Telecom Tutorials by Samir Amberkar

Inside PS - MS PDP Activation [Under GPRS]
» GPRS description - 23.060 «
» Layer 3 - 24.007 «
» AT commands - 27.007 «

Next procedure would be MS initiated PDP Activation.

User invokes an application which would like to access PDN say, Browser application (or PPP entity in MS if MS is connected to external computer) is invoked for Internet access. Application layer configures PDP context with AT command +CGDCONT. MN layer stores the configuration for later use and replies AT OK. Now Application need to send +CGACT=1 for PDP activation.

MN

SN

SM

GMM

LLC

GRR

MN receives AT command +CGACT=1. It then sends request to SM to activate PDP. QoS, APN and other parameters are taken from what application has configured via +CGDCONT. SMREG-PDP-ACT-REQ SM need OK from GMM, so it sends establish request to GMM. GMMSM-EST-REQ MS is already GPRS attached, so GMM just sends the confirmation to SM. If not attached, attach procedure will be performed. GMMSM-EST-CNF SM need to send pdp activation request to network. GMMSM-UNITDATA-REQ [SM PDU=Activate PDP Context Req] LL-UNITDATA-REQ [SM PDU=Activate PDP Context Req] GRR-DATA-REQ [LLC UI frame] At this time, One-phase access procedure will be performed to send LLC PDU. Network SM layer will do further procedure and PDP context will be activated. Refer SM article. Downlink TBF procedure will be performed for network to send SM PDP accept message. GRR-DATA-IND [LLC UI frame] LL-UNITDATA-IND [SM PDU=Activate PDP Context Accept] GMMSM-UNITDATA-IND [SM PDU=Activate PDP Context Accept] SM need to inform SNDCP and pass on PDP context parameters to SNDCP. SNSM-ACT-IND [NSAPI, QoS, compression parameters] SNDCP need to trigger XID procedure for negotiating compression parameters. Compression parameters received in SNSM-ACT-IND will be those passed by MN to SM. SNDCP may request LLC to establish connection or may trigger LLC XID procedure (LL-XID-REQ). LL-EST-REQ [SNDCP PDU=XID request, TLLI] GRR-DATA-REQ [LLC SABM frame, SAPI=data] One-phase access procedure will be performed to send LLC PDU. Network LLC and SNDCP layers respond to these requests. Network performs downlink TBF procedure to send the responses to MS. GRR-DATA-IND [LLC UA frame] LLC gets negotiated values for N201, timers etc. LL-EST-CNF [SNDCP PDU=XID confirmation, TLLI] SNSM-ACT-RES SNDCP is ready now. PDP context activation is complete, SM now inform it to MN. MN receives negotiated values for PDP address, QoS, Compression etc. It sends AT OK to application control entity. SMREG-PDP-ACT-CNF Application user entity can now start sending N PDUs (e.g. IP packets) to SNDCP.

References: GPRS in Practice by McGuiggan

Copyright © Samir Amberkar 2010

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