6.8 PS Domain Session Management Procedures


6.8.1  Basic Concepts of Session Management

1. SM function overview

The purpose of SM is to set up, modify and release the PS domain bearer. As a major component of the Connection Management layer in the 3GPP protocol, the SM is located between the Mobility Management plane and the user plane, and uses the connectionless data transfer service provided by the GMM sublayer to provide connection management service to the upper layer, i.e., the user plane. It is used to set up, modify and release the tunnel between the SGSN and the GGSN on one hand, and to control the setup, modification and release of the Radio Access Bearer between the SGSN and the RNC/MS on the other hand.

2. Terminologies

1)      PDP CONTEXT/PDP ADDRESS
The PDP context saves all tunnel forwarding information of the user plane, including user plane IP address of RNC/GGSN, tunnel identifier and QoS.
Each piece of GPRS subscription data comprises one or more PDP addresses, each PDP address comprises one or more PDP Context descriptions of MS, SGSN and GGSN, and each PDP Context can exist in one of two states: Inactive and active. The state transition chart is shown in the following figure. The PDP state indicates whether the data with the PDP address can be transferred. Inactive sessions do not contain routing information and cannot complete data transfer. All the PDP Contexts of the subscriber are associated with his/her MM Context.
2)      NSAPI
The NSAPI is used in the MS to identify a PDP service access point, and in the SGSN/GGSN to identify a session. Its value is the RAB ID used by the access layer to identify the subscriber RAB.
3)      APN resolution
The APN (Access Point Name) is of the standard domain name format. It comprises two parts: Network Identity (NI) and Operator Identity (OI). It is used in the GGSN to identify the ISP of a specific external network and a service, and in the SGSN to get the GGSN address corresponding to it via the DNS resolution.
4)      QoS negotiation
While setting up a packet transport route, the SM must specify the QoS to be satisfied by the route. During the SM process, the MS, RNC, SGSN and GGSN conduct QoS negotiation to keep the consistency of the QoSs they provide. The QoS negotiation algorithm is to get the minimum from the maximum QoS that can be provided by the subscribed QoS and SGSN and the QoS satisfied by other nodes.

3. Position of SM in the protocol stack

4. SM-related functional entities

1)      RAB management
RABM (RAB Management) is used to establish, modify, release and reestablish the RAB.
The RAB consists of two parts: The GTP tunnel between the RNC and the SGSN, and the Radio Bearer between the RNC and the MS. RAB ID uniquely identifies an RAB of the subscriber.
The RAB establishment, modification, release and reestablishment are completed via the RAB ASSIGNMENT procedure.

Description:
Ø    The SGSN sends the RAB Assignment Request (SGSN ADDR, TEIDs, and QoS) to the RNC, requesting it to establish, modify or release the RAB(s). It can specify the RAB radio priority and whether preemption and queuing are allowed in the assignment parameters.
Ø    RNC establishes, modifies or releases the radio bearer.
Ø    The RNC sends the RAB Assignment Response message to the SGSN. If the assignment fails due to the QoS, then the QoS requirement should be lowered and the assignment request should be resent.
If the QoS changes during RAB reestablishment, the SGSN-initiated PDP Context Modification procedure will be executed to notify the QoS to both the MS and the GGSN.
2)      Tunnel management
The major task of tunnel management is to establish a GTP tunnel between the SGSN and the GGSN. Tunnel management includes tunnel establishment, modification and deletion as well as the network-initiated PDP Context activation procedure.
The SM implements session management through the PDP Context activation, modification and deactivation procedures. The PDP Context activation procedure is to establish the packet transport routes of the user plane; the PDP Context modification procedure is to modify the QoS and TFT of the active PDP Context, as well as modify the tunneling route between the SGSN and the GGSN in the case of RAU changes; the PDP Context deactivation procedure is to release the active PDP Context.
After the RNC initiates the RAB or Iu release procedure, the SGSN may keep these active PDP Contexts but not deactivate them. When the subscriber initiates the SERVICE REQUEST procedure for RAB reestablishment, the data transport procedure can be recovered immediately.

