In order for the GPRS MS to receive or transmit data the end user needs to perform a two-step procedure, GPRS attach (Figure 21) ND PDP context activation (Figure 22)
At GPRS attach a logical link is established between MS and SGSN, the GPRS attachment procedure in Figure 23.
MS sends message to SGSN: “attach request”.
If the MS is not known by the SGSN it asks the old SGSN about the IMSI and triplets.
If the MS is not known by the old SGSN it sends an error message to the new SGSN and the new SGSN asks MS about IMSI.
SGSN authenticates MS.
Update HLR (if new SGSN service area).
Update MSC/VLR (only necessary if new LA).
SGSN tell the MS about new TLLI (Temporary Location Link ID).