The following nodes are involved in GPRS call flow. MS – BTS – BSC – SGSN – GGSN & HLR.
The interface between BSC and SGSN is called Gb interface. Now a days we are commonly using TDM or IP interface for GB interface.
For GPRS access the following scenario’s are happening.
1. GPRS Location Update – in this the SGSN contacts with the HLR and then do the LU. It includes checking the subscriber information, Authentication etc.
2. PDP context activation. In this SGSN contacts with the GGSN and a tunnel connectivity is obtained between MS and GGSN. GGSN connects with external ISP.
With this above process MS will be in contact with ISP and the user will be able to access GPRS.
One more thing i want to add in the PDP context activation, one more node is involved in this step – DNS server. The DNS server provide the IP of GGSN by converting the APN (access point) into IP.
For a detailed GPRS call flow you may refer the book “GPRS in Practice – A Companion to the Specifications ” by McGuiggan P. This is an excellent book to know the internals of GPRS call flow along with the relevant 3GPP specs reference.
GPRS Call flows can be found in 3GPP specifications. Lots of information via the usual Google search.
Depending on the features that are activated a GGSN can have the capability to act as a Router, Firewall and a Gateway. To confirm the end user’s details it can send RADIUS Packet Data requests to a AAA node.
With 3G and 4G there is now the notion of a multi-functional APN that points to a basket of Services / bundles that can be consumed.
VBR/ Wallis Dudhnath
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