Signaling flow [Under GPRS]
Let us have a brief look at overall way of working for GPRS.
1) In addition to registration to MSC/VLR, MS need to register with SGSN. This process is known as GPRS attach. This in fact can be combined with GSM registration. AT command to instruct Protocol Stack to do GPRS attach is "AT+CGATT". Once attached, MS also does "Routing Area update" as it moves from one RA to another. Mobility information is stored by SGSN. These procedures include "authentication" procedure. SGSN takes help of HLR for authentication. During attach, MS is provided with P-TMSI (Packet TMSI) - which is used for identification of MS in later procedures. IMSI is for first identification. GMM layer is in charge of these procedures.
2) To use GPRS connection, a "Session" need to be created. Session setup is triggered by AT command, "AT+CGACT". A Session is identified by "PDP Context". PDP Context has information about type of PDN (IP, X.25 etc.), Address (Nil in case of dynamic addressing) etc. PDP context also has associated "QoS (Quality of Service)" parameters. QoS parameters are simply characteristics of connection like peak/mean data rate, reliability, delay that can afforded etc. Before triggering PDP context activation using "AT+CGACT", UI should inform PS about PDP context with commands, AT+CGDCONT and AT+CGQREQ. PDP context activation (and deactivation) can either be requested by MS or initiated by SGSN (or GGSN via SGSN). SM layer takes care of session/pdp context management.
3) SGSN may reject requested QoS or assign different QoS, based on current user load and subscription. During PDP activation, GGSN does the work of talking to PDN and getting Address for identifying the MS in PDN. Once PDP is activated, MS can start doing data transfer.
4) The Access stratum procedures for signaling and transmission are conceptually same. In signaling plane, LLC work on GMM/SM PDUs and in transmission plane it work on SNDCP PDUs. For both data transfers, MS request allocation from network, waits for allocation and then uses allocated time slots to transmit the data. These data/packet flow is identified by TLLI (Temporary Logical Link Identity) and TFI (Temporary Flow Identity). TLLI is extracted from P-TMSI. TFI is dynamic and changes for every allocation. The radio (point-to-point) link and access resolution procedures are taken care by RLC/MAC. LLC does the ciphering and provide a reliable link at MS-SGSN level (it include flow control for transmission data - i.e. SNDCP PDUs).
References: GPRS in Practice by McGuiggan, GPRS whitepaper from Christian Bettstetter et al
© Copyright Samir Amberkar 2010
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