Page 187 - From GMS to LTE

Universal Mobile Telecommunications System (UMTS) and High-Speed Packet Access (HSPA) 173

UE S-RNC MSC

RRC Connection Setup (cp. Figure 3.14)
RRC Initial Direct Transfer Terminal contacts
(CM Service Request, CS) SCCP CR
SCCP CC the network
RANAP Initial UE Message
(CM Service Request)
MM Authentication Request
MM Authentication Response

RANAP Security Mode Command Network performs
RRC Security Mode Cmd security procedures
RRC Security Mode Complete
RANAP Security Mode Complete
CC Setup (MSISDN)
CC Call Proceeding
RANAP RAB Assignment Request Radio
bearer Call is
Radio Resource Allocation (cp. Figure 3.36) establish- established
RANAP RAB Assignment Complete ment with the
CC Alerting other party
CC Connect Connection accepted
CC Connect Acknowledge
conversation starts

Figure 3.35 Messaging for a mobile‐originated voice call (MOC).

At this point, only a signaling connection exists between the mobile device and the radio
network, which is not suitable for a voice call. Thus, the MSC requests the establishment
of a speech path from the RNC via a RAB Assignment Request message. The RNC pro-
ceeds by reserving the required bandwidth on the Iub interface and instructs the Node‐B
to allocate the necessary resources on the air interface. Furthermore, the RNC also estab-
lishes a bearer for the speech path on the Iu(cs) interface to the MSC. As a dedicated radio
connection was already established for the signaling in our example, it is only modified by
the Radio Resource Allocation procedure (Radio Link Reconfiguration). The reconfigura-
tion includes, for example, the allocation of a new spreading code as the voice bearer
requires a higher bandwidth connection than does a slow signaling connection. If the RNC
has performed the signaling via the FACH (Cell‐FACH state), it is necessary at this point to
establish a DCH and to move the mobile device over to a dedicated connection. Figure 3.36
shows the necessary messages for this step of the call establishment.
Simultaneous with the establishment of the resources for the traffic channel in the
radio network, the MSC tries to establish the connection to the called party. This is
done, for example, via ISUP signaling to the Gateway MSC for a fixed‐line destination,
as described in Section 1.4. If the destination is reachable, the MSC informs the caller
by sending Call Control ‘Alerting’ and ‘Connect’ messages.