Using the Internet from Home: The Physical Layer Chapter 4 Copyright 2001 Prentice Hall Revision 2:...

Using the Internet from Home:The Physical Layer

Chapter 4

Copyright 2001 Prentice HallRevision 2: July 2001

2Orientation

Using the Internet from Home– There are other applications– There are other ways to access the Internet

Chapter 3– Upper layers: HTTP, TCP, IP, and PPP– Governed by messages

This Chapter (4)– Physical layer standards– Transmit one bit at a time– Direct connection host-router and router-router

3Analog Transmission

In analog transmission, the state of line can vary continuously and smoothly among an infinite number of states– States can be signal strengths, voltages, or other

measurable conditions– Human voice is analog; telephone mouthpiece

generates analogous electrical signal

Time

Strength

4Digital Transmission

Time is divided into fixed-length clock cycles– For modems, there are a few thousand clock cycles per

second– For LANs, there are millions of clock cycles per second

The line is kept in one of only a few possible states (conditions) during each time period– cycle; this is why the signal must be kept constant

At the end of each time period, the line may change abruptly to another of these few states– Can also stay the same

5Digital Versus Binary Transmission

Digital transmission: a few states Binary transmission: exactly two states (1 and 0)

– Binary is a special case of digital

Digital Binary

Two StatesFew States

0

1

6Digital Versus Binary Transmission

Sender and Receiver associate one or more bits with each state– Simplest case: High state = 1, Low state = 0

– If four states, might have the following: Highest = 11 Second highest = 10 Next highest = 01 Lowest = 00

7Number of States versus Number of Bits Represented per Clock Cycle

2Bits per clock cycle=Number of states– For 1 bit per clock cycle,– 2 states are required (One for 1, one for 0)– 21=2– Binary

1

00 0 0

1

Clock Cycle

States


2Bits/clock cycle=States/clock cycle– For 2 bits per clock cycle, 4 states are required (22=4)– For 3 bits per clock cycle, 8 states are needed (23=8)– For 4 bits per clock cycle, 16 states are needed (24=16)

3 (11)

2 (10)

1 (01)

0 (00)00

01

10 11

Clock Cycle

States


2Bits per clock cycle=States/clock cycle– With 4 states, send two bits per clock cycle (22=4)– With 8 states, send 3 bits per clock cycle (23=8)– With 16 states, send 4 bits per clock cycle (24=16)

3 (11)

2 (10)

1 (01)

0 (00)00

01

10 11

Clock Cycle

States

10Bits and Baud

Baud Rate = Number of clock cycles/sec– In this example, 4 baud (not 4 bauds/second)– Note: Number of clock cycles, not actual line changes

Bit Rate = Number of bits/second– In this example, 8 bits/second

00

01

10

01

1 Second

Possible Change Not Made

11Equations

For Each Clock Cycle– 2Bits per clock cycle = Number of possible states (Eq. 1)

Overall– Bit rate = Baud Rate * Bits per clock cycle (Eq. 2)

Example– Baud rate of 10,000 with four possible states– Bits per clock cycle = 2 (22=4) (Eq. 1)– Bit rate = 10,000 * 2 (Eq. 2)– Bit rate = 20,000 bps

12Transmission Speeds

Bit: A single 1 or 0 Bits per second (bps)

– Factors of 1,000 (not 1,024 as in memory)– kilobits per second (kbps)--Note lower case k– megabits per second (Mbps)– gigabits per second (Gbps)– terabits per second (Tbps)– petabits per second (Pbps)

Occasionally given in bytes per second (Bps)– Bits per second / 8– Uncommon

100101001 ...

