GSM PHASE 2+
`GENERAL PACKET RADIO SERVICE GPRS:
`ARCHITECTURE, PROTOCOLS,
`AND AIR INTERFACE
`
`CHRISTIAN BETTSTETTER, HANS-JÖRG VÖGEL, AND JÖRG EBERSPÄCHER
`TECHNISCHE UNIVERSITÄT MÜNCHEN (TUM)
`
`ABSTRACT
`The General Packet Radio Service (GPRS) is a new bearer service for GSM that greatly
`improves and simplifies wireless access to packet data networks, e.g., to the Internet.
`It applies a packet radio principle to transfer user data packets in an efficient way between
`mobile stations and external packet data networks. This tutorial gives an introduction to GPRS.
`The article discusses the system architecture and its basic functionality. It explains the offered
`services, the session and mobility management, the routing, the GPRS air interface including
`channel coding, and the GPRS protocol architecture. Finally, an interworking example between
`GPRS and IP networks is shown.
`
`T
`
`he impressive growth of cellular mobile telephony as
`well as the number of Internet users promises an
`exciting potential for a market that combines both
`innovations: cellular wireless data services. Within the
`next few years, there will be an extensive demand for wireless
`data services. In particular, high-performance wireless Inter-
`net access will be requested by users.
`Existing cellular data services do not fulfill the needs of
`users and providers. From the user’s point of view, data rates
`are too slow and the connection setup takes too long and is
`rather complicated. Moreover, the service is too expensive for
`most users. From the technical point of view, the drawback
`results from the fact that current wireless data services are
`based on circuit switched radio transmission. At the air inter-
`face, a complete traffic channel is allocated for a single user
`for the entire call period. In case of bursty traffic (e.g., Inter-
`net traffic), this results in a highly inefficient resource utiliza-
`tion. It is obvious that for bursty traffic, packet switched
`bearer services result in a much better utilization of the traffic
`channels. This is because a channel will only be allocated
`when needed and will be released immediately after the trans-
`mission of the packets. With this principle, multiple users can
`share one physical channel (statistical multiplexing).
`In order to address these inefficiencies, two cellular packet
`data technologies have been developed so far: cellular digital
`packet data (CDPD) (for AMPS, IS-95, and IS-136) and the
`General Packet Radio Service (GPRS). GPRS is the topic of
`this paper. It was originally developed for GSM, but will also
`
`be integrated within IS-136 (see [1]). We treat GPRS from
`the point of view of GSM.
`GPRS is a new bearer service for GSM that greatly
`improves and simplifies wireless access to packet data net-
`works, e.g., to the Internet. It applies a packet radio principle
`to transfer user data packets in an efficient way between GSM
`mobile stations and external packet data networks. Packets
`can be directly routed from the GPRS mobile stations to
`packet switched networks. Networks based on the Internet
`Protocol (IP) (e.g., the global Internet or private/corporate
`intranets) and X.25 networks are supported in the current ver-
`sion of GPRS.
`Users of GPRS benefit from shorter access times and high-
`er data rates. In conventional GSM, the connection setup
`takes several seconds and rates for data transmission are
`restricted to 9.6 kbit/s. GPRS in practice offers session estab-
`lishment times below one second and ISDN-like data rates up
`to several ten kbit/s.
`In addition, GPRS packet transmission offers a more user-
`friendly billing than that offered by circuit switched services. In
`circuit switched services, billing is based on the duration of the
`connection. This is unsuitable for applications with bursty traffic.
`The user must pay for the entire airtime, even for idle periods
`when no packets are sent (e.g., when the user reads a Web
`page). In contrast to this, with packet switched services, billing
`can be based on the amount of transmitted data. The advantage
`for the user is that he or she can be “online” over a long period
`of time but will be billed based on the transmitted data volume.
`
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`
`To sum up, GPRS improves the uti-
`lization of the radio resources, offers
`volume-based billing, higher transfer
`rates, shorter access times, and simpli-
`fies the access to packet data networks.
`GPRS has been standardized by
`ETSI (the European Telecommunica-
`tions Standards Institute) during the
`last five years. It finds great interest
`among many GSM network providers.
`At the moment field trials are being
`carried out, and it is expected that
`GPRS will be implemented in various
`countries by the middle of 2000 (see,
`e.g., [2][3] for Germany).
