WXET1143

Lecture2: Basic Communication

Communication using electricity Since electricity was discovered, scientist

have researched on ways to use the electrical signal for communication.

Digital communication historical stages: Properties of signals on wires Sending bits and organizing them Error detection and correction

Signal on wires Electrical signal reflects from the end of the

metal wire the same way light reflects from mirror – requires terminator device

It loses energy as it passes along a wire – length of interconnecting limited or use amplifier

Electrical signal in a wire emits electromagnetic radiation that can interfere with signals in nearby wires – high speed network uses cable that encloses the wire in a metal shield

Information coding When properties of signals understood,

encoding of the information are studied. Modulation

Transmits voice Uses a modulator

Basic electrical signal oscillating back and forth (carrier)

Second signal (generated by human voice) to change the carrier signal

Demodulator Reverses the function

MODEM: Modulator-Demodulator

Principal of modulation still in use in modern communication systems

Modem contains both modulator (to send info) and a demodulator (for arriving info).

Transmission line

Modem1 Modem2

Computer A

Computer B

MODEM: Modulator-Demodulator

Illustration of modem that use modulation to send data across a transmission line.

When a computer interacts with a modem, it send and receives digital data, the modem encodes the data for transmission.

Two-way traffic Modem permits data to be sent between

them in both directions. Modems either use two carrier signals or

agree to take turns sending data. In either case, data appears to flow in both

directions simultaneously.

Two-way traffic In short:

A modem is a device needed for communication across a dial-up telephone connection on for long distance communication across a wire.

A modem supports two-way communication because it contains a modulator for the signal being sent and a demodulator for the signal being received.

Character code Researchers also studied transmission of

digital information. Found ways to encode basic values of bit in

an electrical signal. +ive voltage to encode 1 -ive voltage to encode 0

Devised a sequence of bits to represent each letter and digit.

Character code This type of encoding is similar to Morse

Code. But each character is assigned a code with

the same number of bits. A sequence of seven 0’s and 1’s to each letter

ASCII – American Standard Code for Information Interchange

Character code To summarize

Many networks use the ASCII code when sending textual information in digital form.

ACII assigns a 7-bit code to each letter and digit. Most users never see ASCII because it is an

internal detail that remains hidden.

Error detection Natural phenomena like lighting can cause

random electrical signals that distorts the carrier signal.

Also when wire carrying the signal passes through a string magnetic field.

Even a bit of change is crucial when passing digital information.

Error detection Remember:

When electrical signals to communicate digital information, electrical or magnetic interference can cause the value of one or more bits to be changed.

To detect and correct errors they added an extra bit to the character’s code. Add 1 if code has odd number of 1’s Add 0 if code has odd number of 0’s This is called a parity bit

Error detection Example:

Letter E – 1000101 (parity bit=1) Letter S – 1010011 (parity bit=0)

To make it work, receiver must test parity of each incoming character.

Examines all and declares error if wrong number of bits turned on.

But does not guarantee that all problems will be detected.

Error detection The point is:

Adding a parity bit to each character code before transmission can help the hardware detect errors that occur when transmitting the character across a network.

However, parity alone is not sufficient to detect all possible errors.

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