8.8 ISDN

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The two types of ISDN channels are the circuit-switched bearer (B) channels and the packet-switched data control (D) channel. With ISDN PRI, each channel takes 64 Kbps of bandwidth. True ISDN functionality is characterized by intelligent linkage between the transport facility (T1) and CPE. In addition to performing routine call setup and tear-down, B channel routing takes place under D channel control on a call-by-call basis.

As stated previously, the ability of the T1 multiplexer to provide interoperability-with both proprietary and DS1/DS0-framed products means that users can more easily build hybrid networks composed of private and public elements. It also means that the multiplexer can be used to improve the efficiency and economy of ISDN.

The multiplexer can be set up to treat the B channels of ISDN as bit-oriented (proprietary) subaggregates. When B channels are routed by the D channel through a public network element such as the Class 4 CO switch, multiple voice or data channels and supervisory information can be embedded within each 64-Kbps B channel destined for a remote corporate location. Because the 4ESS does not route the D channel (SS7 takes over from there), embedding supervisory information within the B channel extends private network management and control through the public ISDN network to remote corporate locations. In effect, this allows the public network to be used as a private network. Thus, control does not have to be sacrificed for the sake of cost savings.

Another way in which the multiplexer can enhance ISDN is through ADPCM. Because they are clear channels, each B channel can carry ADPCM compressed voice at 32, 24, and 16 Kbps. In addition to cost savings, this gives network administrators tremendous flexibility in configuring voice services between corporate locations that do not come with ISDN alone.

The multiplexer’s support of ISDN also greatly improves disaster recovery; it is faster and more economical to implement than, for example, a carrier’s reserved service. Under this subscription arrangement, a dedicated T1 line is brought into service only after a network administrator verbally requests it with a telephone call. Several hours may elapse before the reserved line is finally connected. With a PRI-equipped multiplexer, dynamic B channel call setup for disaster recovery can be accomplished in a matter of seconds under stored program control.

With a PRI-equipped multiplexer, the requirement for dedicated lines as alternate paths is no longer necessary as a hedge against disaster. Through the multiplexer’s dynamic restoration capabilities, data from failing lines may be switched to B channels on the multiplexer’s PRI. The PRI’s D channel routes the individual B channels to their destinations through the public ISDN network. Instead of paying for a dedicated link, the user is billed only for the bandwidth used under ISDN.

Some carriers have expanded PRI to include the higher capacity 384 Kbps and 1.536 Mbps channels, known as the H0 and H11 channels, respectively. These are marketed under different brand names, such as AT&T’s Switched 384 and Switched 1536 services. This gives subscribers the means to implement “wideband” dynamic disaster recovery with their PRl-equipped multiplexers. Support for this wideband function can be added as a software-expandable option without requiring the expense of a major hardware upgrade.

For organizations that do not see the need for ISDN, a multiplexer that is DCScompatible and capable of supporting FT1 can provide flexible bandwidth allocation, integrate voice and data, and extend network management and control to remote locations. The PRI can be added later when the need for ISDN is justified. In supporting both ISDN and FT1, the multiplexer further extends the options available for building hybrid networks. For example, the multiplexer can combine FT1 for dedicated circuits with primary rate ISDN for switched circuits over the same T1 access facility, saving the organization as much as 40% on local-access charges for the two services if purchased separately.

Dedicated data traffic can be carried over the PRI access link to a carrier’s ISDN service node, where it is routed to the appropriate destination via connections programmed into the network DCS. Voice traffic can be carried over the same link but directed to the Class 4 CO for switching through the public network.



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LANs to WANs(c) The Complete Management Guide
LANs to WANs: The Complete Management Guide
ISBN: 1580535720
EAN: 2147483647
Year: 2003
Pages: 184

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