Combined GPRS/GSM Attach

Combined GPRS/GSM Attach Figure 5-6 depicts a simple
GPRS attach scenario that would apply to a Class-C MS. In the case of a
Class-A or Class-B MS, the MS may want to simultaneously attach to the
GSM network and the GPRS network. In this case, the MS can attach to
the MSC/VLR during the GPRS attach procedure. This assumes, of course,
that the network supports a combined attach (which it broadcasts in System
Information messages) and that the network includes the Gs interface.
If, for example, a Class-B MS is powered up and needs to attach to both
the GSM and GPRS services, then the sequence would be as depicted in
Figure 5-6. For the sake of brevity, we have omitted some air interface signaling,
which would be the same in the example of Figure 5-6, as already
shown in Figure 5-5.

In this case, the MS instigates an attach to the SGSN, but it also indicates
that it wants to perform a GSM attach. In this case, the new SGSN,
in addition to performing the procedures required of a GPRS attach, also
interacts with the VLR to initiate a GSM attach. Specifically, we note the
use of the BSSAP+messages Location Update Request and Location
Update Accept between the SGSN and the VLR. The Location Update
Request message from the SGSN is similar to the equivalent message that
would be received from an MS that performs a normal GSM location
update. Therefore, the MSC/VLR performs similar mobility management
functions (see also Figure 3-10 in Chapter 3) such as performing a MAP
Update Location to the HLR. One difference in this scenario, however, is the
fact that the MSC/VLR does not attempt to authenticate the MS itself, as
the authentication has already been performed by the SGSN.
Note that in Figure 5-5 and Figure 5-6 certain optional functions have
not been shown. These functions include an International Mobile Equipment
Identity (IMEI) check and the allocation of a new Packet Temporary
Mobile Subscriber Identity (P-TMSI).