New

13Wire Propagation Effects

Propagation Effects– Signal changes as it travels– If change is too great, receiver may not be able to

recognize it

Distance

OriginalSignal

FinalSignal

14Wire Propagation Effects: Attenuation

Attenuation: Signal Gets Weaker as it Propagates– May become too weak for receiver to recognize

SignalStrength

Distance

15Wire Propagation Effects: Distortion

Distortion: Signal changes shape as it propagates– Adjacent bits may overlap– May make recognition impossible for receiver

Distance

16Wire Propagation Effects: Noise

Noise: Thermal Energy in Wire Adds to Signal– Noise floor is average noise energy– Random energy, so brief noise spikes sometimes occur

SignalStrength

Time

Noise

Spike

Noise Floor

17Wire Propagation Effects: Noise

Noise: Thermal Energy in Wire Adds to Signal– If noise spikes become as large as the signal, they are

likely to cause errors, switching 1s and 0s or just distorting the signal so that it cannot be received

SignalStrength

Time

Signal

Noise

Spike

Error

18Wire Propagation Effects

Noise and Attenuation– As signal attenuates, gets closer to noise floor– Smaller spikes can harm the signal– So noise errors increase with distance, even if the

average noise level is constant

SignalStrength

Distance

Signal

Noise Floor

19Wire Propagation Effects: SNR

Want a high Signal-to-Noise Ratio (SNR)– Signal strength divided by average noise strength– As SNR falls, errors increase

SignalStrength

Distance

Signal

Noise FloorSNR

20Wire Propagation Effects: Noise & Speed

Noise and Speed– As speed increases, each bit is briefer– Noise fluctuations do not average out as much– So noise errors increase as speed increases

One BitNoiseSpike

Average NoiseDuring Bit

Low Speed(Long

Duration)

One BitNoiseSpike

Average NoiseDuring Bit

High Speed(Short

Duration)

OK Error

21Wire Propagation Effects: Interference

Interference– Energy from outside the wire (nearby motors, other

wires, etc.)– Adds to signal, like noise– Often intermittent (comes and goes), so hard to diagnose– Often called electromagnetic interference (EMI)

SignalStrength

Signal

Interference

New

22Wire Propagation Effects: Cross-Talk Interference

Cross-Talk Interference– Often, there are multiple wires in a bundle– Each radiates some of its signal– Causes “cross-talk” interference in nearby wires

23Wire Propagation Effects:Cross Talk

Wire Usually is Twisted– Usually, several twists per inch– Interference adds to signal over half twist, subtracts

over other half– Roughly cancels out– Simple but effective

Single Twist

Interference- +

Signal

24Wire Propagation Effects:Cross Talk

Terminal Cross-Talk Interference– Wire must be untwisted at ends to fit into connectors– So cross-talk interference is high at termination– Problems severe if untwist more than about 1.25 cm

(1/2 inch)– Usually the biggest propagation effect

TerminalCross Talk

25Practical Issues in Propagation Effects

Distance limits in standards prevent serious propagation effects– For instance, usually 100 meters maximum for ordinary

copper wire– If stay within limits, usually no serious problems

Problems usually occur at connectors– Crossed wires– Poor connections– Cross-talk interference

New

26Wire Media: UTP

Unshielded Twisted Pair (UTP)– Ordinary copper wire

– Twisted several times per inch to reduce interference

– Pair of wires needed for a complete electrical signal

– Unshielded: nothing but plastic coating No protection from interference such as a wrap-

around foil covering

27Wire Media: UTP

Unshielded Twisted Pair (UTP)– Business telephone wiring traditionally comes in 4-pair

UTP wire bundles

– Used in LAN wiring to use existing building wiring technology

Jacket

28Wire Propagation: RJ-45

RJ-45 connector terminates a UTP bundle– Slightly wider than RJ-11 residential telephone

connector

– Width needed for 8 wires

RJ-45Connector

RJ-45Jack

29Wire Media: UTP to the Desktop

UTP– Dominant for line from desktop to first hub or switch– Inexpensive to buy and install– Rugged: can take punishment of office work– Easily 100 Mbps, 1 Gbps with careful insulation

UTP

First Hub or Switch

30Wire Media: Optical Fiber

Optical Fiber– Glass core, surrounding glass cladding– Light source turned on/off for 1/0– Total internal reflection at boundary– Almost no attenuation

LightSource

Cladding

Core

Reflection


Limited by Distortion– Light entering at different angles travels different

distances (different number of reflections)

– Called different modes

– Light from successive bits becomes mixed over long distances

LightSource

Mod B


Multimode Fiber– Wide core makes easy to splice (50 or 62 microns)

– Many angles for rays (modes)

