Fall 2005 Part II: Data Transmission The basics of media, signals, bits, carriers, and modems

Qutaibah Malluhi Computer Science and Engineering Qatar University 1

Fall 2005 Transmission Media Overview of transmission media used in network systems

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Sending Data

 At sender: encode

data

as

energy

and transmit energy  At receiver: decode energy at destination back into data  Energy can be electrical, light, radio, sound, etc.

– Each form of energy has different properties and requirements for transmission – Transmitted energy is carried through some sort of medium: copper, glass, air, ... Medium data 10110 … 110 Sender (encodes data) Receiver (decodes data) data 10110 … 110 3

Physical Layer and Data Transmission

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Copper Wire

 Primary medium to connect computers – – – Inexpensive easy to install low resistance to electric current => signal travels further  Interference: How to minimize?

 Twisted pair  Coaxial cable (Coax)  Shielded twisted pair 5

Twisted Pair

 One wire for signal and other as ground reference  Receiver uses the difference between signal – Receiver is immune to interference if both wires are affected equally  Twisting – – – limits electromagnetic energy these wires emit protects the wires from external interference Two wires has same distance to noise source. One is closer in one twist, the other is closer in the second twist.

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UTP and STP

 Unshielded Twisted Pair (UTP) – – Classified into 7 categories based on cable quality Category 1 is the lowest quality (phone) and 5 is the most common today (100 Mbps Ethernet)  Shielded Twisted Pair (STP) 7

Coaxial Cable

 Can carry higher frequencies that TP  Improved performance through shield  Particularly useful for wires that pass next to equipment generating EM fields 8

Glass Fibers

 Thin glass fiber carries light with encoded data  Plastic jacket allows fiber to bend (some!) without breaking  Fiber is very clear and designed to reflect light internally for efficient transmission 

Light emitting diode

(LED) injects light into fiber. Light sensitive receiver at other end translates light back into data  Applications: – Backbone networks (cost-effective wide-bandwidth) – – Backbone of TV cable networks (coax for connection to premise) Some LANS (100baseFX and 1000base-X Ethernets) 9

Glass Fibers: More Details

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Glass Fiber Adv. and Disadv.

 Advantages – Much higher bandwidth » Signal generation and not the medium is the bottleneck today – – No electromagnetic interference Further signal distances (less signal attenuation) » 50 KM for fiber vs. 5 KM for copper – – Better encoding of bits (store more bits in signal) More immune to tapping  Disadvantages – – – Special installation equipment » difficulty of joining fibers Unidirectional (but requires single wire – no ground) difficulty to locate fiber problems 11

Connectors

Twisted Pair Connectors Coax Connectors Coax Connectors 12

Radio

 Data transmitted using radio waves. Conceptually similar to radio, TV, pagers  Energy travels through the air rather than copper or glass – Physical connection is not required – Antennas are needed  Omni-directional (travels in all directions)  Can travel through walls and through an entire building  Can be long distance or short distance  Long distance with satellite relay  Short distance: wireless computer network 13

Wireless Network

 Wireless bridge and net interface 14

Microwave

 High frequency radio waves – 2-300 GHz  Higher speed than lower frequency RF signals   Unidirectional, for point-to-point communication Can not penetrate structures – clear path is required – Line of sight transmission  Antennas mounted on towers relay transmitted data 15

Infrared

 Infrared light transmits data through the air  Similar to technology used in TV remote control  Can propagate throughout a room (bouncing off surfaces), but will not penetrate walls  Becoming common in personal digital assistants 16

Laser

 Unidirectional, like microwave  Higher speed than microwave  Uses laser transmitter and photo-sensitive receiver at each end Point-to-point, typically between buildings  Can be adversely affected by weather 17

Satellites

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Summary

 Copper wire is mature technology, rugged and inexpensive; maximum transmission speed is limited  Glass fiber:  Higher speed  More resistant to electro-magnetic interference  Spans longer distances  Requires only single fiber  More expensive; less rugged  Radio and microwave don't require physical connection  Radio and infrared can be used for mobile connections  Laser also does not need physical connection and supports higher speeds 19