Packet Switched data call in GSM Uplink (PS call flow)
This article covers Packet Swiched data, PS call flow between Mobile(UE) and network for uplink(to upload data). It covers messages exchanged for GPRS attach and PDP context activation procedures between mobile and SGSN/VLR/HLR/GGSN. It include channels(PPCH,PRACH,AGCH,PDCH) used at layer 1 to carry various messages over the air. This article assumes that initial frequency and time synchronization is done between UE and Network as described in GSM tutorial in tutorial section.
Here RACH is sent by mobile(UE) mobile to network(BTS) and network will assign slot(single/multiple) for packet transfer. GPRS attach procedures are completed by mobile station for location update and to complete security check. To establish internet connectivity IP address need to be assigned to the Mobile and connectivity with APN need to be established. This is done using PDP context activation procedures.
As described in GSM protocol stack, messages flow between both mobile and network at various layers(layer 3,layer 2,layer 1(physical layer). The message flow is self explanatory to establish the Packet switched uplink data call in GSM.
PS call flow in the downlink
PS call flow for gsm mobile in the downlink, refer Downlink PS data call