Your Guide for PDP context activation procedure in 2G

For the MS to be able to send and receive data to and from the PDN, the MS should establish a PDP with the GGSN. This is done using the 'PDP context activation' procedure.

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PDP

At the end of this procedure,

  • 'PDP' should be established on the MS, the SGSN, and the GGSN.
  • the 'PDP context' is where the MS, the SGSN, and the GGSN stores the PDP information.
  • An 'IP address' should be assigned to this 'PDP'. This is because the MS will use this 'IP address' to deal with the PDN.
  • In addition, the GGSN should assign a Quality of Service 'QoS' for this 'PDP'.

Information

There are some terms in the 'PDP activation procedure' that we will understand well in following lessons. But temporarily this is a small hint on each of them,
APN, is a logical name for the PDN the MS connects to.
PDP address, is the IP address the GGSN assigned to the PDP context.
PDP type, is the type of the IP address the GGSN assigned to the PDP. If it is IPv4, or IPv6, or IPv4v6.
NSAPI and TI, are identifiers for the PDP context.
QoS, is the Quality of service granted for the PDP context.
TEID, is a local identifier assigned by the nodes that uses GTP protocol for the GTP links  between those nodes.
Direct Tunnel, (exist only in 3G), is a direct link between the RNC and the GGSN for user plan data. The SGSN may decide to create it to offload the load of sending data from itself.

Advice

After you understand above terms well in the following lessons, come back again to below lessons and give them a re-read, to fully understand them.
Your guide for PDP context activation procedure in 2G
Your guide for PDP context activation procedure in 3G

PDP context activation procedure in 2G

PDP context activation procedure in 2G
PDP context activation procedure (2G)

1. 'Active PDP context request' (MS --> SGSN)

The MS sends 'Active PDP context request' to the SGSN including (APN , PDP type , QoS ). In additon this message will include the 'NSAPI' and 'TI' generated from the MS for this 'PDP context'.

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  • The SGSN is already having the MM context for this MS. This is because the SGSN stored this MM context during the 'Attach' procedure.
  • The SGSN validates the requested parameters from the MS against the MS subscription data exist in the 'MM context' of this MS.
  • In case the MS is allowed to access the requested APN , PDP type, QoS,  the SGSN will pass the request to the GGSN without any change.
  • In case the MS requested parameters higher or different than that exist in its subscription data, the SGSN will change the requested parameters to the ones that exist in the MS subscription data, before sending the request to the GGSN.

2. 'Create PDP context' (SGSN <-->GGSN)

a. The SGSN sends 'Create PDP context request' to the GGSN, including the APN, PDP type, QoS the MS should have.

b. The GGSN replies to the SGSN with 'Create PDP context response', including the 'IP address' and the 'QoS' which the GGSN assigned to this 'PDP context'.

3. BSS packet flow context procedure (SGSN <--> BSC)

In the PDP context activation procedure in 2G, The SGSN establishes a 'packet flow' with the BSC, 

  • If the BSC supports this, a 'packet flow context' will be created on the BSC, the aggregate QoS negotiated will be stored in this 'packet flow context'. this will let the BSC to downgrade the QoS negotiated if its resources is congested.

a. The SGSN sends 'Create BSS packet flow context request' to the BSC, informing the BSC with the QoS assigned to the MS.

b. The BSC replies with 'Create Packet flow context accept'. In case the BSC downgraded the QoS, the BSC will inform the SGSN in this message.

4. 'Update PDP context' (SGSN <--> GGSN)

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a. In case the BSC downgraded the QoS negotiated during previous step, the SGSN informs the GGSN about the new QoS attributes by sending an 'Update PDP Context Request' to the GGSN.

b. The GGSN replies with 'Update PDP context response'.

5. 'Active PDP context accept' (SGSN --> MS)

Finally, the SGSN sends 'Active PDP context accept' to the MS, telling the MS that the PDP context activation completed.

  • This message will include the 'PDP address', and the 'negotiated QoS' of the 'PDP context'. In addition to the  'Packet flow ID', and the
    'Radio priority'
  • The 'Packet flow ID' is an identifier to the 'packet flow context'.
  • The MS uses the 'radio priority' to decide access to the radio interface in the periods of congestion.