Cisco Cisco ASR 5000
SGSN in a 2.5G GPRS Network
How the 2.5G SGSN Works ▀
SGSN Administration Guide, StarOS Release 18 ▄
How the 2.5G SGSN Works
In compliance with 3GPP specifications, the 2.5G SGSN supports standard operational procedures such as: attach,
detach, PDP activation.
detach, PDP activation.
For GPRS and/or IMSI Attach
The following illustrates the step-by-step call flow indicating how the 2.5G SGSN handles a GPRS/IMSI attach
procedure.
procedure.
Figure 14. GPRS/IMSI Attach Procedure
1. An Attach Request message is sent from the UE to the SGSN by the BSS over the Gb interface. This is Typically
a Frame Relay connection.
2. The SGSN identifies UE and determines IMSI. Depending on whether or not the UE is already attached, this
could be a simple database lookup or it could require the SGSN to communicate with an SGSN that may have
been previously handling the call.
been previously handling the call.
3. The SGSN communicates with the HLR to authenticate the UE.
4. Once the UE has been authenticated, the SGSN communicates with the EIR to verify that the equipment is not
4. Once the UE has been authenticated, the SGSN communicates with the EIR to verify that the equipment is not
stolen.