5-8 Call control procedures
297-2521-106 Standard 23.01 March 2001
Table STDPRTCT
The pretranslator name (PRAX in Figure 5-3) is used to index into table
STDPRTCT to locate the pretranslator subtable to use for the call. The
pretranslator PRAX contains an entry for the dialed digits and, in this case,
marks the call as call type ONNET. On the DMS-250 switch, ONNET calls
are considered to be private. Marking the call type as ONNET or OFFNET
changes the numbering plan indicator on the outgoing side of the call to
either PRIVATE or E.164, respectively.
Table HNPACONT
The serving translation scheme (904) obtained from AUTHCODM is for
translations through table HNPACONT. At this point, translations proceed
normally and terminate the call to the trunk group D2SPRI. Because the
pretranslator marked the call as ONNET, the outgoing numbering plan
indicator of the called party number is marked as PRIVATE.
B-channel allocation and negotiation
When a call is initiated and SETUP message is sent over the PRI interface to
the network, the network attempts to allocate the B-channel requested in the
incoming SETUP message. If this is successful, the network attempts to
route the call based on the called party number information element (in the
SETUP message).
B-channel negotiation occurs when the two ends of a PRI interface negotiate
the B-channel to be used for a call. The B-channel requested in the SETUP
message maps one-to-one with a particular trunk member in the PRI trunk
group. The network selects that channel (if it is available to make the
connection) and marks it busy; that particular B-channel is no longer
available for other calls.
B-channel negotiation happens at two points:
• B-channel selection at the originating interface (the interface where the
user customer premise equipment originates the call)
• B-channel selection at the destination interface (the interface where the
network originates the call)
Two different methods are available for B-channel selection in the SETUP
message common to originating and terminating interfaces. These are:
• requiring a specific channel, with no acceptable alternative
• preferring a specific channel, but accepting an alternative
However, if the B-channel requested in the SETUP message is not available
to make the connection, the DMS-250 switch processes the call according to
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