什么情况下发modify pdp context request

对于业务，有四种，interactive、background、streaming、conversational
一开始的时候使用了一种业务，比如视频，后来又要WAP下载，这两种业务需要不同的QoS，因此才会要求修改。
 
4   PDP Context Modification
An SGSN can, due to subscriber data changes, modify parameters that were negotiated during the PDP context activation procedure. For example, the QoS and radio priority parameters can be modified.

The following PDP context modification procedures are supported: 

MS-Initiated PDP Context Modification 
SGSN-Initiated PDP Context Modification 
GGSN-Initiated PDP Context Modification 
RNC-Initiated PDP Context Modification
When an SGSN is requested to modify a PDP context, it transparently forwards the protocol configuration option received from the MS to the GGSN, or in the other direction, from the GGSN to the MS. 

To determine the PLMN in which the MS is located, the Mobile Country Code (MCC) and Mobile Network Code (MNC) where the MS is registered are forwarded to the GGSN. When using the GPRS Tunneling Protocol version 1 (GTPv1), the IMEISV and the radio access technology employed (GSM or WCDMA) are also forwarded. The forwarding of MCC, MNC, IMEISV, and radio access technology is handled through Access Aware Core Edge (AACE). The function of AACE is partly restricted when the SGSN is configured to support multiple PLMNs. In this case, the SGSN does not indicate a change of MCC and MNC at an Intra-SGSN RAU.

4.1   MS-Initiated Modification
An MS might request PDP context modification to change the negotiated QoS. WCDMA System MS might also request it to reestablish preserved RABs.

4.1.1   Traffic Case
The MS-Initiated PDP Context Modification procedure is shown in Figure 4. 



Figure 4   MS-Initiated PDP Context Modification Procedure

The following steps describe the MS-Initiated PDP Context Modification procedure:

The MS sends a Modify PDP Context Request message to the SGSN. 
The SGSN sends the GGSN an Update PDP Context Request message, which optionally includes the protocol configuration option.
The GGSN responds with an Update PDP Context Response message to the SGSN. For no response, see Section 4.5. 
For GSM, the SGSN exchanges information related to ongoing user data transmission with the BSS, and initiates modification of the Packet Flow Context, see Section 6.
For WCDMA Systems, the SGSN and RNC exchange information for modifying the RABs between the MS and the SGSN, see Section 7.2. 

For WCDMA Systems, the radio bearer between the RNC and the MS is modified, or released and reestablished, see Section 7.2. 
The SGSN sends a Modify PDP Context Accept message to the MS. 
4.2   SGSN-Initiated Modification
The SGSN can initiate a PDP Context Modification procedure, for example, if the MS performs a RAU to a new RNC or BSC offering a different QoS.

In GSM, the BSC can also request the SGSN to initiate a PDP Context Modification, for example, due to load changes.

4.2.1   Traffic Case
The SGSN-Initiated PDP Context Modification procedure is shown in Figure 5.



Figure 5   SGSN-Initiated PDP Context Modification Procedure

The following steps describe the SGSN-Initiated PDP Context Modification procedure:

The SGSN sends an Update PDP Context Request message to the GGSN, for example, to negotiate a new QoS.
The GGSN responds with an Update PDP Context Response message to the SGSN. For no response, see Section 4.5. 
For GSM, the SGSN exchanges information related to ongoing user data transmission with the BSS, and initiates a modification of the Packet Flow Context, see Section 6.
The SGSN sends a Modify PDP Context Request message to the MS. 
The MS responds with a Modify PDP Context Accept message to the SGSN. For no response, see Section 4.5.
For WCDMA Systems, the SGSN and RNC exchange information on RAB modification for payload traffic between the MS and the SGSN, see Section 7.2. 
For WCDMA Systems, the radio bearer between the RNC and the MS is either modified, or released and reestablished, see Section 7.2. 
4.3   GGSN-Initiated Modification
When radio resources are released due to inactivity, out of coverage or circuit-switched calls, the core network preserves the streaming and conversational bearers by modifying the Maximum Bit Rate (MBR) and Guaranteed Bit Rate (GBR) to zero in the SGSN and GGSN. Thus, to prevent the GGSN from dropping downlink packets due to the modification of bit rates, the SGSN optionally supports that a GGSN initiates a PDP Context Modification procedure when downlink packets arrive.

Note that this procedure is a supplement to the existing 3GPP standard and requires a GGSN with corresponding support, for example Ericsson GGSN R4.

The GGSN-Initiated Modification procedure is only applicable when Operator QoS Control is activated. In case the feature is deactivated, the GGSN signaling is ignored by the SGSN.

4.3.1   Traffic Case
The GGSN-Initiated PDP Context Modification procedure is illustrated in Figure 6.



Figure 6   GGSN-Initiated PDP Context Modification Procedure

The following steps describe the GGSN-Initiated PDP Context Modification procedure:

Upon reception of downlink packets for preserved streaming and conversational bearers, the SGSN receives an Update PDP Context Request message with a modified MBR and GBR to the SGSN.
The SGSN sends a Modify PDP Context Request message to the MS. 
The MS responds with a Modify PDP Context Accept to the SGSN. When no response, see Section 4.5. 
For GSM, the SGSN and BSS exchange information related to ongoing user data transmission, and initiates a modification of the Packet Flow Context, see Section 6.
For WCDMA Systems, the SGSN and RNC exchange information on RAB modification for payload traffic between the MS and the SGSN, see Section 7.2. 

For WCDMA Systems, the radio bearer between the RNC and the MS is either modified, or released and reestablished, see Section 7.2. 
The SGSN sends an Update PDP Context Response message to the GGSN. 
4.4   RNC-Initiaded Modification
For congestion and coverage reasons, the RNC is allowed to initiate a modification of MBR and GBR for existing streaming and conversational bearers. More specifically, the RNC sends an RAB Modify Request message to the SGSN, triggering an SGSN-Initiated PDP Context Modification procedure as described in Section 4.2. For more information on RAB modification, see Section 7.2. 

4.5   Modification Reject
If the SGSN does not receive an Update PDP Context Response message from the GGSN within a time specified by the Gn_T3-ResponseUpdate parameter, the SGSN retransmits the Update PDP Context Request message to the GGSN a number of times specified by the Gn_N3-RequestsUpdate. If no response appears after last retransmission the modification procedure is rejected. Also when the Update PDP Context Response message indicate the request is not accepted, the modification procedure is rejected. At rejection, if the modification procedure was initiated by the MS, the PDP context remains active with the same QoS, except for when the GGSN lacks a PDP context. If the modification procedure was initiated by the SGSN, the PDP context is deactivated at rejection. 

If, during an SGSN- or GGSN-initiated modification procedure, the SGSN does not receive a Modify PDP Context Accept message from the MS within a time specified by the T3386 parameter, the SGSN retransmits the Update PDP Context Request message. If no response appears after last retransmission the PDP context is deactivated.