`This article provides an introduc-
`tion to GPRS. We assume that the
`reader is familiar with the basic con-
`cepts of cellular networks. A brief
`overview of the GSM system can be
`found in [4]. In addition, there exists a
`variety of books on GSM, e.g.,
`[5][6][7]. The structure of the paper is as follows. First we
`describe the GPRS system architecture and discuss the funda-
`mental functionality. We then describe the offered services
`and the Quality of Service parameters. Afterward we show
`how a GPRS mobile station registers with the network, and
`how the network keeps track of its location. An example of
`how packets are routed in GPRS is given. Next, the physical
`layer at the air interface is explained, and we discuss the con-
`cept of multiple access, radio resource management, and the
`logical channels and their mapping onto physical channels.
`We then consider GPRS channel coding, and follow this with
`a discussion of the GPRS protocol architecture. Finally, we
`give an example of a GPRS-Internet interconnection.
`
`SYSTEM ARCHITECTURE
`GENERAL GSM CONCEPT
`In order to understand the GPRS system architecture, let
`us review the general GSM system concept and GSM address-
`ing [5].
`
`GSM System Architecture — Fig. 1 shows the system archi-
`tecture of a GSM public land mobile network (PLMN) with
`essential components [5]. A GSM mobile station is denoted as
`MS. A cell is formed by the radio area coverage of a base
`transceiver station (BTS). Several BTSs together are con-
`trolled by one base station controller (BSC). The BTS and
`BSC together form the base station subsystem (BSS). The
`combined traffic of the mobile stations in their respective cells
`is routed through a switch, the mobile switching center
`(MSC). Connections originating from or terminating in the
`fixed network (e.g., ISDN) are handled by a dedicated gate-
`way mobile switching center (GMSC). GSM networks are
`structured hierarchically. They consist of at least one adminis-
`trative region, which is assigned to a MSC. Each administra-
`tive region is made up of at least one location area (LA). A
`location area consists of several cell groups. Each cell group is
`assigned to a BSC.
`Several data bases are available for call control and net-
`work management: the home location register (HLR), the vis-
`ited location register (VLR), the authentication center
`(AUC), and the equipment identity register (EIR).
`For all users registered with a network operator, permanent
`data (such as the user’s profile) as well as temporary data
`
`n FIGURE 1. GSM system architecture with essential components.
`
`(such as the user’s current location) are stored in the HLR. In
`case of a call to a user, the HLR is always first queried, to
`determine the user’s current location. A VLR is responsible
`for a group of location areas and stores the data of those users
`who are currently in its area of responsibility. This includes
`parts of the permanent user data that have been transmitted
`from the HLR to the VLR for faster access. But the VLR may
`also assign and store local data such as a temporary identifica-
`tion. The AUC generates and stores security-related data such
`as keys used for authentication and encryption, whereas the
`EIR registers equipment data rather than subscriber data.
`
`GSM Addresses and Identifiers — GSM distinguishes
`explicitly between user and equipment and deals with them
`separately. Besides phone numbers and subscriber and equip-
`ment identifiers, several other identifiers have been defined;
`they are needed for the management of subscriber mobility
`and for addressing of all the remaining network elements.
`The international mobile station equipment identity
`(IMEI) uniquely identifies a mobile station internationally. It
`is a kind of serial number. The IMEI is allocated by the
`equipment manufacturer and registered by the network opera-
`tor who stores it in the EIR.
`Each registered user is uniquely identified by its interna-
`tional mobile subscriber identity (IMSI). It is stored in the
`subscriber identity module (SIM) (see Fig. 1). A mobile sta-
`tion can only be operated if a SIM with a valid IMSI is insert-
`ed into equipment with a valid IMEI.
`The “real telephone number” of a mobile station is the
`mobile subscriber ISDN number (MSISDN). It is assigned to
`the subscriber (his or her SIM, respectively), such that a
`mobile station set can have several MSISDNs depending on
`the SIM.
`The VLR, which is responsible for the current location of
`a subscriber, can assign a temporary mobile subscriber identity
`(TMSI) which has only local significance in the area handled
`by the VLR. It is stored on the network side only in the VLR
`and is not passed to the HLR.
