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 Teardown Ind
2)
The SGSN receives the
deactivation request from the MS and sends the Delete PDP Context Request
message (TEID, NSAPI and Teardown
Ind
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 Teardown
Ind
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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