Page 117 - From GMS to LTE

General Packet Radio Service (GPRS) and EDGE 103

2.8 GPRS Mobility Management and Session
Management (GMM/SM)

Apart from forwarding data packets between GPRS subscribers and the Internet, the
GPRS network is also responsible for the mobility management of the subscribers and
the SM to control the individual connections between subscribers and the Internet. For
this purpose, signaling messages and signaling flows have been defined that are part of
the GMM/SM protocol.

2.8.1 Mobility Management Tasks
Before a connection to the Internet can be established, the user has to first connect to
the network. This is similar to attaching to the circuit‐switched part of the network.
When a subscriber wants to attach, the network usually starts an authentication proce-
dure, which is similar to the GSM authentication procedure. If successful, the SGSN
sends an LU message to the HLR to update the location information of that subscriber
in the network’s database. The HLR acknowledges this operation by sending an Insert
Subscriber Data message back to the SGSN. As the name of the message suggests, it not
only acknowledges the LU but also returns the subscription information of the user to
the SGSN so that no further communication with the HLR is necessary as long as the
subscriber does not change location. The SGSN, subsequently, will send an Attach
Accept message to the subscriber. The attach procedure is complete when the subscriber
returns an Attach Complete message to the SGSN. Figure 2.27 shows the message flow
for this procedure.
If the subscriber was previously attached to a different SGSN, the procedure is somewhat
more complex. In this case, the new SGSN will ask the old SGSN for identification
information of the subscriber. Once the subscriber has authenticated successfully, the
SGSN will send the LU message as above to the HLR. As the HLR knows that the
subscriber was previously attached to a different SGSN, it sends a Cancel Location
message to the old SGSN. It later returns the Insert Subscriber Data message to the
new SGSN.
It is also possible to do a combined GSM/GPRS attach procedure in case the Gs inter-
face is available. To inform the mobile device of this possibility, the network broadcasts
the GPRS network operation mode on the BCCH. Should the mobile device thus send
a request for a combined attach from the SGSN, it is the task of the new SGSN to inform
the new MSC of the location of the subscriber. The new MSC will then send an update
location to the HLR for the circuit‐switched part of the network. The HLR will then
cancel the location in the old MSC and send an insert subscriber data back to the new
MSC. Once all operations have been performed, the new MSC sends back an LU accept
to the SGSN, which will then finally return the Attach Accept message to the mobile
device. Although this message flow is more complex from the point of view of the core
network, it allows the mobile device to attach to both circuit‐ and packet‐switched net-
work parts with only a single procedure. This speeds up the process for the mobile
device and reduces the signaling load in the radio network.
Once the attach procedure is complete, the mobile device is authenticated and known
to the network. In the circuit‐switched part of the network, the user can now go ahead
and establish a voice call by dialing a number. In the GPRS packet‐switched part of the