6.8.2  PDP Context Activation


The PDP Context activation includes the MS-initiated and the network-initiated PDP Context activation procedures as well as the secondary activation procedure. This section only describes the MS-initiated PDP Context activation procedure.

      1.    MS-initiated PDP Context activation

1)      The MS sends the Activate PDP Context Request (with NSAPI, TI, PDP Type, PDP Address, Access Point Name and QoS Requested) to the SGSN. The PDP Address indicates the dynamic or static address. If it indicates dynamic address, it is set to null.
2)      The RAB assignment procedure is executed.
3)      The SGSN uses the PDP Type (optional), PDP Address (optional), Access Point Name (optional) and PDP Context subscriber data to verify the validity of the Activate PDP Context Request message.
The SGSN assigns a TEID to the PDP Context. If the dynamic address is to be used, the SGSN requests the GGSN to allocate a dynamic address. The SGSN selects an APN according to a certain algorithm and then sends the request to establish a PDP Context to the GGSN.
The GGSN allocates a dynamic address for the PDP Context, as well as the charging ID and negotiated QoS. If the MS requests that an IP address be allocated for the external network part, the address will be set to 0.0.0.0. Later after the external network is allocated with an IP address, the GGSN-initiated PDP Context modification procedure will then be executed.
4)      Upon receipt of the Create PDP Context Response message (with NSAPI, PDP ADDR, GGSN ADDR, TEID and QoS) from the GGSN, the SGSN sends the address and OoS information via the Activate PDP Context Accept message to the MS.  

      2.      The improvement of MS-initiated PDP Context activation procedure in the later version.


In the early R99 version, if the GGSN has downgraded the QoS in the activation procedure, it won’t notify this to the RNC. So the resource occupied on the two sides of the SGSN will be inconsistent and the resources at the air interface may be even much more than the resources allocated in the core network, thus causing the waste of air interface resources. In the later version (later R99/R4/R5/R6), the activation procedure was optimized. The SGSN first interacts with the GGSN to establish a GTP tunnel and then an RAB. After then, an optional PDP Context Modification procedure may be executed to synchronize the resources on the two sides of the SGSN, if the QoS is downgraded during the RAB establishment procedure.

6.8.3  PDP Context Modification


The PDP Context modification procedures include the MS-initiated, SGSN-initiated and the GGSN-initiated PDP Context modification procedures, as well as the SGSN-initiated PDP Context modification procedure due to RAB/Iu release. This section only describes the MS-initiated and SGSN-initiated ones. The modification parameters include QoS Negotiated, Radio Priority, Packet Flow Id, PDP Address (in case of the GGSN-initiated modification procedure) and TFT (in the case of MS-initiated modification procedure).

1. SGSN-initiated PDP Context modification

1)      The SGSN sends the Update PDP Context Request (with TEID, NSAPI, QoS Negotiated, Trace Reference, Trace Type, Trigger Id and OMC Identity) to the GGSN for QoS negotiation.
2)      The GGSN conducts the QoS negotiation and sends the Update PDP Context Response (with TEID, QoS Negotiated and Cause) to the SGSN.
3)      The SGSN selects the Radio Priority and Packet Flow Id according to the QoS. It then sends to MS the Modify PDP Context Request (with TI, QoS Negotiated, Radio Priority and Packet Flow Id).
4)      The MS accepts the QoS by sending to the SGSN the Modify PDP Context Accept message or rejects the QoS by initiating the PDP Context deactivation procedure.
5)      The RAB assignment procedure is executed to modify the RAB.
6)      If BSS tracing is to be started, the Invoke Trace message (with Trace Reference, Trace Type, Trigger Id and OMC Identity) will be sent.

