Modem

ContentsIntroduction to Modem...............................................................................................................................2

The Orgin of Modem...................................................................................................................................3

Classification of Modem..............................................................................................................................5

* Short Haul.............................................................................................................................................5

* Voice Grade (VG)..................................................................................................................................5

* Wideband.............................................................................................................................................6

* Leased, Private......................................................................................................................................6

* Dial up...................................................................................................................................................6

* Two and Four-Wires Lines....................................................................................................................6

* Half Duplex...........................................................................................................................................7

* Full Duplex............................................................................................................................................7

* Simplex.................................................................................................................................................8

* Asynchronous Modems........................................................................................................................8

* Synchronous Modems..........................................................................................................................9

Types of Modem..........................................................................................................................................9

Difference between Internal and External Modem.................................................................................9

How it works:.............................................................................................................................................10

Modulation Techniques.........................................................................................................................11

Speed variation from 1960 to till now!......................................................................................................14

Future Trend..............................................................................................................................................15

1

2

Introduction to Modem

A modem (modulator-demodulator) is a device that modulates an analog carrier signal to

encode digital information, and also demodulates such a carrier signal to decode the transmitted

information. The goal is to produce a signal that can be transmitted easily and decoded to

reproduce the original digital data. Modems can be used over any means of transmitting analog

signals, from driven diodes to radio.

The most familiar example is a voice band modem that turns the digital data of a personal

computer into analog audio signals that can be transmitted over a telephone line, and once

received on the other side, a modem converts the analog data back into digital.

Modems are generally classified by the amount of data they can send in a given time, normally

measured in bits per second (bit/s, or bps). They can also be classified by Baud, the number of

times the modem changes its signal state per second. For example, the ITU V.21standard

used audio frequency-shift keying, aka tones, to carry 300 bit/s using 300 baud, whereas the

original ITU V.22 standard allowed 1,200 bit/s with 600 baud using phase-shift keying.

3

The Orgin of Modem

News wire services in 1920s used multiplex equipment that met the definition, but the modem

function was incidental to the multiplexing function, so they are not commonly included in the

history of modems.

Modems grew out of the need to connect teletype machines over ordinary phone lines instead of

more expensive leased lines which had previously been used for current loop-based

teleprinters and automated telegraphs. George Stibitz connected a New Hampshire teletype to a

computer in New York City by a subscriber telephone line in 1940.

Mass-produced modems in the United States began as part of the SAGE air-defense system in

1958, connecting terminals at various airbases, radar sites, and command-and-control centers to

the SAGE director centers scattered around the U.S. and Canada. SAGE modems were described

by AT&T's Bell Labs as conforming to their newly published Bell 101 dataset standard. While

they ran on dedicated telephone lines, the devices at each end were no different from commercial

acoustically coupled Bell 101, 110 baud modems.

The sending modem modulates the data into a signal that is compatible with the phone line, and

the receiving modem demodulates the signal back into digital data. Wireless modems convert

digital data into radio signals and back.

Modems came into existence in the 1960s as a way to allow terminals to connect to computers

over the phone lines. A typical arrangement is shown below:

In a configuration like this, a dumb terminal at an off-site office or store could "dial in" to a

large, central computer. The 1960s were the age of time-shared computers, so a business would

4

often buy computer time from a time-share facility and connect to it via a 300-bit-per-second

(bps) modem.

A dumb terminal is simply a keyboard and a screen. A very common dumb terminal at the time

was called the DEC VT-100, and it became a standard of the day (now memorialized in terminal

emulators worldwide). The VT-100 could display 25 lines of 80 characters each. When the user

typed a character on the terminal, the modem sent the ASCII code for the character to the

computer. The computer then sent the character back to the computer so it would appear on the

screen.

When personal computers started appearing in the late 1970s, bulletin board systems (BBS)

became the rage. A person would set up a computer with a modem or two and some BBS

software, and other people would dial in to connect to the bulletin board. The users would

run terminal emulators on their computers to emulate a dumb terminal.

People got along at 300 bps for quite a while. The reason this speed was tolerable was because

300 bps represents about 30 characters per second, which is a lot more characters per second

than a person can type or read. Once people started transferring large programs and images to

and from bulletin board systems, however, 300 bps became intolerable. Modem speeds went

through a series of steps at approximately two-year intervals:

5

Classification of Modem

Classifying Modems according to: Range

* Short Haul

Short haul modems are cheap solutions to systems of short ranges (up to 15 km), which use

private lines and are not part of a public system. Short haul modems can also be used, even if the

end-to-end length of the direct connection is longer than 15 km, when both ends of the line are

served by the same central office in the telephone system. These lines are called "local loops".

Short haul modems are distance-sensitive, because signal attenuation occurs as the signal travels

through the line.

Short haul modems tend to be cheaper than other modems for two reasons:

(1) No circuitry is included in them to correct for differences between the carrier

frequency of the demodulator and the frequency of the modulator.

(2) Generally no circuitry is included to reduce/correct for noise rejection, which is less

of a problem over short distances than over long distances.

There are two main types of short haul modems:

1. Analog modems, using a simple modulation method, without sophisticated devices for

error control or equalizers.

