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Therefore the ATM address of any devices manufactured by Cisco will begin 0x47.00.91.
The ¬rst 13 bytes of an ATM address are the network identi¬er, and Cisco uses the MAC
address to form a unique identi¬er, pre¬xing it with 0x81.00.00.00 to extend the MAC
address beyond its normal 6 bytes.
Consider that the switch has a MAC address of 0x00.60.70.3E.F3.8C, and a device
with a MAC address of 0x00.60.70.27.33.8A is connected to it, then the switch will
automatically con¬gure the following ATM addresses:
Switch:
0x47.00.91.81.00.00.00.00.60.70.3E.F3.8C.00.60.70.3E.F3.8C.00
Device:
0x47.00.91.81.00.00.00.00.60.70.3E.F3.8C.00.60.70.27.33.8A.00


7.13.5 UMTS signalling transport
Earlier the user planes for the UMTS network interfaces Iu and Iub were shown, illustrat-
ing the use of AAL2 and AAL5 for traf¬c transport. As has been seen, all signalling uses
486 UMTS TRANSMISSION NETWORKS




Iub Iu
Core
Network


Base Station Control User
RNC
data data



AAL5 AAL2

Circuit
ATM
Core
Control User Control User Control User User
data data data data data data data Physical

Control User
data data
AAL5 AAL2 AAL5 AAL2 AAL5 AAL2 AAL5

ATM ATM ATM
AAL5 AAL5
Iub Iu
Physical Physical Physical Iu Packet
ATM
Core

Physical


Figure 7.65 Control and user planes in UMTS

AAL5. The protocol stack can now be extended to show both user (traf¬c transfer) and
control (signalling) planes for both interfaces. This is illustrated in Figure 7.65. Notice
that there is no ATM used over the air interface and that signalling between the UE and
the UTRAN or CN is considered higher layer and is all seen at the network interface as
user data, and is thus transported over AAL2 on the Iub interface. It is only separated
out into actual traf¬c (e.g. DTCH) and user signalling (e.g. DCCH/CCCH) at the logical
layer above the UMTS MAC layer. This user signalling is all part of the radio resource
control (RRC) protocol as discussed in Chapter 6.


7.13.6 UNI3.x signalling
UNI3.0 is the original signalling protocol de¬ned by the ATM Forum, to provide basic
functions for call setup, quality of service, etc. as required by a host connecting to an ATM
network. It was upgraded to UNI3.1 to make it compatible with the ITU-T recommenda-
tions in Q.2931. The control plane protocol stack for signalling is shown in Figure 7.66.
Some of the main signalling functions de¬ned in ITU-T recommendation Q.2931 are:

• Point to point connection setup and release
• VPI/VCI selection and assignment
7.13 ATM SIGNALLING 487



ATM Forum
ITU-T
UNI 3.0, UNI 3.1
Q.2931
UNI 4.0 Signalling
Service Specific Connection Oriented Protocol
(SSCOP)

AAL5


ATM layer


Physical layer


Figure 7.66 ATM control plane

• QoS class request
• Calling party identi¬cation
• Simple error handling.


7.13.7 Connection establishment
ATM describes two different types of virtual circuits, permanent (PVC) and switched
(SVC). The permanent circuit is essentially like a leased line and entries are made in the
network switches to indicate its route. PVCs have been enhanced by the introduction of
soft PVCs where connection routes can be dynamically chosen. The SVC is established
dynamically each time it is required in the same way that a circuit switched network
creates a physical path for a phone call. To initiate an SVC, a connection setup phase is
required. Referring to the ATM reference model shown earlier in Figure 7.6, the setup is
handled by the control plane. The signalling protocol used is de¬ned by the ITU-T and
known as Q. 2931 (roughly equivalent to UNI3.1 as de¬ned by the ATM Forum).
The procedure for establishing a connection has two phases. First, a number of cells
are sent on virtual path 0, circuit 5, which is reserved for this purpose. The cells contain
a request to open a new virtual circuit. The reason they are sent on this VCI = 5 is that
since this circuit is only used for this purpose, it can be allocated minimum bandwidth,
hence reducing overheads. Once a successful reply is received for the ¬rst phase, the new
virtual circuit is established and now requests for connection setup are sent on this new
circuit. The operation can be summarized as follows:

1. request a virtual channel to be established;
2. use this channel to send request for connection.

In some cases, a user may have virtual paths established on a permanent basis through
an arrangement with the carrier. Then the user is free to allocate their own virtual circuits,
since this operation would be transparent to the network switches which are routing based
on the VPI. Alternatively, the virtual paths can be set up dynamically.
488 UMTS TRANSMISSION NETWORKS


Figure 7.67, shows the ¬‚ow of connection setup through the network, with each request
and reply being relayed through the switches. Table 7.23 summarizes the various mes-
sages used.
To close the virtual circuit, the party that wishes to quit sends a release request, which
is acknowledged by the other party, as shown in Figure 7.68 and Table 7.24.

source destination
host switch switch host
setup
setup
setup
call
call
proceeding
roceeding
p
connect
time




connect
connect
connect
connect ack
connect ack
ack



Figure 7.67 Connection setup

Table 7.23 Setup messages
Message Interpretation
Sent by host Sent by network
Setup Establish this circuit Call incoming
Call proceeding Call seen Attempting call request
Connect Call accepted Call request accepted
Connect ack Accept acknowledged Call acknowledged


host switch switch host
release
release
release
time




release
complete
release
release
complete
complete


Figure 7.68 Connection release

Table 7.24 Connection release messages
Message Interpretation
Sent by host Sent by network
Release Release this call Host wants to release
Release complete Release acknowledged Release acknowledged
7.13 ATM SIGNALLING 489


So far, all that has been considered is a point-to-point connection with only a sender
and a receiver present. However, a facility to allow multicasting must also be provided,
where one host is sending to multiple destinations. This is done by ¬rst establishing a
point-to-point. Subsequently, an add party signal is sent which attaches other destinations
to the current virtual circuit.


7.13.8 Signalling message structure
A signalling message is based around the Q.931 message format. It consists of a header and
a variable number of information elements (IEs). The format is illustrated in Figure 7.69.
The format of the header is as shown in Figure 7.70.


Message header

IE

IE

...

IE


Figure 7.69 ATM signalling message structure


Bit
7 6 5 4 3 2 1 0 Byte
1
Protocol discriminator
0 0 0 0 Call reference value length 2
Flag 3
Call reference value
Call reference value (continued) 4
Call reference value (continued) 5
Message type 6
Message type (continued) 7
Message length 8
Message length (continued) 9

Infomation elements etc.
(variable length)


Figure 7.70 ATM signalling header
490 UMTS TRANSMISSION NETWORKS


The protocol discriminator is used to distinguish UNI control messages from other
messages. Two common values are:
08 “ Q.931messages
09 “ Q.2931 messages
The call reference value is 3 bytes long and identi¬es the call to which this message
is related, since a user may have many calls active at any given time. The ¬rst bit, the
¬‚ag, is used to indicate whether the message is coming from or going to the originator
of the call:
Flag = 0: to call originator
Flag = 1: from call originator
The next 2 bytes indicate the type of message being sent. A sample of some common
message types are shown in Figure 7.71.

Value Type

Call establishment messages


00000010 Call proceeding


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