the most important interface in network planning. Gn interface: This is present between the SGSN and SGSN/GGSN of the same network. It provides data and signalling for intra-system functioning. Gd interface: This is present between the SMS-GSMC/SMS-IWMSC and SGSN, pro- viding for better use of the SMS services. Gp interface: This lies between the SGSN and the GGSN of other public land mobile networks. Therefore it is an interface between the two GPRS networks. This interface is highly important considering its strategic location and functions that include security, routing etc. Gs interface: This is present between the SGSN and MSC/VLR. Location data handling and paging requests through the MSC are handled via this interface. SMSGMSC SMS-IWMSC Other PLMN Figure 5.3 GPRS interfaces PROTOCOL STRUCTURE IN A GPRS NETWORK G interface: As this is an interface between the SGSN and HLR, all the subscriber information can be assessed by the SGSN from the HLR. Gf interface: This interface gives the SGSN the equipment information that is present in the EIR. G; interface: This lies between the GGSN and external networks. This is not a standard interface, as the specification will depend on the type of interface that will be connected to the GPRS network. 5.4 PROTOCOL STRUCTURE IN A GPRS NETWORK The protocol structure in GPRS is quite different from that for GSM, as shown in Figure 5.4. Application SNDCP SNDCP UDP/TCP UDP/TCP BSSGP BSSGP N/W Services N/W Services GSM RF GSM RF Figure 5.4 GPRS protocol stack 5.4.1 MS Protocols Two kinds of physical layer have been defined in the GSM 05 specifications: the RF layer and the link layer. The physical RF layer performs modulation/demodulation of the in- put signal (information) apart from FEC (forward error correction) coding, interleaving and congestion detection. The physical link layer provides the services that are required to trans- fer this information over the air interface; these include coding, framing, synchronisation, monitoring of radio quality, power control procedures