e used by external PDNs, including the GGSNs. A physical SGSN node must have network interfaces inside all of the external transport networks in which it is connected (as e.g. the interfaces j5.1 and j6.1 in the internal IP network and the FR network). The corresponding protocol stack is the one that is defined for each transport network, including transmission protocols and physical media. In addition, it must handle protocol stacks that belong to the GPRS network itself, and which define the network interfaces between, e.g. an SGSN and a GGSN (j2.1). This protocol stack defines all protocol relations between the logi- cal GPRS nodes By making the distinction between physical and logical GPRS nodes, each net- work interface will exhibit a unique set of protocols. In this case, the network inter- face between SGSN and GGSN will be equivalent to a reference point, as long as the logical nodes are not further disintegrated into different logical networks. The GPRS logical network can therefore be depicted as shown in Fig. 4.14. Only one instance of each type of logical node is included. O RP7 SNDCP GPRS logical network logical GSM RF-CSP BSSGP (RP1) (RP4) BSSGP SNDCP (RP2) (RP5) Int. IP-CSP FR-CSP BSSAP+ Modified MAP IP applications X.25 applications +TCAP Ext. CSP Ext. CSP SCCP-CSP (CNL only) SCCP-CSP (CNL only) (RP3) (RP8) (RP3) (RP8) Fig. 4.14 The GPRS logical network For the reader who wants to learn more about the specifics of GPRS, the stand- ards for protocols of the logical network model are: MAC: ETSI GSM 04.60 RLC: ETSI GSM 04.60 LLC: ETSI GSM 04.64 GMM: (Not finished, see instead ETSI GSM 03.60) 4 Physical Network Modelling SNDCP: ETSI GSM 04.65 IP: RFC 791 (IPv4) X.25: ITU-T X.25 NWS (Network Service): ETSI GSM 08.16 BSSGP: ETSI GSM 08.18 UDP: RFC 786 GTP: ETSI GSM 09.60 BSSAP+: ETSI GSM 09.18 Modified MAP: ETSI GSM 09.02 Considering the network service interfaces that are defined by the protocol stacks, the GPRS logical network from Fig. 4.14 can be separated into five differ- ent logical networks: 1. 