The packet data protocol (PDP) context is a connection between a mobile station and the end address that goes through the SGSN and GGSN. It includes identifying information about the mobile customer used by each server or device to properly forward the call data to the next hop in the carrier network, typically using a GTP tunnel between the SGSN and GGSN.
When a mobile customer has an active voice or data connection open, both the SGSN and GGSN have the PDP context information for that customer and session.
When a mobile phone attempts to communicate with an address on an external packet network, either an IP or X.25 address, the mobile station that phone is connected to opens a PDP context through the SGSN and GGSN to the end address. Before any traffic is sent, the PDP context must first be activated.
The information included in the PDP context includes the customer’s IP address, the IMSI number of the mobile handset, and the tunnel endpoint ID (TEID) for both the SGSN and GGSN. The TEID is a unique number, much like a session ID on a TCP/IP firewall. All this information ensures a uniquely identifiable connection is made.
Since one mobile device may have multiple connections open at one time, such as data connections to different internet services and voice connections to different locations, there may be more than one PDP context with the same IP address making the extra identifying information required.
The endpoint that the mobile phone is connecting to only knows about the GGSN — the rest of the GPRS connection is masked by the GGSN.
Along the PDP context path, communication is accomplished in using three different protocols.
- The connection between the Mobile Station and SGSN uses the SM protocol.
- Between SGSN and GGSN GTP is used.
- Between GGSN and the endpoint IP is used.
FortiOS Carrier is concerned with the SGSN to GGSN part of the PDP context — the part that uses GTP.