This procedure assumes that a data link connection providing services described in Recommendation Q.920/I.440 [45] may not exist before the first layer 3 message (SETUP) is transferred across the interface. However, reliable data link connections must be established by each of the users (terminals and/or NT2s) at the interface before they respond to the SETUP message.
Data link connections may be established by the TA, TE or NT2 as soon as a TEI is assigned (either locally or by automatic assignment procedure), and may be retained indefinitely. This may be recommended as a network option.
The SETUP message offered on point-to-point data link shall be delivered to layer 2 using a DL-DATA request primitive. No use shall be made of the DL-UNIT-DATA request primitive other than for operation using the broadcast capability of the data link layer.
The call reference contained in all messages exchanged across the user-network interface shall contain the call reference value specified in the SETUP message delivered by the network. In selecting a call reference, the dummy call reference shall not be used in association with the basic call.