ISDN network is a digital communication system that is capable of producing the maximum transmission speed of 1.4Mbps. 128Kbps speed is however more common in the digital technology. It's a international standard for sending data, voice, video over regular digital telephone lines. ISDN uses UTP (unshielded twisted pair cable) for transmission.
• Primary Rate Interface (PRI) -- It consists of 23 B-channels and one D-channel (for U.S.) or 30 B-channels and one D-channel (for Europe).
The actual version of ISDN employs base band transmission. Another version, called B-ISDN, uses broadband transmission, which is able to support transmission rate of 1.5 Mbps. B-ISDN mainly requires fiber optic cables.
To access the B channel, it is necessary to subscribe the ISDN phone line. Customers are also required to have some special ISDN devices, which are known as terminal adapters, which are used to communication with the telephone company switch or other ISDN devices.
How ISDN Works
In an analog network, a regular telephone line that is provided by the telephone company supports a single transmission channel, which normally can carry only one service, voice, data or video at a single time. With ISDN, this same pair telephone line is logically divided into the multiple channels. A typical line has two channels.
The first type of channel is called B channel. This channel can carry about 64Kbps of data. Typical ISDN line has 2 B channels. One channel is used for voice and other channel is used for data communication. This data/voice transmission process occurs on the regular one pair copper wire.
The second type of channel is used for link and call setup. This channel is known as D channel or Delta Channel.
The third channel has only 16Kbps of bandwidth.
There is an upper limit of the speed in the commonly used dial up modems, which is 56kbps. But due to the quality and other factor, the maximum attainable speed is 45kbps.
ISDN allows multiple digital channels to operate simultaneously through the same regular one pair copper wire. If the telephone company supports the digital connections then a change can occur. The digital signals instead of the analogue signals, transmitted across the telephone line. In the digital scheme, there is much more data transfer rate than the analog lines.
A separate telephone line is required for using the fax, telephone, computer, router and live video conferencing systems. A separate line is required for each device. ISDN network lines can handle the multiple devices on the single line. Up to eight devices such as, computer, faxes, cash registers, credit card readers or other devices can directly be connected to a single ISDN line, all these devices can work simultaneously.
V.34 modems typical take 30-60 seconds to establish a connection and on the other end an ISDN call usually takes less than 2 seconds to establish a connection.
There are some disadvantages of the ISDN lines, which I have discussed below.
1. ISDN is more expensive than the Plain old telephone system.
2. The telephone company and the ISDN user both are required to have the specialized digital devices.
ADSL VS ISDN
There is a noticeable difference between the two services.
• ISDN provides two voice channels or one 128Kbps data channel while ADSL is basically a data pipe.
• The power for ADSL is sent by carrier over copper wire; as with the local phone service. The line works even when local power fails. This is an advantage over the ISDN lines, which requires the local power supply.