1 physical layer propagation chapter 3 (revised august 2002) copyright 2003 prentice-hall panko’s...

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Physical Layer Propagation

Chapter 3 (Revised August 2002)

Copyright 2003 Prentice-HallPanko’s Business Data Networks and Telecommunications, 4th edition

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The Physical Layer

Chapter 2 – StandardsStandards above the physical layer

Chapter 3 – The Physical Layer

Real connections between machines

No messages

Propagation effects that change the signal as it propagates over the transmission medium

Perspective

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Figure 3.1: Signal and Propagation

Sender

Transmitted Signal

Transmission Medium

Received Signal(Attenuated &

Distorted)Because of

Propagation Effects

Receiver

Propagation

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Analog, Binary, and Digital

Analog and binary data and analog, binary, and digital signals

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Figure 3.2: Analog and Binary Data

Binary Data

1101011000011100101

Analog Data

Smoothly changing among an infinite number of states (loudness levels,

etc.)

Two states:One state represents 1

The other state represents 0

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Quiz

Which is Analog? Which is Binary?

Gender

Clock

On/OffSwitch

Thermometer

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Figure 3.3: Binary Data and Binary Signal

15 Volts(0)

0 Volts

-15 Volts(1)

TransmittedSignal

0 0

1

There are two states (in this case,voltage levels).

One, (high) represents a 0. The other (low) represents a 1.

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Figure 3.3: Binary Data and Binary Signal

15 Volts(0)

Clock Cycle

0 Volts

-15 Volts(1)

TransmittedSignal

0 0

1

Time is divided into clock cyclesThe State is held constant

within each clock cycle.It can jump abruptly at the end of

each cycle. One bit is sent per clock cycle.

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Figure 3.4: Binary Data and Digital Signal

10

11

00

01 01

11

10

01

00Client PC Server

In binary transmission, there are two states.In digital transmission, there are few states (in this case, four).

With four states, two information bits can be sent per clock cycle.

00, 01, 10, and 11 Binary transmission is a special case of digital transmission.

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Quiz

Which is Analog? Which is Digital?

Calendar Clock

NumberOf

Fingers

Audio CD

On/OffSwitch

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Figure 3.4: Baud Rates for Digital Signals

10

11

00

01 01

11

10

01

00Client PCServer

Suppose that the clock cycle is 1/10,000 second.Then the baud rate is 10,000 baud (10 kbaud).

The bit rate will be 20 kbps (two bits/clock cycle times 10,000 clock cycles per second).

(The bit rate gives the number of information bits per second.)

Baud Rate =# of Clock Cycles/Second

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Bit Rate versus Baud Rate

Number of Possible States

Bits per ClockCycle

2 (Binary)

4

8

16

1

2

3

4

If a Baud Rate is 1,200 Baud,Bit Rate is

1,200 bps

2,400 bps

3,600 bps

4,800 bps

Each Doubling of States Gives One More Bit per Clock Cycle

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Perspective

Analog Data Smooth changes among an infinite number of

states—like hands going around an analog clock

Digital Data Few states

In a digital clock, each position can be in one of ten states (the digits 0 through 9)

Binary Data Two states (a special case of digital)

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Figure 3.5: Using a Modem to Send Binary Data Over an Analog Transmission Line

Computer

Modem

Telephone

PSTN

Modulated AnalogSignal

1 0 1 1

Amplitude (Loudness or Intensity) Modulation

1010010101

Binary Data

1011 becomes loud-soft-loud-loud

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Figure 3.5: Using a Modem to Send Binary Data Over an Analog Transmission Line

Computer

Modem

Telephone

PSTN

Modulated AnalogSignal

1 0 1 1

Amplitude (Loudness or Intensity) Modulation

1010010101

DemodulatedBinary Data

Loud-soft-loud-loud becomes 1011

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Figure 3.6: Sending Analog Data Over a Digital Line

AnalogData

Source

DigitalTransmission

Line

Encoding

Decoding

Digital Signal110010101

(Binary Example)

Digital Signal100001101

(Binary Example)

Many Time PeriodsSo Fairly Smooth

Codec

Analog Data

Analog Data

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Data and Signals: Modems Vs. Codecs

Analog Line Signal Digital Line Signal

Analog data Codec

Digital data (including binary

data)Modem

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UTP Media and Propagation Effects

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Figure 3.7: 4-Pair Unshielded Twisted Pair Cable with RJ-45 Connector

Single Twisted Pair

Jacket

Four pairs (each pair is twisted)

There is insulation around each wire.

