Creating a PDP context

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

1. The Mobile Station (MS) sends a PDP activation request message to the SGSN including the MS PDP address, and APN.
2. Optionally, security functions may be performed to authenticate the MS.
3. The SGSN determines the GGSN address by using the APN identifier.
4. The SGSN creates a down link GTP tunnel to send IP packets between the GGSN and SGSN.
5. The GGSN creates an entry in its PDP context table to deliver IP packets between the SGSN and the external packet switching network.
6. The GGSN creates an uplink GTP tunnel to route IP-PDU from SGSN to GGSN.
7. The GGSN then sends back to the SGSN the result of the PDP context creation and if necessary the MS PDP address.
8. The SGSN sends an Activate PDP context accept message to the MS by returning the negotiated PDP context information and if necessary the MS PDP address.
9. Now traffic can pass from the MS to the external network endpoint.

A GGSN creates a PDP context

1. The network receives an IP packet from an external network.
2. The GGSN checks if the PDP Context has already been created.
3. If not, the GGSN sends a PDU notification request to the SGSN in order to initiate a PDP context activation.
4. The GGSN retrieves the IP address of the appropriate SGSN address by interrogating the HLR from the IMSI identifier of the MS.
5. The SGSN sends to the MS a request to activate the indicated PDP context.
6. The PDP context activation procedure follows the one initiated by the MS. See “A Mobile Station creates a PDP context”.
7. When the PDP context is activated, the IP packet can be sent from the GGSN to the MS.