Modems
What Does A Modem Do?
The Traditional Answer:
It enables computers to exchange data using the ordinary telephone system.
The More Modern Answer:
Why Are Traditional Modems Needed?
Computer data is digital - 0s and 1s.
Ordinary telephones convert sound into an analog electrical signal - a wave pattern. Connecting the I/O port of a PC to a telephone set will not work. The telephone system cannot handle the stream of 0s and 1s - they are not the correct kind of signal.
The job of the modem is to convert the digital computer data into an analog waveform that can be passed through the telephone system. This is called modulation.
At the receiving end we use another modem. This does the opposite job. It takes the analog signal from the phone and changes it to digital 0s and 1s. This is called demodulation.
modem = modulator / demodulator
Internal and External Modems
Internal modems are cards which plug into an expansion slot on the PC. Typically they have two RJ-11 telephone sockets on the end. The ‘phone cable from the wall jack plugs into one and a telephone handset may be plugged into the other.
An external modem is a little box that sits next to the computer. It is has an RS-232 socket that allows it to be connected to a serial port of the PC, and an RJ-11 socket to allow it to be connected to the phone cable.
Standards For Traditional Modems
To allow different modems to talk to each other successfully there must be agreed standards.
In 1981 IBM made the original "Personal Computer". All modern PCs are "IBM PC Compatible". This means they operate in the same way and use the same software.
At about the same time a company called Hayes Microcomputer Products started selling the "Hayes Smartmodem". In the same way as the IBM PC this became an industry standard. Most modems sold today are "Hayes Compatible".
Soon manufacturers realized that by using clever data compression tricks they could speed the rate at which modems work. The ITU - International Telecommunications Union - have published a series of compression standards. At the time of writing the most common standard is called V.34. This gives a maximum transmission rate of 28,800 bits per second or 3,600 bytes per second. (Compare this with an Ethernet which can send 10,000,000 bits per second.)
Synchronous and Asynchronous Modems
The traditional modems described above work asynchronously. This means that bytes of data are sent through the ‘phone system one at a time. Each byte is preceded by start bits and stop bits. There is no fixed timing, the bits are sent when the sender is ready to send and the receiver is ready to receive.
Synchronous modems work in a different way. Here, larger blocks of data, called frames, are sent with control bits only at the beginning and end of the block. This is a much faster method. For this to work, the sender and receiver must be synchronized - they must use exactly the same timings.
The problem with synchronous transmission is that the quality of the connecting link must be high. Sometimes synchronous modems can be made to work over ordinary ‘dial-up’ telephone lines, but often they will not. More often they are used with special ‘dedicated’ telephone links, permanently joining two sites.
What Kind of Connection?
1. Public Dial-Up Telephone Lines
In our homes we normally connect our asynchronous modems to traditional ‘dial-up’ analogue telephone lines. This is cheap and convenient, but because the quality of these lines is low, transmission speeds are low.
(Some telephone companies actually use digital signals within their ordinary public telephone system. With this higher quality system, synchronous modems can be used, and speeds of up to 56 Kbps can be achieved.)
2. ISDN - Integrated Services Digital Network
This is a new system, intended, eventually to replace the world’s current telephone network. It is a dial up system, but uses more modern cables and switches. This means that digital signals can be sent without first modulating to analog.
To make voice calls over an ISDN line, a new telephone handset is needed. This set must convert voices to digital bit streams before they are put on to the cable.
To send data over an ISDN line we still use device called a modem, but now the name is misleading. There is no longer any modulation or demodulation. The ‘modem’ now is simply an interface between phone line and computer. It handles the speed differences, flow control, error correction, etc..
A typical ISDN line can transmit data at around 128 Kbps.
To read about Etisalat’s ISDN service go to :
3. Dedicated Leased Lines
With a dial-up service, the connection between sender and receiver is temporary. It is made by the switches in the exchange and exists only for the duration of the call.
An alternative is to pay for a permanent connection, which is open 24 hours a day, 7 days a week. The advantage of this is that the connection passes through fewer switches in the exchange so the quality of the connection is better. (It may also be cheaper if the line is heavily used.)
Many telephone companies offer dedicated lines at a range of qualities and speeds. Etisalat offers connection ranging from ‘voice grade’, up to 100 Mbps - ten times faster than ordinary Ethernet. To use a high speed dedicated line, special cables will have to be run from the ‘phone company to your office.
AAW uses a 64 Kbps leased line to talk to HCT in Abu Dhabi.
Look at: http://www.etisalat.co.ae/s_leased.htm
Remote Login To A Network
It is possible for a user in one city to login to a network in a different city using only a modem and a telephone line. Windows NT Server has a feature called RAS - Remote Access Service. A remote workstation running RAS client software can connect to the network server by modem and use the server as if it was connected to the local network.
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