Introduction
ISDN is a digital network that is capable of producing maximum transmission speeds of about 1.4Mbps. Speeds of 128Kbps are however more common in this digital technology.
ISDN is an international communications standard for sending voice, video, and data over digital telephone lines or normal telephone wires.
ISDN uses UTP (Unshielded Twisted Pair Wiring) and carries digital signals.
UTP - multiple, individually insulted wires that are twisted together into pairs without any outer shielding.
Most ISDN lines offered by telephone companies give you two lines at once, called B channels. It is three times the data rate provided by today's fastest modems.
How ISDN Works
In an analog network, a two-wire loop from the telephone company's local central office to the customer's premises, supports a single transmission channel, which can carry only one service--voice, data, or video--at a time.
With ISDN, this same pair of twisted copper wires is logically divided into multiple channels.
A typical ISDN line has 2 types of channels.
The first type of channel is called a Bearer or B channel. This channel can carry about 64Kbps of data. A typical ISDN line has 2 B channels where one channel can be used for a voice call, while the other, can be used for data transmission. This process occurs on one pair of copper wire.
The second type of channel is used for link management and call setup. This channel is known as the Signal or D channel (often referred to as the Delta channel).
The third channel has only 16Kbps of bandwidth.
Types of ISDN Service
There are two basic types of ISDN service:
- Basic Rate Interface (BRI)
- Primary Rate Interface (PRI).
BRI consists of two 64Kbps B channels and one 16Kbps D channel for a total of 144Kbps. This basic service is intended to meet the needs of most individual users.
To access BRI service, it is necessary to subscribe to an ISDN phone line. Customers will also need special equipment to communicate with the phone company switch and with other ISDN devices. These devices include ISDN Terminal Adapters (sometimes called, incorrectly, "ISDN Modems") and ISDN Routers.
Advantages of ISDN
Speed
Using a dial-up connection, there is an upper limit to the amount of information that an analog telephone line can hold. Currently, it is about 56Kbps. Commonly available modems have a maximum speed of 56Kbps, but are limited by the quality of the analog connection and routinely go about 45 Kbps.
ISDN allows multiple digital channels to be operated simultaneously through the same regular phone wiring used for analog lines. The change comes about when the telephone company's switches can support digital connections. Therefore, the same physical wiring can be used, but a digital signal, instead of an analog signal, is transmitted across the line. This scheme permits a much higher data transfer rate than analog lines.
Multiple Devices
One line each was required for a telephone, fax, computer, bridge/router, and live video conferencing system. A separate line was required for each device using normal dial-up connection.
ISDN can handle multiple devices on a single line. Up to eight telephones, computers, workstations, faxes, credit card readers, cash registers or other devices can be directly attached to a single ISDN line, all in use simultaneously.
Connection Time
V.34 modem typically takes 30-60 seconds to establish a connection; an ISDN call usually takes less than 2 seconds ie. Fast dial-up establishment.
Disadvantages
- More expensive than the Plain Old Telephone System (POTS)
- The telephone company and the remote computer both need specialized equipment