`
`GPRS SYSTEM ARCHITECTURE
`
`In order to integrate GPRS into the existing GSM archi-
`tecture, a new class of network nodes, called GPRS support
`nodes (GSN), has been introduced [8]. GSNs are responsible
`for the delivery and routing of data packets between the
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`IEEE Communications Surveys • http://www.comsoc.org/pubs/surveys • Third Quarter 1999, vol. 2 no. 3
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`packet data networks. It converts the
`GPRS packets coming from the SGSN
`into the appropriate packet data pro-
`tocol (PDP) format (e.g., IP or X.25)
`and sends them out on the corre-
`sponding packet data network. In the
`other direction, PDP addresses of
`incoming data packets are converted
`to the GSM address of the destination
`user. The readdressed packets are
`sent to the responsible SGSN. For
`this purpose, the GGSN stores the
`current SGSN address of the user and
`his or her profile in its location regis-
`ter. The GGSN also performs authen-
`tication and charging functions.
`In general, there is a many-to-
`many relationship between the
`SGSNs and the GGSNs: A GGSN is
`the interface to external packet data
`networks for several SGSNs; an
`SGSN may route its packets over different GGSNs to reach
`different packet data networks.
`Fig. 2 also shows the interfaces between the new network
`nodes and the GSM network as defined by ETSI in [8].
`The Gb interface connects the BSC with the SGSN. Via
`the Gn and the Gp interfaces, user data and signaling data are
`transmitted between the GSNs. The Gn interface will be used
`if SGSN and GGSN are located in the same PLMN, whereas
`the Gp interface will be used if they are in different PLMNs.
`All GSNs are connected via an IP-based GPRS backbone
`network. Within this backbone, the GSNs encapsulate the PDN
`packets and transmit (tunnel) them using the GPRS Tunneling
`Protocol GTP. There are two kinds of GPRS backbones:
`
`n FIGURE 2. GPRS system architecture.
`
`mobile stations and the external packet data networks (PDN).
`Fig. 2 illustrates the system architecture.
`A serving GPRS support node (SGSN) is responsible for
`the delivery of data packets from and to the mobile stations
`within its service area. Its tasks include packet routing and
`transfer, mobility management (attach/detach and location
`management), logical link management, and authentication
`and charging functions. The location register of the SGSN
`stores location information (e.g., current cell, current VLR)
`and user profiles (e.g., IMSI, address(es) used in the packet
`data network) of all GPRS users registered with this SGSN.
`A gateway GPRS support node (GGSN) acts as an inter-
`face between the GPRS backbone network and the external
`
`n FIGURE 3. GPRS system architecture and routing example.
`
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`• Intra-PLMN backbone networks connect GSNs of the
`same PLMN and are therefore private IP-based networks
`of the GPRS network provider.
`• Inter-PLMN backbone networks connect GSNs of differ-
`ent PLMNs. A roaming agreement between two GPRS
`network providers is necessary to install such a backbone.
`Fig. 3 shows two intra-PLMN backbone networks of differ-
`ent PLMNs connected with an inter-PLMN backbone. The
`gateways between the PLMNs and the external inter-PLMN
`backbone are called border gateways. Among other things,
`they perform security functions to protect the private intra-
`PLMN backbones against unauthorized users and attacks. The
`illustrated routing example will be explained later.
`The Gn and Gp interfaces are also defined between two
`SGSNs. This allows the SGSNs to exchange user profiles
`when a mobile station moves from one SGSN area to another.
`Across the Gf interface, the SGSN may query the IMEI of
`a mobile station trying to register with the network.
`The Gi interface connects the PLMN with external public
`or private PDNs, such as the Internet or corporate intranets.
`Interfaces to IP (IPv4 and IPv6) and X.25 networks are sup-
`ported.
`The HLR stores the user profile, the current SGSN address,
`and the PDP address(es) for each GPRS user in the PLMN.
`The Gr interface is used to exchange this information between
`HLR and SGSN. For example, the SGSN informs the HLR
`about the current location of the MS. When the MS registers
`with a new SGSN, the HLR will send the user profile to the
`new SGSN. The signaling path between GGSN and HLR (Gc
`interface) may be used by the GGSN to query a user’s location
`and profile in order to update its location register.
`In addition, the MSC/VLR may be extended with functions
`and register entries that allow efficient coordination between
`packet switched (GPRS) and circuit switched (conventional
`GSM) services. Examples of this are combined GPRS and
`non-GPRS location updates and combined attachment proce-
`dures. Moreover, paging requests of circuit switched GSM
`calls can be performed via the SGSN. For this purpose, the
`Gs interface connects the data bases of SGSN and MSC/VLR.
