Your Guide for PDP context activation procedure in 3G

For the MS to be able to send and receive data to and from the PDN, which is the network the MS would like to connect to,  the MS should establish a PDP with the GGSN. This is done using the 'PDP context activation' procedure.
The data between the MS and the PDN, will move into this PDP between the MS and the GGSN.

Advice

In order to well understand this topic, you need to be familiar with
GPRS Network Architecture.

PDP Establishment between the MS and the GGSN
PDP Establishment between the MS and the GGSN

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At the end of this procedure,

  • 'PDP' is established between the MS and the GGSN.
  • The GGSN assigns an IP address for 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'.
  • 'PDP context' should be established on the MS, the SGSN, and the GGSN.
  • This 'PDP context' is where the MS, the SGSN, and the GGSN stores the PDP information.

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 that the GGSN assigned for the PDP.
TEID, is a local identifier for the GTP links. The nodes that uses GTP protocol, assigns this identifier for the GTP links  between those nodes.
Direct Tunnel, 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.

'PDP Context Activation' Signaling

PDP context activation procedure
PDP context activation procedure

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

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

  • The SGSN is already having the subscription information for this MS. This is because the SGSN stored this subscription information during the 'Attach' procedure.
  • The SGSN validates the requested parameters from the MS against the MS subscription information.
  • 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 parameters that the MS requested, to the ones that exist in the MS subscription information, 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. Then the GGSN replies to the SGSN with 'Create PDP context response', including the 'PDP address' and the 'QoS' that the GGSN assigned to this 'PDP context'.

3. 'RAB establishment' (MS<-->RNC <--> SGSN)

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a. The SGSN sends 'RAB assignment request' to the RNC, in order to instruct it to setup the Radio Bearers 'RB' with the MS.

  • This message will include 'RAB ID', which is an identifier to the RAB. In addition to the QoS information.
  • The 'RAB ID' is unique over an 'Iu connection' between the RNC and the SGSN.
  • If the SGSN decided to create 'direct tunnel'  for this connection, the SGSN will send the IP address/ TEID of the GGSN to the RNC.

b. After that the RNC creates the Radio Bearers 'RB' with the MS.

c. At last, the RNC replies the SGSN with 'RAB assignment response', telling the SGSN it completed the RAB establishment.

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

a. The SGSN sends 'Update PDP context request' to the GGSN, updating it with QoS accepted by the RAN,

  • If the SGSN decided to create direct tunnel between the RNC and the GGSN, the 'Update PDP context request' sent to the GGSN will contain the user plan 'IP address' and the TEID of the RNC.

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 'IP address' and the 'QoS' that the GGSN assigned for the 'PDP'.

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