27.6 ISDN CONNECTIONS

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The four types of connections supported in ISDN are:

Circuit-switched calls over B channel
Semipermanent connections over B channel
Packet-switched calls over B channel
Packet-switched calls over D channel

ISDN supports the following types of calls: circuit-switched and packet-switched calls over the B channel and packet-switched calls over the D channel.

Circuit-switched calls: Circuit-switched calls involve both B and D channels. The procedure is as follows:

User requests a circuit over D channel.
User is informed of the circuit establishment over D channel.
Data is transferred over B channel.
Call is terminated through signaling on D channel.

Semipermanent connection: For semipermanent connection, only layer 1 functionality is provided by the network interface. No call control protocol is required because a connection already exists.

Packet-switched calls over B channel: The procedure for establishing packet-switched calls is as follows:

The user makes a request over the D channel for a connection on B channel using the call control procedure.
The connection is set up, and the user is notified on the D channel using the call control procedure.
The user sets up a virtual circuit to another user using X.25 call establishment procedure on B channel.
The user terminates the virtual circuit using X.25 on B channel.
After one or more virtual calls on the B channel, the user signals through the D channel to terminate connection to the packet-switching node.

Packet-switched calls over the D channel: Low-speed data is sent over the D channel, for which X.25 level 3 protocol using LAPD frames is employed.

In ISDN, signaling information between switches is exchanged through a standard called Signaling System No. 7 (SS7).

Signaling in ISDN

Layer 3 protocol for signaling is defined in I.451 recommendations. Messages are exchanged between the user and the switch over the D channel for establishing, controlling, and terminating a call. Signaling between the switches is through an SS7 signaling network, which was discussed in the previous chapter.

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