Physical Layer Propagation: UTP and Optical Fiber Chapter 3 Updated January 2009 XU Zhengchuan Fudan University

Physical Layer Propagation:UTP and Optical Fiber

Chapter 3Updated January 2009

XU Zhengchuan

Fudan University

2

Orientation

• Chapter 2

– Data link, internet, transport, and application layers

– Characterized by message exchanges

• Chapter 3

– Physical layer (Layer 1)

– There are no messages—bits are sent individually

– Concerned with transmission media, plugs, signaling methods, propagation effects

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

– Wireless transmission is covered in Chapter 5

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 to read the received signal

4

• Test Your Understanding

• P 141

Data Representation

6

Binary-Encoded Data

• Computers store and process data in binary representations

– Binary means “two”

– There are only ones and zeros

– Called bits

1101010110001110101100111

7

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…

8

Binary-Encoded Data

• Some data are inherently binary

– 48-bit Ethernet addresses

– 32-bit IP addresses

– Need no further encoding

9

Figure 3-2: Arithmetic with Binary Numbers

Binary Arithmetic for Whole Numbers (Integers)(Counting Begins with 0, not 1)

Integer012345678

Binary01

1011

100101110111

1000

“There are 10 kinds of people—those who understand binary and those who don’t”

10

Figure 3-2: Arithmetic with Binary Numbers, Continued

Binary Arithmetic for Binary Numbers

1 0 0 1 1 +1+0 +1 +0 +1 +1=0 =1 =1 =10 =11

Basic Rules

11

Figure 3-2: Arithmetic with Binary Numbers, Continued

Binary Decimal 1000 8 +1 +1=1001 =9 +1 +1=1010 =10 +1 +1=1011 =11 +1 +1=1100 =12

Examples

12

Figure 3-3: Binary Encoding for Alternatives

Encoding Alternatives(Product number, region, gender, etc.)(N bits can represent 2N Alternatives)

Number of BitsIn Field (N)

12348

16…

Number of AlternativesThat Can be Encoded

with N bits2 (21)4 (22)8 (23)

16 (24)256 (28)

65,536 (216)…

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

13


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

14

Powers of 2

Bits Alternatives

1 2

2 4

3 8

4 16

5 32

6 64

7 128

8 256

10 1,024

16 65,536

Each additional bit doubles the number of possibilities

Start with one you know and double or halve until you 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, and 16 bits

15


• 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 ice cream?

– How many bits do you need to represent 6 sales districts?

16

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 shifted values)

• 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

17

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

18

Figure 3-5: Binary Coding for Graphics Image

• Pixels

– 1. Screen is divided into small squares called pixels (picture elements)

– 2. Each pixel has three dots—red, green, and blue. Sometimes a black dot too

3. JPEG stores one byte per color (24 bits total)

This gives 256 intensity levels for each color or16.8 million colors

overall (2563)

19


• P 146

• 3 c

Signaling

21

Figure 3-6: Data Encoding and Signaling

Data“Now is the …”Male or Female

GraphicsHuman Voice

Binary-Encoded

Data1101010

BinaryEncoding

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.

22

Figure 3-7: On/Off Binary Signaling

Off=0

On=1

On=1

Off=0

On=1

Off=0

On=1

LightSource

Optical Fiber

ClockCycle

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

23

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 in

232 serial ports.

In a clock cycle,

3 to 15 volts represents a zero

-3 to -15 volts is a ONE

24

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

1Despite a 50% drop in voltage,

the receiver will still knowthat the signal is a zero

25

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 few bits per clock cycle—2, 4, 8, 16, 32, …

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

• Note that all binary transmission (2 states) is digital (few states)

• But not all digitaltransmission isbinary

11

10

01

00

10

11

00

0101ClockCycle

26


• P 149

• 4 a, b, c

27

Figure 3-10: 4-State Digital Signaling

11

10

01

00Client 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 cycle

This increases the bit transmission rate per clock cycleIt reduces error resistance because differences between states are smaller

Box

28

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

29

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 rate is 1,000 baud

– Bit rate is then the number of clock cycles per second times the number of bits sent per clock cycle

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

Box

30


• Equation 3-2: States = 2Bits

– Bits is the number of bits to be sent per clock cycle

– States is the number of states needed to send that many bits

• Doubling the number of states transmits one more bit per clock cycle.

