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What is the purpose of the MODEM in reference to analog and digital signals?
Wiki User
∙ 9y ago
A MODEM (i.e. MOdulator-DEModulator) is a device used to interface a digitally encoded signal with a communication channel designed only to carry analog signals. It does this by using some form of modulation of an analog signal that the communication channel is capable of faithfully transmitting within its available bandwidth.
As the name implies a MODEM consists of two main sections:
- a modulator, converts an input digitally encoded signal to an output modulated analog signal which is sent out through the communication channel
- a demodulator, converts an input modulated analog signal which came in through the communication channel to an output digitally encoded signal
There have been many different types of MODEMs designed and used over the years to connect to different types of analog communication channels with different types of modulation and supporting different digital data rates and encodings. The first MODEMs were designed by the Telephone Company (i.e. The Bell System/A.T.&T.) to connect remote user computer terminals to early timesharing computers via existing Long Distance Direct Dial Voice Lines (which have a bandwidth of only 3KHz and block DC) and had either a maximum digital data rate of either 110 bps full duplex (i.e. both directions active at the same time), 300 bps full duplex, or 1200 bps half duplex (i.e. only one direction active at a time using a handshake protocol to manage switching directions) depending on the MODEM you leased (Bell 101, Bell 103, or Bell 202). Back then you paid your phone company a monthly lease charge for the MODEM and roughly a $3 per minute Long Distance Toll Charge (as part of your monthly phone bill) as well as roughly a $400 per CPU hour charge (to the timesharing company you had an account with). Some later types of MODEMs are:
- 2400 bps full duplex telephone MODEMs
- 4800 bps full duplex telephone MODEMs
- 9600 bps full duplex telephone MODEMs
- 14400 bps full duplex telephone MODEMs
- 28800 bps full duplex telephone MODEMs
- 33600 bps full duplex telephone MODEMs
- 56000 bps full duplex telephone MODEMs
- FAX telephone MODEMs
- WiFi router radio MODEMs
- BlueTooth radio MODEMs
- Cell phone radio MODEMs
- DSL phone line MODEMs to phone company ISP
- Cable MODEMs to cable TV company ISP
While the earlier lower speed MODEMs always operated at the same speed in both directions, all of the high speed MODEMs only operate at their full speed in the "down stream" direction and use a slower speed in the "up stream" direction, to take advantage of the fact that the user typically transmits much less data "up" to the computer/server than the computer/server transmits "down" to the user, which permits a significant reduction in bandwidth demands placed on the limited capacity analog communication channel.
Wiki User
∙ 9y ago
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