsulation and tunneling, which encapsulates packets at tge source of a

tunnel, deliver the packets through the tunnel and decapsulated them at the destination.

! Compression and ciphering

! Domain service functions, which resolves logical GSN names to their IP addresses

Logical link management maintain the communication channel between and MS and the GSM network across the radio interface, which includes: ! Logical link establishment ! Logical link maintenance ! Logical link release

Radio resources management allocates and maintains radio communication path, which include: ! Um management, which determines the amount of radio resources to be

allocates for GPRS usage ! Cell selection, which enables the MS to select the optimal cell for radio

communication ! Um-tranx, which provides packet data transfer capability, such as medium

access control, packer multiplexing, packet discrimination, error detection and correction, flow control across the radio interface between the MS and the BSS

! Path management, which maintain the communication path between the BSS and the serving GSNs

Mobility management keep track of the current location of an MS. Three different scenarios can exist when the MS enters a new cell and possibly a new routing area: cell update, routing area update, and combined routing area and location update. Network management functions provided mechanisms to support OA&M functions related to GPRS. Further details regarding GPRS functional groups can be found in [3]. In addition GPRS Support Nodes implementation in the GSM architecture, protocols have been developed to support this data concept. On the other hand, mobile terminals are still under design process since standards with respect to BSS and MS have not been globally finalized. The main goal of this standardization is to obtain a general infrastructure in where GSNs and circuit switched network entities such as MSC, VLR, HLR and SMS-center are internetworking efficiently as the concepts of roaming, QoS and remote access are applied. A summary of GSNs functionalities is described below in order to facilitate a profound understanding: Serving GSN (SGSN) ! Node serving multiple MS and BSS ! Establishes mobility management context ! Interfaces with HLR to perform security functions and access control ! Each controls GPRS services in a particular geographical coverage area. This

may also be considered similar to routers with additional functionality for GPRS Gateway GSN (GGSN) ! Serves as a gateway between the GPRS network and other packet networks (i.e.

IP/ X.25)

! It is responsible for routing data to the MS at their current points of attachment to the network

! Stores SGSN routing info for each GPRS user ! Stores specific info on IP/X.25 address ! One PLMN may contain multiple GGSNs

GSNs that make up the PLMN are interconnected via an IP backbone, which contains standard IP routers. Additional GSM PLMN network elements that play a role in providing GPRS service are the GSM Home Location Register (HLR), Mobile Switching Center (MSC), Visitor Location Register (VLR), Base Station Controller (BSC), and Base Station Transceiver (BTS). Thus, BSS (BSC + BTSs) of a GSM PLMN must be modified in order to support packet data transmission (GPRS) suggesting that the HLR must be enhanced to include GPRS subscription data including quality of service (QoS), statically allocated PDP addresses, and roaming permissions. Further, the MSC and VLR can also be modified to allow coordination between voice and packet data transmission, so that the system becomes more efficient.

