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What is a Modem?
by John Anthony


The word Modem is an acronym for Modulate-Demodulate.
A modem converts (modulates) digital signals into analog signals that can be sent over telephone lines.
The Modem at the receiving end converts (demodulates) the analog signals back into digital signals.

Digital devices like PCs and printers produce outputs that can be in one of two distinct states, 0 or 1.
Like a stair step, they jump from one level to the other, never stopping in between.

In modern digital communications the digital stream of 1s and 0s are modulated onto phone lines by modems using amplitude, frequency and phase modulation.

The receiving modem uses a compatible demodulation scheme to convert the modulated signal back into the original sequence of 1s and 0s.

The Dial-up Modem
Dial-up modems send their data over telephone land lines, the same lines that wired telephones used before cell phones were invented and edged them out of predominate use.

In operation your computer connects to the modem which is connected to a telephone jack and calls a remote modem that is connected to a server at your ISP (Internet Service Provider). When you open a browser and request a web page the server responds to the request and sends your browser the requested page.

The down side of the dial-up modem is three fold.
  1. They need to call and connect to other end.
  2. You can't use the line for a voice call when the modem is in use.
  3. Maximum data rate of 56Kbits/sec is relatively slow.

Dial-up modems still find some use in rural and remote areas that have no broadband service.

Broadband modems
DSL and cable modems are two types of broadband modem which for the most part have displaced Dial-up modems.

Broadband connections are always on.
They do not require a dial-up connection and they move data very fast.

DSL modems run on a dedicated telephone land line between the telephone office and your home. You can make a telephone call at the same time the DSL modem is in use without interference.
This happens because the telephone uses the lower part of the telephone bandwidth and the modem uses the upper portion of the bandwidth.

Since broadband modems are always on there is need to make and wait for a connection to the ISP server. YoU just open your browser, request a web and the server sends it to you.

DSL modems comes in two types:
  • ADSL (Asymmetric DSL) where the download speed (1.5 to 9 MB/sec) is much faster that the upload speed (16 to 640 KB/sec).

  • SDSL (Symmetric DSL) where the upload and download speeds are the same typically 3 MB/sec each way.

Cable modems run on TV cable lines which many homes already have.

Some of the TV channels are used for upstream data transmission while others carry downstream data transmission.

Cable modems support speeds about twice that of SDSL.

Cable technology is based on shared bandwidth meaning the speed available to you fluctuates depending on the number of subscribers using the network at the same time. rule

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