Using “D” Messages for Layer 3
• 8-16 •
The H.110 Basic Rate ISDN Board
8.9 HOLD & RETrieve
An active ISDN call can be placed on hold to free up a B-channel for use
by another call. The held call remains active and retains its call reference,
but the connection is broken. The connection can be restored by retrieving
the call and assigning a new B-channel for the circuit. This does not have
to be the same B-channel as was originally used.
A terminal connected to an NT port can initiate a hold by sending a HOLD
message. This results in the “D” response message
DHxxrr
, where xx is
the B-channel and rr is the call reference. The call reference is important
as it is the only way a call can be retrieved once it is placed on hold. Once
a call has been placed on hold, it is the responsibility of the application to
retain the call reference until the call is retrieved or released. To confirm
the hold, the application would send a message of the form
DHxxA
to
acknowledge the hold. If it did not want to accept the hold, it would send
a message of the form
DHxxRcc
, where cc is the reason the hold was
rejected. Once the HOLD ACK message has been sent, the B-channel is
available for a new call.
To retrieve the held call, the terminal would send a RETrieve message.
This would cause a “D” response message of the form
DGxxrr
, where xx
is the B-channel and rr is the call reference. If the terminal did not choose
a B-channel, the board would assign one if available. To allow the
retrieval, the application would send the command
DGxxArr
. To reject
the retrieval, the application would send the command
DGxxRccrr
where
cc is the cause for rejecting the retrieval.
When the port on the H.110 BRI Board is a TE, it can request that a call
be placed on hold in a similar manner. The application would send a
command of the form
DHxx
where xx is the B-channel of the call to be
held. If the network responds with a HOLD ACK, the board would send
a response message of the form
DHxxArr
where rr is the call reference.
A rejection would result is a response message of the form
DHxxRccrr
,
where cc is the cause for rejection.