Using “D” Messages for Layer 3• 8-2 •
The H.110 Basic Rate ISDN Board
or exchanging configuration information such as SPID’s. Additional
messages are defined for maintenance or management purposes including
NOTIFY, STATus, STATus ENQuiry, FACILITY and REGister.
In addition to the basic codes for the various message types, each Q.931
message consists of a number of information elements. These information
elements are used to contain items such as the called and calling numbers,
whether a call is speech or data, the B channel involved, and other
information about the progress of a call. For some messages such as the
SETUP message a large number of information elements are possible,
many of which are optional or only used for certain types of calls. As an
example a typical SETUP message from a central office switch contains
eight information elements and ninety octets of information.
Each active call has a call reference that is part of every Q.931 message
dealing with that call. This allows for the juggling of multiple calls on an
interface. The call reference is granted by the originating party and remains
in effect until the call is finally released, no matter what happens in the
interim. However, a call reference is only valid at that particular interface
in messages between the network termination and the terminal equipment.
Q.931 also describes a state model for the progress of a call. The states are
defined by which type of message has been received or sent. State
information may be exchanged through STATus messages as a check on
the operation of the terminal and to insure that the terminal and network are
in agreement on the condition of a particular call. For each state of a call,
only certain message types are appropriate.
8.2 “D” Command & Response Messages
The Q.931 message set can be complicated, with many options and
information elements to deal with. On the H.110 Basic Rate ISDN Board,
an application can directly access the information field of a Q.931 message
by using the Layer 2 support level. In some cases this may be desirable
when dealing with equipment that does not support National ISDN 1 or has
non-standard features or messages. However, the H.110 BRI Board does
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