2. MS-initiated PDP Context modification

The purpose of the MS-initiated PDP Context modification procedure is to change the QoS or TFT of the PDP Context.
1)      The MS sends the Modify PDP Context Request message (with TI, QoS Requested and TFT) to the SGSN, so as to request for changing the PDP Context.
2)      The SGSN sends the Update PDP Context Request (with TEID, NSAPI, QoS Negotiated, Trace Reference, Trace Type, Trigger Id and OMC Identity) to the GGSN for QoS negotiation.
3)      The GGSN conducts the QoS negotiation and sends the Update PDP Context Response (TEID, QoS Negotiated and Cause) to the SGSN.
4)      The RAB assignment procedure is executed to modify the RAB.
5)      The SGSN sends the Modify PDP Context Accept message to the MS.

6.8.4  PDP Context Deactivation


The PDP Context deactivation procedures include the MS-initiated, SGSN-initiated and GGSN-initiated PDP Context modification procedures. This section only describes the MS-initiated and the SGSN-initiated ones.

1. MS-initiated PDP Context deactivation

1)      The MS sends the Deactivate PDP Context Request message (with TI and Teardown Ind) to the SGSN, where Teardown Ind indicates whether to deactivate the PDP Context and specifies the active PDP Context for the TI shared address.
2)      The SGSN receives the deactivation request from the MS and sends the Delete PDP Context Request message (TEID, NSAPI and Teardown Ind) to the GGSN to delete the GGSN PDP Context.
3)      The GGSN sends the Delete PDP Context Response message (with TEID) to the SGSN.
4)      Upon receipt of the Delete PDP Context Response message, the SGSN sends the Deactivate PDP Context Accept message to the MS.
5)      The SGSN invokes the RAB assignment procedure to release the RAB.

2. SGSN-initiated PDP Context deactivation

The SGSN-initiated deactivation procedure is often triggered by the MM release or various exceptions such as PDP Context inconsistency between the MS, the SGSN and the GGSN, or RAB reestablishment failure or resource shortage.
1)      The SGSN sends the Delete PDP Context Request message (with TEID, NSAPI and Teardown Ind) to the GGSN, where Teardown Ind indicates whether to deactivate the PDP Context and specifies the active PDP Context for the TI shared address.
2)      The GGSN sends the Delete PDP Context Response message (with TEID) to the SGSN.
3)      Upon receipt of the Delete PDP Context Response message from the GGSN, the SGSN sends the Deactivate PDP Context Request message to the MS to delete the MS PDP Context. If the PDP Context deactivation procedure is triggered by DETACH, this Deactivate PDP Context Request message will not be sent.
4)      The SGSN receives the Deactivate PDP Context Accept message from the MS.
5)      The SGSN initiates the RAB assignment procedure to release the RAB.

6.8.5  Reservation Procedure and RAB Reestablishment


When the RNC initiates the RAB release and Iu release procedures, the PDP Context may not be released, rather, the PDP Context will be reserved without any change for RAB reestablishment in the later Service Request procedure.

1. MS-initiated service request for RAB reestablishment

When the MS has the uplink data transport needs while the PDP Context is active but the RAB does not exist, the MS will initiate the Service Request procedure to reestablish the RAB for the active PDP Context. The procedure is shown in the following figure.
1)      The RRC connection must be established first if there is no existing RRC connection.
2)      The MS sends the Service Request message (with P-TMSI, PAI, CKSN and Service Type) to the SGSN. In the message, the Service Type parameter indicates data.
3)      The security procedure is executed.
4)      The SGSN sends the Service Accept message to the MS, so as to reestablish the RAB for each active PDP Context of the subscriber with RAB released.
5)      If the QoS changes during RAB reestablishment, the SGSN-initiated PDP Context Modification procedure will be executed to notify the QoS to both the MS and the GGSN.
6)      The MS transmits the uplink data.

2. SGSN-initiated service request for RAB reestablishment


When the SGSN receives the downlink signaling or data packet and finds that the subscriber is in the PMM-IDLE state, it needs to initiate the paging procedure. Upon receipt of the paging message, the MS sends the Service Request message with the Service Type indicating “Paging Response”. If the Service Request procedure is caused because the SGSN receives a data packet, the RAB Assignment procedure will be invoked to reestablish the RAB. 

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