Networking / Beginners

Integrated Services Digital Network (ISDN)

In the past, the phone network consisted of an interconnection of wires that directly connected telephone users via an analog-based system. This system was very inefficient because it did not work well for long-distance connections and was very prone to "noise." In the 1960s, the telephone company began converting this system to a packet-based, digital switching network. Today, nearly all voice switching in the United States is digital; however, the customer connection to the switching office is primarily still analog.

The Integrated Services Digital Network (ISDN) is a system of digital telephone connections that enables data to be transmitted simultaneously end to end. This technology has been available for more than a decade and is designed to enable faster, clearer communications for small offices and home users. It came about as the standard telephone system began its migration from an analog format to digital. ISDN is the format portion of the digital telephone system now being used to replace analog systems.

ISDN History

The concept of ISDN was introduced in 1972. The concept was based on moving the analog-to-digital conversion equipment onto the customer's premises to enable voice and data services to be sent through a single line. Telephone companies also began using a new kind of digital communications link between each central office. A T1 link could carry twenty-four 64 Kbps voice channels, and it used the same amount of copper wire as only two analog voice calls. Throughout the 1970s, the telephone companies continued to upgrade their switching offices. They began rolling out T1 links directly to customers to provide high-speed access. The need for an efficient solution was greater than ever.

In the early 1990s, an effort was begun to establish a standard implementation for ISDN in the United States. The National ISDN 1 (NI-1) standard was defined by the industry so that users would not have to know the type of switch they were connected to in order to buy equipment and software compatible with it.

Because some major office switches were incompatible with this standard, some major telephone companies had trouble switching to the NI-1 standard. This caused a number of problems in trying to communicate between these nonstandard systems and everyone else. Eventually, all the systems were brought up to standard. A set of core services was defined in all basic rate interfaces (BRIs) of the NI-1 standard. The services include data-call services, voice-call services, call forwarding, and call waiting. Most devices today conform to the NI-1 standard.

A more comprehensive standardization initiative, National ISDN 2 (NI-2), was recently adopted. Now, several major manufacturers of networking equipment have become involved to help set the standard and make ISDN a more economical solution. The NI-2 standard had two goals: to standardize the primary rate interface (PRI) as NI-1 did for BRI, and to simplify the identification process. Until this point, PRIs were mainly vendor-dependent, which made it difficult to interconnect them. Furthermore, a standard was created for NI-2 for identifiers.

ISDN Channels

An ISDN transmission circuit consists of a logical grouping of data channels. With ISDN, voice and data are carried by these channels. Two types of channels, a B channel and a D channel, are used for a single ISDN connection. Each channel has a specific function and bandwidth associated with it. The bearer channels, or B channels, transfer data. They offer a bandwidth of 64 Kbps per channel.

The data channel, or D channel, handles signaling at 16 Kbps or 64 Kbps. This includes the session setup and teardown using a communications language known as DSS1. The purpose of the D channel is to enable the B channels to strictly pass data and not have to worry about signaling information. You remove the administrative overhead from B channels using the D channel. The bandwidth available for the D channel depends on the type of ISDN service; BRIs usually require 16 Kbps and PRIs use 64 Kbps. Typically, ISDN service contains two B channels and a single D channel.

H channels are used to specify a number of B channels. The following list shows the implementations:

  • H0 384 Kbps (6 B channels)
  • H10 1472 Kbps (23 B channels)
  • H11 1536 Kbps (24 B channels)
  • H12 1920 Kbps (30 B channels, the European standard)

ISDN Interfaces

Although B channels and D channels can be combined in any number of ways, the phone companies created two standard configurations. There are two basic types of ISDN service: BRI and PRI.

  • Basic Rate Interface (BRI) BRI consists of two 64 Kbps B channels and one 16 Kbps D channel, for a total of 144 Kbps. With BRI, only 128 Kbps is used for data transfers, while the remaining 16 Kbps is used for signaling information. BRIs were designed to enable customers to use their existing wiring. Because this provided a low-cost solution for customers, it is the most basic type of service intended for small business or home use.
  • Primary Rate Interface (PRI) PRI is intended for users that need greater bandwidth. It requires T1 carriers to facilitate communications. Normally, the channel structure contains twenty-three B channels plus one 64 Kbps D channel for a total of 1536 Kbps. This standard is used only in North America and Japan. European countries support a different kind of ISDN standard for PRI. It consists of 30 B channels and one 64 Kbps D channel, for a total of 1984 Kbps. A technology known as non-facility associated signaling (NFAS) is available to enable support of multiple PRI lines with one 64 Kbps D channel.

To use BRI services, you must subscribe to ISDN services through a local telephone company or provider. By default, you must be within 18,000 feet (about 3.4 miles) of the telephone company's central office to use BRI services. Repeater devices are available for ISDN service to extend this distance, but these devices can be very expensive. Special types of equipment are required to communicate with the ISDN provider switch and with other ISDN devices. You must have an ISDN terminal adapter and an ISDN router.

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