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4 Physical LayerOur goals: know what is the

physical layer incharge of

Know how the physicallayer achieves itsduties

Overview:type of media used to transmit

informationconnection of hosts to mediumhow signal is transmittedefficient use of mediaspecification of a connection

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4 Physical Layer

4.1 Introduction4.2 Topologies4.3 Physical Media4.4 Transmission Techniques4.5 Multiplexing4.6 Making Connections

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Introduction

Physical layerDefines the mechanical, electric and functionalcharacteristics of the interconnection to the physicalmedia.

Establishes the interface with the link layer.Specifies: Transmission media & topology Signal details and encoding & decoding Generation & extraction of preambles to

synchronize devices Transmission & reception of bits Connectors and plugs Multiplex and modulation

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4 Physical Layer


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Topologies

Physical arrangement ofstations on mediumPoint to point twostations

such as between tworouters / computersMulti point multiplestations

traditionallymainframe computerand terminals

now typically a localarea network (LAN)

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Topologies

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Topologies

BusAll the hosts are connected to an only channel, the bus,ended in both sides by its characteristic impedance.Any host can send info to bus access algorithm

required to avoid collisions.Sent info is broadcasted in both directions to arrive toall the hosts connected to the bus.Malfunction of a host does not affect the other hosts.

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Topologies

TreeBus generalizationAll the main tree branches begin in the same point, theHeadendFrom the main branches, other branches can arise

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Topologies

RingHosts distributed along a closed loopEach hosts repeats info to the next hostInfo flows only in one directionAlgorithm to insert info is requiredOne of the hosts has to control (monitor) the networkMalfunction of a host breaks the ring. It can be solved

with a double-ring

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Topologies

StarEach hosts is connected to a central switching nodewith a point-to-point link

Central node controls the networkMalfunction of a host does not affect the other hostsNetwork throughput limited by central node congestionCurrently is the most used in the shape of tree-of-stars

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Topologies

Cellular

Wireless communication antennas requiredHosts are distributed in a covering area.Each covering area depends on a central node (accesspoint, repeater...)Malfunction of a hosts does not affect the other hostsNetwork throughput is limited by congestion of centralnode.Common in Wi-Fi networks and mobile telephony

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4 Physical Layer


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Physical MediaPropagate bits between transmitter and receiver pairsthrough the physical path between them.Physical media can be:

Guided (wired) Simplicity point-to-point Energy confined in the medium High installation time Att 10kd, with k= f(f) and d distance

Unguided (wireless) Limited directivity Interferences and multipath Shared medium spectrum regulation Quick establishment Att dn, with d distance and n 2

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Physical Media

Key factors for maximum range and throughput:BWAttenuationDistortion

CostInterference sCrosstalkNumber ofreceivers

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Physical media: overview

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Physical Media: twisted pairA pair is made up of two insulatedcopper wires, isolated between themand surrounded by a protective jacket.The two wires are twisted.They may be...

Unshielded

Shielded with a metallic coat to reduceelectromagnetic interferences it needs morespaces and is more expensive.

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Physical Media: twisted pairUnshielded Twisted Pair (UTP)

It is made up of four pairsof 100 Categories are defined in norm EIA/TIA 568American norm aboutwires is AWG (Ame-rican Wire Gauge)The higher theidentifier number isthe less the diameterof the wire is.

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Physical Media: twisted pairFoiled Twisted Pair (FTP or ScTP)

It is made up of four wire pairssurrounded by a metallic coat to get protected againstelectromagnetic interferences

Shielded Twisted Pair (STP)It is made up of four wire pairssurrounded by a metallic coatto get protected againstelectromagnetic interferencesEach pair is also surrounded by a metallic coatIt is the more robust against interferences

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Physical Media: twisted pair

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Physical Media: twisted pairMaximum attenuation (dB) per 100 m...

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Made up of a cylindrical copper wire and a concentricmetallic coat separated by an isolating material andcovered by a protective jacket.It is bidirectional.Two types are commonly used:

Base band coax of 50 , typical BW 100 MHz Thick (RG-11) 5-10 mm , yellow coat Thin (RG-58) 5 mm , black coat

Broad band coax of 75 , typical BW 400MHz; frequently used in CATV.

