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11-4 Internetwork Design Guide

Introduction

Central Ofﬁce Considerations

When designing your ISDN network, you need to consider the central ofﬁce switches through which

your ISDN connections will be made. Telephone companies use a variety of central ofﬁce switches,

each with unique characteristics that affect the way in which you conﬁgure your router equipment.

This section discusses the following characteristics of central ofﬁce switches:

• Switch Type

• TEI Negotiation (BRI only)

• Service Proﬁle Identiﬁers (SPIDs) (BRI only)

• Signaling System 7

• ISDN Interfaces

• Calling Line Identiﬁcation

Switch Type

Some manufacturers of ISDN central ofﬁce switches (also known as local exchange equipment)

divide the local exchange into two functions: local termination and exchange termination. The local

termination function primarily deals with the transmission facility and termination of the local loop.

The exchange termination function deals with the switching portion of the local exchange. First, the

exchange termination function demultiplexes the bits on the B and D channels. Next, B channel

information is routed to the ﬁrst stage of the circuit switch, and D-channel packets are routed to

D-channel packet separation circuitry.

Several companies manufacture ISDN-compatible central ofﬁce switches. Today, the AT&T 5ESS

and the NorTel DMS-100 are the two principal ISDN switches in North America. Until the current

release of National ISDN-1 software, incompatibility between the AT&T and NorTel switches

meant, for example, that AT&T ISDN telephone sets could not be used with a NorTel switch.

The 5ESS was introduced in 1982 and can provide up to 100,000 local loops. Approximately

1600 5ESS switches are in use worldwide, serving close to 40 million lines. In the United States,

over 85 percent of the BRI lines in service terminate at a 5ESS-equipped central ofﬁce.

The NorTel DMS-100 switch family is intended to deliver a wide range of telecommunication

services. The DMS-100, introduced in 1978, can terminate up to 100,000 lines. The DMS-10 is a

smaller version of the DMS-100 and supports up to 10,800 lines. The DMS-200 is intended for

switching ofﬁces in the toll network, equal-access end ofﬁces, or access tandem switch applications.

The DMS-250 is a toll switch for specialized common carriers requiring tandem switch operation.

The DMS-300 is intended for international gateway operations.

Although AT&T and NorTel have deployed the most ISDN switches, there are other ISDN switch

manufacturers. Table 11-2 lists the ISDN-capable switches that are used in Australia, Europe, Japan,

and New Zealand, where ISDN switches are country speciﬁc. The Keyword column lists the

keyword that is used with the isdn switch-type command to conﬁgure the router for the type of

switch that the router connects to. Your telephone company can tell you the type of switch that is

located in the central ofﬁce to which your router will connect.

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