3-1© 2001 by Prentice Hall

Local Area Networks, 3rd EditionDavid A. Stamper

Part 2: Hardware

Chapter 3

Hardware Introduction and LAN Media

3-2© 2001 by Prentice Hall

Chapter Preview

• What makes up a LAN system• Several of the Leading LAN media• Characteristics of LAN media• Error sources, detection, and

correction

In this chapter you will study:

3-3© 2001 by Prentice Hall

Three Major LAN Components

• LAN software• Topology• The media access control (MAC)

protocol

3-4© 2001 by Prentice Hall

Things to Consider When Building a LAN

• A variety of media—twisted-pair wires, coaxial cable, fiber optic cable, and several lesser used wireless media

• Three basic topologies—ring, bus, and star• Two basic media access control protocols—

contention and token passing• Hardware from many vendors• Network operating systems from several vendors• Network utilities• Application software

3-5© 2001 by Prentice Hall

Two Major Classes of LAN Media

• Conducted Media– uses a conductor like a wire or a fiber optic cable to move

the signal from sender to receiver– includes twisted-pair wires, coaxial cables, and fiber optic

cables

• Wireless Media– uses radio waves of different frequencies or infrared light

broadcast through space– does not need a wire or cable conductor to transmit signals

3-6© 2001 by Prentice Hall

Conducted Media• Twisted-Pair Wires

– Twisted-pair wires are classified in several ways• by American wire gauge (AWG) rating• by shielding, either unshielded twisted-pair (UTP) or shielded twisted-

pair (STP)• by categories that define the wire’s rated acceptable speed and error

characteristics

• AWG Rating– The AWG rating is a measure of the thickness of the copper conductor in

the cable. The higher the AWG rating, the smaller the diameter of the wire.– Twisted-pair wiring for LANs have an AWG rating of 22-26.

3-7© 2001 by Prentice Hall

Conducted Media (cont.)• UTP and STP

– Shielded twisted-pair (STP) • These wires have a metal foil or wire mesh wrapped around

individual wire pairs with a metal braided shield around the twisted-pair wire bundle itself.

• Twisting pairs of wires helps eliminate interference from neighboring wires; the metal shielding helps prevent ambient distortion like heavy-duty motors, electrical or magnetic fields, and fluorescent lights.

– Unshielded twisted-pair (UTP)• These wires have no protective metal covering. UTP wires are more

susceptible to environmental noise that can disrupt the signal.• UTP is used because it is cheaper than STP, and it may safely be

used in environments where external disruptions are rare.

3-8© 2001 by Prentice Hall

Twisted-Pair Wire Category Summary

1

2

3

4

5

Maximum Data RateCategory

1 Mbps

4 Mbps

10 Mbps

16 Mbps

100, 155, and 1,000 Mbps

Telephones

Token Ring LANs

Ethernet LANs

Token ring LANs

Ethernet, fast ethernet, and gigabit ethernet LANs, CDDI LANs and asynchronous transfer mode (ATM)

Typical UseCost (Relative to Category 1)

1

1.5

2

3

4

3-9© 2001 by Prentice Hall

Coaxial Cable

• Most early microcomputer-based LAN implementations used coaxial cable as the medium.

• Coaxial cable comes packaged in a variety of ways, but essentially it consists of one or two central data transmission wires surrounded by an insulating layer, a shielding layer, and an outer jacket.

• Coaxial cable is most commonly used in two types of LANs, ethernet and ARCNET.

3-10© 2001 by Prentice Hall

A Single Conductor Coaxial Cable

Outer Insulation Mesh Shielding Insulation Conductor

3-11© 2001 by Prentice Hall

Fiber Optic Cable

• Fiber optic cables come in two varieties, multimode and singlemode, each with a different way of guiding the light pulses from source to destination.

• Fiber optic links for very short distances cost more than wires, but as distance or the required transmission rate increases, fiber optic cables become cost effective.

• Fiber optic cables will not corrode, so they can be used in environments unsuited for copper media.

3-12© 2001 by Prentice Hall

Views of a Fiber Optic Cable

Plastic Covering

Glass Cladding

Glass Conductor

3-13© 2001 by Prentice Hall

Wireless Media

• Broadcast Radio– When broadcast radio is used with local area networks, cables

connecting each microcomputer are eliminated.

• Microwave Radio– For networks where installation of conducted media is difficult

or too expensive, microwaves provide a high-speed medium alternative.

• Spread Spectrum Radio– Its reliability in environments where signal interference is likely

makes SSR well suited for LAN transmissions.

3-14© 2001 by Prentice Hall

Wireless Media (cont.)

• Infrared Transmission– Infrared transmission is a line-of-sight technology. It can

be used to provide LAN connections between buildings and also is the medium used in some wireless local area networks.