– Short propagation distance (usually 200 m to 500 m)

LightSource

Mod B


Single Mode Fiber– Narrow core difficult to splice (5 or 8 microns)

– Only one angle for rays (one mode), so (almost) no distortion

– Longer propagation distance (usually up to 2 km for LAN fiber, longer for long-distance fiber)

– Narrow core makes fiber fragile and difficult to splice

Mod B


Optical Fiber– High speeds over long distances

200 m to 2 km– Costs more than UTP, but worth it on long runs – Good for all links between hubs and switches within

and between buildings in a site network

OpticalFiber

35Wire Media: UTP and Optical Fiber

The emerging pattern: UTP from first hub or switch to desk, Fiber everywhere else on site

36Wire Media: Coax

Coaxial Cable– Used in cable TV, VCRs– Central wire, external concentric cylinder– Outer conductor wrapped in PVC

Screw-On Connector

InnerWire

Outer Conductor Wrapped in PVC

37Wire Media: Coaxial Cable

Coaxial Cable

– Installed widely today in old 10 Mbps Ethernet LANs

– Not being used in new installations

Optical fiber more cost-effective for long links

UTP more cost-effective for desktop links

38PC 232 Serial Ports

Ports– Connectors at back of PC– Plus related internal electronics to send/receive

PC 232 Serial Port– Follows EIA/TIA 232 standards

39PC 232 Serial Ports: 9-Pin and 25-Pin Ports

9 pins or 25 pins

Parallel ports have 25 holes

Pins

Holes

9-pin Serial Port

25-pin Serial Port

25-pin Parallel Port

40232 Serial Ports: Send on One Pin Each Way

9-Pin 232 Serial Ports– PC sends on Pin 3 (modem

receives)

– PC receives on Pin 2 (modem sends)

– Pin 5 is a signal ground defining zero volts

PC Modem

41232 Serial Ports: Send on One Pin Each Way

9-Pin 232 Serial Ports– Other pins are control signals

to tell other side when it may transmit

– Or tell PC what modem is hearing on the line (ringing, modem carrier signal) PC Modem

42Serial and Parallel Transmission

Serial: one wire, one bit per clock cycle*– Second (ground) wire needed for circuit is not shown

1 0

OneBit inClockCycleOne

OneBit inClockCycleTwo

*For simplicity, we assume binary transmission (2 possible states/clock cycle)


Parallel– N bits per second on N wires– Parallel is faster than serial

1101100

1101100

0 0

Eight BitsIn Clock

Cycle One

Eight BitsIn Clock

Cycle Two


Parallel Transmission– N bits per second on N wires– N=8 in this example, but this is not the only possibility– N can also be 4, 16, 32, etc.

1101100

1101100

0 0


Parallel Transmission is Only for Short Distances– Usually up to about 2 meters (6 feet)– Wire propagation speeds vary– Over long distances, bits from different clock cycles

overlap

11

01

10

00

11

0110

00

46PC 232 Serial Ports: Voltages

For sending data– One is -3 to -15 volts (Yes, one is low)– Zero is +3 to +15 volts (Yes, zero is high)– Binary (only two possible states)

+15v

-15v

0

1

0

47PC 232 Serial Ports

PC 232 serial ports are binary because there are only two states (voltage levels)

PC 232 serial ports are serial because data is sent on only one wire at a time

These are separate things– One does not require the other

48Duplex

Full-duplex transmission: both sides can transmit simultaneously– Even if only one sends, still full-duplex line– Even if neither is sending, still full-duplex line

A B

Time 1Both can send

Both do

A B

Time 2Both can sendOnly A does

A B

Time 3Both can sendNeither does

49Duplex

Half-duplex transmission: only one can transmit at a time; must take turns– Still half duplex if neither transmits

A B A B

Time 1Only one side

Can sendA does

Time 2Only one side

Can sendNeither does

50Duplex

Duplex is a Characteristic of the Transmission System, Not of Use at a Given Moment

– In full duplex, both sides can transmit at once; in half duplex, only one side can transmit at a time

– Still full duplex system if only one side or neither side actually is transmitting at a moment