Figure 6. The GPRS logical node architecture

Signaling and Data Transfer Interface

TE MT BSS SGSN GGSN

MSC/VLR

SMS-GMSC SMS-IWMSC

SMS-C

HLR

TE

PDN

GGSN

Other PLMN

CGF

EIR

Billing System

Signaling Interface

R U

Gn

Gp

Gf

Ga

Gb Gn

SGSN

GsGrA

Gc

Gd

Gi

PSTNE

C

The figure above depicts the GSM architecture in a logical architecture. It is referred to as logical architecture because it subscribes logical separation of functions added by the GPRS network (GGSN and SGSN). The objective is to implement one or more logical entity on a single physical component thus reducing network complexity. Besides the GGSN and SGSN, the other nodes are the same as the existing GSM network. HLR, BSS and EIR have been modified to support GPRS. Further, optional enhancements to MSC/ VLR and SMS-GMSC/SMS-IWMSC are also being considered by GPRS specification standards with the sole objective As seen, the MS and the BSS communicate via the Um interface. The BSS and the SGSN are connected by the Gb interface using frame relay. Within the same GPRS network, SGSNs/GGSNs are connected through the Gn interface. When SGSN and GGSN are in different GPRS networks, they are interconnected via the Gp interface. The MSC/VLR communicates with the BSS using the existing using GSM A interface, ad with the SGSN using the Gs interface. The HLR connects to the SGSN via the Gr interface, and to the GGSN via the Gc interface. Both Gr and GC follow the GSM Mobile Application Part (MAP) protocol, which is used in he B, C, D, E, F and G interfaces in terms of networking signaling. The HLR and VLR are connected through the existing GSM D interface, Interfaces A, GS, Gr, Gc, and D are used for signaling, without involving user data transmission in GPRS. Note that the A interface is used for both signaling and voice transmission in GSM. Interfaces Um, Gb, Gn, Gp, and Gi are used for both signaling and transmission in GPRS. of gaining efficient resources sharing between packets and circuit switches. GPRS Protocols On the network level, GPRS supports IP and X.25 protocols to be used by an end-to-end application. The GPRS packet delivery to the MS terminal constantly faces packet traffic as it travels to its destination such traffic is referred as the transport of data which consists on flow control, error detection, error correction and error recovery are all included under the same suite of protocols.

Figure 6. GPRS transmission plane

Figure 6 illustrates the GPRS transmission plane, which consists of a layered protocol structure for user information transfer and the associated control procedures mentioned above. As seen from Figure 6, the IP or X.25 packets are forwarded through the GPRS PLMN network using dedicated protocols. Albeit, the GPRS network seems to have different protocol structures, they all merge into one IP backbone. Note that in GPRS, IP is used as the network layer to transport independent packets between the SGSN and GGSN thus the GPRS Tunnel Protocol (GTP) tunnel the PDU through the GPRS backbone network by adding routing information. The GPR utilizes either TCP or UDP depending on whether a reliable connection is needed i.e. for X.25 or IP packets. Further, Ethernet, ISDN, or ATM based protocols may be used for L2 (Data Link Control Layer) depending on the operator’s network architecture. Between the SGSN and MS, the Subnetwork Dependent Convergence Protocol (SNDCP) maps the network level protocol characteristics onto the underlying logical link control and provides functionalities like multiplexing of the network layer messages onto a single virtual logical connection, encryption, segmentation and compression. In other words, SNDCP main task is to carry network layer protocol data units (X.25/IP) in a transparent way. [5] Between the MS and the BSS, the data link layer has been separated into two distinct sublayers: the Logical Link Control Layer (LLC) and the Radio Link/Medium Access Control Layer (RLC/MAC) sublayers. The LLC layer is the higher sublayer of the data link layer and operates above the RLC/MAC and across the Gb and the Um interface, proving a logical link between the MS and its SGSN [6]. Protocol functionality is based on LAPD [7] used within the GSM signaling plane with support for point-to-multipoint transmission. Typical LLC functions compromise ciphering, flow control, and sequence control. In addition, of the LLC protocol is used in acknowledged mode, it provides detection and recovery transmission errors, as whereas in unacknowledged mode it signals unrecovered errors. LLC is used by SNDCP for the transfer of network layer

packet data units (PDUs), by the SMS protocol to transfer SMS messages, and by GPRS mobility management to transfer control data. The RLC/MAC protocol layer located in the PCU provides services for information transfer over the physical layer of the GPRS radio interface. It defines the procedures that enable multiple MSs to share a common transmission medium, which may consist of several physical channels. The functions of the RLC protocol include segmentation and reassembling of the LLC PDUs. It can be operated in either acknowledged or acknowledged mode in accordance with the requested QoS. In acknowledged mode, checksum-based detection of erroneous RLC PDUs and retransmissions of them s deployed. The MAC layer itself is derived from a slotted ALOHA protocol and operates between the MS and BTS. It is responsible for access signaling procedures for the radio channel governing the attempts to access the channel by the Ms, and the control of that access by the network side. It realizes the different between logical channels needed to share the common transmission medium by several MSs in response to services requests. It also allows one MS to use multiplexing of several MSs over one physical channel. The physical layer is split into a Physical Link Sublayer (PLL) and a Physical RF Sublayer (RFL). The physical link sublayer (PLL) provides a number of physical channels to the RLC/MAC layer. In other words, it provides services for information transfer over a physical channel between the MS and he network. The PLL uses the services of the RFL sublayer. PLL functionalities includes forward error correction (FEC), rectangular interleaving, Procedures for detection, physical link congestion, and monitoring of radio link signal quality (i.e. power control). The RFL is part of a complete GSM system that delivers a range of services including GPRS and is part of the Um interface. The RFL performs the modulation and demodulation of the physical waveforms and conforms to the traditional GSM RF layer, which detailed recommendations are found in the GSM 05 series. The RFL functionalities include Carrier frequencies and GSM radio channel structures, modulation of transmitted waveforms and the raw data rates of GSM channels, and the transmitter and receiver characteristics and performance requirements. In the network, the LLC is split between the BSS and SGSN. The BSS functionality is called LLC Relay. Between the BSS and SGSN, the BSS GPRS protocol (BSSGP) conveys routing and QoS related to the information and operates above Frame Relay (FR). The GPRS relay function conveys LLC packet data units (PDUs) between the Um and Gb interfaces. In the SGSN, this functions relays Packet Data Protocol (PDP) PDUs between the Gb and Gn interfaces, The Gb and Gn relay function add a sequence numbers to PDP PDUs received from the SNDCP. And form the Gi reference point, respectively. To transparently transport PDP PDUs between external networks and MSs, the PDP PDUs are encapsulated and decapsulated for routing. Figure 7 illustrates the GPRS signaling plane, which gives a more detailed concept of the functionalities and internetworking applications between GSM and GPRS network elements.

