Networking Explained, 2nd Edition

Chapter 11

Integrated Services Digital Network (ISDN)

The integrated services digital network (ISDN) represents the overhaul and redesign of

our conventional telephone network from an analog system to an end-to-end digital net-

work. This new, completely digital-based network is capable of transmitting voice and

data communications over a single telephone line using inexpensive and conventional

twisted-pair cable (i.e., standard copper telephone wire). A brief overview of various fun-

damental ISDN concepts was provided in Chapter 7 as part of our discussion of WAN

technologies and services. In this chapter, we extend this discussion and provide more

detailed information about ISDN. An outline of the major topics we present follows:

• History of ISDN (Questions 1–3)

• General Overview and Components (Questions 4–6)

• Channel Types (Question 7)

• BRIs, PRIs, and SPIDs (Questions 8–9)

• Line Sets and Feature Sets (Questions 10–12)

• ISDN Protocols (Questions 16–18)

• AO/DI and B-ISDN (Questions 19–20)

• Alternative Implementation Strategies (Question 21)

1. What is ISDN?

The integrated services digital network is a carrier service that is offered by telephone

companies (telcos) and designed to transmit voice and nonvoice (e.g., computer data, fax,

video) communications on the same network. The advantage ISDN offers over other ser-

vices is that separate connections are not needed for these different transmissions. Thus,

instead of having a telephone line for voice communications, a second telephone line for

fax or computer dialup connections, and a coaxial cable link for video communications, a

single ISDN connection will support all of these transmissions. That is, ISDN integrates

all of these services into a single system. ISDN service (and hence, an ISDN connection)

is completely digital from end-to-end. This represents both a departure from and an

improvement in today’s conventional telecommunication services, which use a hybrid of

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366 Networking Explained, Second Edition

analog and digital technologies (Figure 11.1). Furthermore, ISDN’s technology permits

standard twisted-pair wiring to carry circuit- or packet-switched digital data. ISDN also

provides a cost-effective strategy for internetworking. Instead of paying for dedicated

leased lines, remote sites (i.e., user workstations or LANs) can interconnect with other

sites via dialup links.

Digital

Analog

Analog

Digital

Analog

Analog

Digital

Digital

Digital

Digital

Digital

Digital

Digital

Digital

ISDN

Telephone

ISDN

Device

ISDN

Device

ISDN

Telephone

a

Customer Site

Telephone Company Network

Customer Site

Customer SiteCustomer Site

Telephone Company Network

FIGURE 11.1 Today’s conventional telephone network (a) is a hybrid of analog and digital tech-

nologies. Note that a computer connection requires four conversions: digital to analog from the PC,

which is digital, to the customer’s phone system, which is analog; analog to digital from the cus-

tomer’s site to the telephone company’s site, which is all digital; digital to analog from the tele-

phone company’s site to the customer’s site; and analog to digital from the customer’s phone

system to the PC. An ISDN connection (b), however, which uses special ISDN-compatible devices,

is digital from end to end. In the case of the computer connections, no digital-to-analog or analog-

to-digital conversions are necessary, which result in faster connections.