Page 189 - From GMS to LTE

Universal Mobile Telecommunications System (UMTS) and High-Speed Packet Access (HSPA) 175

UE S-RNC SGSN

RRC Connection Setup (cp. Figure 3.14)
RRC Initial Direct Transfer Terminal establishes
(Activate PDP Context, PS) SCCP CR
SCCP CC connection with the
RANAP Initial UE Message network
(Activate PDP Context)
GMM Authentication and Ciphering Request
GMM Authentication and Ciphering Response
RANAP Security Mode Command Network starts
RRC Security Mode Cmd
RRC Security Mode Complete RANAP Security Mode Complete security procedures
GGSN
GTP Create PDP
Context Request
PDP GTP Create PDP
Context Response
Context RANAP RAB Assignment Req. (returns IP address)
establish- Radio Resource Allocation (cp. Figure 3.36)
ment RANAP RAB Assignment Cmpl.
GMM Activate PDP Context Accept (return IP address)
Transfer of User Data

Figure 3.37 PDP context activation.

3.9 UMTS Security

Like GSM, UMTS has strong security measures to prevent unauthorized use and eaves-
dropping on user data traffic and conversations. UMTS also includes enhancements to
overcome a number of weaknesses that have been found over the years in the way GSM
protects networks and users. The following are the main weaknesses:

The GSM circuit‐switched part does not protect the link between the base station
●
and the BSC. In many cases microwave links are used, which are vulnerable to third
party monitoring.
GSM allows man‐in‐the‐middle attacks with equipment that masquerades as a GSM
●
base station.
The CK length used in GSM is 64 bits. Although secure when GSM was first developed
●
in the early 1990s, the length is considered insufficient today.
A number of weaknesses with the A5/1 stream cipher have been detected, as described
●
in Chapter 1, which allow decryption of a voice conversation with the appropriate
equipment.
UMTS addresses these weaknesses in a number of ways. As in GSM, a one‐pass
authentication and key agreement (AKA) procedure is used with immediate activation
of ciphering after successful authentication. The general principle is described in
Chapter 1. When a mobile device attaches to the network after power‐up, it tries to