chapter 3 computer networks (dr.belal)

7/31/2019 Chapter 3 Computer Networks (Dr.Belal)

1/83

Physical LayerPropagation:UTP and Optical Fiber

Chapter 3

Updated January 2007

PankosBusiness Data Networks and Telecommunications, 6th edition


2/83

2

Orientation

Chapter 2 Data link, internet, transport, and application layers

Characterized by message exchanges

Chapter 3 Physical layer (Layer 1)

There are no messagesbits are sent individually

Concerned with transmission media, plugs, signalingmethods, propagation effects

Chapter 3: Signaling, UTP, optical fiber, and topologies

Wireless transmission is covered in Chapter 5


3/83

3

Figure 3-1: Signal and Propagation

Sender Receiver

Transmission Medium

Propagation

TransmittedSignal

Received Signal(Attenuated &

Distorted)

A signal is a disturbance in the media that propagates (travels)down the transmission medium to the receiver

If propagation effects are too large, the receiver will not be able toread the received signal


4/83

Data

Representation


5/83

5

Binary-Encoded Data

Computers store and process data in binaryrepresentations

Binary means two

There are only ones and zeros Called bits

1101010110001110101100111


6/83

6

Binary-Encoded Data

Non-Binary Data Must be Encoded into Binary Text

Integers (whole numbers)

Decimal numbers Alternatives (North, South, East, or West, etc.)

Graphics

Human voice

etc.

Hello 11011001


7/83

7

Binary-Encoded Data

Some data are inherently binary

48-bit Ethernet addresses

32-bit IP addresses

Need no further encoding


8/83

8

Figure 3-2: Arithmetic with Binary

Numbers

Binary Arithmetic for Whole Numbers (Integers)

(Counting Begins with 0, not 1)

Integer0

12345

678

Binary0

11011

100101

110111

1000

There are 10 kinds of people

those who understand binary and those who dont


9/83

9


Numbers, Continued

Binary Arithmetic for Binary Numbers

10 0 1 1 +1

+0 +1 +0 +1 +1

=0 =1 =1 =10 =11

Basic Rules


10/83

10


Numbers, Continued

Binary Decimal1000 8

+1 +1

=1001 =9+1 +1

=1010 =10+1 +1

=1011 =11+1 +1

=1100 =12

Examples


11/83

11

Figure 3-3: Binary Encoding for

Alternatives

Encoding Alternatives

(Product number, region, gender, etc.)

(N bits can represent 2N Alternatives)

Number of Bits

In Field (N)

1234

816

Number of Alternatives

That Can be Encoded

with N bits

2 (21)4 (22)8 (23)

16 (24)

256 (28)65,536 (216)

Each added bit doubles the number of alternatives that can be represented


12/83

12


Alternatives

Bits Alternatives Examples

1 21=2 Male = 0, Female = 1

2 22=4 Spring = 00, Summer = 01,Autumn = 10, Winter = 11

8 28=256 Keyboard characters for U.S.

keyboards. Space=00000000, etc.

ASCII code actually uses 7 bits


13/83

13

Powers of 2

Bits Alternatives

1 2

2 4

3 8

4 165 32

6 64

7 128

8 256

10 1,024

16 65,536

Each additional bitdoubles the number ofpossibilities

Start with one you know

and double or halve untilyou have what you need

E.g., if you know 8 is 256,10 must be 4 times as

large or 1,024.

Memorize for 1, 4, 8, and16 bits


14/83

14


Alternatives

Quiz How many flavors of ice cream can you represent in half

a byte of storage?

How many bits do you need to represent 64 flavors of icecream?

How many bits do you need to represent 6 salesdistricts?


15/83

15

Figure 3-4: ASCII and Extended ASCII

ASCII Code to Represent Text ASCII is the traditional binary code to represent text data

Seven bits per character

27 (128) characters possible

Sufficient for all keyboard characters (including shiftedvalues)

Capital letters (A is 1000001)

Lowercase letters (a is 1100001) Each character is stored in a byte

The 8th bit in a byte normally is not used


16/83

16

Figure 3-4: ASCII and Extended ASCII,

Continued

Extended ASCII

Used on PCs

Uses a full 8 bits per character

28 (256) characters possible

Extra characters can represent formatting in word

processing, etc.