`To exchange messages of the short message service (SMS)
`via GPRS, the Gd interface is defined. It interconnects the
`SMS gateway MSC (SMS-GMSC) with the SGSN.
`
`SERVICES
`BEARER SERVICES AND SUPPLEMENTARY SERVICES
`The bearer services of GPRS offer end-to-end packet
`switched data transfer. There are two different kinds: The
`point-to-point (PTP) service and the point-to-multipoint
`(PTM) service. The latter will be available in future releases
`of GPRS.
`The PTP service [9] offers transfer of data packets between
`two users. It is offered in both connectionless mode (PTP con-
`nectionless network service (PTP-CLNS), e.g., for IP) and
`connection-oriented mode (PTP connection-oriented network
`service (PTP-CONS), e.g., for X.25).
`The PTM service offers transfer of data packets from one user
`to multiple users. There exist two kinds of PTM services [10]:
`• Using the multicast service PTM-M, data packets are
`broadcast in a certain geographical area. A group identi-
`fier indicates whether the packets are intended for all
`users or for a group of users.
`• Using the group call service PTM-G, data packets are
`addressed to a group of users (PTM group) and are sent
`out in geographical areas where the group members are
`currently located.
`
`Probability for
`
`Class
`
`Lost
`packet
`
`Duplicated
`packet
`
`Out of
`sequence
`packet
`
`Corrupted
`packet
`
`1
`
`2
`
`10 –9
`
`10 –4
`
`10 –9
`
`10 –5
`
`10 –2
`10 –5
`3
`n Table 1. Reliability classes.
`
`10 –9
`
`10 –5
`
`10 –5
`
`10 –9
`
`10 –6
`
`10 –2
`
`128 byte packet
`
`1024 byte packet
`
`Class
`
`1
`
`2
`
`3
`
`Mean
`delay
`
`<0.5s
`
`<5s
`
`<50s
`
`4
`
`Best
`effort
`n Table 2. Delay classes.
`
`95%
`delay
`
`<1.5s
`
`<25s
`
`<250s
`
`Best
`effort
`
`Mean
`delay
`
`<2s
`
`< 15s
`
`<75s
`
`Best
`effort
`
`95%
`delay
`
`<7s
`
`<75s
`
`<375s
`
`Best
`effort
`
`It is also possible to send SMS messages over GPRS. In
`addition, it is planned to implement supplementary services,
`such as call forwarding unconditional (CFU), call forwarding
`on mobile subscriber not reachable (CFNRc), and closed user
`group (CUG).
`Moreover, a GPRS service provider may offer additional
`non-standardized services, such as access to data bases, mes-
`saging services, and tele-action services (e.g., credit card vali-
`dations, lottery transactions, and electronic monitoring and
`surveillance systems) [9].
`
`QUALITY OF SERVICE
`
`The Quality of Service QoS requirements of typical mobile
`packet data applications are very diverse (e.g., consider real-
`time multimedia, Web browsing, and e-mail transfer). Support
`of different QoS classes, which can be specified for each indi-
`vidual session, is therefore an important feature. GPRS allows
`defining QoS profiles using the parameters service prece-
`dence, reliability, delay, and throughput [9].
`• The service precedence is the priority of a service in rela-
`tion to another service. There exist three levels of priori-
`ty: high, normal, and low.
`• The reliability indicates the transmission characteristics
`required by an application. Three reliability classes are
`defined, which guarantee certain maximum values for
`the probability of loss, duplication, mis-sequencing,
`and corruption (an undetected error) of packets (see
`Table 1).
`• The delay parameters define maximum values for the
`mean delay and the 95-percentile delay (see Table 2).
`The latter is the maximum delay guaranteed in 95 per-
`cent of all transfers. The delay is defined as the end-to-
`end transfer time between two communicating mobile
`stations or between a mobile station and the Gi interface
`to an external packet data network. This includes all
`delays within the GPRS network, e.g., the delay for
`request and assignment of radio resources and the transit
`delay in the GPRS backbone network. Transfer delays
`outside the GPRS network, e.g., in external transit net-
`works, are not taken into account.
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`• The throughput specifies the maximum/peak bit rate and
`the mean bit rate.
`Using these QoS classes, QoS profiles can be negotiated
`between the mobile user and the network for each session,
`depending on the QoS demand and the current available
`resources. The billing of the service is then based on the
`transmitted data volume, the type of service, and the chosen
`QoS profile.
