MODEMS

M. Lutfar   Rahman

Transfer of small amount of data between two computers can be done by a physical storage device such as a magnetic disk or a magnetic tape. But faster transfer of a large amount of data requires wiring the computers together to enable direct exchange of information. Vast amount of data are now carried by telephone lines and applications of electronic data communications are expanding in our country. Electronic data communications are going to change the way the business operates and the way we work. As more and more people get access to computers there will be greater reliance on the use of telephone lines for data transfer for electronic mail, facsimile, file transfer etc. Computer based systems also can save time and travel costs through the use of data communication through telephone lines.

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Modems are required to set up a communication link between two distant computers through the telephone line.

Communication Modes

The commonly employed communication modes in a modem are ; simplex, half-duplex and full-duplex. The simplex mode provides one-one way communication between two points involving a transmitter at one point and a receiver on the other. The half-duplex provides two-way communication between two points but only one-way at a time. The arrangements uses a receiver and a transmitter at each end. The full-duplex operation provides a two-way communication with both points able to transfer data simultaneously. This mode requires a four-wire telephone line unless when split-frequency band modem is employed. For spilt-frequency band the bandwidth is split into two separate bands one for transmission and the other for reception.

Data transmission rate

Data transmission speed, utilizing conventional telephone lines, is an important parameter for a modem and depending one this speed modems can be categorized as : low speed (0 to 600 bps), medium speed (600 to 4800 bps) and high speed (4800 to 9600 bps) modems.

Data transmission rates are specified as bit rate or baud rate. The bit rate is the actual rate of transfer of data, whereas baud is the unit of signaling. For higher data transmission rate special modulation techniques are used which enable the transmission of multiple number of bits per baud. High speed modes require two, three or four bits per baud to implement bit rates of 2400, 4800 or 9600 bps.

Modulation techniques

Telephone signals are analog in nature, that is these signals rise and fall continuously. Computer data are digital (0, 1) in nature and transmission of data from one computer to another over telephone lines creates special problems. Thus it is necessary to convert a digital signal to analog from in order for a computer to send information over telephone lines. The process of converting a digital signal to its analog form is called modulation. At the receiving end the analog signal is converted back to its digital form and the process of this conversion is known as demodulation.

The major forms of modulation (Fig. 2) used in modems are : amplitude-shift keying (ASK), frequency-shift keying (FSK) and phase-shift keying (PSK). In ASK a single frequency is turned on to represent ” 1″ and it is turned off to represent “0″. ASK is used for very low speed transmission because of its poor noise rejection characteristics. The FSK uses one frequency to represent a “0″ and another frequency to represent a “1″. In the simplest from of PSK, the phase of a sine wave carrier is shifted by 180 degrees to represent a change in the data from a 1 to 0 or from a 0 to a 1. A special device called modem is used at the transmitting end to convert digital signal to its analog form for transmission over telephone lines and another modem at the receiving end converts the analog signal back to the digital form. Thus modems must be present at the transmitting and receiving ends of a communication link. The term modem is formed by combining the terms modulation and demodulation. Modems operating on telephone lines are called voice grade or voice band modems.

For error free data transmission, modems employ sophisticated modulation techniques. The FSK technique is normally used in low speed modems. The receiver and transmitter sections of the modems respond to two different frequencies one for representing O’s and the other for l’s. In PSK data are transmitted as a phase change information. The data are encoded by phase information of the carrier signal (1700 Hz in some modems), that is phase changes define the data. Transmitting multiple phase changes enable higher ata encoding.

The simple PSK has no real advantage over FSK. By using additional phase angles besides 180 degrees, two or three bits can be sent in one baudd. Two bits sent in a single baud are called dibits and three bits sent in a single baud are called tribits. Each pair of bits in the data stream is treated together for dibit encoding, and the value of the two bits determines the amount that the phase of the carrier will be shifted. Common set of phase-shifts used to represent the four possible dibit combinations are :

Dibit values phase-shift (degrees)
00 0
01 90
11 180
10 270

The value of the carrier, here, only

has to change for each two transmitted bits. For example, if the value of two bits taken together is 00, the pahse of the carrier will be shifted by 90 degrees to represent that dibit. Baud rate is the rate at which the carrier changes. In this case it is not the same as the number of bits transmitted per second. Here 1200 bits are transmitted in one second at a rate of 600 baud. This scheme is employed in Bell 212A type modems. For full-duplex mode using PSK scheme two different carrier frequencies (1200 Hz and 2400 Hz in same cases) are normally used.

