In this coming topic, I am going to give some basic introduces on what is Integrated Services Digital Network (ISDN), digital subscriber line (DSL), and universal serial bus (USB) standards. It explains how each standard enables device communication, as well as its advantages and disadvantages.


ISDN overview
Integrated Services Digital Network (ISDN) refers to a set of communication protocols proposed by telephone companies to permit telephone networks to carry data, voice, graphics, music, and video. ISDN was developed to permit faster access over existing telephone systems without the additional call setup time. It enables the simultaneous transmission of voice and data signals using end-to-end digital connectivity.

Because ISDN uses existing phone lines, it requires that the central office be within a certain distance, which limits service availability

With ISDN, bearer channels (B channels) carry voice and data signals. These channels occupy a bandwidth of 64 kilobits per second (kbps). Some switches limit B channels to a capacity of 56 kbps. A data channel (D channel) handles signaling at 16 kbps or 64 kbps, depending on the service type.

Computer applications other than ISDN use the letter “k” or “K” to represent 1024 bits (2^10). In ISDN terminology, however, “k” represents 1000 bits (10^3). Therefore, a 64 kbps channel carries data at a rate of 64,000 bits per second (bps).

There are two basic types of ISDN service – Basic Rate Interface (BRI) and Primary Rate Interface (PRI). BRI consists of two 64 kbps B channels and one 16 kbps D channel. It provides a total capacity of 144 kbps, which is adequate for most individual users.

PRI is intended for users with greater capacity requirements. Typically, the PRI channel structure includes 23 B channels plus one 64 kbps D channel, providing a total capacity of 1536 kbps. In Europe, PRI consists of 30 B channels plus one 64 kbps D channel, for a total capacity of 1984 kbps. It is also possible to support multiple PRI lines with one 64 kbps D channel using Non-Facility Associated Signaling (NFAS).

H channels provide a way to combine B channels. They are implemented as follows:
H0=384 kbps (6 B channels)
H10=1472 kbps (23 B channels)
H11=1536 kbps (24 B channels)
H12=1920 kbps (30 B channels) – International (E1) only

To access a BRI service, a customer needs to subscribe to an ISDN phone line and must be within 18,000 feet (about 5.5 km) of the telephone company’s central office. If a customer is further away from the central office, expensive repeater devices are required. Without these devices, ISDN service may not be available at all. Customers also need special equipment such as ISDN terminal adapters – sometimes incorrectly referred to as
“ISDN modems” – and a router with an ISDN interface to communicate with the phone company switch, and with other ISDN devices.