Converters Text-to-ASCII and Text-to-Extended ASCII

Converters are Readily Available on the Internet


17/83

17

Figure 3-5: Binary Coding for

Graphics Image

Pixels

1. Screen is dividedinto small squarescalled pixels (picture

elements)

2. Each pixel has threedotsred, green, and

blue. Sometimes ablack dot too

3. JPEG stores onebyte per color(24 bits total)

This gives 256intensity levels foreach color or16.8 million colorsoverall (2563)


18/83

Signaling


19/83

19

Figure 3-6: Data Encoding and Signaling

DataNow is the Male or Female

GraphicsHuman Voice

Binary-

Encoded

Data

1101010

Binary

Encoding

Signaling

1.First, data must be

converted to binary, aswe have just seen

2.Second, bits must be covered

Into signals (voltage changes, etc.).

Voltage change, etc.


20/83

20

Figure 3-7: On/Off Binary Signaling

Off=

0

On=

1

On=

1

Off=

0

On=

1

Off=

0

On=

1

Light

Source

Optical Fiber

ClockCycle

During each clock cycle, light is turned on for a one or off for a zero.


21/83

21

Figure 3-8: Binary Signaling in 232 Serial Ports

0 Volts

-15 Volts

15 Volts Clock Cycle

0 0

1

3 Volts

-3 Volts

0

1 This type ofsignaling is used in232 serial ports.

In a clock cycle,

3 to 15 volts represents a

-2 to -15 volts is a zero


22/83

22

Figure 3-9: Relative Immunity to Errors in

Binary Signaling

0 Volts

-15 Volts

15 Volts

TransmittedSignal

(12 Volts)Received

Signal(6 volts)

3 Volts

-3 Volts

0

1

Despite a 50% drop in voltage,the receiver will still knowthat the signal is a zero


23/83

23

Binary and Binary Signaling

In binary signaling, there are two states This can represent a single bit per clock cycle.

In digital signaling, there are a fewbits per clock cycle2,4, 8, 16, 32,

With more states, several bits to be sent per clock cycle

Note that all binary transmission (2 states) is digital (fewstates)

But not all digitaltransmission isbinary

11

10

01

00

1011

00

0101ClockCycle


24/83

24

Figure 3-10: 4-State Digital Signaling

11

10

0100

Client PC Server

10

11

00

0101

Clock Cycle

Digital signaling has a FEW possible states per clock cycle (4 in this slide)This allows it to send multiple bits per clock cycleThis increases the bit transmission rate per clock cycle

It reduces error resistance because differences between states are smaller

Box


25/83

25

Quiz

Which Is Binary? Which Is Digital?

1.Calendar

2.Number

ofFingers

5.Gender

Male or Female

3.On/Off Switch

4.Day of the Week

Box


26/83

26

Figure 3-10: 4-State Digital Signaling, Continued

Equation 3-1:Bit rate = Baud rate * Bits sent per clock cycle

Baud rate is the number of clock cycles per second

If the clock cycle is 1/1000 of a second, the baud rateis 1,000 baud

Bit rate is then the number of clock cycles per secondtimes the number of bits sent per clock cycle

If the three bits are sent per clock cycle, the bit rate is3,000 bps or 3 kbps

Box


27/83

27

Figure 3-10: 4-State Digital Signaling, Continued

Equation 3-2: States =

2Bits

Bits is the number of bits tobe sent per clock cycle

States is the number ofstates needed to send thatmany bits

Doubling the number of

states transmits one morebit per clock cycle.

Rapidly diminishingreturns to adding states

Box

Bits to besent per

clock cycle

Number ofstates

required

1 2

2 4

3 8

4 16


28/83

28

Figure 3-10: 4-State Digital Signaling,

Continued

Example: The clock cycle is 1/100,000 second

The baud rate is 100 kbaud (not kbauds)

You want a bit rate of 500,000 kbps Solution:

You have to send 5 bits per clock cycle (baud)

This will require 32 states States = 2bits

States = 25

States = 32

Box


29/83

29

Figure 3-10: 4-State Digital Signaling,

Continued

Example: Suppose there a system has 8 states

Suppose that the clock cycle is 1/10,000 second

How fast can the system transmit?