Physical Media: coaxial cable

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Made up of a cylindrical guide of silica (core), coveredby another concentric layer of silica (coat) andsurrounded by a protective jacket.Refraction index of core (n1) is higher than that ofcoat (n2) n1>n2, so that fiber is a wave guide thatcarries light pulses (each pulse a bit).High-speed operation: typical point-to-point tx 10-100 GbpsLow error rate: repeaters sp aced far apart; immune toelectromagnetic noise.

Physical Media: fiber optic cable

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If n1 is constant in the whole core diameter stepindex fiber; otherwise graded index fiber.Commonly, two types of fibers are used: Multimodal (65/125 m)

Monomodal (8/125 m)


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Furthermore, different regions of the optical spectrumcan be used to minimize the attenuation: First window 850 nm Second window 1300 nm

Third window 1550 nm


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Physical Media: comparison of cables

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Signal carried in electromagneticspectrum no physical wire.It is bidirectional.Radio link types:

Terrestrial microwave

e.g. up to 45Mbps channels LAN (e.g., Wifi) 11Mbps, 54 Mbps Wide-area (e.g., cellular, 3G) ~ 1 Mbps Satellite k bps to 45Mbps channel (or

multiple smaller channels), 270 ms end-enddelay, geosynchronous versus low altitude

Physical Media: wireless

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Differences from wired link .

decreased signal strength: radio signal attenuates asit propagates through matter (path loss)interference from other sources: standardizedwireless network frequencies (e.g., 2.4 GHz) shared

by other devices (e.g., phone, motors) that interferemultipath propagation: radio signal reflects offobjects ground, arriving ad destination at slightlydifferent timesdispersion of signal: scattering

mobility of users. make communication across

(even a point to point) wirelesslink much more difficult


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Frequency ranges used in wireless communication:

2 GHz to 40 GHz Microwave, highly directional,point to point, satellite30 MHz to 1 GHz Omnidirectional, broadcastradio3 x 10 11 Hz to 2 x 10 14 Hz Infrared, local

Antennas are requires to transmit and receiveinformation

Bandwidth, radiation pattern, antenna'sgain, type of antenna


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Terrestrial MicrowavesUsed for long haul telecommunications and shortpoint-to-point linksRequires fewer repeaters but line of sightUse a parabolic dish to focus a narrow beamonto a receiver antenna1-40 GHz frequenciesHigher frequencies giv e higher data ratesMain source of

loss is attenua-tion given bydistance, rainfalland alsointerference


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Satellite MicrowavesSatellite is relay stationReceives on one frequency, amplifies or repeatssignal and transmits on another frequency

eg. uplink 5.925-6.425 GHz & downlink 3.7-4.2 GHz

Typically requires geo-stationary orbit: height of 35,784 km, spaced at least 3-4 apart

Typical uses: Television

Long distance telephone Private business networks Global positioning


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Broadcast radioRange is 3 kHz to 300 GHzUse broadcast radio, 30MHz - 1GHz, for:FM radio, UHF and VHF televisionOmnidirectionalStill need line of sightSuffers from multipath interferenceReflections from land, water, other objects


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Mobile telephonyPhysical Media: wireless

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Broadband wireless systemsPhysical Media: wireless

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SNR: signal-to-noise ratiolarger SNR easier toextract signal from noise (agood thing)

SNR versus BER tradeoffs given physical layer: increase power -> increaseSNR->decrease BER

given SNR: choose physicallayer that meets BERrequirement, giving highestthruput

SNR may change withmobility: dynamically adaptphysical layer (modulation

technique, rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

B E R

10-1

10 -2

10 -3

10 -5

10 -6

10 -7

10 -4


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Line-of-sight transmissionFree space loss loss of signal with distanceAtmospheric Absorption from water vapour andoxygen absorptionMultipath multiple interfering signals from

reflectionsRefraction bending signal away from receiver


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Multiple wireless senders and receivers createadditional problems (beyond multiple access):

AB

C

Hidden terminal problem

B, A hear each otherB, C hear each otherA, C can not hear each other

means A, C unaware of theirinterference at B

A B C

As signalstrength

space

Cs signalstrength

Signal attenuation:B, A hear each otherB, C hear each otherA, C can not hear each otherinterfering at B


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PhysicalMedia:

wireless

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Physical Media: selection criteria

Cost Different types of costs Initial cost what does a particular type of

medium cost to purchase? To install? Maintenance / support cost

ROI (return on investment) if one medium ischeaper to purchase and install but is not costeffective, where are the savings?

Expandability and distance Certain media lend themselves more easily to

expansion Dont forget right-of-way issue

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Physical Media: selection criteriaSpeed

Two different forms of speed: Propagation time to send first bit across medium

This speed depends upon the medium Airwaves and fiber are speed of light Copper wire is two thirds the speed of light

Data transfer time to transmit rest of message This speed is measured in bits per second

Environment Many types of environments are hazardous to certainmedia

Security If data must be secure during transmission, it is

important that the medium not be easy to tap

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4 Physical Layer


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Transmission techniquesLine-coding and modulation: techniques used to adaptdata, analogical or digital, to the transmission channel

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Transmission techniques: basebandEncoded signal is introduced directly to thetransmission medium.The whole bandwidth of the medium is used only onesignal can be transmitted at a time.Adequate to short-distance transmissions.Interface devices and repeaters are very cheap.Non-adequate for environments with high noise level orelectromagnetic interferences.

Line-codes are used to encode signals.

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Transmission techniques: broadbandThe signal is modulated over an analogical carrier several carrier can be transmitted at a time.Transmission is unidirectional two channels required,one for sending and another for receiving purposes.Adequate for interconnection of devices with a highperformance index.Interface devices are expensive.Two configurations are used

One cable (split) two carriers, the low freq.

is the uplink and the high freq. is the downlink;head-end must apply a frequency conversion.

Two cables (dual) two connections, one percable, using the same carrier frequency.

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Transmission techniques: broadband- Split cable - Dual cable

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Transmission techniques:encoding vs. modulation

h

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Transmission techniques:modulation and quatization

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Transmission techniq ues: line-codesDigital data Digital signals

Assessment:Max speedMin BER

Limited spectrumNo DC / HFSelf-synchron.Immunity to

noise & interfError-detectionLow costLow complex.

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Transmission techniques: line-codesNRZI (Non Return to Zero Invertive)

'1' inverts polarity and 0 maintains polarityUsed in USB, but it inserts an additional '0' bit after 6consecutive '1' bits

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Transmission techniques: line-codesAMI (Alternate Mark Inversion)

'0' is encoded as 0 V as in unipolar encoding'1' is encoded alternately as +V and -VLong sequences of zero bits result in no transitions and

a loss of synchronization p ulse-stuffing

Pseudoternary code isdual version of AMIImprovements made with...

B8ZS (USA-T1) B6ZS (USA-T2)

B3ZS (USA-T3) HDB3 (E-carrier)

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Transmission techniques: line-codesScrambling

Replace sequences thatproduce constant voltageThese filling sequencesmust produce enoughtransitions to sync, berecognized by receiver &replaced with original, besame length as original

Design goals: no DC compo-nent, no long sequences ofnull line signal, no reduc-tion in data rate, give

error detection capability

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Transmission techniques: line-codesManchester

'1' is represented by high level between 0-T/2 and bylow level between T/2-T '0' is represented by low level between 0-T/2 and byhigh level between T/2-T Idle state is represented by a continuous high levelIn any tx there is always zero-mean level and midtransitions help extracting synchronism of signal

Used in IEEE-802.3 (ethernet) and RFID

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Transmission techniques: line-codesDifferential Manchester

There is always a transition at t=T/2 for '1' and '0'valuesThere is also another transition at t=0 for '0' value

There is not transition at t=0 for '1' valueUsed in IEEE-802.5 (token ring)

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Transmission techniques: line-codes4B/5B

Maps the 16 possible values of 4-bit groups (4B) to asubset of 16 codes of 5-bit groups (5B) within the 32possible codesEach group of 5 bits is chosen so that there will be atleast two transitions per groupThe unused characters can actually be used to detecterrors in the data stream or to send control sequencesUsed in 'Fiber distributed data interface' (FDDI) andin IEEE-802.3u (100BASE-TX)

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Transmission techniques: line-codes4B/5B

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Transmission techniques: line-codesMLT-3 (Multi-Level Threshold-3)

It alternates 3 voltage levels cyclicallyEach bit is codified with a presenceor absence of transition

'1' changes voltage level tothe next level

'0' maintains voltage level

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Transmission techniques: line-codes8B/6T

It is a ternary code (+V, 0, -V)Data to transmit is grouped in 8-bit blocksEach 8-bit block is mapped to a group of 6 ternarysymbolsEach ternary group is transmitted round-robin over 3different channelsUsed in Ethernet 100-Base-T4

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Transmission techniques: line-codesPortion ofthe 8B/6T encodingtable

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4 Physical Layer


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MultiplexingUnder simplest conditions, medium can carry only onesignal at any moment in timeFor multiple signals to share a medium, medium mustsomehow be divided, giving each signal a portion of thetotal bandwidthThe multiplexor is attached to a high-speed comm. lineA corresponding multiplexor, or demultiplexor, is onthe end of the high-speed line and separates themultiplexed signalsCurrent techniques include:

Frequency/Wavelength division multiplexing Time division multiplexing

Code division multiplexing

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MultiplexingFDM Frequency Division Multiplexing

Assignment of non-overlapping frequency ranges toeach user or signal on a mediumThus, all signals are transmitted at the same time,each using different frequenciesA multiplexor accepts inputs and assigns frequencies toeach device

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MultiplexingFDM Frequency Division Multiplexing

Analog signaling is used in older systems; discreteanalog signals in more recent systemsBroadcast radio and television, cable television, andcellular telephone systems use frequency division mux.This technique is the oldest multiplexing techniqueSince it involves acertain level of analogsignaling, it may besusceptible to noise

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MultiplexingWDM Wavelength Division Multiplexing

Wavelength division multiplexing multiplexes multipledata streams onto a single fiber-optic lineDifferent wavelength lasers (called lambdas) transmitthe multiple signalsEach signal carried on the fiber can be transmitted ata different rate fromthe other signalsDense wavelength division multiplexing combinesmany (30, 40, 50 ormore) onto one fiber

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MultiplexingTDM Time Division Multiplexing

Sharing of the signal is accomplished by dividingavailable transmission time on a medium among usersDigital signaling is used exclusivelyTime division multiplexing comes in two basic forms:

Synchronous time division multiplexing Statistical time division multip lexing

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MultiplexingSyTDM Synchronous Time Division Multiplexing

The original time division multiplexingThe multiplexor accepts input from attached devices ina round-robin fashion and transmits the data in a never-ending patternT-1 and SONET telephone systems are commonexamples of synchronous time division multiplexing

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MultiplexingSyTDM Synchronous Time

Division MultiplexingIf one device generatesdata at faster rate thanother devices, then themultiplexor must either sample the incoming datastream from that device more often than to theothers, or buffer the faster incoming streamSo that the receiver may stay synchronized with the

incoming data stream, the transmitting multiplexor caninsert alternating 1s and 0s into the data stream

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If a device has nothing to transmit, the multiplexormust still insert something into the multiplexed stream

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The T-1 multiplexor stream is a continuous series offramesNote how each frame contains the data (one byte) forpotentially 24 voice-grade telephone lines, plus onesync bitIt is possible to combine all 24 channels into onechannel for a total of 1.544 Mbps

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Similar to T-1, SONET incorporates a continuousseries of framesSONET is used for high-speed data transmissionTelephone companies have traditionally used a lot ofSONET but this may be giving way to other high-speedtransmission servicesSDH is the European equivalent to SONET

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MultiplexingStTDM Statistical Time Division Multiplexing

A statistical multiplexor transmits the data fromactive workstations onlyIf a workstation is not active, no space is wasted in themultiplexed streamA statistical multi-plexor accepts theincoming datastreams andcreates a framecontaining the datato be transmitted

