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ITU-T Q.931 1998 Edition, May 1998
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ISDN USER-NETWORK INTERFACE LAYER 3 SPECIFICATION FOR BASIC CALL CONTROL
Includes all amendments and changes through Erratum , February 2003
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Scope of this Recommendation

The procedures currently described in this Recommendation are for the control of circuit-switched connections, user-to-user signalling connections and packet-switched connections. The transport of other message-based information flows on the D-channel is a subject for further study and will be included in related Recommendations.

NOTE 1 – The term "layer 3" is used for the functions and protocol described in this Recommendation (see Recommendation Q.930/I.450). The terms "data link layer" and "layer 2" are used interchangeably to refer to the layer immediately below layer 3.

NOTE 2 – Alignment of the functions and protocol with those of OSI network layer is for further study.