Solution:

With four states, 3 information bits can be sent per clock

cycle (8=2X) [Equation 3-2] X=3

With a clock cycle of 1/10,000, baud rate is 10,000 baud

The bit rate will be 30 kbps (3 bits/clock cycle times10,000 clock cycles per second). [Equation 3-1]

Box


30/83

UTP PropagationUnshielded Twisted Pair wiring


31/83

31

Figure 3-12: 4-Pair UTP Cord with RJ45

Connector

3.RJ-45

Connector

2.8 Wires

Organizedas 4

TwistedPairs

Industry Standard Pen

1.UTP Cord

UTP Cord


32/83

32

RJ-45 Jacks and Connectors

RJ-45Jack

RJ-45Jack

RJ-45Jack

RJ-45 Connectors


33/83

33

Figure 3-11: Unshielded Twisted Pair

(UTP) Wiring, Continued

UTP Characteristics

Inexpensive and to purchase and install

Dominates media for access links between computersand the nearest switch


34/83

34

Figure 3-13: Attenuation and Noise

Power

Distance

1.Signal

Signals in UTP attenuate withpropagation distance.

If attenuation is too great, thesignal will not be readable by thereceiver.


35/83

35

Figure 3-14: Decibels

Attenuation is Sometimes Expressed inDecibels (dB)

The equation for decibels is

dB = 10 log10(P2/P1)

Where P1 is the initial power and P2 is the finalpower after transmission

If P2 is smaller than P1, then the answer will benegative


36/83

36

Figure 3-14: Decibels, Continued

Example Over a transmission link, power drops to 37% of its

original value

P2/P1 = 37/100 = .37 (37%/100%) LOG10(0.37) = -0.4318

10*LOG10(0.37) = -4.3 dB (negative, reflecting

power reduction through attenuation)

In calculations, the Excel LOG10 function can beused


37/83

37

Figure 3-14: Decibels, Continued

There are two useful approximations

3 dB loss is a reduction to very nearly1/2 theoriginal power

6 dB loss is a decrease to 1/4 the original power 9 dB loss is a decrease to 1/8 the original power

10 dB loss is a reduction to very nearly1/10 the

original power 20 dB loss is a decrease to 1/100 the original power


38/83

38

Figure 3-13: Attenuation and Noise, Continued

Power

Distance

Noise Floor

Noise

Noise SpikeSignal

Signal-

to-NoiseRatio (SNR)

Noise is random unwanted energy within the wire

Its average is called the noise floorRandom noise spikes cause errors--

A high signal-to-noise ratio reduces noise error problemsAs a signal attenuates with distance, damaging noise spikes

become more common

Error


39/83

39

Limiting UTP Cord Length

Limit UTP cord length to 100 meters Limits attenuation to being a negligible problem

Limits noise problems being a negligible problem

Note that limiting cord lengths limits BOTH noise andattenuation problems

100 Meters MaximumCord Length


40/83

40



Electromagnetic Interference (EMI) (Fig. 3-15)

Electromagnetic interference is electromagneticenergy from outside sources that adds to the signal

From fluorescent lights, electrical motors,microwave ovens, etc.

The problem is that UTP cords are like long radioantennas.

They pick up EMI energy nicely

When they carry signals, they also send EMIenergy out from themselves


41/83

41

Figure 3-15: Electromagnetic

Interference (EMI) and Twisting

Interference on the Two Halves of a Twist Cancels Out

TwistedWire

ElectromagneticInterference (EMI)


42/83

42

Figure 3-16: Crosstalk Interference and Terminal

Crosstalk Interference

Untwistedat Ends Signal

Terminal CrosstalkInterference

Crosstalk Interference

Terminal crosstalk interferenceNormally is the biggest EMI problem for UTP

Figure 3 16: Crosstalk Interference


43/83

43

Figure 3-16: Crosstalk Interferenceand Terminal Crosstalk Interference,Continued

EMI is any interference

Signals in adjacent pairs interfere with one another(crosstalk interference). This is a specific type of EMI

Crosstalk interference is worst at the ends, where thewires are untwisted. This is terminal crosstalkinterferencea specific type of crosstalk EMI

EMI

Crosstalk InterferenceTerminal CrosstalkInterference

Figure 3 11: Unshielded Twisted Pair


44/83

44



Electromagnetic Interference (EMI) (Fig. 3-15)

Terminal crosstalk interference dominatesinterference in UTP

Terminal crosstalk interference is limited to anacceptable level by not untwisting wires more than ahalf inch (1.25 cm) at each end of the cord to fit intothe RJ-45 connector

This reduces terminal crosstalk interferenceto a negligible level.