Box

Bits to be sent per

clock cycle

Number of states

required

1 2

2 4

3 8

4 16

31


• 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 bps

• Solution:

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

– This will require 32 states

• States = 2bits

• States = 25

• States = 32

Box

32


• 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 times 10,000 clock cycles per second). [Equation 3-1]

Box

33


• P 151

UTP PropagationUnshielded Twisted Pair wiring

35

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

36

RJ-45 Jacks and Connectors

RJ-45Jack

RJ-45Jack

RJ-45Jack

RJ-45 Connectors

37

Figure 3-11: Unshielded Twisted Pair (UTP) Wiring, Continued

• UTP Characteristics

– Inexpensive and to purchase and install

– Dominates media for access links between computers and the nearest switch

38


• P 154

39

Figure 3-13: Attenuation and Noise

Power

Distance

1.Signal

Signals in UTP attenuate with propagation distance.

If attenuation is too great, the signal will not be readable by the receiver.

40

Figure 3-14: Decibels

• Attenuation is Sometimes Expressed in Decibels (dB)

• The equation for decibels is

– dB = 10 log10(P2/P1)

– Where P1 is the initial power and P2 is the final power after transmission

– If P2 is smaller than P1, then the answer will be negative

41

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 be used

42

Figure 3-14: Decibels, Continued

• There are two useful approximations

• 3 dB loss is a reduction to very nearly 1/2 the original 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 nearly 1/10 the original power– 20 dB loss is a decrease to 1/100 the original power– …

43

Figure 3-13: Attenuation and Noise, Continued

Power

Distance

Noise Floor

Noise

Noise SpikeSignal

Signal-to-Noise

Ratio (SNR)

Noise is random unwanted energy within the wireIts average is called the noise floor (噪声基底 )Random noise spikes (噪声毛刺 ) cause errors--A high signal-to-noise ratio reduces noise error problemsAs a signal attenuates with distance, damaging noise spikes become more common

Error

44

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 and attenuation problems

100 Meters MaximumCord Length

45


• P 157-158

46


• Electromagnetic Interference (EMI) (Fig. 3-15)– Electromagnetic interference (电磁干扰 ) is

electromagnetic energy 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 radio antennas.

• They pick up EMI energy nicely

• When they carry signals, they also send EMI energy out from themselves

47

Figure 3-15: Electromagnetic Interference (EMI) and Twisting

Interference on the Two Halves of a Twist Cancels Out

TwistedWire

ElectromagneticInterference (EMI)

48

Figure 3-16: Crosstalk Interference and Terminal Crosstalk Interference (交互干扰 )

Untwistedat Ends Signal

Terminal CrosstalkInterference

Crosstalk Interference

Terminal crosstalk interferenceNormally is the biggest EMI problem for UTP

49


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

– Terminal crosstalk interference dominates interference in UTP

– Terminal crosstalk interference is limited to an acceptable level by not untwisting wires more than a half inch (1.25 cm) at each end of the cord to fit into the RJ-45 connector

– This reduces terminal crosstalk interference to a negligible level.

1.25 cm or 0.5 inches

50

UTP Limitations

• Limit cords to 100 meters

– Limits BOTH noise AND attenuation problems to an acceptable 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 but they usually reduce the problems to negligible levels

51


• P 160

52

Figure 3-17: Serial Versus Parallel Transmission

One Clock Cycle1.Serial

Transmission(1 bit per clock cycle)

2.Parallel

Transmission(1 bit per clock cycle

per wire pair)4 bits per clock cycle

on 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.

53


• P 161

54

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, the standard is ISO/IEC 11801

– The two sets of standards are close but not identical

55


• 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

56


• 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

57


• Category 7 STP

– Shielded twisted pair (STP) rather than unshielded twisted pair (UTP)

• Metal foil shield around each pair to reduce crosstalk interference

• Metal mesh around all four pairs to reduce crosstalk from other cords

– STP is expensive and awkward to lay

– Can 10 Gbps Ethernet to 100 meters

58


• P 164

Optical Fiber Transmission

Light through Glass

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

60

Figure 3-19: UTP in Access Lines and Optical Fiber in Trunk Lines

WorkgroupSwitch

UTPAccess

Line

UTPAccess

LineUTP

AccessLine

2.UTP dominates access lines

between stations andtheir workgroup switches

1.Workgroup

Switches LinkComputers tothe Network

61

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

62


• P 165

• 14-15

63

Figure 3-20: Optical Fiber Transceiver and Strand

Transceiver

5.Perfect internal reflection at

core/cladding boundary;No signal loss, so low attenuation

4.LightRay

3.Cladding （镀层）

125 micron diameter

2.Core8.3, 50or 62.5microndiameter

1.(Transmitter/Receiver)

