Page 106 - From GMS to LTE

92 From GSM to LTE-Advanced Pro and 5G

GSM: Ciphering
between MS and BTS

Circuit-switched
TRAU MSC

BSC

PCU SGSN
Packet-switched

GPRS: Ciphering between MS and SGSN

Figure 2.17 Ciphering in GSM and GPRS.

As IP over different transport protocols has become common since the initial stand-
ardization of GPRS, the 3GPP GPRS standards were later extended with an option to
replace Frame Relay with IP on the Gb interface. In practice, this option is typically used
by network operators today.
While ciphering for circuit‐switched traffic is terminated in the BTS, ciphering for
packet‐switched traffic is terminated in the SGSN as shown in Figure 2.17. This has
a number of advantages. In GPRS, the mobile device and not the network has control
over cell changes during data transfers. If ciphering were done on the BTS, the net-
work would first have to supply the ciphering information to the new BTS before the
data transfer could resume. As this step is not necessary when the ciphering is termi-
nated in the SGSN, the procedure is accelerated. Furthermore, the user data remains
encrypted on all radio network links. From a security point of view, this is a great
improvement. The link between BTS and BSC is often carried over microwave links,
which are not very difficult to intercept. The drawback of this solution is that the
processing power necessary for ciphering is not distributed over many BTS but con-
centrated on the SGSN.

Signaling Plane Management
The SGSN is also responsible for the management of all subscribers in its area. All
protocols and procedures for user management are handled on the signaling plane.
To be able to exchange data with the Internet, it is necessary to establish a data session
with the GPRS network. This procedure is called PDP context activation and is part of
the session management (SM) tasks of the SGSN. From the user point of view, this
procedure is invoked to get an IP address from the network.
Subscribers can change their location in a mobile network frequently. When this hap-
pens the SGSN needs to change its routing of packets to the radio network accordingly.
This task is done by the GPRS mobility management (GMM) sublayer.
When a subscriber leaves the area of the current SGSN, GMM also contains proce-
dures to change the routing for a subscriber in the core network to the new SGSN. This
procedure is called inter‐SGSN routing area update (IRAU).