Information Technology Reference
In-Depth Information
protocols. At present it is normally set to
0001000
. After the second byte the call reference
length value is defined. This is used to identify particular calls with a reference number. The
length of the call reference value is defined within the second byte. As it contains a 4-bit
value, up to 16 bytes can be contained in the call reference value field. The next byte gives
the message type and this type defines the information contained in the proceeding field.
Protocol discriminator
Call reference
value length
0
0
0
0
Call reference value
0
Mesage type
Other information
(as required)
Figure G.10
Signalling message structure
There are four main types of message: call establish, call information, call clearing and
miscellaneous messages. Table G.6 outlines the main messages. Figure G.11 shows an ex-
ample connection procedure. The initial message sent is the setup. This may contain some of
the following:
•
Channel identification - identifies a channel with an ISDN interface.
•
Calling party number.
•
Calling party subaddress.
•
Called party number.
•
Called party subnumber.
•
Extra data (2-131 bytes).
After the calling TE has sent the setup message, the network then returns the setup ACK
message. If there is insufficient information in the setup message then other information
needs to flow between the called TE and the network. After this the network sends back a
call proceeding message and it also sends a setup message to the called TE. When the called
TE detects its TEI address and SAPI, it sends back an alerting message. This informs the net-
work that the node is alerting the user to answer the call. When it is answered, the called TE
sends a connect message to the network. The network then acknowledges this with a connect
ACK message, at the same time it sends a connect message to the calling TE. The calling TE
then acknowledges this with a connect ACK. The connection is then established between the
two nodes and data can be transferred.
To disconnect the connection the disconnect, release and release complete messages are
used.