Figure 7 GPRS signaling plane GPRS signaling plane consists of protocols for control and support of the transmission plane functions. Among these protocols, the GPRS-specific protocols include SNDCP, LLC, RLC, MAC, BSSGP, BSSAP+ and GTP. PLL, RFL GMM/SM, and MAP are derived from the GSM architecture. However these protocols have been modified to accommodate GPRS functionalities. TCAP, SCCP, MTP are SS7 layers. The other protocols are standard data protocols.

Figure 8. PDU segmentation The segmentation corresponding to the different protocol layers is shown in Figure 8, which demonstrates a graphical PDU application as it is transmitted across the GPRS interface, segmented and encapsulated in several subprotocols PDUs resulting in considerable header overhead. There, it is noted that as the packet moves to is next layer, the header overhead will increase and as it goes through the signaling protocol stack the overhead will have reached 20-30 percent of GPRS air interface capacity. Figure 9, shows that Radio Block Structures for user data and control. Each radio Block consists of MAC header, RLC Data Block or RLC/MAC Control Block and Block Check Sequence (BCS), thus that it is always carried by normal four bursts. The MAC header consists of the Uplink state Flag (USF), the Block Type Indicator (T), and the Power Control (PC) fields. The RLC Data Block consists of the RLC Header and the RLC Data. The RLC/MAC Control Block contains the RLC/MAC signaling information elements. Furthermore, Channel coding schemes is currently specified under GSM 05.3, however enhancements of such are still under development of standards. The four coding schemes are shown in Table 2 and are defined for the Radio Block carrying RLC data blocks.

Table 2. GPRS Coding Schemes

Figure 10 presents a more general and sophisticated graphical representation of GPRS architecture with respect to the OSI model.

Figure 10. GPRS in relation to the OSI model

It is also necessary to mention that there are three essential applications that are used interchangeably throughout the GPRS network in order to facilitate GPRS service. These applications are: Mobility Management (MM) context, PDP context, and Quality of Service (QoS) profile. The MM context consits of the MM state and other MM-related information store in the MS and SGSN and is function is to specify the MM activities of an MS. Such states are defined as follows: ! Idle state, if the MS is not attached to the GPRS MM. ! Standby state, if the MS is attached to the GPRS MM. ! Ready state, if the location information for the MS has been identified of cell

level. The PDP contexts are stored in the MS, HLR, SGSN and GGSN, which contain mapping and routing information packet transmission between the MS and GGSN. Note that for each GPRS communication made on a MS, a PDP context is created to characterize the session. In other words, a MS may have several PDP contexts if the terminal supports several IP addresses. The QoS profile is maintained in the PDP context to indicate radio and network resources required for data transmission. The QoS attributes include: ! Precedent class ! Delay Class ! Reliability class ! Peak throughput class ! Mean throughput class.

