While FortiOS Carrier is concerned mostly with the SGSN to GGSN part of the PDP Context, knowing the steps involved in creating a PDP context helps understand the role each device, protocol, and message type plays.
Both mobile stations and GGSNs can create PDP contexts.
A Mobile Station creates a PDP context
- The Mobile Station (MS) sends a
PDP activation requestmessage to the SGSN including the MS PDP address, and APN.
- Optionally, security functions may be performed to authenticate the MS.
- The SGSN determines the GGSN address by using the APN identifier.
- The SGSN creates a down link GTP tunnel to send IP packets between the GGSN and SGSN.
- The GGSN creates an entry in its PDP context table to deliver IP packets between the SGSN and the external packet switching network.
- The GGSN creates an uplink GTP tunnel to route IP-PDU from SGSN to GGSN.
- The GGSN then sends back to the SGSN the result of the PDP context creation and if necessary the MS PDP address.
- The SGSN sends an
Activate PDP context acceptmessage to the MS by returning the negotiated PDP context information and if necessary the MS PDP address.
- Now traffic can pass from the MS to the external network endpoint.
A GGSN creates a PDP context
- The network receives an IP packet from an external network.
- The GGSN checks if the PDP Context has already been created.
- If not, the GGSN sends a
PDU notification requestto the SGSN in order to initiate a PDP context activation.
- The GGSN retrieves the IP address of the appropriate SGSN address by interrogating the HLR from the IMSI identifier of the MS.
- The SGSN sends to the MS a request to activate the indicated PDP context.
- The PDP context activation procedure follows the one initiated by the MS. See “A Mobile Station creates a PDP context”.
- When the PDP context is activated, the IP packet can be sent from the GGSN to the MS.