Page 186 - From GMS to LTE

172 From GSM to LTE-Advanced Pro and 5G

The core network is not aware of UTRAN registration areas. Furthermore, even sin-
gle cells have been abstracted into so‐called service areas. This is in contrast to a GSM/
GPRS network, where the MSC and SGSN are aware of the location area and even the
cell ID in which the mobile device is located during an active connection. In UMTS, the
location area does not contain single cells but one or more service areas. It is possible to
assign only a single cell to a service area to allow better pinpointing of the location of a
mobile device in the core network. By this abstraction it was possible to clearly separate
the location principles of the core network, which is aware of location areas, routing
areas and services areas, and the radio network, which deals with UTRAN registration
areas and single cells. Core network and radio network are thus logically decoupled.
The mapping between the location principles of core and radio network is done at the
interface between the two networks, the RNC.

3.8 UMTS CS and PS Call Establishment

To establish a circuit‐switched or packet‐switched connection, the mobile device has to
contact the network and request the establishment of a session. The establishment of
the user data bearer is then performed in several phases.
As a first step, the mobile device needs to perform an RRC connection setup proce-
dure, as shown in Figure 3.35, to establish a signaling connection. The procedure itself
was introduced in Figure 3.14. The goal of the RRC connection setup is to establish a
temporary radio channel that can be used for signaling between the mobile device, the
RNC and a core network node. The RNC can choose either to assign a dedicated chan-
nel (Cell‐DCH state) or to use the FACH (Cell‐FACH state) for the subsequent exchange
of messages.
If a circuit‐switched connection is established, as shown in Figure 3.35, the mobile
device sends a CM Service Request DTAP message (see Section 1.4) over the estab-
lished signaling connection to the RNC, which transparently forwards the message to
the MSC. DTAP messages are exchanged between the RNC and the MSC via the con-
nection‐oriented Signaling Connection and Control Part (SCCP) protocol. Therefore,
the RNC has to establish a new SCCP connection before the message can be forwarded.
Once the MSC has received the CM Service Request message, it verifies the identity
of the subscriber via the attached TMSI or IMSI. This is done in a challenge and response
procedure similar to GSM. In addition to the mobile device authentication already
known from GSM, a UMTS network has to authenticate itself to the user to protect
against air interface eavesdropping with a false base station. Once the authentication
procedure has been performed, the MSC activates ciphering of the radio channel by
issuing a Security Mode command. Optionally, the MSC afterward assigns a new TMSI
to the subscriber, which, however, is not shown in Figure 3.35 for clarity. Details of the
UMTS authentication and ciphering process are described in Section 3.9.
After successful authentication and activation of the encrypted radio channel, the
mobile device then proceeds to inform the MSC of the exact reason for the connection
request. The CC Setup message contains, among other things, the telephone number
(MSISDN) of the destination. If the MSC approves the request, it returns a
Call Proceeding message to the mobile device and starts two additional procedures
simultaneously.