The percent class specifies three transmission priority levels. During congestion, the packets with lower priorities are discarded. The Delay class specifies four delay levels. Such levels are characterized by maximum speed constraints. For example in a 128-octect transfer, the expected transfers speeds for delay are less than 0.5, 5, 50 seconds respectively for levels 1-3 and for level 4 delay is best-effort transmission without specifying speed limitation. The Reliability class defines residual error rates for data loss, out-of-sequence delivery, and corrupted data. There are five reliability classes. Class 1 supports acknowledgment for GTP mode, LLC frame mode, and RLC block mode, and the LLC data are protected. Class 5 does not support acknowledgement, and LLC data are not protected. Peak throughput class specifies the expected maximum data transmission rate and it contains nine classes ranging form 8 kbps to 2048 kbps. Finally, the Mean throughput class specifies the average data transmission rate. Within this class there are 19 classes ranging from best effort to 111 kbps in order to meet GPRS transmission standards. GPRS Air-Interface Protocol The air interface known as the Um interface is considered as one of the most important aspects of GPRS because it mainly determines that performance of GPRS.

Here, the air interface protocol is concerned with applications related to the physical, Medium Access Control (MAC) and Radio Link Control (RLC) protocol layers. By utilizing efficient protocol such selective ARQ, the RLC/MAC sublayers allow efficient multiuser multiplexing on the shared packet data channels (PDCHs), which are physical channels dedicated to packet data traffic that travel across the network. A cell consists of one or more shared PDCHs, which are taken form a common pool of physical channel allocated to that cell, in addition to traffic channels (TCHs) which are used to decrease or prevent overload in the network. Furthermore, the allocation of PDCHs and TCHs is performed dynamically based on capacity and demand in the network. Since each network handle massive number of users, a multiframe structure is needed for the PDCH to facilitate paging groups and possibly blocks for broadcasting of the GPRS system information. The multiframe structure is derived from the GSM concept, thus that for GPRS system consists of 51 and 52 TDMA frames. Either multiframe type is used based on network infrastructure. As the data flow through the network, the network layer protocol data units are transmitted between the MS and SGSN using the LLC protocol. This process includes, head, data compression, segmentation and encryption. Then the LLC frame is segmented into RLC data blocks, which are formatted into the physical layer. Each block contains 4 normal bursts in consecutive TDMA frames. GPRS Logical Channels As mentioned earlier, GPRS uses the same TDMA/FDMA structure as GSM to form its physical channels. Table 4 provides the GPRS logical channels and their specific functions. Note that, for the uplink and downlink direction many frequency channels with bandwidth of 200kHz are defined through FDMA. Each 200 kHz frequency channel is subdivided into 51 or 52 TDMA frames with length duration of 4.615 ms. Each TDMA frame is split up into eight equally time slots with a duration of 0.576 ms. Since GPRS are GSM share the same frequency bands, each time slot can be assigned either to GPRS for transmitting packet-switched data, or to GSM for handling circuit switch voice calls. Note that time slots used by GPRS are called packet data channels (PDCH). The basic transmission of a PDCH is called a radio block. To transmit a radio block 4 consecutive TDMA frames are needed. A PDCH is structured in multiframes comprising 52 TDMA frames, which correspond to duration of 240 ms. Every 13th burst called the Idle burst is not used to transmit data, thus leaving 12 radio blocks in one multiframe. This implies tat the mean transmission time for a radio block is approximately 20 ms. The capacity of a radio block is approximately 456 bits, but die to forward error correction fewer payload bits are transmitted. Basically, the structure of the radio block and the number of bits depend on the message type and coding scheme, which is listed in Table 2. Depending on the message type transmitted in one radio block, the sequence of a radio block forms a logical channel.

Table 4. GPRS logical channels

PBCCH Group ! Packet Broadcast Control Channel (PBCH) transmits system information to al GPRS

terminal in a cell. PCCCH Group ! Packet Common Control Channel (PCCCH) is used to initiate packet transfer or ti

respond to paging messages. ! Packet Paging Channel (PPCH) is used to page an MS prior to downlink packet

transfer. ! Packet Access Grant Channel (PAGCH) is used in the packet transfer establishment

phase to send resource assignment to an MS prior to the packet transfer. ! Packet Notification Channel (PNCH) is used to send a Point to Multipoint-Multicast

(PTM-M) notification to a group of MSs prior to a PTM-M packet transfer. PDTCH Group ! Packet data Transfer Channel (PDTCH) is allocated for data transfer. One MS may

use more than one PDTCH in parallel (multislot operation) for individual packet transfers.

