Using “D” Messages for Layer 3
• 8-17 •
The H.110 Basic Rate ISDN Board
The call would be retrieved by the application sending a command of the
form
DGxxrr,
where xx is an available B-channel and rr is the call
reference of the held call. This message should not be sent if a B-channel
is not available. If the call is retrieved the network will send a RETrieve
ACK message which will result in a response message of the form
DGxxArr.
If the network sends a RETrieve REJect message, the response
message would be
DGxxRccrr
, where cc is the cause for the rejection.
In the examples above, it is the terminal that initiates the hold. It is also
possible for the network side to place a call on hold to free the B-channel.
However, only the terminal side can retrieve a call.
It is possible to release a held call by sending a RELease COMplete. This
might be done if the far end disconnects. In this case, the “DR” command
or response message would include not only the cause, but the call
reference. It would be of the form
DRxxccrr
, where rr is the call
reference.
8.10 EKTS, CACH EKTS, and AT&T Custom
Electronic Key Telephone Service or EKTS and Call Appearance Call
Handling Electronic Key Telephone Service of CACH EKTS are ISDN
services which allow a group of ISDN BRI station sets emulate a key
telephone system. These services provide a means for the handling of
multiple calls as well as for the appearance of the same call on multiple
station sets. The procedures for Basic EKTS are similar to those for Basic
Call Handling, and require no special setup. CACH EKTS uses an
information element called the call appearance to associate calls with a
specific button on the station set. To use CACH EKTS, the protocol level
for the port must be set to “E”, and a call appearance argument must be
present in SETUP commands. This argument takes the form
A=ca
where
ca is the call appearance.
The AT&T Custom Protocol predates National ISDN-1, but is still
deployed on many AT&T switches and PBXs. In some ways it resembles
CACH EKTS in that it uses call appearances for handling calls. While