Demand access B-channel connections are controlled using the D-channel signalling procedures for call establishment described in 5.1 using the messages defined in 3.2 with the following exceptions:
a) the procedures for overlap sending specified in 5.1.3 do not apply;
b) the procedures for call proceeding and overlap sending specified in 5.1.5.2 do not apply;
c) the procedures for notification of interworking at the origination interface specified in 5.1.6 do not apply;
d) the procedures for call confirmation indication specified in 5.1.7 do not apply;
e) the procedures for call connection specified in 5.1.8 apply as follows:
- upon accepting the access connection, the network shall send a CONNECT message across the user-network interface to the calling user and enter the Active state;
- this message indicates to the calling user that an access connection to the packet handler has been established;
- on receipt of the CONNECT message, the calling user shall stop timer T310, if running, may optionally send a CONNECT ACKNOWLEDGE message, and shall enter the Active state;
f) the procedures for call rejection specified in 5.1.9 apply as follows:
- when unable to accept the access connection, the network shall initiate ISDN access connection clearing at the originating user-network interface as described in 5.3;
g) the procedures for transit network selection specified in 5.1.10 do not apply.
The specific B-channel to be used as a demand connection is selected using the channel selection procedures described in 5.1.2 and summarized in Table 6-1.
For a demand connection to an ISDN PH, the Bearer capability information element included in the SETUP message shall be coded with:
- information transfer capability set to unrestricted digital information;
- transfer mode set to packet mode;
- information transfer rate set to00000;
- user information layer 2 protocol set to Recommendation X.25, link layer;
- user information layer 3 protocol set to Recommendation X.25, packet layer.
Note - Octets 5a, 5b, 5c and 5d shall not be included.
The demand access connection can then be used to support packet communications according to X.25 link layer and X.25 packet layer procedures as specified in 6.3.
Some ISDNs may require the Calling party number and the Calling party subaddress information elements to be included in the SETUP message to select a specific user profile.