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MultiplexingStTDM Statistical Time Division Multiplexing

To identify each piece of data, an address is included

If the data is of variable size, a length is also included

More precisely, thetransmitted framecontains a collection ofdata groups

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MultiplexingCDM Code Division Multiplexing

Also known as code division multiple access (CDMA)An advanced technique that allows multiple devices totransmit on the same frequencies at the same timeEach mobile device is assigned a unique 64-bit codeTo send a binary 1, a mobile device transmits the codeTo send a binary 0, a mobile device transmits theinverse of the code

To send nothing, a mobile device transmits zerosReceiver gets summed signal, multiplies it by receivercode, adds up the resulting values

Interprets as a binary 1 if sum is near +64 Interprets as a binary 0 if sum is near -64

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Multiplexing: comparison

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4 Physical Layer

4.1 Introduction4.2 Topologies4.3 Physical Media

4.4 Transmission Techniques4.5 Multiplexing4.6 Making Connections

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Making connections

Classify data exchange as: Simplex (one-way)

just one-way transmission available Half duplex (two-way alternate)

only one station may transmit at a time requires one data path

Full duplex (two-way simultaneous)

simultaneous transmission and receptionbetween two stations requires two data paths separate media or frequencies used for

each direction or echo cancelling

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Making connectionsTiming problems require a mechanism to synchronizethe transmitter and receiver:

receiver samples stream at bit intervals if clocks not aligned and drifting will sample at

wrong time after sufficient bits are sentThree solutions to synchronizing clocks:

asynchronous transmission synchronous transmission

isochronous transmission

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Making connectionsAsynchronous Transmission

A type of connection defined at the data link layerTo transmit data from Tx to Rx, an asynchronousconnection creates a one-character package a frameAdded to the front of the frame is a start bit, while astop bit is added to the end of the frameOptional parity bit can be added to detect errorsIt is simpleand cheapOverhead of 2 or3 bits per char

Good for data with large gaps

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Making connectionsAsynchronous

TransmissionAsynchronous connectionsmaintainsynchronizationby using smallframes with aleading start

bit

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Making connectionsSynchronous Transmission

A synchronous connection creates a large frame thatconsists of header and trailer flags (to indicate start &end of block), control information, optional addressinformation, error detection code, and dataA synchr. connection is more elaborate but transfersdata in a more efficient manner (less overhead)Clocks must be synchronized

can use separate clock line or embed clock signal in data

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Making connectionsIsochronous Transmission

A third type of connection defined at the data linklayer used to support real-time applicationsData must be delivered at just the right speed (real-time) not too fast and not too slowTypically an isochronous connection must allocateresources on both ends to maintain real-timeUSB and Firewire can both support isochronous

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Making connectionsHardware interface

The process of providing all the proper interconnec-tions between a computer and a peripheral or link iscalled interfacingConnecting a device such as a modem (or DCE - datacircuit-terminating equipment or data communicatingequipment) to a computer (or DTE - data terminalequipment).The connections between the DTE and DCE are theinterchange circuits.

k

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Making connectionsHardware interface

There are four possible components to an interface std: Electrical component: deals with voltages, line

capacitance, and other electrical characteristics Mechanical component: deals with items such as

the connector or plug description Functional component: describes the function of

each pin or circuit that is used in a particularinterface

Procedural component: describes how theparticular circuits are used to perform anoperation

k

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Making connectionsHardware interface: USB

The USB (Universal Serial Bus) interface is a modernstandard for interconnecting a wide range of peripheraldevices to computersSupports plug & play and can daisy-chain multiple devicesUSB 1.0 12 Mbps; 2.0 480 Mbps; 3.0 4.8 Gbps

M ki i

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Making connectionsHardware interface: USB

The USB interface defines all four components The electrical component defines two wires

VBUS and Ground to carry a 5-volt signal, whilethe D+ and D- wires carry the data and signalinginformation

The mechanical component precisely defines thesize of four different connectors and uses onlyfour wires (the metal shell counts as one moreconnector)

The functional and procedural components arefairly complex but are based on the polled bus

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