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Figure 3.7: 4-Pair Unshielded Twisted Pair Cable with RJ-45 Connector

A length of UTP is called a cord.

There is no metal shielding aroundThe individual pairs or around the entire Cord. Hence the name unshielded UTP

UTP Cord

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Figure 3.7: 4-Pair Unshielded Twisted Pair Cable with RJ-45 Connector

The cord terminates in an 8-pinRJ-45 connector, which plugsinto an RJ-45 jack in the NIC, switchOr wall jack.

Pin 1 on left of Jack

RJ-45Jack

8-PinRJ-45

Connector

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Figure 3.7: 4-Pair Unshielded TwistedPair Cable with RJ-45 Connector

RJ-45Connector

UTP Cord

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Figure 3.7: 4-Pair Unshielded TwistedPair Cable with RJ-45 Connector

WithRJ-45

Connector

4 PairsSeparated

Pen

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Figure 3.8: Noise and Attenuation

Distance

Signal Noise Floor(average)

Noise

Power

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Figure 3.8: Noise and Attenuation

Distance

Signal Noise Spike

Noise

Power

DamageNoise Floor(average)

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Figure 3.8: Noise and Attenuation

Distance

Signal

Noise Floor(average)

Noise

Signal-to-Noise

Ratio(SNR)

PowerSNR = Signal Power / Noise PowerIf SNR is high, noise errors are rare

As signal travels, it attenuates, and noise errors increase

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Noise and Attenuation

The TIA/EIA-568 standard recommends that UTP runs be kept to 100 meters

If this distance limit is observed, problems with noise and attenuation usually are minor

Low-tech solution, but it works well.

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Figure 3.9: Twisting Wire Paris to ReduceElectromagnetic Interference (EMI)

Interference On the Two Halves of a Twist Cancels Out

Interference

TwistedWire

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Figure 3.10: Crosstalk Electromagnetic Interference (EMI) and Terminal Crosstalk Interference

Signal

Crosstalk Interference

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Figure 3.10: Crosstalk Electromagnetic Interference (EMI) and Terminal Crosstalk Interference

Untwistedat Ends Signal

Terminal CrosstalkInterference

Crosstalk Interference

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Figure 3.10: Crosstalk Electromagnetic Interference (EMI) and Terminal Crosstalk Interference

EMI is any interference Signals in adjacent pairs interfere with one another

(crosstalk interference) is a specific type of EMI.

Crosstalk interference is worst at the ends, where the wires are untwisted. This is terminal crosstalk interference—a specific type of crosstalk EMI.

Solution: untwist wires for connector no more than 1.25 cm (0.5 in). Does not eliminate terminal cross-talk interference

but makes it negligible

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Limiting UTP Propagation Problems

Two simple things can limit UTP propagation problems Limit cord distances to 100 meters to control

attenuation and noise effects

Limit the untwisting of wires at the connectors to 1.25 cm (0.5 inch) to control terminal crosstalk interference.

If these rules are followed strictly, propagation problems should be negligible

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Figure 3.11: Serial versus Parallel Transmission

SerialTransmission

(1 bit per clock cycle)

Parallel Transmission(1 bit per clock cycle

per wire pair)4 bits per clock cycle

on 4 pairs

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Figure 3.11: Serial versus Parallel Transmission

Serial Transmission: one bit per clock cycle if binary transmission

Parallel Transmission with N wire pairs: N bits per clock cycle if binary transmission Not limited to four wire pairs (can be 2, 8, 100, etc.)

The advantage of parallel transmission is that it is faster than serial transmission

Only works over very short distances.

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Optical Fiber Media and Propagation Effects

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Figure 3.12: Optical Fiber Cabling

LightSource(LED orLaser)

Cladding

Core

LightRay

Reflection at Core/Cladding Boundary

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Figure 3.13: Wavelength Division Multiplexing (WDM) in Optical Fiber

LightSource 2

LightSource 1

Optical Fiber Core

Multiple Light Sources Transmit on Different WavelengthsEach Carries a Separate Signal

More Capacity Per Fiber

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Figure 3.14:Full-Duplex Optical Fiber Cord

Switch Router

Fiber Cord

Fiber Cord

A pair of fibers is needed for full-duplex (simultaneous 2-way) transmission.Each fiber carries a signal in only one direction.