2. Line drivers increase the digital signal, which transmit to the communication channel and

do not transmit the carrier signal, as conventional modems

* Voice Grade (VG)

Voice-grade modems are used for unlimited destination, using a moderate to high data rate.

These modems are expensive and their maintenance and tuning are sophisticated.

Communication channels are leased lines and dial-up.

Voice-band telephone network is used for data transmission. A user-to-user connection may be

either dedicated or dialed. The links in the connection are the same in the two cases, and the only

6

difference for the user is that for some impairments (particularly attenuation and delay

distortion), a dedicated (private or leased) line is guaranteed to meet certain specifications,

whereas a dialed connection can only be described statistically.

* Wideband

Wideband modems are used in large-volume telephone-line multiplexing, dedicated computer-

to-computer links. These modems exceed high data rates.

Classifying Modems according to: Line Type

* Leased, Private

Leased, private or dedicated lines (usually 4-wire) are for the exclusive use of "leased-line"

modems - either pair (in a simple point-to-point connection) or several (on a multidrop network

for a polling or a contention system). If the medium is the telephone network, their transmission

characteristics are usually guaranteed to meet certain specifications, but if the link includes any

radio transmission, the quality of it may be as variable as that of a switched (i.e. nondedicated)

line.

* Dial up

Dial-up modems can establish point-to-point connections on the PSTN by any combination of

manual or automatic dialing or answering. The quality of the circuit is not guaranteed, but all

phone companies establish objectives. The links established are almost always 2-wire because 4-

wire dialing is tedious and expensive.

* Two and Four-Wires Lines

A four-wire (4W) line is a pair of two-wire (2W) lines, one for transmitting and one for

receiving, in which the signals in the two directions are to be kept totally separate. Perfect

separation can be maintained only if the four-wire configuration is sustained from transmitter to

receiver. The lines may be combined in a 4W/2W network (often called a hybrid or a hybrid

7

transformer) at any point in the signal path. In this case impedance mismatches will cause

reflections and interference between the two signals.

Classifying Modems according to: Operation Mode

* Half Duplex

Half duplex means that signals can be passed in either direction, but not in both simultaneously.

A telephone channel often includes an echo; this renders the channel half-duplex. Echo

suppressors are slowly being replaced by echo cancellers, which are theoretically full-duplex

devices.

When a modem is connected to a two-wire line, its output impedance cannot be matched exactly

to the input impedance of the line, and some part of its transmitted signal (usually badly

distorted) will always be reflected back. For this reason half- duplex receivers are disabled

(received data is clamped) when their local transmitter is operative.

Half-duplex modems can work in full-duplex mode.

* Full Duplex

Full duplex means that signals can be passed in either direction, simultaneously. Full duplex

operation on a two-wire line requires the ability to separate a receive signal from the reflection of

the transmitted signal. This is accomplished by either FDM (frequency division multiplexing) in

which the signals in the two directions occupy different frequency bands and are separated by

filtering, or by Echo Canceling (EC).

The implication of the term full-duplex is usually that the modem can transmit and receive

simultaneously at full speed. Modems that provide a low-speed reverse channel are sometimes

called split-speed or asymmetric modems.

Full duplex modems will not work on half-duplex channels.

8

* Simplex

Simplex means that signals can be passed in one direction only. A remote modem for a

telemetering system might be simplex and a 2-wire line with a common unidirectional amplifier

is simplex.

Echo suppressor are slowly being replaced by ECs, which allow a certain amount of double-

talking and do not require "capture" time for any one talker to assume control of the connection.

Classifying Modems according to: Synchronization

* Asynchronous Modems

Most of the modems that operate in slow and moderate rates, up to 1800 bps, are asynchronous

(using asynchronous data). Asynchronous modems operate in FSK modulation and use two

frequencies for transmission and another two for receiving. Asynchronous modems can be

connected in different options to the communication media:

Using 2-wire or 4-wire interface.

Using switched lines or leased lines.

Using interface to call unit/automatic answer, when dialing-up.

9

* Synchronous Modems

Synchronous modems operate in the audio domain, at rates up to 28800 bps in audio lines, used

in telephones systems (using synchronous). The usual modulation methods are the phase

modulation and integrated phase and amplitude (at higher rates than 4800 bps).

In synchronous modems, equalizers are used, in order to offset the misfit of the telephone lines.

These equalizers are inserted in addition to the equalizers, which sometimes already exist in the

telephone lines.

Types of Modem Internal Modem

External Modem

Difference between Internal and External Modem

Internal Modem External Modem

This is circuit board that is plugged into one of the computer’s expansions.

Separate device has to be attached externally

It cannot be detached from the computer. Can be easily detached from computer

Tedious to configure & install Very easy to install

If internal modem gets stuck, you have to switch off the computer and reinstall.

If the gets stuck, just switch off the modem and switch it on.

