Using “D” Messages for Layer 3
• 8-8 •
The H.110 Basic Rate ISDN Board
8.3.5 Progress Indicator
Progress indicators are used to indicate something about the nature of the
interworking of a call. This can be information about the fact that the call
may not be an end to end ISDN call, or it may indicate the presence of
inband information such as call progress tones or announcements. This
element is present only in commands to NT ports or responses on TE ports
as it is never found in messages sent by a user. The codes for a progress
indicator are:
C
Call is not end to end ISDN, call progress information may be
available inband
D
Destination address is non-ISDN
O
Origination address is non-ISDN
I
Inband information or appropriate pattern now available
W
Delay in response at destination interface
N
no progress indicator
8.3.6
Signal
The Signal data element is used to inform a terminal that a condition exists
in which a user should be alerted by an audible tone or signal. There are
two instances in which the Signal element is used. One is in a SETUP
message to a terminal indicating the presence of a terminating call. In this
instance the Signal takes the place of ringing on an analog line to alert the
user of an incoming call. The second instance is in messages to a terminal
originating a call to inform the user of the progress of the call. This takes
the place of the traditional inband call progress tones such as dial tone,
busy, ringback, and reorder. The messages that the Signal may appear in
are ALERTing, CALL PROCeeding, PROGress, CONNect, DISConnect,
RELease, and RELease COMplete. The Signal element may be present at
the same time that the Progress Indicator indicates inband signaling and
tones are present in band. The signal codes in “D” messages are: