Introduction to Networking

Spring 2002Computer Network Applications Analog Devices Maintain an exact physical analog of (some form of) information. Ex: Phonograph Cassette tape recorder Early telephone systems Radio Television

Spring 2002Computer Network Applications Drawbacks of Analog Representation Cannot handle all ranges of input Adds noise Signal loss Distorts the input

Spring 2002Computer Network Applications Digital Representation Uses numbers to record information Ex: CDs Telephones Computers

Spring 2002Computer Network Applications Using Digital to Recreate Analog An Analog-to-Digital (A-to-D) converter transforms an analog signal into a sequence of numbers. Conversely, a Digital-to-Analog (D-to-A) converter transforms a numerical value into an analog signal A-to-D 1.41,3.14

Spring 2002Computer Network Applications More about Digital Representation: Morse Code The telegraph was used for communicating information; It used two basic symbols for encoding information: “.” and “_”. Ex of Morse codes: A._ B _... C _._. E. T _

Spring 2002Computer Network Applications Binary Encoding Computers transmit & receive digital information over a network Bit (Binary Digit) 0 or 1 Byte = 8 bits Actually, all electronic devices use bits to encode information. Advantage: easy to distinguish between two (opposed) states; Disadvantage: verbose

Spring 2002Computer Network Applications Examples of Binary Coding Binary representation of decimal numbers 2  10 8    1111 Binary representation of alphabet symbols (ASCII encoding) A  B  What about coding colors, for example red, green, yellow? How many bits are needed?

Spring 2002Computer Network Applications Digital Representation of Information Numbers & Text Images Sound Video InputProcessOutput = pitch, volume time

Spring 2002Computer Network Applications Information Coding Modulation uses two devices: A modulator uses two signals: A basic, regulate, agreed upon signal called carrier; The signal denoting the information to be transmitted; A demodulator which retrieves the info It measures how much the signal deviates from the carrier.

Spring 2002Computer Network Applications Modem Device used to send/receive information via a network (usually consisting of phone lines); Contains both a modulator and a demodulator; A dial-up modem dials a phone number and answers a call. AB modem

Spring 2002Computer Network Applications Sending Information on Wires Electrical signals: reflect from the end of the wire  network wires require a terminator device; lose energy as they pass through a wire  to send a message along a long wire one needs a signal amplifier; emit electromagnetic radiation that can interfere with signals in nearby wires  limits the distance between adjacent wires.

Spring 2002Computer Network Applications Detecting Errors Electric and magnetic interference may disrupt wire signals and data can be damaged. Parity bit: an extra bit added to every character so that the total number of 1 bits is even. Ex: parity bit for A ( ) is 0; and the parity bit for E ( ) is 1. When a character (or a byte) is received; the 1 bits are counted; if the # is not even an error had occurred. Detects the error only in a limited number of cases. Ex: if is transmitted and is received the error is not detected.

Spring 2002Computer Network Applications Detecting Errors (cont.) Checksum The sum of the bytes contained by a message is appended at the end of the message; Ex: the checksum of the message ‘1 3 5’ is 9 At the message arrival, the receiving software sums all the bytes except the last; If the sum != checksum, an error has occurred. What if the checksum was damaged?

Spring 2002Computer Network Applications Network Transmission Media Electricity: Twisted pair, coaxial cable Light: Fiber optics Radio waves Coaxial cable Example: Cable TV Shield Radio or Micro Waves Example: Cellular phones glass or plastic Fiber Optic Cable antenna Ex. of twisted pair: Local phone lines

Spring 2002Computer Network Applications Wiring Telephone Twisted Pair Unshielded and susceptible to noise Not for higher data rates or long distances the rate of transfer is very low (4Mbps) Inexpensive Coaxial Cable Central core with shield around it Shield insures radio frequency noise is not generated High data rates at long distances (140Mbps),

Spring 2002Computer Network Applications Wiring (cont.) Fiber Optic Light signals transmitted by light emitting diodes are immune to electrical and magnetic noise Advantages: Tremendous data transfer rates (10,000 Mbps) immune to electrical interference can reliably transmit signals over extremely long distances highly resistant to corrosion impossible to tap undetected small in size Disadvantages: very expensive difficult to install

Spring 2002Computer Network Applications Fiber Optics A thin fiber optic cable can carry as much data as 900 single copper wires, with minimal interference, and superior tensile strength.

Spring 2002Computer Network Applications Radio Waves Electromagnetic waves operating at radio frequency; 100MBps Satellite can transmit data over longer distances but propagation delay is significant (540 msec) (100Mbps) Microwave high frequency radio signal sent through the air

Spring 2002Computer Network Applications Frequency Spectrum All waves behave similarly Sound Radio Micro Light Frequency differences Amount of data Distance Interference / Noise ELFVLFLFMFHFVHFUHFMicrowaveOptical 1001K100K1M10M100M1G10G Hertz Navy/submarines TV: 220M MHz AM: 550K KHz Public Safety: 150M MHzPublic Safety: 460M MHz Cellular phones: 800 MHz Cordless phones (some): 900 MHz PCS ET: 2 GHz TV: 54M MHz FM: 88M MHz

Spring 2002Computer Network Applications Transmission Speeds How about an image of 1,000,000 bits?