Download Network Troubleshooting

SECTION II
TROUBLESHOOTING LOCAL-AREA NETWORKS
362
ATM
B it
8
7
0
6
0
5
0
4
0
3
1
2
0
10
1
0
1
P r o to c o l d is c r im in a to r v a lu e fo r Q .2 9 3 1 m e s s a g e s
B it
8 7 6 5 4 3 2 1
0 0 0 0 0 0 0 0
to
0 0 0 0 0 1 1 1
R e s e rv e d
0 0 0 0 1 0 0 0
Q .9 3 1 / I.4 1 5 u s e r n e tw o r k c a ll c o n tr o l
0 0 0 0 1 0 0 1
Q .2 9 3 1 u s e r n e tw o r k c a ll/c o n n e c tio n c o n tr o l
0 0 0 1 0 0 0 0
to
0 0 1 1 1 1 1 1
O th e r L a y e r 3 p r o to c o ls
(X .2 5 e tc .)
0 1 0 0 0 0 0 0
to
0 1 0 0 1 1 1 1
N a tio n a l u s e
0 1 0 1 0 0 0 0
to
1 1 1 1 1 1 1 0
O th e r L a y e r 3 p r o to c o ls
(X .2 5 e tc .)
Figure 10.44 The protocol discriminator field
Call Reference
The call reference serves to associate Q.2931 messages with a given connection.
When a new connection is established, all messages concerning that connection
have the same call reference value. When the connection has been cleared down,
the call reference is released and can be used again. The same call reference can
be used by two connections within an ATM virtual channel only if the respective
connection setups take place in opposite directions. The length of the call
reference field is measured in bytes; the default length is 3 bytes. The call
reference flag identifies the sending and receiving stations. The station that
indicates the connection always sets this flag to 0 in its messages, while messages originating from the receiving station have the flag set to 1. The call
reference value 0 is known as the global call reference and refers to all connections within a signaling virtual channel.
Message Type
This field indicates the message type. All message types except SETUP
ACKNOWLEDGE and INFORMATION are also supported by the corresponding
ATM Forum specification UNI 4.0 (see Figure 10.46).