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 Modem 1 Modem 2 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 – (parity bit=1) Letter S – (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.  Checksum –