Inter-SGSN Routing Area Update

In GPRS, each PDU to or from the MS is passed individually and no permanent
resource is established between the SGSN and MS. Thus, if a subscriber
moves from the service area of one SGSN to that of another, it is not
necessary for the first SGSN to act as an anchor or relay of packets for the
duration of the PDP context. This is fortunate as the PDP context could last
for a long time. Thus, no direct equivalent of a handover, as it is known in
circuit-switching technology, takes place, where the first MSC acts as an
anchor until a call is finished. Nonetheless, as an MS moves from one SGSN
to another during an active PDP context, special functions need to be
invoked so that packets are not lost as a result of the transition.


The process is illustrated in Figure 5-8, where an MS moves from the
service area of one SGSN to that of another during an active PDP context.
The MS notices, from the PBCCH (or BCCH), that it is in a new routing
area. Consequently, it sends a routing area update to the new SGSN.
Among the information elements in the message are the TLLI, the existing
P-TMSI, and the old Routing Area Identity (RAI). Based on the old RAI, the
new SGSN derives the address of the old SGSN and sends an SGSN Context
Request message to the old SGSN. This is a GTP message, passed over
an IP network between the two SGSNs.
The old SGSN validates the P-TMSI and responds with an SGSN Context
Response message, with information regarding any PDP context and
MM context currently active for the subscriber, plus the subscriber’s IMSI.
PDP context information includes GTP sequence numbers for the next
PDUs to be sent to the MS or tunneled to the GGSN, the APN, the GGSN address for control plane signaling, and QOS information. The old SGSN
stops the transmission of PDUs to the MS, stores the address of the new
SGSN, and starts a timer.
The MM context sent from the old SGSN to the new one may include
unused triplets, which the new SGSN will use to authenticate the subscriber.
If the old SGSN has not sent such triplets, then the new SGSN can fetch
triplets from the HLR in order to perform authentication and ciphering.
The new SGSN responds to the old one with the GTP message, SGSN
Context Acknowledge. This indicates to the old SGSN that the new one is
ready to take over the PDP context. Consequently, the old SGSN forwards
any packets that may have been buffered at the old SGSN so that the new
SGSN can forward them. The old SGSN continues to forward to the new
SGSN any additional PDUs that are received from the GGSN.
The new SGSN sends an Update PDP Context request to the GGSN to
inform the GGSN of the new serving SGSN for the PDP context.The GGSN
responds with the Update PDP Context Response message. Any subsequent
PDUs from the GGSN to the MS are now sent via the new SGSN.
The new SGSN then invokes an Update GRPS Location operation
towards the HLR. This causes the HLR to send a MAP Cancel Location to
the old SGSN. Upon receipt of the Cancel Location, the old SGSN stops the
timer and deletes any information regarding the subscriber and the PDP
context.
Once the MAP Update Location procedure is complete, the new SGSN
accepts the Routing Area update from the MS, which the MS acknowledges
with a Routing Area Complete message. The new SGSN proceeds to send
and receive PDUs to and from the MS.
In a combined GSM/GPRS network, it is common for location area
boundaries and routing area boundaries to coincide. In such a case, an
inter-SGSN routing area update might also coincide with the need to perform
a location update towards a new MSC/VLR. In that case, the SGSN
can communicate with the MSC over the Gs interface and can trigger a
location update at the MSC in much the same manner as shown in Figure
5-6 for a combined GSM/GPRS Attach.