5.1 Broadcast Connections.

Broadcast Connections may only be used in conjunction with the Unacknowledged Service. This type of connection is required to support the point to multipoint connection.

The point to multipoint connection involves the presentation of a call at the terminating end to a number of competing TEs connected to the S-Bus, viz. the multipoint configuration. TEs, which compete for the incoming call, support the same service features as indicated by their SAPI value but only one TE will be offered the call as part of the Layer three signalling procedure described below.

Broadcast connections must be used, therefore, if the terminating end has such a multipoint configuration, i.e. more than one TE on the S-Bus. A broadcast message is presented to all the TEs on the S-Bus which support a similar service as indicated by their SAPI value, viz. signalling (SAPI=0), OAM (SAPI=63) or Packet communication conforming to the X25 protocol (SAPI=16).

Layer 2 broadcast connections are used to communicate Layer three information in the form of UI frames. These carry the layer three messages such as the Setup message and present it to the set of terminating user interfaces between layers 2 and 3 with the same SAP value, i.e. ones which support the same type of service.

A number of TEs may be in a position to accept the incoming call, i.e. may have the necessary addressing (especially if no addressing information additional to the Called Subscriber Number, accompanies the Setup message as an Information Element) qualifications, and also conform to the compatibility checking which may include bearer service as well as both high layer and/or low layer compatibility checking.

A number of TEs may respond positively to the incoming call with Layer 3 messages, such as Call Proceeding, Alerting, Connect.

(This will occur within the framework of D channel contention and prioritisation schemes. Prioritisation schemes may ensure that the TE which answered the last call is placed at the end of the queue to deal with calls it is eligible to answer. However, the remainder of TEs which are eligible to answer the incoming call may respond with Call Proceeding, Alerting or Connect messages).

The TE which is the first to receive the Connect Acknowledge message is given the call.

All other users (TEs) will receive the Release message, i.e. only one physical connection is established between a TE on the originating side with one TE in the multipoint configuration at the terminating end. This single physical connection is established with the support of the layer 2 broadcast addressing scheme.

Note 1: In the case of a broadcast message layer 3 addressing information such as subaddressing or DDI information, if either is present, will be included in the Setup message and sent to the Layer 2 to 3 SAP interface of the TEs on the S-Bus which support the service of the incoming call. The addressing information can then be checked for compatibility with the addressing information associated with the TEs on the S-Bus.

Note 2: If one TE is equipped to respond to incoming addressing information, such as subaddressing or DDI, then all TEs on the same S-Bus must similarly be equipped with addressing information.

The outcome of the broadcast connection is that a B channel connection is established with the support of one logical link set up by means of LAPD.

In summary, Broadcast messages are sent over the D channel to prepare, at the SAP interface, TEs to respond to the Layer 3 Setup message carried by the Layer 2 Broadcast message.

5.2 Point to Point Connections.

In this case there is only one ISDN termination at the terminating end.

A contention issue similar to that which may apply for broadcast connections may arise in a point to point connection because a number of logical links may be established with the one TE (using point to point connections) in cases where the TE can support more than one type of call, i.e. where it is assigned a number of TEIs.

In this case Layer three, compatibility checking will determine qualifying applications and again the first of these to respond will be given the call.

Also note that the terminating user interface may be an NT2 such as a ISPBX. In this case the DDI Information Element may be used with the Setup message to address the specific extension required if this facility is available and supported by the ISPBX.

A point to point connection may be used to support both acknowledged and, in principle, unacknowledged services.