physical layer issues theoretical underpinning –or, bandwidth 101 media characteristics –optical...

Physical Layer Issues

• Theoretical Underpinning– or, Bandwidth 101

• Media Characteristics– Optical Fiber– Coax– Copper Wire (Twisted Pair)– Wireless

• Other Useful Ideas

Signals

• Propagation - {how fast does the signal travel in that media, esp. compared to light?}

• Frequency - {number of oscillations per second of the electromagnetic field of the signal}

• Bandwidth - {the width/size, in Hz, of the signal -- usually defined by where most of the energy is}

• Data Rate - {the number of bits per second. Distinct from, but related to, frequency and bandwidth}

• Baud - {Changes per second in the signal. Limited by bandwidth.}

Freq/BW/DR

Power

Frequency

BW

FREQ

{see Fig 2-1}

1

0

0 1 1 0 0 0 1 0

TTime

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Harmonic number

0.50

0.25

rms

ampl

itude

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Harmonic number

0.50

0.25

rms

ampl

itude

1 harmonic

1

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Harmonic number

0.50

0.25

rms

ampl

itude

2 harmonics

1

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Harmonic number

0.50

0.25

rms

ampl

itude

4 harmonics

1

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Harmonic number

0.50

0.25

rms

ampl

itude

8 harmonics

1

0

Maximum Data Rates• Nyquist:

– DataRate <= 2*BandWidth * log2 V

where ‘V’ is the number of values which are encoded into

the signal. In the On/Off, 0/1 world, V = 2. Your highspeed

modem has V = 16.

• Shannon:

– The real world is noisy, so Nyquist was an optimist.

• Marti:

– Complexity costs money and adds fragility.

So be choosy.

DR ~ 2 * BW {Max by Theory}

DR ~ 1/2 * BW {Practical}

<- In an On/Off world (V = 2)

Physical Effects

Bandwidth Limits - {Signals consist of many (infinite) different sine waves, not all of which can be carried by the media}

Dispersion - {Particularly for multimode fiber, different parts of thesignal may move at different speeds, thus changing the shape ofthe signal at the receiver}

Jitter - {Imperfect clock synchronization along the transmission path}

Noise - {Unwanted, external energy that may corrupt the signal}

Media

• Optical Fiber {pg 87} Multimode Single Mode

• Coax {pg 84,85} Broadband Baseband

• Twisted Pair {pg 83} Shielded Unshielded

Cost and PerformanceMedia Types:

UTP

Coax Baseband Broadband

Fiber Multimode Single Mode

IncreasingBandwidth

IncreasingCost

But remember, cost includes --material--LABOR--electronics

Biggest part of installation cost

DistancesMedia Types:

UTP

Coax Baseband Broadband

Fiber Multimode Single Mode

Typically 100m +/-

200m-500mup to 40km

depends on power budget;can be 100s of km*

* Most LANs use 2km between devices

Fiber Facts

CoreCladding

Protective Coating

Core Cladding50 125 Multimode (microns)62.5 125 " "8 to 10 n/a Singlemode

Fiber Facts, cont.

"photons"

Modes == Different paths thru core

Since the photons travel at the same speed, but for different distances,the energy is spread out, or dispersed, at the receiver

Fiber is specified as XX Mhz-km. So a specification of 800Mhz-kmmeans you could have a bandwidth of 400Mhz over a 2km distanceor 1.6Ghz over a 0.5km distance.

Dispersion has two components: modal and material

Traditional Baseband

Coax

Terminator

Transceiver

Transceiver Cable

Host

CATV Systems

Headend

Amplifier

Splitter

NetworkInterfaceUnit

Translator

"Forward" Signal"Return" Signal

Twisted Pair

• Just copper wire where each two wires (“pairs”) have been twisted around each other in the cable. {Phone wire}

• Rejects common mode noise

• Minimizes antenna characteristics

• Shielded or Unshielded refers to a ground sheath around the whole cable.

• Cat 3 vs Cat 4 vs Cat 5

Design

Backbone

Distribution

Daisy Chain Home Run Bus

Riser Systems

Wireless Overview

• Radio– Mobile– Cellular

• Microwave

• Satellite

• Infrared/Laser

Modulation

ASK - Amplitude Shift Keying {varying signal strength}

FSK - Frequency Shift Keying {varying signal frequency}

PSK - Phase Shift Keying {don't ask!}

{NB the above three methods are usually applied to signal carriers}

PCM/PWM - Pulse Code Modulation/ Pulse Width Modulation{good for fiber}

Others

"Modification of a transmitted signal to encode information (bits)"

Basic Network Concepts

Circuit switching - {basic TELCO service. Guaranteed responsebecause resources are guaranteed. Inefficient for some applications}Virtual-circuit packet-switching - {divide the info into packets tomultiplex}Datagram packet-switching - {like the US Mail....}

Connectionless vs Connection-oriented{At the Link layer, do we do acknowledgements? At the network layer,doall the packets have to follow the same route?}

Multiplexing - {single media, multiple independent 'circuits'}{putting multiple 'sessions' on a single media}

Switching

• Circuit Switching– Guaranteed resource– No size limit on information sent

• Packet Switching - Divides the information into packets; restricts sizes; also sharing of resources

– Virtual Circuit // Connect-oriented– Datagram // Connection-less

Multiplexing

TDM - {time division multiplexing} {low overhead, inefficient}

FDM - {frequency division multiplexing}

STDM - { statistical time division multiplexing} {some overhead, more efficient, may FAIL}

Multiplexing Examples

TDM

STDM

ABCD

BADCBADCBADCBAD

2400 2400

2400 2400

9600

4800

ABCD

BACACBDCBDBABBD

FDM

Multiplexing w/ Packets

Like STDM, except NO "ROUND ROBIN"

Framing & Synchronization

Synchronous - Sender and receiver somehow share a common clock.good for longer runs of data; more efficient but requires the clock signalsomehow be sent along with the data

Asynchronous - Sender and receiver use different clocks so data runshave to be shorter. Doesn't require the extra clock signal

Synchronous vs Asynchronous - Framing & clocksIsochronous vs Aperiodic - Characteristics of traffic {beware ofconfusion as each writer may mix terms}

{Here synchronization refers to the sender's and receiver's clocks}{Frames are packets added signal needed to transmit them on Physical Layer}

Specific Framing

Bit Stuffing - Used to ensure special framing and/or control characters are not sent in the data. A problem because the 'clock' isusually continuous but data may not be there, so we have to know whenthe line is idle and when a frame starts {frame delimiter}

Manchester Encoding - Example of combining clock with data toform a single signal -- no separate line is required. It does require twicethe bandwidth of the original signal

ManchesterEncoding

Ensures for each bit there is a clock transition. Data values (0 or 1)are encoded by positive or negative clock transitions in the middle ofthe bit time. Transitions are made at bit edges if needed so that thecorrect transition can be made in the middle of the bit.

TELCO Architecture

Users

Users

Users

Users

Trunks

CO

CO

CO

CO

TELCO Trunking

Older, Analog World: Frequency Multiplexing

Current World: Digitizing & Time Multiplexing

ISDNIntegrated Services Digital Network

"Think of defining ISDN by looking at the two pairs of words"

BRI • 2B+D Basic Rate Interface B = 64kbps each D = 16kbp for TELCO signaling

PRI • 23B+D

ISDN Nets

"User"

GTE

AT&T

Definition points

BISDN

•Not broadband in the sense of FDM, but rather:

“So fast, we can carry a lot of stuff."

RF LANsEthernet (or slower) over microwave

Pushed by Apple and Motorola

PCS

An umbrella for several new digital cellular services.