`
`SIMULTANEOUS USAGE OF
`PACKET SWITCHED AND CIRCUIT SWITCHED SERVICES
`In a GSM/GPRS network, conventional circuit switched
`services (speech, data, and SMS) and GPRS services can be
`used in parallel. Three classes of mobile stations are defined
`[9]:
`• A class A mobile station supports simultaneous operation
`of GPRS and conventional GSM services.
`• A class B mobile station is able to register with the net-
`work for both GPRS and conventional GSM services
`simultaneously. In contrast to an MS of class A, it can
`only use one of the two services at a given time.
`• A class C mobile station can attach for either GPRS or
`conventional GSM services. Simultaneous registration
`(and usage) is not possible. An exception are SMS mes-
`sages, which can be received and sent at any time.
`
`SESSION MANAGEMENT,
`MOBILITY MANAGEMENT, AND ROUTING
`In this section, we describe how a mobile station (MS) reg-
`isters with the GPRS network and becomes known to an
`external packet data network (PDN). We show how packets
`are routed to or from mobile stations, and how the network
`keeps track of the current location of the user.
`
`ATTACHMENT AND DETACHMENT PROCEDURE
`
`Before a mobile station can use GPRS services, it must reg-
`ister with an SGSN of the GPRS network. The network checks
`if the user is authorized, copies the user profile from the HLR
`to the SGSN, and assigns a packet temporary mobile sub-
`scriber identity (P-TMSI) to the user. This procedure is called
`GPRS attach. For mobile stations using both circuit switched
`
`and packet switched services it is possible to perform com-
`bined GPRS/IMSI attach procedures. The disconnection from
`the GPRS network is called GPRS detach. It can be initiated
`by the mobile station or by the network (SGSN or HLR).
`
`SESSION MANAGEMENT, PDP CONTEXT
`
`To exchange data packets with external PDNs after a suc-
`cessful GPRS attach, a mobile station must apply for one or
`more addresses used in the PDN, e.g., for an IP address in
`case the PDN is an IP network. This address is called PDP
`address (Packet Data Protocol address). For each session, a
`so-called PDP context is created, which describes the charac-
`teristics of the session. It contains the PDP type (e.g., IPv4),
`the PDP address assigned to the mobile station (e.g.,
`129.187.222.10), the requested QoS, and the address of a
`GGSN that serves as the access point to the PDN. This con-
`text is stored in the MS, the SGSN, and the GGSN. With an
`active PDP context, the mobile station is “visible” for the
`external PDN and is able to send and receive data packets.
`The mapping between the two addresses, PDP and IMSI,
`enables the GGSN to transfer data packets between PDN and
`MS. A user may have several simultaneous PDP contexts
`active at a given time.
`The allocation of the PDP address can be static or dynam-
`ic. In the first case, the network operator of the user’s home-
`PLMN permanently assigns a PDP address to the user. In the
`second case, a PDP address is assigned to the user upon acti-
`vation of a PDP context. The PDP address can be assigned by
`the operator of the user’s home-PLMN (dynamic home-
`PLMN PDP address) or by the operator of the visited net-
`work (dynamic visited-PLMN PDP address). The home
`network operator decides which of the possible alternatives
`may be used. In case of dynamic PDP address assignment, the
`GGSN is responsible for the allocation and the activation/
`deactivation of the PDP addresses.
`Fig. 4 shows the PDP context activation procedure. Using
`the message “activate PDP context request,” the MS informs
`the SGSN about the requested PDP context. If dynamic PDP
`address assignment is requested, the parameter PDP address
`will be left empty. Afterward, usual security functions (e.g.,
`authentication of the user) are performed. If access is granted,
`the SGSN will send a “create PDP context request” message
`to the affected GGSN. The latter creates a new entry in its
`PDP context table, which enables the GGSN to route data
`packets between the SGSN and the external
`PDN. Afterward, the GGSN returns a confir-
`mation message “create PDP context response”
`to the SGSN, which contains the PDP address
`in case dynamic PDP address allocation was
`requested. The SGSN updates its PDP context
`table and confirms the activation of the new
`PDP context to the MS (“activate PDP context
`accept”).
`GPRS also supports anonymous PDP con-
`text activation. In this case, security functions as
`shown in Fig. 4 are skipped, and thus, the user
`(i.e., the IMSI) using the PDP context remains
`unknown to the network. Anonymous context
`activation may be employed for pre-paid ser-
`vices, where the user does not want to be iden-
`tified. Only dynamic address allocation is
`possible in this case.
