1

Part II: Data Transmission

The basics of media, signals, bits, carriers, and modems

Fall 2005

Qutaibah MalluhiComputer Science and Engineering

Qatar University

2

Fall 2005

Transmission Media

Overview of transmission media used in network systems

3

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, ...

Sender(encodes data)

Receiver(decodes data)

data10110 … 110

data10110 … 110

Medium

4

Physical Layer and Data Transmission

5

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

6

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.

7

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)

8

Coaxial Cable

Can carry higher frequencies that TP Improved performance through shield Particularly useful for wires that pass next to

equipment generating EM fields

9

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)

10

Glass Fibers: More Details

11

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

12

Connectors

Twisted Pair Connectors

Coax Connectors

Coax Connectors

13

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

14

Wireless Network

Wireless bridge and net interface

15

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

16

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

17

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

18

Satellites

19

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