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RANAP RANAP 11
Response
Uplink Direct Transfer
RRC RRC
Alerting 12
Direct Transfer
RANAP RANAP
Alerting
Uplink Direct Transfer
RRC RRC
Connect Direct Transfer
RANAP RANAP
Connect
13
Direct Transfer
RANAP
RANAP
Downlink Direct Transfer Connect Acknowledge
RRC RRC
Connect Acknowledge
Circuit Call Established 14


Figure 6.134 Mobile terminated voice call establishment

1. After the user is paged, once again a signalling connection must be established. The
procedure is the same as described above for that prior to a location update. All that
will be different is the establishment cause. In this case, the cause is terminating
conversational call.
2. The UE then sends an initial connection management (CM) service request paging
response to the circuit core. This message will indicate the IMSI/TMSI of the UE.
Recall that all UMTS has introduced is a new access layer, so for R99, the basic
structure and operation of the core networks are unchanged. These L3 procedures
are the same as would be seen for a GSM call establishment, and it is the
underlying access bearer establishment where the changes are signi¬cant. Please
refer to Chapter 3 on GSM for more details. The paging response will be followed
by authentication and security procedures as previously described.
6.22 UMTS CALL LIFE CYCLE 409


3. Now, the core network informs the RNC of the permanent NAS UE identity of the
user, which is the IMSI, through the common ID update message. This is so as to
tie the RRC connection for this UE to its permanent identity for such procedures as
paging coordination.
4. The L3 procedure for call establishment continue with the call control (CC) setup
and call con¬rmed messages. These are Q.931 basic call control messages. For
further details, refer to Chapter 3.
5. The core network now requests the SRNC to set up a radio access bearer using the
RAB assignment request, which will contain all the QoS information for the call. It
is this message that will result in both Iub and Iu bearers being established as
described in the following steps. In this case, since a voice call is being made, there
will be three RAB sub¬‚ows set up for the different classes of AMR CODEC bits,
classes A, B and C. The RAB assignment request also contains the AAL2 address
and binding ID for coordination with AAL2 signalling over the Iu interface.
6. An AAL2 bearer is established across the Iu interface for this call.
7. On the UTRAN side, a bearer must now be set up for the call. The ¬rst stage of this
is to inform the BTS. Since a radio link which is being used for signalling already
exists between the UE and RNC, the procedure is to recon¬gure this link to now
carry data as well. Since timing is critical for a voice call, it is necessary that there
is a synchronized procedure for recon¬guring the link. This means that initially the
BTS is passed all the new transport and physical layer information in a radio link
recon¬guration prepare message, and responds with a radio link recon¬guration
ready message. At this stage, it has not committed the link, since the AAL2
transport bearer is not ready. The radio link recon¬guration ready also contains the
AAL2 address and binding ID of the new transport bearer to be established.
8. An AAL2 bearer is established across the Iub interface for this call.
9. Since the AAL2 bearer is ready, the RNC can now commit the recon¬gured radio
link, radio link con¬guration commit.
10. The RNC now needs to establish new radio bearers between it and the UE with the
radio bearer setup. For a voice call, three RBs are set up for the three classes of
AMR bits. Since the different classes require different QoS, each of these RBs have
a separate transport channel, only being concatenated at the physical layer into a
CCTrCH. Both the new/modi¬ed transport and physical layer parameters are passed
in this message also. The UE responds with a radio bearer setup complete.
11. Now that the connection on the UTRAN side is complete, the SRNC will ¬nalize
the process with the RAB assignment response.
12. The Q.931 signalling continues with the alerting message from the UE, indicating
that the phone is ringing.
13. Once the connection is made on the UE side, i.e. the user answers the phone, the
connect message is sent, with the network responding with the connect
acknowledge.
14. At this point, all the connections have been established and the call proceeds, with
speech ¬‚owing across the bearers.
410 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM




RRC Connection
Setup




L3 setup with
authentication



Radio link
prepared for
voice call


Called party's
phone is ringing




Figure 6.135 Sample Iub trace of part of a mobile originated call. Reproduced by permission
of NetHawk Oyj

Figure 6.135, shows a sample trace ¬le of part of a mobile originated call, captured
on the Iub interface. First, there is an RRC connection setup procedure followed by the
L3 CM service request and subsequent authentication procedures. Then the radio link is
recon¬gured for the new bearer, with the transport bearer established by AAL2 signalling
ERQ/ECF. Finally in this sample, the alerting message is seen, indicating that the called
party™s phone is now ringing.