Light Source850 nm,

1,310 nm,and 1,550 nm

Strand（股）

64

Figure 3-22: Two-Strand Full-Duplex Optical Fiber Cord with SC and ST Connectors

A fiber cord has two-fiber strands

for full-duplex (two-way) transmission

SC Connectors

ST Connectors

TwoStrands

Cord

65

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)

66

Figure 3-23: Frequency and Wavelengths

1.Amplitude

Power,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

67

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

68


• P 169

• 16

69

Figure 3-24: Carrier Fiber and LAN Fiber

• LAN Fiber

– Uses multimode fiber, which has a “thick” core diameter of 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 （多模光纤） with 850 nm signaling cannot span the kilometer distances needed by carriers, but can span the 200-300 meters needed in LAN fiber cords

70

Figure 3-24: Multimode and Single-Mode 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（模态散射） . 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

71


• P 172

• 17

72


• LAN Fiber

– All multimode fiber today is graded-index multimode fiber

• The index of refraction （折射级数） decreases from the center of the core to the core’s outer edge.

HigherIncidence ofRefraction

Lower

73


• 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

Mode 2 (Slowed)

Mode 1 (Speeded Up Near Edge of Core)LowerModal

Dispersion

74


• 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 speed–distance product

– With greater mobile bandwidth, can go faster, farther, or some combination of the two

75


• LAN Fiber

– Example: 1000BASE-SX Ethernet

• Uses inexpensive 850 nm light

• With 62.5 micron fiber and 160 MHz-km modal bandwidth, 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

76


• LANs and WAN carriers use different types of fiber

• Carrier Fiber

– Carrier fiber must span long distances

– This requires expensive long-wavelength laser light sources (1,310 and 1,550 nm)

– It also requires expensive “single-mode” fiber with a very narrow core (8.3 microns)

77

Figure 3-24: Multimode and Single-Mode 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 propagate—the one traveling straight through

No modal dispersion (discussed earlier), so can span long distances without this distortion

Expensive but necessary in WANs

78


• P 174

• 18

79


• Carrier Fiber

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

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

• At 1,310 nm, attenuation is lower—about 0.8 dB/km

• At 1,550 nm, attenuation falls even lower—about 0.2 dB/km

– Longer wavelengths carry farther but cost more

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

80


• Noise and Electromagnetic Interference (EMI) Are Not Problems for Either LAN or Carrier Fiber

– Noise from moving electrons cannot interfere with light signals

– EMI would have to be light signals

• Wrapping the cladding in an opaque covering prevents light from coming in

81


Corporate LAN Multimode Fiber

Carrier (WAN) Single-Mode Fiber

Needed Distance Only 200-300 meters

Many kilometers

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

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

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

Typical Core 50/62.5 microns ($) 8.3 microns ($$$)

Propagation Limit Modal Distortion Attenuation

Is Modal Bandwidth Important?

Yes No. Only attenuation matters

82


• P 174

• 19

Topology

84

Figure 3-26: Major Topologies

• Topology

– Network topology refers to the physical arrangement of a network’s computers, switches, routers, and transmission lines

– Topology is a physical layer concept

– Different network (and internet) standards specify different topologies

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

85

Figure 3-26: Major Topologies, Continued

Star (Modern Ethernet)

Example:Pat Lee’s House

in Chapter 1a

86


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

87


Mesh (Routers, Frame Relay, ATM)

Multiple alternative paths between two

computers

AB

CD

PathABD

PathACD

88


Ring (SONET/SDH)

89


• P 176

• 20

90


Bus Topology (Broadcasting)Used in Wireless LANs

Topics Covered

92

Topics Covered

• Binary Data Encoding

• Inherently binary data (IP addresses, etc.)

• Integers (binary arithmetic)

• Alternatives (N bits can represent 2N Alternatives)

• Text (ASCII and Extended ASCII)

• Graphics (pixels, bits per pixel color)

• …

• For transmission the sender converts bits to signals (on/off, voltage levels, etc.)

93

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 two states) is binary

• In the box: bit rates and baud rates

94


• 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, and terminal crosstalk interference

• Limit wire unwinding to 1.25 cm (a half inch) to limit terminal crosstalk interference

– Serial versus parallel transmission

95


• 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

96


• Multimode Optical Fiber Distance Increases With …

– 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

97


• Topologies

– Organization of devices and transmission links

– Physical layer concept

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

Documents

Physical Layer Propagation: UTP and Optical Fiber Chapter 3 Updated January 2009 XU Zhengchuan Fudan University