1.25 cm or 0.5 inches


45/83

45

UTP Limitations

Limit cords to 100 meters

Limits BOTH noise AND attenuation problems to anacceptable level

Do not untwist wires more than 1.25 cm (a half

inch) when placing them in RJ-45 connectors Limits terminal crosstalk interference to an acceptable

level

Neither completely eliminates the problems butthey usually reduce the problems to negligiblelevels

Fi 3 17 S i l V P ll l


46/83

46

Figure 3-17: Serial Versus ParallelTransmission

One Clock Cycle1.Serial

Transmission(1 bit per clock cycle)

2.ParallelTransmission

(1 bit per clock cycleper wire pair)

4 bits per clock cycleon 4 pairs

1 bit

1 bit

1 bit

1 bit

1 bit

Parallel transmission increases speed.But it is only workable over short distances.

Parallel is not 4. It is more than one.


47/83

47

Figure 3-18: Wire Quality Standards

Wiring Quality Standards Rated by Category (Cat) Numbers

Category Standards are Set by ANSI/TIA/EIA and

ISO/IEC In the United States, the TIA/EIA/ANSI-568 governs UTP

and optical fiber standards

In Europe and many other parts of the world, thestandard is ISO/IEC 11801

The two sets of standards are close but not identical


48/83

48


UTP Categories 3 and 4 Early data wiring, which could only handle Ethernet

speeds up to 10 Mbps

UTP Categories 5 and 5e Most wiring installed today is Category 5e (enhanced)

Cat 5e and Cat 5 can handle Ethernet up to 1 Gbps

Most wiring sold today is Cat 5e


49/83

49


UTP Category 6 Relatively new

No better than Cat 5 or Cat 5e at 1 Gbps

Developed for higher Ethernet speeds of 10 Gbps

But can only span 55 meters at that speed

Book says cannot be used. This is an error.

Category 6A (Augmented)

Able to carry Ethernet signals at 10 Gbps up 100 meters

The book said 55 meters, but this is an error

Errors


50/83

50


Category 7 STP

Shielded twisted pair (STP) ratherthan unshielded twisted pair (UTP)

Metal foil shield around each pair to reduce crosstalkinterference

Metal mesh around all four pairs to reduce crosstalkfrom other cords

STP is expensive and awkward to lay

Can 10 Gbps Ethernet to 100 meters


51/83

Optical FiberTransmission

Light through Glass

Better than UTP:More Easily Spans Longer Distances at High Speeds

Figure 3 19: UTP in Access Lines and


52/83

52

Figure 3-19: UTP in Access Lines andOptical Fiber in Trunk Lines

WorkgroupSwitch

UTPAccess

Line

UTPAccess

Line UTPAccess

Line

2.UTP dominates access lines

between stations andtheir workgroup switches

1.Workgroup

Switches Link

Computers tothe Network

Figure 3 19: UTP in Access Lines and


53/83

53

Figure 3-19: UTP in Access Lines and

Optical Fiber in Trunk Lines, Continued

1.Core switches

connectother switches

Core Switch

Core Switch

Core Switch

CoreFiber Trunk Fiber Trunk

FiberTrunk Fiber Trunk

FiberTrunk

2.Fiber dominates trunk lines

between switches


54/83

54

Figure 3-20: Optical Fiber Transceiver and Strand

Transceiver

5.Perfect internal reflection atcore/cladding boundary;

No signal loss, so low attenuation

4.LightRay

3.

Cladding125 micron diameter

2.

Core8.3, 50or 62.5microndiameter

1.(Transmitter/Receiver)

Light Source

850 nm,1,310 nm,

and 1,550 nm

Strand

Fi 3 22 T St d F ll D l O ti l Fib


55/83

55

Figure 3-22: Two-Strand Full-Duplex Optical Fiber

Cord with SC and ST Connectors

A fiber cord has

two-fiber strandsfor full-duplex (two-way) transmission

SC Connectors

ST Connectors

TwoStrands

Cord

Figure 3 22: Pen and Full Duplex Optical Fiber


56/83

56

Figure 3-22: Pen and Full-Duplex Optical Fiber

Cord with SC and ST Connectors

SC Connectors(Push in and Snap)

ST Connectors(Bayonet: Push in and Twist)

Fi 3 23 F d W l th


57/83

57

Figure 3-23: Frequency and Wavelengths

1.

