Page 118 - From GMS to LTE

104 From GSM to LTE-Advanced Pro and 5G

MS SGSN HLR

Attach request (IMSI)

Send authentication info
(IMSI)

Send authentication info ack.
(Authentication triplets)
Authentication and ciphering
Request, ciphering on/off
Authentication and ciphering
response (SRES)
Update location
Insert subscriber data

Insert subscriber data ack.
Update location ack.

Attach accept
Attach complete

Figure 2.27 GPRS attach message flow.

network, the mobile device can now go ahead and establish a data session. This so‐called
PDP context activation procedure is described in the next paragraph.
Figure 2.28 shows an example of a GPRS Attach message that was traced on the Gb
interface. Some interesting parameters are highlighted in bold. As can be seen in the
message, the mobile device not only informs the network about its identity, but it also
includes its capabilities, such as its multislot capabilities and which frequency bands it
supports (850, 900, 1800, 1900 MHz). Although standards evolve quickly, mobile device
developers often only implement a subset of functionality at the beginning and add
more features over time in new software versions or even only in new models. This
flexibility and thus fast time to market are only possible if networks and mobile devices
are able to exchange information about their capabilities.
A good example of such an approach is the multislot capability. Early GPRS mobile
devices were able to aggregate only two downlink timeslots and use only a single one in
the uplink. Current mobile devices support up to five timeslots in the downlink and
three in the uplink (multislot class 32).
Once the mobile device is attached, the network has to keep track of the location of
the mobile device. As discussed in Chapter 1, this is done by dividing the GSM network
into location areas. When a mobile device in idle mode changes to a cell in a different
location area, it has to perform a so‐called location update (LU). This is necessary so