Broadband service

For most of the last 100 years the connection between the subscribers and their telephone exchange was copper twisted pair buried in the pavement or distributed overhead on poles. The bandwidth that copper provided was some 3kHz, limited by analog techniques. Thus, in the early 1990s, companies began to invest in alternative connections to the home to deliver digital-based services.

· ISDN ( Integrated Services Digital Network ). As you have seen the modem was a big breakthrough in computer communications converting digital information into an analog signal to travel through the public phone network. Yet, modems have a maximum speed of 56 Kbps and are limited by the quality of the analog connection. ISDN allows multiple digital channels to be operated simultaneously through the same regular copper-wire telephone lines (POTS) used for analog lines. Since the system is digital, it permits a much higher data transfer rate. Most recently, ISDN has largely been displaced by faster, less expensive, and easier to set up and maintain broadband service: DSL and cable modem.

· DSL is a catchall name for all types of Digital Subscriber Lines that exploit unused spectrum capacity of copper telephone lines: new wiring is not necessarily required DSLs use a special modem which is supplied by the company that offers DSL. The two types used mainly are: asymmetric (ADSL), for residential hookup, and symmetric (ASDL) for small businesses. SDLs allow the user to use the phone and fax lines while being connected to the Internet (except for SDSL) and to leave his Internet connection open 24 hours a day. As with ISDN, this connection works better when the user is closer to the provider’s central office.

· Cable modem. A cable modem operates over cable TV lines, which provide greater bandwidth than POTS lines. Unlike the cable TV infrastructure, which broadcasts TV signals in just one direction (from the cable TV company to people’s homes), cable modems handle two-way transmission. For many home users, cable offers a fast, always-on Internet access at an affordable price. It has a number of disadvantages, too. Not all homes are yet wired for cable TV; since a user has to share bandwidth with other users, as with DSL, performance and speed will degrade the greater the number of subscribers in a given area.

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