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25. Appendix E: Cable and DSL modems

25.1 Introduction

This HOWTO only deals with the common type of analog modem used to connect PC's to ordinary analog telephone lines. There are also higher speed analog modems that use special types of lines: cable and DSL modems. There is also the ISDN "modem" which uses digital signals. While this HOWTO doesn't cover such modems, some links to documents that do may be found at the start of this HOWTO. The next 3 sub-sections: DSL, Cable, and ISDN, briefly discuss such modems. For both DSL and Cable modems, the basic QAM modulation method is similar to ordinary analog analog modems. See Combination Modulation

25.2 Digital Subscriber Line (DSL)

DSL (often ADSL) uses the existing twisted pair line from your home/office to the local telephone office. This can be used if your telephone line can accept significantly higher speeds than an ordinary modem would use. It replaces the analog-to-digital converter at the local telephone office with one which can accept a much faster flow of data (in a different format of course). The spectrum of the twisted pair line is divided up into various channels. Each channel uses QAM modulation like ordinary modems do. Data is sent over multiple channels. The device which converts the digital signals from your computer to the analog signal used to represent digital data on what was once an ordinary telephone line, is a DSL modem. The DSL modem is often external (takes up space on your desk) and connects to your computer via either an ethernet port or a USB port.

25.3 Cable Modems

The coaxial cables that provide for cable television in homes have additional bandwidth not used for television, mostly at frequencies higher than used for cable TV. This extra bandwidth may be used for connecting computers to ISP's. However, many computers need to share the same cable. The spectrum of the free bandwidth is split up into channels (frequency division multiplexing) and each channel is given time slots to which individual computers are assigned (time division multiplexing). The cable modem converts the digital date from your computer (from a network card: NIC) to the required analog signal, and only broadcasts within it's assigned time slots on it's assigned channel.


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