Article 2

This article discuss's five ways in which net managers, can improve their customers access to the internet, while still keeping costs down. These are ISDN, cable modems, asymmetric digital sub-scriber line (ADSL), satellite services and wireless cable services. Some of these services can even serve as the physical underpinnings for other technologies like frame relay and ATM.

ISDN is the slowest of the above mentioned technologies. ISDN carries three separate digital signals over a standard two wire phone line. This is the BRI (basic rate interface). One channel is used for control and signalling information and the remaining two "B" channels can be used for voice or data or both. Speed per B channel is 56 or 64 kbit/s. The two channels can be aggregated for a total speed of 128 kbit/s. Another variety of ISDN is PRI (primary rate interface). It can handle 23 or 30 bearer channels and can operate at 1.544 and 2.048 Mbit/s. It is mainly used by large companies and ISP's (Internet service providers). Routers and bridges can be used to link multiple systems to an ISDN line.

Cable modems are still in trial deployment stage. The physical medium being used here is the coaxial cable, meant to carry analog television signals, to transmit data at rates up to 10Mbit/s. The two main problems with this are coaxial cables are very noisy and are analog based, requiring different coding techniques. These techniques include quadrature phase-shift key modulation, spread spectrum encoding and orthogonal frequency division multiplexing. Although there is not an industry standard, major companies, including AT&T, HP, Intel, etc., are working at it.

ADSL was technology developed by the telephone companies to deliver video over existing copper telephone lines. As such it offers several times the bandwidth and speed of ISDN lines. ADSL basically offers three channels over the same line. One for telephone conversations, one for user to internet communication (16-640 kbit/s) and the third for Internet to user communication (1.544-9 Mbit/s). The main problem is that range is limited in that for a transmission speed of 9Mbit/s the subscriber and the central office link have to be within 9,000 ft. of each other.

Connecting via satellite requires a basic kit, which costs around $700, including a satellite dish, hardware and an ISA adapter card. The two main problems are that users need a modem and connection to the Internet to upload information, meaning that the dish is good only for downloading information. Secondly, the top rate of transmission is around 400 kbit/s, which is relatively slow. Satellite connection is best used by companies which download graphics or data intensive information off the Net.

Lastly, the other wireless method is through cable services which use microwave networks to transmit TV. CAI is the leading business to offer such a service allowing people access to its 100 channel cable network. By the end of this year it also plans to offer a wireless modem which will allow users to receive data at the rate of 27 Mbit/s. As more competitors enter the market the price for the leased wireless modems should decrease significantly.

synopsis of an article by Audrey Daum in Data Communications

© Ranjit Sandhu, 1997