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Using the overlap_rcv feature of L4L3mENABLE_PROTOCOL
November 2009 328
Using the overlap_rcv feature of
L4L3mENABLE_PROTOCOL
What is Overlap Receive?
When an ISDN call is received from the Public Telephone Network,
the caller information arrives in a Q.931 SETUP message. Typically,
this SETUP message contains all the information necessary for the
user to properly route, switch and/or complete the call: information
about the type of call (voice, modem, data, and so on), caller ID
information, and so on.
Overlap Receive mode occurs when the incoming SETUP message
contains no CALLED party information, or incomplete CALLED
party information. The CALLED party information is necessary to
complete a circuit-switched call, because it identifies the complete
number that the originator is dialing.
The ITU Q.931 Recommendation defines a SENDING_COMPLETE
Information Element as an indicator of when a SETUP message has
complete CALLED party number information. If a SETUP message
is received and there is no SENDING_COMPLETE info element in
the message, the user is expected to enter Overlap Receive mode.
RESTart ACKnowledge
nothing nothing
SEGMENT
nothing nothing
CONGestion CONtrol
nothing nothing
INFOmation
INFO_REQUEST INFO
NOTIFY
nothing UNIVERSAL
STATus
nothing STATUS_IND
STATus ENQuiry
nothing nothing
FACILITY
UNIVERSAL UNIVERSAL
Table 14. Q.931 Message Comparison Table (Continued)
Q.931Message BSMI Message(L4L3m) BSMI Message (L3L4m)