The data stream is divided into groups of three bits for tribit encoding. The following table shows one common set of phase-shifts used to represent the eight possible tribit combination :

tribit values Phase-shift (degrees)
001 22.5
000 67.5
010 112.5
O11 157.5
111 202.5
110 247.5
100 292.5
010 337.5

The Bell 208 modems use this tribit scheme to transmit data at 4800 bps.

High speed modems employ a combination of amplitude modulation and phase modulation called by the name quadrature amplitude modulation. For example, a standard 9600 bps modem uses twelve phase angles four of which has four amplitude values (Fig.3)

Acoustic modems

Two different types of modems are : acoustic modems and direct-con-nect modems. An acoustic modem consists of a small box with two doughnut-shaped speaker cups made of spongy material. It is also known as acoustic coupler. The acoustic coupler is connected to the computer and a telephone hand-set is placed on top of two speaker cups. An acoustic coupler is used for applications which require special communication needs. A standard telephone hand-set can also be placed on top of the coupler. For using an acoustic modem, the telephone number of the remote computer is first dialed and the telephone hand-set is placed on the coupler when the answering tone is heard.

Direct-connect modems

These modems can be connected directly to the telephone lines. Most of the direct-connect modems can be used for voice as well as computer communications. Two types of direct-connect modems are : external modems and internal modems.

An external modem is completely a self-contained unit and is plugged into a serial port the back of a computer. An external modem, a small box in appearance, also requires external electrical power supply. An external modem, attached to a computer by a cable, is connected to the telephone line. A number of status lights of the modems indicated the activities to the modem. Such activities include: test mode, auto answer, carrier detect, send data, receive data, ring indie action etc.

Interfacing

Most of the personal computers have a built-in communication adapter. This adapter is called an RS-232C connector or serial port. The RS-232C (also known as EIA-232D) refers to the type of cable that is used to connect the serial port and a modem. If the computers do not have a built-in communication adapter, a board should be added to the computer for connecting a modem.

An internal modem is an expansion card that can be purchased and installed in the expansion slot on the mother board of the computer. Internal modem boards contain the electronic circuits for the communication adapter. Some personal computers are supplied with internal modem built into them. The cost of an internal modem is less than that for an external one.

Some modems are smart or intelligent in the sense that they do more than translate information. Such a modem can test itself, connect directly to telephone line, dial the phone number of another computer, answer a telephone call, adjust to the rate of data transmission. A normal telephone line is good enough for data transmission at low speeds. However, most business organizations transmit data at very high speeds and require special noise free dedicated or leased telephone lines.

Uses

A modem can be used for sending and receiving electronic mail, accessing electronic bulletin board, utilizing public data networks (yet to be installed in Bangladesh) and many other applications. Modems are necessary to connect two computers at a distance. If two computes are closed together and both have serial ports then a modem is not required to transfer data or exchange messages between them. The connection between two computers without modem is called a null-modem connection or direct connection.

Selection of Modems

Modems are manufactured following CCITT standards. The standards relating to modems start with the letter V. Examples are : V.26 which is a 2400 bps modem, V.27 which is a 4800 bps modem, V.32 which is upto 9600 bps modem etc. In the USA most of the modems follow Bell Telephone standards. Examples of these standards are type 103, 202, 208. 212 A modems.

The basic qualities of a modem are: registration with FCC (in USA), industry standard (such as 1200 bps. 2400 bps), compatibility with the computer etc. The manual of the computer should be consulted before purchasing a modem. An internal modem draws power from the computer and if the power supply of the computer is enough (150 watts or more) then an internal modem may be installed. However, external modems are plugged into external power source and thus draw no power from the computer.

Cost of modems varies considerably depending upon the transmission speed (1200 bps, 2400 bps, 9600 bps etc.) and other features. Most no-line services and bulletin boards do not ordinarily require speeds greater than 2400 bps. For sending and receiving vast quantities of data through on-line services expensive high speed modems can be justified. However higher speed modems are susceptible to electrical noise and interference. While purchasing a modem a balance between cost and features are generally taken into account. Such features, among other, include : automatic fallback that is automatic adjustment to transmission rate to match the remote system, auto dial, automatic hang-up on carrier loss for incoming calls etc. *

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