04.653GPPGeneral Packet Radio Service (GPRS)Mobile Station (MS) - Serving GPRS Support Node (SGSN)Release 1999Subnetwork Dependent Convergence Protocol (SNDCP)TS
The present document describes the functionality of the GPRS SNDCP. The overall GPRS logical architecture is defined in 3GPP TS 03.60 . Location of the SNDCP in GPRS protocol stack can be seen in Figure 1.
Figure 1: GPRS protocol stack
Network layer protocols are intended to be capable of operating over services derived from a wide variety of subnetworks and data links. GPRS supports several network layer protocols providing protocol transparency for the users of the service. Introduction of new network layer protocols to be transferred over GPRS shall be possible without any changes to GPRS. Therefore, all functions related to transfer of Network layer Protocol Data Units (N‑PDUs) shall be carried out in a transparent way by the GPRS network entities. This is one of the requirements for GPRS SNDCP.
Another requirement for the SNDCP is to provide functions that help to improve channel efficiency. This requirement is fulfilled by means of compression techniques.
The set of protocol entities above SNDCP consists of commonly used network protocols. They all use the same SNDCP entity, which then performs multiplexing of data coming from different sources to be sent using the service provided by the LLC layer (Figure 2). The Network Service Access Point Identifier (NSAPI) is an index to the PDP context (see 3GPP TS 03.60 ) of the PDP that is using the services provided by SNDCP. One PDP may have several PDP contexts and NSAPIs. However, it is possible that each allocated NSAPI is used by separate PDP. Each active NSAPI shall use the services provided by the Service Access Point Identifier (SAPI) in the LLC layer. Several NSAPIs may be associated with the same SAPI.
Since the adaptation of different network layer protocols to SNDCP is implementation dependent, it is not defined in the present document.
Figure 2: Example for multiplexing of different protocols