3-15© 2001 by Prentice Hall

The Frequencies of Various Wireless Media

1016

1015

1014

1013

1012

1011

1010

109

108

107

106

105

104

103

102

101

X rays, gamma raysUltraviolet lightVisible lightInfrared lightMillimeter waves

MicrowavesUHF televisionVHF televisionVHF TV (high band)FM radioVHF TV (low band)Short-wave radioAM radio

Very low frequency

Frequency (Hz )

3-16© 2001 by Prentice Hall

LAN Media Selection Criteria

• Cost• Speed or

Capacity• Availability• Expandability• Error Rates

• Security• Distance• Environment• Application• Maintenance

3-17© 2001 by Prentice Hall

Media Selection Criteria• Cost

– The costs associated with a given transmission medium include not only the costs of the medium but also ancillary fees, such as the costs for additional hardware like repeaters that might be required.

• Speed– Response time– Aggregate data rate

• Expandability– Some LAN media, for example, coaxial cable, are easier to expand

than others, for example, fiber optic cables.

3-18© 2001 by Prentice Hall

Media Selection Criteria (cont.)• Error Rates

– The propensity for error influences not only the quality of the transmission but also its speed.

• Security– Although most of the hacker incidents reported relate to wide area

networks, similar concerns occur on LANs.

• Distance– Before deploying a medium, LAN designers need to determine the

distances that need to be covered and ensure that the wiring configuration or wireless configuration does not exceed the distance limitations of the technology being used.

3-19© 2001 by Prentice Hall

Media Selection Criteria (cont.)

• Environment– The constraints of environment can eliminate certain

types of media.

• Application– In some applications, the characteristics of the required

equipment may dictate the type of medium and interfaces to be used.

3-20© 2001 by Prentice Hall

Characteristics of Common LAN Media

Unshielded twisted-pair

Shielded twisted-pair

Coaxial Cable

Fiber optic cable

Broadcast radio

Spread spectrum radio

Microwave radio

Infrared light

Common Speeds (Mbps)

Medium Type

1, 4, 10, 16, 100, 1000

1, 4, 10, 16, 100, 1000

10, 16, 50

10, 16, 50, 100, 1000, 2000

2

2, 10, 16

5.7

4, 10, 16

Less capable than other conducted media

Better than unshielded; less capable than fiber optic or coaxial cables

Good; less capable than fiber optic cable

Excellent

Subject to interference

Good

Subject to interference

Objects can block transmission

Error Characteristics

3-21© 2001 by Prentice Hall

Error Sources

• White Noise– White noise, also referred to as thermal noise and Gaussian noise,

result from the normal movements of electrons and is present in al transmission media at temperatures above absolute zero.

• Impulse Noise– In LANs, it can be caused by lightning striking the medium, by jarring

loose connections, or by transient electrical impulses such as those occurring on a shop floor.

• Crosstalk– Crosstalk occurs when signals from one channel distort or interfere

with the signals of a different channel.

3-22© 2001 by Prentice Hall

Error Sources (cont.)

• Echo– Echo is essentially the reflection or reversal of the signal

being transmitted.

• Attenuation– Attenuation is the weakening of a signal as a result of

distance and characteristics of the medium.

3-23© 2001 by Prentice Hall

Error Detection

• Parity Check– A parity check (also known as vertical redundancy check [VRC])

involves adding a bit—known as the parity bit—to each character during transmission.

• Longitudinal Redundancy Check (LRC)– With LRC, an additional, redundant character called the block check

character (BCC) is appended to a block of transmitted characters, typically at the end of the block.

• Cyclic Redundancy Check (CRC)– A CRC can detect bit errors better than either VRC or LRC or both. The

transmitting station generates the CRC and transmits it with the data.

3-24© 2001 by Prentice Hall

Error Detection (cont.)

• Sequence Checks– Sequence check numbers can be assigned to each block of

data so that the ultimate receiver can determine that all blocks have indeed arrived, and the blocks can be put back into proper sequence.

• Error Correction Codes– Some error-detection schemes allow the receiving station

not only to detect errors but also to correct some of them. Such codes are called forward error-correcting codes, the most common of which are called Hamming codes.

3-25© 2001 by Prentice Hall

Error Correction

• Message Acknowledgment– The mechanism used to effect retransmission is the positive or

negative acknowledgment, often referred to as ACK and NAK, respectively.

• Retry Limit– To cut down on continual retransmission of messages, a retry

limit—typically between 3 and 100—can be set. A retry limit of five means that a message received in error will be retransmitted five times; if it is not successfully received by the fifth try, the receiving station either disables the link or disables the sending station itself.