SC, ST, or otherconnector

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Optical Fiber Cabling

STConnectors(Popular)

SCConnectors

(Recommended)

Two fiber cords for full-duplex (two-

way) transmission

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Figure 3.15: Multimode & Single-Mode Fiber

LightSource

Core

Cladding

Multimode Fiber

Modes

Light only travels in one of several allowed modesLight travels faster at the edges to speed modes going the farthest

Multimode fiber must keep its distance short or limit modal distortionMultimode fiber goes a few hundred meters and is inexpensive to lay

It is dominant in LANs

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Figure 3.15: Multimode & Single-Mode Fiber

LightSource

Core

Cladding

Graded Index of Refraction(Decreasing from Center)

Graded Index Multimode Fiber

Modes

Signals Travel FastestOn Outside of Core

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Figure 3.15: Multimode & Single-Mode Fiber

LightSource

Single Mode Fiber

Cladding

Core

Single Mode

Core is so thin that only one mode can propagate.

No modal dispersion, so can span long distances without distortion.

Expensive, so not widely used in LANs. Popular in WANs

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Multimode and Single-Mode Fiber

Multimode Limited distance (a few hundred meters)

Inexpensive to install

Dominates fiber use in LANs

Single-Mode Fiber Longer distances: tens of kilometers

Expensive to install

Commonly used by WANs and telecoms carriers

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Radio Transmission and Propagation Effects

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Figure 3.16: Omnidirectional and Dish Antennas

Dish Antenna

Concentrates incomingand outgoing signalsSignals can travel far

Omnidirectional Antenna

No need to pointto sender or receiverRapid attenuation

with distance

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Figure 3.17: Radio Wave

Amplitude

Wavelength

FrequencyMeasured in Hertz (Cycles per Second)

2 Cycles in one Second, so 2 Hz

Wavelength * Frequency = Speed of Propagation

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Figure 3.18: The Frequency Spectrum, Service Bands, and Channels

Channel 4

Channel 3

Channel 5

Channel 2

Channel 1

FrequencySpectrum

(0 Hzto infinity)

ServiceBand

0 Hz

A service band has a specificpurpose, such as FM radio orcellular telephony.

Service bands are divided intochannels. Signals sent in different channels do not interfere with oneanother.

Channels with wider bandwidthscan carry signals faster.

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Shannon’s Law

Here W = maximum possible speed in channel

B = bandwidth (highest frequency minus lowest frequency)

S/N = signal to noise ratio

Wide bandwidth (broadband) gives high speed

Small bandwidth (narrowband) gives low speeds

W = B * Log2 (1 + S/N)

Figure 3.18: The Frequency Spectrum, Service Bands, and Channels

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LaptopComm. Tower

Figure 3.19: Wireless Propagation Problems

Inverse SquareLaw Attenuation

Very Rapid Attenuation with DistanceCompared to Wires and Fiber

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LaptopComm. Tower

ShadowZone:

No Signal

Figure 3.19: Wireless Propagation Problems

MultipathInterference

Signals Arriving at SlightlyDifferent Times Can Interfere

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Golden Zone

At lower frequencies, there is little total bandwidth.

At very high frequencies, propagation is poor.

Mobile devices tend to work in the “golden zone” from the high megahertz to the low gigahertz range.

Frequencies in the golden zone are limited and in high demand.

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Topology

Transmit

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Figure 3.20: Major Topologies

A network technology’s topology is the order in which stations are connected to one another via media.

Point-to-Point

The Simplest Topology

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Figure 3.20: Major Topologies

Star (Modern Ethernet) Extended Star or Hierarchy(Modern Ethernet)

RootSwitch

Only one possible path between two

stations

Switch

Switch

Switch

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Figure 3.20: Major Topologies

Mesh (Routers, Frame Relay, ATM)

Multiple alternative paths between two

stations

AB

CD

PathABD

PathACD

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Figure 3.20: Major Topologies

Ring (802.5, FDDI, SONET/SDH)

Only one possible path between two

stations

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Figure 3.20: Major Topologies

Daisy Chain Bus(Ethernet 10Base2)

Multidrop Line Bus(Ethernet 10Base5)

All stations hear each transmissionOnly one possible path between two stations

Transmit Transmit

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Recap

Analog, Binary, and Digital: Data and Signals

Transmission Media UTP (limit distance and wire untwisting) Optical Fiber (multimode for most LAN use) Radio (freedom but weird propagation and limited

spectrum)

General Concepts Propagation effects Full duplex Serial versus parallel transmission