ISDN L2 and L3 Protocols

ISDN user-network interface L2 and L3 specifications also are referred to as Digital Subscriber Signaling System No. 1 (DSS1). L2 provides error-free and secure connections for two endpoints across the ISDN reference configuration. L3 provides the mechanism for call establishment, control, and access to services. The L2 protocol for ISDN is Q.920/921, and the L3 protocol is Q.930/931. Q.932 enables general procedures for accessing and controlling supplementary services.

The specifications for L2 are referred to as Link Access Procedures on the D channel (LAPD). This protocol provides the reliable transfer of frames between the local exchange and the TE. The specifications for Q.920 and Q.921 are extensive and are available from the Q series of ITU recommendations.

The specifications for L3 define the messages that pass between the local exchange and the TE. These messages are used for call setup, call supervision, call teardown, and supplementary services. The next section discusses the specifics of ISDN messaging.

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