Establishment of a UMTS Speech Call

The procedure for the establishment of a basic speech call in UMTS is
shown in Figure 6-23 (NBAP messaging has been omitted). The process
begins with an access request from the UE. This access request is sent
either on the RACH transport channel or the CPCH transport channel.The
message sent is a request to establish an RRC connection, which must be
done before signaling transactions or bearer establishment can take place.
The RRC Connection Request includes an indication of the reason for the
connection request.

The RNC responds with an RRC Connection Setup message. This message
will be sent on the CCCH logical channel (typically mapped to the
FACH transport channel). At the discretion of the RNC, the RRC Connection
Setup message may or may not allocate a DCH transport channel to
the UE. If a DCH transport channel is allocated, then the RRC Connection
Setup message indicates the scrambling code to be used by the UE in the
uplink. The channelization code is determined by the UE and is indicated
on the uplink itself. Recall, for example, that a DPCCH is associated with a
DPDCH. The DPDCH contains the TFCI that contains spreading factor
information and enables the UTRAN to determine the channelization code
for the DPDCH. If the RNC does not allocate a DCH, then further signaling
is carried out on the FACH in the downlink and on the RACH or CPCH in
the uplink.

The UE responds to the RNC with the message, RRC Connection Setup
Complete. This message is carried on the uplink DCCH logical channel,
which is mapped to the RACH, CPCH, or DCH transport channel. Next, the
UE issues a message destined for the core network. This is sent in an RRC
Initial Direct Transfer message. The payload of a direct transfer message is
passed directly between the UE and the core network. In the case that a signaling
relationship has not been established between the UE and core network,
then the RRC message Initial Direct Transfer is used. This indicates
to the RNC, and subsequently to the core network, that a new signaling
relationship needs to be established between the UE and the core.
The RNC maps the Initial Direct Transfer message to the RANAP Initial
UE message and sends the message to the core network. In this case, the
message is passed to the MSC. The choice of MSC or SGSN is made based
upon header information in the Initial Transfer message from the UE. The
payload of the Initial Direct Transfer message is mapped to the payload of
the RANAP Initial UE message to the MSC.

Next, the MSC will initiate security procedures. This begins with authentication,
which uses a challenge-response mechanism similar to that used
in GSM. One difference, however, is that the UE and network authenticate
each other. Not only does the network send a random number to the UE to
which a correct response must be received, but it also sends a network
authentication token (AUTN), which is calculated independently in the
USIM and the HLR. The AUTN must match what the network is expecting.
The authentication request is sent to the UE using the direct transfer messaging
of RANAP and the RRC protocol.

Assuming that the AUTN is acceptable, the UE responds with an
authentication response message, which contains a response that the MSC
checks. This message is also carried using the direct transfer capabilities of
RANAP and RRC.

Next, the core network will instigate encryption (ciphering) and integrity
procedures. This is similar to the ciphering that is performed in GSM, with
the addition that integrity assurance is also enabled. This capability
enables the network or UE to verify that signaling messages from the other
entity have not been maliciously altered. Ciphering and integrity procedures
are initiated by the core network, but are executed between the UE
and UTRAN. Therefore, the MSC sends the RANAP Security Mode Command
message to the RNC. In turn, the RNC sends the RRC Security Mode
Command message to the UE. The UE responds to the RNC with the RRC
message, Security Mode Complete, and the RNC responds to the MSC with
the RANAP message, Security Mode Complete.

At this point, the actual call establishment information such as the
called party number data is sent in a Setup message from the UE to the
MSC using direct transfer signaling. Provided that the call attempt can be
processed, MSC responds with the Call Proceeding message, much like is
done in GSM. Next, it is necessary to establish a Radio Access Bearer (RAB)
for transport of the actual voice stream from the user.

A RAB is a bearer between the UE and the core network for the transport
of user data, either speech or packet data. It is mapped to one or more
radio bearers on the air interface. Each RAB has its own identifier that is
used in signaling between the UE and the network. A RAB establishment
is requested by the core network through a RANAP RAB Assignment
Request message.

Based on the information in the RAB Assignment Request, the RNC may
set up a new radio bearer for the UE to use, or it may reconfigure any existing
bearer that the UE has active. The RNC uses either the RRC message
Radio Bearer Setup or the Radio Bearer Reconfiguration to instruct the UE to use the new or reconfigured radio bearers. The UE responds with either
Radio Bearer Setup Complete or Radio Bearer Reconfiguration Complete.
The RNC, in turn, responds to the MSC with the RANAP message RAB
Assignment Complete. Now a bearer path exists from the UE through to
the MSC. Note that the establishment of the bearer path also requires the
establishment of a terrestrial facility between the Node B and RNC and
between the RNC and MSC. The details of this establishment have not been
shown in Figure 6-23. Suffice it to say that the transport bearer (using
AAL2) will be established through the transport user control plane and the
ALCAP previously described.

The remainder of the call establishment is quite similar to call establishment
in GSM. It involves Alerting, Connect, and Connect Acknowledge
messages carried over direct transfer signaling.
It should be noted that speech service in the 3GPP Release 1999 architecture
is still a circuit-switched service. Although the speech is actually
packetized for transfer over the air and is also packetized as it is carried
over the Iub and Iu interfaces, a dedicated bearer is established for the
duration of a call, even when discontinuous transmission is active and no
speech packets are being sent.