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.1 GPRS network architecture.(Source: [2] © ETSI 2001.)via GSM uses the base station system (BSS), while the later interface into the 3G network uses the UTRAN connections.Home location register (HLR),visitor location register (VLR), and equipment identification register (EIR)indicate home, visitor, and equipment identification as in GSM, while SMSdenotes the GSM short messaging service.The key GPRS additions are thesupport GPRS service node (SGSN) and the gateway GPRS support node(GGSN) that are usually IP routers with additional specialized software.There is a significant functional difference between the GSNs for 2.5G and3G systems, so a prefix 2 (as in 2-SGSN) indicates that they are for interface to the BSS of a 2G network, while 3-SGSN would indicate full UMTSoperation with a different physical layer.The BSS is itself essentially a 2GBTS that has been upgraded with a packet control unit (PCU) and newsoftware.In terms of the interfaces shown in Figure 5.1, this chapter is primarilyconcerned with Um and Gb from MS to BSS and BSS to SGSN, respectively.The overall structure of the GPRS protocol stack for GSM mode is shown inFigure 5.2.98ApplicationIPIPRelayGTP-UQoSSNDCPSNDCPGTP-UinLLCLLCIntegratedRelayUDPUDPRLCBSSGPRLCBSSGPIPIP3GNetworksMACMACNetworkNetworkL2L2serviceserviceGSM RFGSM RFL1bisL1bisL1L1UmGbGnGiMSBSSSGSNGGSNFigure 5.2 GPRS protocol stack.(Source: [2] © ETSI 2001.)GPRS99In the case of UMTS, the operation of the mobile uses UMTS func-tions at the Uu interface (see Chapter 3).This chapter covers the protocolsthat are used between the MS, BSS, and SGSN.The GPRS Tunneling Pro-tocol (GTP) [3] at interface Gn is described in Section 8.1.3.Mobility Management and AddressingA mobile can exist in one of three MM states for GSM:• Idle: In this state the mobile is not attached to GPRS mobility man-agement and is unreachable, but it is able to choose a PLMN andselect or deselect a cell.• Standby: This state is reached from the idle state by the mobile per-forming a GPRS attach.In this state it is able to receive paginginformation and requests for both GPRS and CS operation.It isalso able to initiate PDP context activation or deactivation.It movesfrom standby state to ready state upon data transmission.• Ready: In this state the mobile is able to send or receive data as wellas activating or deactivating PDP contexts.If a ready state activitytimer expires, it moves back to standby and then to idle if it sends aGPRS detach.Information about the mobile is stored in the HLR with which it is regis-tered and also by the SGSN.The MS has to identify itself in the GPRSattach and does so by means of either its IMSI or a TMSI.The IMSI consistsof a three-digit mobile country code (MCC), a two- or three-digit mobilenetwork code (MNC), and a mobile subscriber identity number (MSIN)within the network, so that the total IMSI does not exceed 15 digits [4].ATMSI is a 32-bit local identifier allocated by either the VLR or SGSN towhich the MS is temporarily attached.A TMSI is used for both circuit andPS services, and these are distinguished by the first 2 bits, where 11 indicates the PS case.A VLR may also allocate a four-octet local mobile subscriberidentity (LMSI).An MS that is attached to the PS services is also assigned a three-octet packet TMSI (P-TMSI) by the SGSN.In addition, the MS orSGSN assigns a temporary logical link identifier (TLLI) on the basis of theP-TMSI (if there is one) or according to a specific set of rules [4] for use in the logical link control (LLC) (see Section 5.2.3) and RLC/MAC layer.Its home PLMN (HPLMN) operator also assigns the MS an IPaddress.This can either be a permanent static address for IPv4 or IPv6 ora dynamic address.Similarly, a visited PLMN (VPLMN) can provide IP100QoS in Integrated 3G Networksaddresses.The choice of whether static or dynamic addresses are to be used is made by the HPLMN operator.Each GPRS support node (GSN) also has an address.This consists of a2-bit type field with values 0 and 1 for IPv4 and IPv6, respectively, an IPaddress length field, and the actual IP address.An access point name (APN)is also assigned to a GGSN and consists of a mandatory APN network identi-fier to identify the external network to which the gateway is attached and an optional APN operator identifier to show which PLMN it belongs to.TheAPN is resolved to the IP address by the GPRS domain name server (DNS).GPRS uses two types of PDP: IP itself and Point-to-Point Protocol(PPP) [5].PDP contexts are established between the MS and network forspecific PDP types, addresses, QoS requirements, and network service accesspoint identifier (NSAPI) (see Section 5.2.3).A single PDP address may havemore than one context (e.g., for different QoS requirements) in Release 4(but not in the 1997–8 version of GPRS), and if so, uses a traffic flow tem-plate (TFT) [2, 6] to identify all but at most one of the contexts.The TFTconsists of one to eight different packet filters for the PDP address [ 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