! Packet Associated Control Channel (PACCH) is used to convey signaling information related to a given MS such as Acknowledgements (ACK) and Power Control (PC) information. It also carrier resource assignment and reassignment messages either for the allocation of a PDTCH or further occurrences of a PACCH.

To finalize the GPRS logical channels, it is important to know that GPRS radio interface consists of asymmetry and independent uplink and downlink channels. For example, in a certain TDMA time slot, a PDCH uplink may carry data from one MS and the downlink may do the same thing but for other MS.

Future of GPRS In the wired world, data network performance issues, namely network congestion are often addresses by over provisioning the network. The renaissance of today's wireless Internet is surely accelerating innovation and competition between markets around the world. However, the architecture, protocols, services and wireless technologies that constitute wireless internet are still under reformation and are subject to great debate. GPRS is one of the key players in this revolution and most likely will become the triumphant path in order to escalate to third generation. As it has been explained above, GPRS lack of certain features however, these drawbacks are being discussed under different standards but mainly under ETSI consortium. The idea is to offer support for seamless mobility, paging, and service quality at an efficient cost effectiveness, inherent flexibility, and scalability among IP networks. Furthermore, the development of IP-centric mobile communication networks presents a number of challenges that go beyond the existing capabilities of GPRS and third generation networks. That is why a number of new initiatives are taking place in order to address some of these challenges as well as to propose enhancements to GPRS technology, thus all types of data are supported including the concept of Voice over IP (VoIP). In other words, the key goal of 2.5G and 3G systems is to increase the data throughput capabilities for both the individual and the network without loosing cost effectiveness, application flexibility, and transparency of IP technologies. Recall that in 2G systems such as Time Division Multiple Access (TDMA) based on Global System for Mobile Communication (GSM), the primary goal of this increased capacity was to allow more wireless voice users on the network. In 2.5G systems, GSM/GPRS designers and operators focused on the network architecture so that a more increased capacity could be supported while offering wireless internet services. In addition, they have also realized that in order to accomplish this tremendous task, without sacrificing the network capacity, they must incorporate the mean of Quality of Service (QoS), which includes managing user data throughput, data integrity, and delay on a per service basis. As the standards are being discussed, the concept of QoS has adopted new norms and definitions so that the ability of a service provider to satisfy a customer's application requirements is met. Furthermore, new mechanisms are currently being discussed to assure QoS for customer applications. Some of the key factors when analyzing QoS are data integrity, precedence, delay and throughput in addition to QoS fundamental standards, QoS algorithms, and QoS infrastructure and implementation perspectives. In other words, QoS must be provided in every network so that the user can have a reasonable streaming experience. In a wireless application QoS is intended to be pervasive throughout the network thus that the SGSN is responsible for controlling GPRS bearer service QoS, and the BSS system is responsible for the combination of radio bearer and Um bearer services. The radio bearer requires resources over the air. Here, the BSC allocates radio resources based on the request from the core network elements. The Um bearer service requires QoS for IP and ATM transport. In addition to these QoS bearers’ concepts, it is also

