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Sub-Network Dependent Convergence Protocol (SNDCP) [Under GPRS]
» MS-SGSN; SNDCP - 04.65 «
» GPRS description - 23.060 «

SNDCP is top most layer of user plane GPRS protocol stack. Network layer corresponding to PDN with which connection is to be managed lie above SNDCP. Main function of SNDCP is to buffer N PDUs (Network PDU), segment N PDU, add header to each segment (for other side to assemble it) and then give it to LLC for transmission. Receiving side does the reverse procedure.

SNDCP also does compression/decompression. The idea is to reduce amount of data that is required to be transmitted over air. SNDCP need to be aware of certain details about PDN protocol for compression related functions.

To do these functions SNDCP need to be aware of PDP contexts and corresponding information like NSAPI, pdp type, QoS etc. This information is given by SM layer during PDP context activation procedure.

Before transferring N PDUs, SNDCP does XID with peer entity for negotiating compression (PCOMP - Protocol COMPression and DCOMP - Data COMPression) parameters.

Peer SNDCP layers keep track of N PDUs by having N PDU number included in the SNDCP header of the first segment of an N PDU. This is useful during inter SGSN RA update when partially transmitted and partially received N PDUs need to be retransmitted and re-received respectively.

Below diagram shows SNDCP and related SAPs.

References: GPRS in Practice by McGuiggan

Copyright © Samir Amberkar 2010§ §

SM « GPRS Index » Inside PS - SAP diagram