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ISDN is a network access architecture that offers the ability to integrate voice, video, and data traffic over the same circuit connection. Unlike the analog signaling methods, ISDN employs a CCS signaling type for call setup and channel management. Although ISDN takes the same physical link, it is on a different logical link that is called a D channel and is therefore considered to be CCS.

Two main types of channels are associated with N-ISDN, B channel and D channel. The B channels are used for the transmission of bearer services to include voice, video, and data. The D channel is used as the vehicle for transporting the signaling traffic. A BRI circuit contains two 64-kbps B channels and a 16-kbps D channel, and a PRI circuit contains either 23 or 30 B channels and a full 64-kbps D channel. BRI service is typically deployed as a home user service, and PRI is deployed as an enterprise level service.

ISDN is considered a local loop technology, which means that ISDN is only present between the subscriber and the local ISDN switch. The back end of the switch connected to the PSTN can be any number of communication technologies. The protocols can be converted back into ISDN at either end of the connection.

Three layers are associated with ISDN:

  • Layer 1 = I.430 and I.431 (BRI and PRI respectively)

  • Layer 2 = Q.921

  • Layer 3 = Q.931

I.430 and I.431 specify the physical layer attributes for the ISDN circuit, including electrical and mechanical communication methods. Q.921 specifies the Layer 2 functionality of an ISDN circuit. Q.921 is designed to provide an error free link for the transmission of Q.931 messages and to ensure link integrity. Q.921 provides addressing in the form of TEIs. TEIs identify the end device to the ISDN network switch. BRI circuits have a range of TEIs that can be assigned because they are typically deployed in a point-to-multipoint configuration. PRI circuits always have a TEI=0 because they are deployed over T- and E-carrier facilities in a point-to-point configuration. SAPIs identify the points at which Q.921 can access and communicate with Q.931 messages.

Q.931 handles call control and circuit management within the ISDN network. Q.931 is a message-based signaling format that allows the user or network side to invoke a large number of functions for each call. Addressing on the ISDN network is done through a set of phone numbers (E.164 address) and sub-addresses. The sub-address identifies between two terminals that are using the same phone number. It does not have to be used in the event that there are two different phone numbers allocated.

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