The call offering procedure is performed using the layer 3 messages and procedures of clause 5. The call offering procedure is integrated into the circuit-switched call control procedures, signalled on the D-channel, with the channel selection being accomplished by means of the channel selection procedure if offered as a network option.
As described in clause 5, the network selects the first user which responds to the call offering with a CONNECT message. When the selected user has requested that the X.25 call be set up over a new B-channel, the network will indicate that the channel is acceptable by returning a CONNECT ACKNOWLEDGE message to the user. If multiple terminals have responded positively to the SETUP message, the network shall clear each of the non-selected terminals with a RELEASE message containing cause No. 26, non-selected user clearing.
When the selected user has requested that the X.25 call be set up over an established B-channel or the D-channel, the network shall respond to the CONNECT message with a RELEASE message containing cause No. 7, call awarded and being delivered in an established channel. The network shall also return a RELEASE message containing cause No. 26, non-selected user clearing to any other positively responding terminals. The network will then deliver the X.25 virtual call over the selected channel.
Note 1 - There is no time significance between the delivery of the RELEASE message and the incoming call packet, i.e. either may occur first.
Note 2 - The network shall send the RELEASE message(s) and the user(s) shall respond with RELEASE COMPLETE.
If the channel indicated by the first positively responding user is not available, the network will use Q.931 call clearing procedures to clear the call with cause No. 6, channel unacceptable. If the channel indicated in the SETUP message is not acceptable to the user, the user will clear the call with a RELEASE COMPLETE message containing cause No. 34, no circuit/channel available or cause No. 44, requested circuit/channel not available.
On the basis of a network option or subscription agreement, the network may choose the access channel or access channel type (e.g. B or D) for a particular incoming packet call.
When the Channel indication information element indicates Channel indication = No channel, Exclusive, and D-channel indication - Yes, then the Bearer capability information element should be coded as follows:
- information transfer capability set tounrestricted digital information;
- transfer mode set to packet mode;
- information rate set to packet mode (00000);
- layer 2 protocol set to Recommendation Q.921;
- layer 3 protocol set to Recommendation X.25, packet layer.
In all other cases, the Bearer capability information element should be encoded as follows:
- information transfer capability set to either
a) unrestricted digital information; or
b) restricted digital information;
- transfer mode set to packet mode;
- information rate set to packet mode (00000);
- layer 2 protocol set to Recommendation X.25, link layer;
- layer 3 protocol set to Recommendation X.25, Packet layer.
There exists an understanding that if the terminal responds with D-channel indication set (see Table 6-3), the Layer 2 protocol to be used is Recommendation Q.921 (LAPD).
Network requested channel and user response Incoming access for packet-mode
Channel indicated in the SETUP message network to user direction |
Allowable network response (network-user) |
||
Information channel selection |
Preferred or exclusive |
D-channel indicator (Note 1) |
|
Bi |
Exclusive |
No |
Bi |
Yes |
Bi, D |
||
Bi |
Preferred |
No |
Bi, Bi', Bj |
Yes |
Bi, Bi', Bj, D |
||
No channel |
Preferred |
No |
Bj |
Yes |
BJ, D |
||
Exclusive |
Yes |
D |
|
Bi Indicated (idle) B-channel Bi' Any other idle B-channel (not permitted in response to broadcast call offering) Bj An established B-channel under the user's control (a semi-permanent B-channel which is allocated to the user may be indicated if the user subscribes to the unconditional notification class) D The D-channel NOTE 1- D-channel indicator shall be encoded "0" to indicate No and "1" to indicate Yes. NOTE 2- All other encodings are invalid. |
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The channel selection procedure for incoming calls is independent of the type of channel selected at the calling end. In this respect, any combination of channel type used at each end is possible, provided the user rates and available bandwidth are compatible.
The channel selection principle to be used in the procedure is shown in Table 6-3.
Note 3 - When the incoming SETUP message is sent on a broadcast data link with a Channel identification information element which indicates an idle B-channel and preferred, the called user is not permitted to respond with a different idle B-channel in the response. The option to respond with a different idle channel is restricted to point-to-point call offerings.
Note 4 - Networks providing packet-mode call offering shall provide Q.931 signalling procedures for packet-mode calls on SAPI=0. For an interim period, some networks, by subscription agreement,may offer SAPI=16 broadcast call offering procedures for providing Q.931 signalling. This option shall use all Q.931 procedures for packet-mode calls with the following restriction: All calls will be offered as D-channel exclusive and will not provide channel selection procedures. Terminals implementing SAPI=16 procedures should also implement SAPI=0 procedures for portability.