6.22.5 Handover control
During the course of the call, the user is free to move around. One of the key aspects
of the WCDMA system is the ability to perform soft handovers, where the new connec-
tion is made before the existing connection is dropped. Soft handover also provides the
advantage that diversity may be used to provide better quality signals while maintain-
ing minimum power levels. Consider that during the call established in the last phase,
the user moves towards an adjacent cell employing the same frequency and performs an
intra-RNC soft handover.

1. Once the measurement criteria of the neighbouring cell is ful¬lled, the UE sends a
measurement report to the SRNC. A common criteria may be that the power of the
CPICH pilot channel in the cell has exceeded a de¬ned threshold, Figure 6.136.
6.22 UMTS CALL LIFE CYCLE 411


New Current
UE SRNC
BTS BTS

Connection Established
Measurement Report
RRC RRC 1
Radio Link Setup Request
NBAP NBAP
2
Radio Link Setup Response
NBAP NBAP
ERQ
A2SIG A2SIG
3
ECF
A2SIG A2SIG

L1 synchronisation
Radio Link Restore Indication
NBAP NBAP 4
Active Set Update (branch addition)
RRC RRC
5
Active Set Update Complete
RRC RRC


Active Set Update (branch deletion)
RRC RRC
6
Active Set Update Complete
RRC RRC
RL Deletion Request
NBAP NBAP
7
RL Deletion Response
NBAP NBAP
REL
A2SIG A2SIG
8
RLC
A2SIG A2SIG



Figure 6.136 Intra-RNC soft handover


2. The SRNC performs an evaluation of the report and decides that the new cell should
be added to the active set. It then initiates a radio link setup procedure with the new
BTS. For a softer handover, this procedure passes the required parameters to the
same BTS, but for the new cell.
3. An AAL2 bearer is established across the Iub interface for this new branch as
described previously in this scenario. If this was a softer handover procedure, there
would be no requirement for a new AAL2 bearer to be established, since the signals
are combined at a single BTS so only one transport bearer is required.
4. Once the new BTS has achieved L1 synchronization with the UE, it sends a radio
link restore indication message to the SRNC.
5. The SRNC informs the UE that the cell it was monitoring should now be placed in
the active set with an active set update message, and the UE replies with an active
set update complete. The UE is now in soft handover, and is using two active radio
connections.
6. Sometime later as a result of another measurement report, the SRNC evaluates that
the initial branch is now no longer contributing anything and should be removed
from the active set. Once again, an active set update message is sent to inform the
UE to remove the ¬rst link from the active set.
412 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM


7. The RNC then requests the BTS to delete the radio link, radio link deletion
request/radio link deletion response.
8. The RNC releases the AAL2 connection, REL/RLC.

If this were an inter-RNC soft handover, the same NBAP commands between the SRNC
and the BTS would be seen also through RNSAP on the Iur interface.


6.22.6 Circuit call termination
After some time, the user hangs up the phone.

1. Once the user hangs up, the UE sends the disconnect request to the network
(Figure 6.137).
2. The network responds with the release message, con¬rmed with the release
complete message.
3. The core network then releases the Iu connection, Iu release/Iu release complete,
which has the effect of closing the entire connection to the core, both the radio
access bearers and the signalling connection.
4. The AAL2 transport bearer is also released.
5. The UTRAN side now deletes the RRC connection, the radio link and the AAL2
transport bearer, as previously described for the location update.

UE BTS SRNC 3G MSC

Circuit Call Established
Uplink Direct Transfer
RRC RRC
Disconnect 1
initial UE message
RANAP RANAP
Disconnect
Direct Transfer
RANAP RANAP
Release
Downlink Direct Transfer
RRC RRC
Release
2
Uplink Direct Transfer
RRC RRC
Release Complete Direct Transfer
RANAP RANAP
Release Complete
Iu Release Command
RANAP RANAP
3
Iu Release Complete
RANAP RANAP
REL
A2SIG A2SIG
4
RLC
A2SIG A2SIG
Connection Release
RRC RRC
Connection Release Complete
RRC RRC
RL Deletion Request
NBAP NBAP
5
RL Deletion Response
NBAP NBAP
REL
A2SIG A2SIG

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