applicable in the core network where the packet switched core network is based on IP network, thus IP QoS mechanisms such as RSVP must be used in the backbone bearer. Note that the external network is the Internet, and therefore IP based QoS mechanisms to provide external bearer QoS. GPRS QoS classes have been specified previously and will not be mentioned in this section since its modifications are under standard revisions. Moreover, QoS signaling refers to how QoS requests are exchanged between entities. To provide end-to-end QoS, QoS requests need to be exchanged between entity domains. At this point two main procedures are being considerate to manage QoS in GPRS. One is session setup in where the QoS requests are exchanged in signaling messages, and second is traffic exchanges in where each packet can be market with the required QoS class for that packet. As a packet transverses the network, each node in the network treats the packet according to the QoS marking in the packet header. Each values of this QoS field maps to predefined bandwidth, delay, and packet error rate and priority requirements. Overall, during the initial deployment of 2.5 and then 3G the wireless Internet will be predominant flavor of service for public as well as for private networks, albeit it will take a while for the networks to mature the new services to be developed and deployed. Therefore, the typical services will mirror those seen on the public Internet today, such as web browsing, email, e-commerce, m-commerce and all the other mentioned earlier in this paper. GPRS conclusion In this article it has been examined the concept, evolution, challenges and future views that concerns GPRS development and its considerations for 3G systems. It is captured that GPRS is an evolutionary path chosen by the Universal Wireless Communication Consortium (UWCC) among many others with the purpose to let GPRS evolve to the next step, which will lead towards the convergence of GSM and TDMA systems for its next generation. Thus, these transformations will offer GPRS a wide range through a single packet network. IP networks allow the combination of data, voice and other multimedia traffic on a single packet network, thus as a function of next generation networks application such VoIP will become possible. However, converging networks to facilitate VoIP will create technical challenges in both system infrastructure and VoIP network. These include Internetworking of IP & SS7 networks, Voice quality, Call Generation, Gateway controllers, Media (MG) and Signaling Gateways (SG), Transport (IP, ATM), Signaling interoperability (i.e. H.323, SIP, SIP-T, MGCP, MEGACO/H.248, ISUP, and ISDN), among other elements. In spite of foreseeable technical difficulties mobile communication has become one of the most dynamic sectors of activity. That is why there is so much effort to continue with the development and enhancements of GPRS especially since the current market holds some 700 million mobile subscribers of which some 70 percent use GSM digital

technology. The endeavors of developing a vast of astounding services by the need to continuously widen the user’s choice in the terms of access, service, and content, anywhere, anytime, and by means of terminal equipment will expand throughout the world, even thought full potential of knowledge based-applications and services has not yet been unleashed despite the wide availability of the mobile communication and the parallel phenomenal development of the Internet growing rapidly. Therefore, it is inferred that GPRS growth will come predominately upon the new approaches and development of other Internet applications exclusively QoS and the evolution of IPv6. The perspective of today’s information society call for a multiplicity of small devices, including IP-enable mobile terminals, home appliances, PDAs, personal computers and so on, to be globally connected. To cope with these complex connectivity requirements, current mobile and wireless systems and architectural concepts must evolved. However, a ubiquitous obstacle preventing these new concept is that currently there are not information devices capable of roaming across a variety of heterogeneous networks and that is a challenge that designers and telecommunication providers need to overcome in order to proceed to future generations. That is why mobile operators are currently extending and expanding their 2G and 2.5G GPRS networks, but with the goal of moving toward more advanced technologies, which made possible by so called 3G and 4G mobile systems where massive investments are being made for new spectrum licenses and network deployments.

References [1] Cai Jian and David J. Goodman. “General Packet Radio Service in GSM, “ IEEE Communication Magazine, October 1997 pp. 122-131 [2] Brasche, Gotz, and Berhand Walke. “Concepts, Services and Protocols of the new GSM Phase 2+ General Packet Radio Services in GSM, “IEEE Communication Magazine, August 1997, pp. 94-104 [3] MOBILE Lifestreams Limited. Data on GPRS. www.mobileGPRS.com [4] GSM world. Topic on GPRS. www.gsmworld.com [5] Seybold, Andrew. “Wireless Data Opportunities.” www.wirelessknowledge.com [6] Advances in Packet Switching, Routing in Optical Networks, QoS and Resources Allocation in the 3G Wireless Networks. IEEE Communication Magazine February 2001 [7] GSM 01.61 GPRS Ciphering algorithm requirements [8] GSM 02.60 Service Description of General Packet Radio Service (GPRS) [9] GSM 03.60 General Packet Radio Interface, Stage 2 [10] GSM 03.64 Overall Description of GPRS Radio Interface; Stage 2 [11] GSM 04.01 Mobile Station Base Station System (MS-BSS) interface; General aspects and principles [12] GSM 04.08 Mobile radio interface layer 3 specifications [13] GSM 05.01 Physical layer on the radio path, General description [14] GSM 05.02 Multiplexing and multiple accesses on the radio path [15] GSM 08.18 General Packet Radio, (GPRS); Base Station System (BSS) – Serving GPRS Support node (SGSN); BSS GPRS Protocol (BSSGP) [16] TS.24.008 “3rd Generation Partnership Project; Technical Specification Group Core Network; Mobile Radio Interface layer 3 specification; Core Network Protocol – stage 3