AmplitudePower,Voltage,

etc.

2.Wavelength

Distance between comparable points in successive cycles(Measured in nanometers for light)

3.Frequency is the number of cycles per second.

1 Hz = 1 cycle per secondIn this case, there are two cycles in 1 second,

so frequency is two hertz (2 Hz).

1 Second

Amplitude

Wave

Li ht W l th


58/83

58

Light Wavelengths

Light signals are measured by wavelength Light wavelengths measured in nanometers (nm)

There are three fiber wavelength windows with

good propagation characteristics 850 nm

1310 nm

1550 nm

Shorter wavelength allows cheaper transceivers

Longer-wavelength light travels farther

Fi 3 24 C i Fib d LAN


59/83

59

Figure 3-24: Carrier Fiber and LAN

Fiber

LAN Fiber

Uses multimode fiber, which has a thick core diameterof 50 or 62.5 microns

Less expensive than single-mode fiber (later) 62.5 micron fiber is more common in the US but does

not carry signals as far as 50 micron fiber

Also uses inexpensive 850 nm transceivers

Multimode fiber with 850 nm signaling cannot span thekilometer distances needed by carriers, but can span the200-300 meters needed in LAN fiber cords

Figure 3-24: Multimode and Single-Mode


60/83

60


Optical Fiber

Multimode Fiber

In thicker fiber, light only travels in one of several allowed modes.Different modes travel different distances and arrive at different times

(See that Mode 1 light takes longer to arrive than Mode 2 light.)

If distance is too long, modes from successive light pulses will overlap.This is modal distortion. If it is too large, signals will be unreadable.

Modal distortion is the main limitation on distance in multimode fiber.

LightSource(UsuallyLaser)

Core

Mode 1ArrivesLater

Mode 2

Figure 3 24: Carrier Fiber and LAN


61/83

61


Fiber

LAN Fiber All multimode fiber today is graded-index multimode fiber

The index of refraction decreases from the center of

the core to the cores outer edge.

HigherIncidence ofRefraction

Lower



62/83

62


Fiber

LAN Fiber Graded-index multimode fiber

Light speed increases when the index decreases

The central mode (Mode 2) is slowed High-angle modes (Mode 1) are speeded up

Modal dispersion between the modes is reduced

Mode 2 (Slowed)

Mode 1 (Speeded Up Near Edge of Core)LowerModal

Dispersion



63/83

63


Fiber

LAN Fiber

UTP quality is measured by category number.

Multimode Fiber Quality

Measured as modal bandwidth (MHz.km or MHz-km)

More modal bandwidth is better

Increases the speeddistance product With greater mobile bandwidth, can go faster,

farther, or some combination of the two



64/83

64


Fiber

LAN Fiber Example: 1000BASE-SX Ethernet

Uses inexpensive 850 nm light

With 62.5 micron fiber and 160 MHz-km modalbandwidth, maximum distance is 220 m

With 62.5 micron fiber and 200 MHz-km bandwidth,

maximum distance is 275 m Some vendors with higher-than-standard modal

bandwidth can carry traffic farther



65/83

65


Fiber

LANs and WAN carriers use different types of fiber

Carrier Fiber

Carrier fiber must span long distances

This requires expensive long-wavelength laser lightsources (1,310 and 1,550 nm)

It also requires expensive single-mode fiber with a verynarrow core (8.3 microns)



66/83

66


Optical Fiber , Continued

LightSource

Single-Mode Fiber

Cladding

Core

Single Mode

Light enters only at certain angles called modes

Single-mode fiber cores are so thin that only one mode can

propagatethe one traveling straight throughNo modal dispersion (discussed earlier), so can span long distanceswithout this distortion

