Page 36 - From GMS to LTE

22 From GSM to LTE-Advanced Pro and 5G

SIM/Mobile device MSC HLR/AuC

Connection establishment
(e.g. location update or call establishment)
MAP: Send authentication triplets
(IMSI)
Send authentication triplets ack.
DTAP: Authentication request (RAND, SRES, Kc)
(RAND)

DTAP: Authentication response
(SRES*)

SRES* = SRES?

Connection is maintained, t
activation of ciphering

Figure 1.16 Authentication between network and mobile device.

To speed up subsequent connection establishments, the AuC usually returns several
authentication triplets per request. These are buffered by the MSC/VLR and are used
during subsequent connection establishments.
In the next step, the MSC sends the RAND inside an Authentication Request message to
the mobile device. The mobile device forwards the RAND to the SIM card, which then uses
*
the Ki and the authentication A3 algorithm to generate a signed response (SRES ). The
*
SRES is returned to the mobile device and then sent back to the MSC inside an Authentication
*
Response message. The MSC then compares SRES and SRES , and if they are equal, the
subscriber is authenticated and allowed to proceed with the communication.
As the secret key, Ki, is not transmitted over any interface that could be eavesdropped
on, it is not possible for a third party to correctly calculate an SRES. As a fresh random
*
number is used for the next authentication, it is also pointless to intercept the SRES and
use it for another authentication. A detailed description of the authentication proce-
dure and many other procedures between the mobile device and the core network can
be found in 3GPP TS 24.008 [14].
Figure 1.16 shows some parts of an authentication request and an Authentication
Response message. Apart from the format of RAND and SRES, it is also interesting
to note the different protocols that are used to encapsulate the message (see
Section 1.4.2).

1.6.5 The Short Messaging Service Center (SMSC)
Another important network element is the Short Messaging Service Center (SMSC),
which is used to store and forward short messages. The SMS was only introduced about
four years after the first GSM networks went into operation, as an add‐on, and has been