`
`ROUTING
`
`n FIGURE 4. PDP context activation.
`
`Fig. 3 gives an example of how packets are
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`routed in GPRS. We assume that the packet data network
`is an IP network. A GPRS mobile station located in PLMN1
`sends IP packets to a host connected to the IP network, e.g.,
`to a Web server connected to the Internet. The SGSN that
`the mobile station is registered with encapsulates the IP
`packets coming from the mobile station, examines the PDP
`context, and routes them through the intra-PLMN GPRS
`backbone to the appropriate GGSN. The GGSN decapsu-
`lates the packets and sends them out on the IP network,
`where IP routing mechanisms are used to transfer the pack-
`ets to the access router of the destination network. The lat-
`ter delivers the IP packets to the host.
`Let us assume the home-PLMN of the mobile station is
`PLMN2. An IP address has been assigned to the mobile by
`the GGSN of PLMN2. Thus, the MS’s IP address has the
`same network prefix as the IP address of the GGSN in
`PLMN2. The correspondent host is now sending IP packets
`to the MS. The packets are sent out onto the IP network
`and are routed to the GGSN of PLMN2 (the home-GGSN
`of the MS). The latter queries the HLR and obtains the
`information that the MS is currently located in PLMN1. It
`encapsulates the incoming IP packets and tunnels them
`through the inter-PLMN GPRS backbone to the appropriate
`SGSN in PLMN1. The SGSN decapsulates the packets and
`delivers them to the MS.
`
`LOCATION MANAGEMENT
`
`The main task of location management is to keep track of
`the user’s current location, so that incoming packets can be
`routed to his or her MS. For this purpose, the MS frequently
`sends location update messages to its current SGSN. If the
`MS sends updates rather seldom, its location (e.g., its current
`cell) is not known exactly and paging is necessary for each
`downlink packet, resulting in a significant delivery delay. On
`the other hand, if location updates happen very often, the
`MS’s location is well known to the network, and the data
`packets can be delivered without any additional paging delay.
`However, quite a lot of uplink radio capacity and battery
`power is consumed for mobility management in this case.
`Thus, a good location management strategy must be a com-
`promise between these two extreme methods.
`For this reason, a state model shown in Fig. 5 has been
`defined for location management in GPRS [11]. A MS can be in
`one of three states depending on its current traffic amount; the
`location update frequency is dependent on the state of the MS.
`
`n FIGURE 6. Intr-SGSN routing area update.
`
`n FIGURE 5. State model of a GPRS mobile station.
`
`In IDLE state the MS is not reachable. Performing a
`GPRS attach, the MS gets into READY state. With a GPRS
`detach it may disconnect from the network and fall back to
`IDLE state. All PDP contexts will be deleted. The STANDBY
`state will be reached when an MS does not send any packets
`for a longer period of time, and therefore the READY timer
`(which was started at GPRS attach) expires.
`In IDLE state, no location updating is performed, i.e., the
`current location of the MS is unknown to the network. An MS
`in READY state informs its SGSN of every movement to a
`new cell. For the location management of an MS in STAND-
`BY state, a GSM location area (LA) is divided into several
`routing areas (RA). In general, an RA consists of several
`cells. The SGSN will only be informed when an MS moves to
`a new RA; cell changes will not be disclosed. To find out the
`current cell of an MS in STANDBY state, paging of the MS
`within a certain RA must be performed (see Fig. 9). For MSs
`in READY state, no paging is necessary.
`Whenever an MS moves to a new RA, it sends a “routing
`area update request” to its assigned SGSN (see Fig. 6). The
`message contains the routing area identity (RAI) of its old
`RA. The base station subsystem (BSS) adds the cell identifier
`(CI) of the new cell, from which the SGSN can derive the new
`RAI. Two different scenarios are possible:
`• Intra-SGSN routing area update (Fig. 6): The MS has
`moved to an RA that is assigned to the same SGSN as the
`old RA. In this case, the SGSN has
`already stored the necessary user profile
`and can assign a new packet temporary
`mobile subscriber identity (P-TMSI) to
`the user (“routing area update accept”).
`Since the routing context does not change,
`there is no need to inform other network
`elements, such as GGSN or HLR.