Expensive but necessary in WANs



67/83

67


Fiber

Carrier Fiber

Main propagation effect for single-mode fiber isattenuation, which is very low

For 850 nm light, attenuation is around 2.5 dB/km

At 1,310 nm, attenuation is lowerabout 0.8 dB/km

At 1,550 nm, attenuation falls even lowerabout0.2 dB/km

Longer wavelengths carry farther but cost more

Carrier fiber uses wavelengths of 1,310 or 1,550 nm



68/83

68

gu e 3 Ca e be a d

Fiber

Noise and Electromagnetic Interference (EMI) AreNot Problems for Either LAN or Carrier Fiber

Noise from moving electrons cannot interferewith light signals

EMI would have to be light signals

Wrapping the cladding in an opaque coveringprevents light from coming in

Figure 3 24: Carrier Fiber and LAN Fiber


69/83

69

Figure 3-24: Carrier Fiber and LAN Fiber

Corporate LAN

Multimode Fiber

Carrier (WAN)

Single-Mode Fiber

Needed Distance Only 200-300meters

Many kilometers

Cost Much Lower ($) Very high ($$$$)

Fiber Type Multimode ($) Single-mode ($$$$)

Wavelength Usually 850 nm ($) Usually 1,310 or1,550 nm ($$$$)

Typical Core 50/62.5 microns ($) 8.3 microns ($$$)Propagation Limit Modal Distortion Attenuation

Is Modal BandwidthImportant?

Yes No. Onlyattenuation matters


70/83

Topology

Figure 3 26: Major Topologies


71/83

71

Figure 3-26: Major Topologies

Topology

Network topology refers to the physical arrangementof a networks computers, switches, routers, andtransmission lines

Topology is a physical layer concept

Different network (and internet) standards specifydifferent topologies

Point-to-Point Topology(Telephone Modem Communication, Private Lines)

Fig re 3 26 Major Topologies Contin ed


72/83

72

Figure 3-26: Major Topologies, Continued

Star (Modern Ethernet)

Example:Pat Lees House

in Chapter 1a

Figure 3-26: Major Topologies Continued


73/83

73


Extended Star or Hierarchy

(Modern Ethernet)

Only one possible pathbetween any two computers

For computers X and Y,the path is XBACDY

A

BC

DE

X

Y

Z

Figure 3 26: Major Topologies Continued


74/83

74


Mesh (Routers, Frame Relay, ATM)

Multiple alternativepaths between two

computers

AB

CD

PathABD

PathACD



75/83

75


Ring (SONET/SDH)



76/83

76


Bus Topology (Broadcasting)Used in Wireless LANs


77/83

Topics Covered

Topics Covered


78/83

78

Topics Covered

Binary Data Encoding

Inherently binary data (IP addresses, etc.)

Integers (binary arithmetic)

Alternatives (N bits can represent 2NAlternatives) Text (ASCII and Extended ASCII)

Graphics (pixels, bits per pixel color)

For transmission the sender converts bits to signals

(on/off, voltage levels, etc.)

Topics Covered Continued


79/83

79

Topics Covered, Continued

Digital Transmission (Box) A few states instead of just two states (binary)

All binary transmission is digital transmission

Only some digital transmission (transmission with twostates) is binary

In the box: bit rates and baud rates

Topics Covered Continued


80/83

80


UTP 4-pair UTP cords and RJ-45 connectors and jacks

Attenuation (often expressed in decibels) and noise

Limit UTP cords to 100 meters

Electromagnetic interference, crosstalk interference, andterminal crosstalk interference

Limit wire unwinding to 1.25 cm (a half inch) to limit

terminal crosstalk interference

Serial versus parallel transmission



81/83

81


Optical Fiber On/off light pulses from transceiver

Core and cladding; perfect internal reflection

Dominates for trunk lines among core switches

2 fiber strands/fiber cord for full-duplex transmission

SC and ST connectors are the most common

Carriers use single-mode fiber and long wavelengths

LANs use multimode fiber and short wavelengths



82/83

82


Multimode Optical Fiber Distance IncreasesWith

Greater Wavelength

850 nm < 1310 nm < 1550 nm windows But larger-wavelength transceivers cost more

Smaller Core Diameter

50 microns > 62.5 microns Greater Modal Bandwidth (MHz.km)

Measure of multimode fiber quality



83/83


Topologies

Organization of devices and transmission links

Physical layer concept

Point-to-point, star, hierarchy, ring, etc.

chapter 3 computer networks (dr.belal)

Documents