– Still half duplex if neither side actually is transmitting at a moment

51Radio Propagation

Broadcast signal– Not confined to a wire

52Radio Waves

When Electron Oscillates, Gives Off Radio Waves– Single electron gives a very weak signal– Many electrons in an antenna are forced to oscillate in

unison to give a practical signal

53Radio Propagation Problems

Wires Propagation is Predictable– Signals go through a fixed path: the wire– Propagation problems can be easily anticipated– Problems can be addressed easily

Radio Propagation is Difficult– Signals begin propagating as a simple sphere– Inverse square law attenuation– If double distance, only ¼ signal strength– If triple distance only 1/9 signal strength

New


Radio Propagation is Difficult– Signals can be blocked by dense objects– Creates shadow zones with no reception

New

ShadowZone


Radio Propagation is Difficult– Signals are reflected– May arrive at a destination via multiple paths– Signals arriving by different paths can interfere with

one another: called multipath interference– Can be constructive or destructive interference– Very different reception characteristics with in a few

meters or centimeters

New

56Radio Propagation: Waves

Waves

Amplitude(strength)

Wavelength(meters)

Frequency in hertz (Hz)Cycles per Second

One Second7 Cycles

1 Hz = 1 cycle per second

1

4

3

2

57Radio Propagation: Frequency Spectrum

Frequency Spectrum– Frequencies vary (like strings in a harp)– Frequencies measured in hertz (Hz)– Frequency spectrum: all possible frequencies from 0

Hz to infinity

0 Hz

58Frequencies

Metric system– kHz (1,000 Hz) kilohertz; note lower-case k

– MHz (1,000 kHz) megahertz

– GHz (1,000 MHz) gigahertz

– THz (1,000 GHz) terahertz

59Radio Propagation: Service Bands

Service Bands– Divide spectrum into bands for services– A band is a contiguous range of frequencies– FM radio, cellular telephone service bands etc.

0 Hz

Cellular Telephone

FM Radio

AM Radio

ServiceBands

60Radio Propagation: Channels and Bandwidth

Service Bands are Further Divided into Channels– Like television channels– Bandwidth of a channel is highest frequency minus

lowest frequency

0 Hz

Channel 3

Channel 2

Channel 1

ServiceBand

ChannelBandwidth


Example– Highest frequency of a radio channel is 43 kHz– Lowest frequency of the radio channel is 38 kHz– Bandwidth of radio channel is 5 kHz (43-38 kHz)

0 Hz

Channel 3

Channel 2

Channel 1

ServiceBand

ChannelBandwidth


Shannon’s Equation– W is maximum possible (not actual) transmission speed

in a channel– B is bandwidth of the channel: highest frequency minus

lowest frequency– S/N is the signal-to-noise ratio

W = B Log2 (1 + S/N)

63Radio Transmission: Broadband

Speed and Bandwidth– The wider the channel bandwidth (B), the faster the

maximum possible transmission speed (W)– W = B Log2 (1+S/N)

MaximumPossible

Speed

Bandwidth

64Telephony is Narrowband

Bandwidth in Telephone Channels is Narrow– Sounds below about 300 Hz cut off to reduce

equipment hum within telephone system

– Sounds above about 3,400 Hz cut off to reduce the bandwidth needed to send a telephone signal

20 kHz300 Hz 3.4 kHz

3.1 kHzRevised

Discussion


Bandwidth in Telephone Channels is Narrow– Signal is placed within a 4 kHz channel

Gives “guardbands”– Compared to 20 kHz channels, allows 5x number of

signals in radio transmission


3.1 kHz

4 kHz Channel

RevisedDiscussion


Narrow Channels Mean Low Speed– Through Shannon’s equation, maximum possible

transmission speed in each telephone channel is only about 35 kbps.

This is narrowband transmission


3.1 kHz

4 kHz Channel

RevisedDiscussion

67Broadband

Two Uses of the Term “Broadband”

Technically, the signal is transmitted in a single channel AND the bandwidth of the channel is large

– Therefore, maximum possible transmission speed is high

Popularly, if the signal is fast, the system is called “broadband” whether it uses channels at all

Using the Internet from Home: The Physical Layer Chapter 4 Copyright 2001 Prentice Hall Revision 2:...

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