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5-10 Call control procedures
Call establishment at the terminating exchange
Call establishment at the terminating exchange occurs when a PRI call goes
out from the DMS-250 switch (node 4 in Figure 5-4) when the switch is the
network.
In any PRI connection, one end of the interface functions as the user and the
other functions as the network. The switch can function as the user or
network in a PRI connection, but is ideally configured as the network.
Figure 5-4
Originating exchange
1
Originating
user
2
User
Basic
rate
interface
PBX
Network call request with SETUP message
The network indicates the arrival of a call to a user by sending a SETUP
message across the terminating PRI interface. The SETUP message contains
information about the call regarding where it should be routed.
Selection of PRI B-channel for termination
The DMS-250 switch performs the following to select a PRI B-channel to
make an outgoing call.
PRI B-channel allocated
When a route is determined and the trunk group selected (within the route
verified as a PRI-type), the call is treated as a PRI termination. This is
described as follows.
To select a B-channel, the next idle member in the trunk group is chosen
according to the SELSEQ field in table TRKGRP. The outgoing PRI trunk
member is marked busy and the B-channel is allocated (made unavailable to
other calls). (The selected trunk member is in effect the B-channel.) This
297-2521-106 Standard 23.01 March 2001
Originating
exchange
(call originates
into switch)
3
Network
PRI
MCI
DMS-250
switch
Network
Terminating
exchange
(call terminates
out of switch)
4
Network
ISUP
MCI
IMT
DMS-250
switch
6
Terminating
user
5
User
Basic
PRI
rate
interface
PBX
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