No additional cable required. Additional cable is required

10

How it works:A modulator converts a digital signal into analog signal. A demodulator converts an analog

signal to digital one. It doesn’t sample a signal to create digital facsimile. It is merely a process

of modulation. The two pc at ends are the DTE, the modem are DCE’s. The DTE create digital

signal and relay it to modem via interface. Modulated signal is received by the demodulation

function of the second modem. The demodulator takes the ASK, FSK, PSK, or QAM signal and

decodes it into a format. Its computer can accept. If the relays the resulting digital signal to

receiving computer via an interface. Each DCE is compatible with its own DTE and with Other

DCE’s.

Today, no one uses dumb terminals or terminal emulators to connect to an individual

computer. Instead, we use our modems to connect to an Internet service provider (ISP), and the

ISP connects us into the Internet. The Internet lets us connect to any machine in the world

(see How Web Servers and the Internet Work for details). Because of the relationship between

your computer, the ISP and the Internet, it is no longer appropriate to send individual characters.

Instead, your modem is routing TCP/IP packets between you and your ISP.

11

The standard technique for routing these packets through your modem is called the Point-to-

Point Protocol (PPP). The basic idea is simple -- your computer's TCP/IP stack forms its

TCP/IP datagrams normally, but then the datagrams are handed to the modem for transmission.

The ISP receives each datagram and routes it appropriately onto the Internet. The same process

occurs to get data from the ISP to your computer.

Modulation Techniques

* AM - amplitude modulation

This technique changes the amplitude of the sine wave. In the earliest modems, digital signals

were converted to analog by transmitting a large amplitude sine wave for a "1" and zero

amplitude for a "0", as shown in figure Modem-3. The main advantage of this technique is that it

is easy to produce such signals and also to detect them. This technique has two major

disadvantages. The first is that the speed of the changing amplitude is limited by the bandwidth

of the line. The second is that the small amplitude changes suffer from unreliable detection.

Telephone lines limit amplitude changes to some 3000 changes per second. The disadvantages of

amplitude modulation causes this technique to no longer be used by modems, however, it is used

in conjunction with other techniques

. 

12

* QAM - quadrature amplitude modulation

This technique is based on the basic amplitude modulation . This technique improves the

performance of the basic amplitude modulation. In this technique two carrier signals are

transmitted simultaneously. The two carrier signals are at the same frequency with a 90 degrees

phase shift. The mathematical form of the transmitted signal will be as follows:

S(t)=A*SIN(Wc*t)+B*COS(Wc*t)

A, B, are the amplitude of the two carrier signals. Each of them can get a value from a known set

of values. In this way a few bits can be transmitted in the period of one symbol time. For

example consider the set of values {1 , 2 , 3 , 4 }. In this example 4 different values can represent

2 bits. During one symbol time 4 bits will be transmitted, "A" will represent 2 bits and another 2

bits will be represented by "B".

* FM - frequency modulation

In this technique the frequency of the carrier signal is changed according to the data. The

transmitter sends different frequencies for a "1" than for a "0" as shown in figure Modem-4.This

technique is also called FSK - frequency shift keying. The disadvantages of this technique are

that again (as it was with amplitude modulation) the rate of frequency changes is limited by the

bandwidth of the line, and that distortion caused by the lines makes the detection even harder

than amplitude modulation. Today this technique is used in law rate asynchronous modems up to

1200 baud only. 

13

* CPM - continuous phase modulation

A modern technique which derives from basic frequency modulation. The only difference is that

in the transition from one symbol to another the phase is continuously changed, there are no

phase steps. Continuous phase means that the transmitted signal bandwidth is limited and faster

data rates can be achieved for the same bandwidth.

* PM - phase modulation

In this modulation method a sine wave is transmitted and the phase of the sine carries the digital

data. For a "0", a 0 degrees phase sine wave is transmitted ( PHI = 0 ). For a "1", a 180 degrees

sine wave is transmitted ( PHI = 180 ) as shown in figure Modem-5. This technique, in order to

detect the phase of each symbol, requires phase synchronization between the receiver's and

transmitter's phase. This complicates the receiver's design. 

14

Speed variation from 1960 to till now!

15

Future Trend

Today in addition external modems, there are internal modems which are included as an

additional board within the computer. There are advantages to each type.

Actual transfer rates are limited due to type of phone lines. Using slower phone trunks,

international circuits where half the normal bandwidth is used, and the slow cellular connections

where it might run at only 14.4 kbps (without compression) although the modem itself enable

28.8kbps.

More and more users are accessing the Internet and on-line services such as Compuserve, so, use

of modems has increased dramatically. The more powerful processors such as Pentium and

PowerPC in workstations and PC's, enable the modem h/w to be less complicated. Part of the

functions done in the DSP or microcontroller might be performed by the host. So, modems might

drop in price.

The advent of semiconductor modems will enable a wide range of applications to be

implemented: Vending machines will call up when they need more goods to vend, or elevators

will call when they require service, and so on.

The last approved standard of V.34 with 28.8 kbits/s speed will enable the Digital Simultaneous

Voice and Data (DSDV) applications. DSVD is a modem specification that lets voice and data to

be shared over a single dial-up connection. The data is multiplexed into packets, much like an

ATM stream.

V.34 is approaching the theoretical speed limit of an analog line - estimated to be in the low

30kbps range. This may be the last of the new modem protocols which doubled the previous

speeds.

16