`• Inter-SGSN routing area update: The
`new RA is administered by a different
`SGSN than the old RA. The new SGSN
`realizes that the MS has changed to its
`area and requests the old SGSN to send
`the PDP contexts of the user. After-
`ward, the new SGSN informs the
`involved GGSNs about the user’s new
`routing context. In addition, the HLR
`and (if needed) the MSC/VLR are
`informed about the user’s new SGSN.
`There also exist combined RA/LA
`updates. These occur when an MS using
`
`IEEE Communications Surveys • http://www.comsoc.org/pubs/surveys • Third Quarter 1999, vol. 2 no. 3
`
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`

`
`GPRS as well as conventional GSM moves to a new LA. The
`MS sends a “routing area update request” to the SGSN. The
`parameter “update type” is used to indicate that an LA
`update is needed. The message is then forwarded to the VLR,
`which performs the LA update.
`To sum up, GPRS mobility management consists of two
`levels: Micro mobility management tracks the current routing
`area or cell of the mobile station. It is performed by the
`SGSN. Macro mobility management keeps track of the mobile
`station’s current SGSN and stores it in the HLR, VLR, and
`GGSN.
`
`AIR INTERFACE — PHYSICAL LAYER
`MULTIPLE ACCESS AND
`RADIO RESOURCE MANAGEMENT PRINCIPLES
`On the physical layer, GSM uses a combination of FDMA
`and TDMA for multiple access. As shown in Fig. 7, two fre-
`quency bands 45 MHz apart have been reserved for GSM
`operation: 890 – 915 MHz for transmission from the mobile
`station, i.e., uplink, and 935 – 960 MHz for transmission from
`the BTS, i.e., downlink. Each of these bands of 25 MHz width
`is divided into 124 single carrier channels of 200 kHz width. A
`certain number of these frequency channels, the so-called cell
`allocation, is allocated to a BTS, i.e., to a cell [5].
`Each of the 200 kHz frequency channels carries eight
`TDMA channels by dividing each of them into eight time
`slots. The eight time slots in these TDMA channels form a
`TDMA frame. Each time slot of a TDMA frame lasts for a
`duration of 156.25 bit times and, if used, contains a data
`burst. The time slot lasts 15/26 ms = 576.9 m s; so a frame
`
`takes 4.613 ms. The recurrence of one particular time slot
`defines a physical channel. A GSM mobile station uses the
`same time slots in the uplink as in the downlink [5].
`The channel allocation in GPRS is different from the origi-
`nal GSM. GPRS allows a single mobile station to transmit on
`multiple time slots of the same TDMA frame (multislot oper-
`ation). This results in a very flexible channel allocation: one to
`eight time slots per TDMA frame can be allocated for one
`mobile station. Moreover, uplink and downlink are allocated
`separately, which efficiently supports asymmetric data traffic
`(e.g., Web browsing).
`In conventional GSM, a channel is permanently allocated
`for a particular user during the entire call period (whether
`data is transmitted or not). In contrast to this, in GPRS the
`channels are only allocated when data packets are sent or
`received, and they are released after the transmission. For
`bursty traffic this results in a much more efficient usage of the
`scarce radio resources. With this principle, multiple users can
`share one physical channel.
`A cell supporting GPRS may allocate physical channels for
`GPRS traffic. Such a physical channel is denoted as packet
`data channel (PDCH). The PDCHs are taken from the com-
`mon pool of all channels available in the cell. Thus, the radio
`resources of a cell are shared by all GPRS and non-GPRS
`mobile stations located in this cell. The mapping of physical
`channels to either packet switched (GPRS) or circuit switched
`(conventional GSM) services can be performed dynamically
`(capacity on demand principle [12]), depending on the current
`traffic load, the priority of the service, and the multislot class.
`A load supervision procedure monitors the load of the PDCHs
`in the cell. According to the current demand, the number of
`channels allocated for GPRS (i.e., the number of PDCHs) can
`
`n FIGURE 7. GSM carrier frequencies, duplexing, and TDMA frames.
`
`8
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`IEEE Communications Surveys • http://www.comsoc.org/pubs/surveys • Third Quarter 1999, vol. 2 no. 3
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`

`
`Group
`
`Channel
`
`Function
`
`Direction
`
`PDTCH
`
`Data traffic
`
`MS «
`
`BSS
`
`PBCCH
`
`PRACH
`PAGCH
`PPCH
`PNCH
`
`PACCH
`
`PTCCH
`
`MS ‹
`
`Broadcast
`control
`Random access MS fi
`MS ‹
`Access grant
`MS ‹
`