Helsinki University of TechnologyHelsinki University of TechnologyS-72.333 Postgraduate Seminar on Radio Communications

Multiple Access MethodsMultiple Access Methods

Er Liu

liuer@cc.hut.filiuer@cc.hut.fiCommunications LaboratoryCommunications Laboratory

16.11.200416.11.2004

Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu (liuer@cc.hut.filiuer@cc.hut.fi))

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Content of presentationContent of presentationProtocol ClassificationConflict-Free Access Protocols

Static AllocationTDMAFDMACDMA

Dynamic AllocationPollingToken passing

Contention Access ProtocolStatic Resolution

AlohaCarrier sensing protocols

Dynamic ResolutionBinary Tree

Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu (liuer@cc.hut.filiuer@cc.hut.fi))

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ProtocolProtocol ClassificationClassification

Multiple Access Protocols

Conflict FreeContention

ALOHAALOHA

Carrier sensingCarrier sensingBinary treeBinary tree

Dynamic Allocation

Token passingToken passingPollingPolling

Static Allocation

FDMAFDMATDMATDMA

CDMACDMA

Static Resolution Dynamic Resolution

Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu (liuer@cc.hut.filiuer@cc.hut.fi))

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ConflictConflict--Free Access ProtocolsFree Access Protocols

Static Allocation = Fixed Assignment:I.e. fixed allocated channel resourceResource can be frequency, or time, or bothPredetermined basis to a single user

Basic access methods:FDMA – Frequency-Division Multiple AccessTDMA – Time-division Multiple AccessCDMA – Code-Division Multiple access

Some other formats:Combination of the basic access methodsImplemented with various multi-user access algorithm

Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu (liuer@cc.hut.filiuer@cc.hut.fi))

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FrequencyFrequency--Division Multiple AccessDivision Multiple AccessBuilt upon FrequencyFrequency--division multiplexingdivision multiplexing schemeIt is the simplest and oldest form of multiplexingA fixed subchannel is assigned to a user terminal and is retained until released by the userAt receiver, the user terminal filters the designated channel out if the composite signal

1 2 3

1, 2, 3

Guard band

t

f

Currently used inCellular mobile telephoneVHF & UHF land-mobile radio systemSatellite networks

Characteristics:Efficient when information is steady flowInefficient when data are sporadic

Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu (liuer@cc.hut.filiuer@cc.hut.fi))

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FDMA FDMA PerformancePerformanceSystem model:

M independent queuesM/G/1 queueing system

Throughput-Delay

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TimeTime--Division Multiple AccessDivision Multiple AccessBuilt upon TimeTime--division multiplexingdivision multiplexing transmission format

Deterministic allocation of time interval – timetime--slotsslots

Time slots are organized into framesT1 channel : 1.544 Mbits/s

Multiplexing 24 PCM encoded voice channel

Each channel is 64 kbits/sEach channel sampled at an 8-kHz rateEach sample is encoded into 8 bits

Used in new digital cellular networkEurope (GSM)Japan (JDC)America (IS-54)

1 2 3

Guard timet

1, 2, 3

fw

Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu (liuer@cc.hut.filiuer@cc.hut.fi))

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TDMA PerformanceTDMA PerformanceSystem model:

M independent queuesM/D/1 queueing system

Throughput-Delay

Comparison between FDMA and CDMA

Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu (liuer@cc.hut.filiuer@cc.hut.fi))

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Hybrid of TDMA and FDMAHybrid of TDMA and FDMA

TDMA and FDMA can be implemented together to get optimized function and performance

For exampleTDMA/FDMA

TDMA/TDD/FDMA

F1 F2 F3 F4

Amplitude

Frequency

5

Tx

23

4

67

8

1

Tx

Tx

Tx

Rx

Rx

Rx

Rx

5

Tx

23

4

67

8

1

Tx

Tx

Tx

Rx

Rx

Rx

Rx

5

Tx

23

4

67

8

1

Tx

Tx

Tx

Rx

Rx

Rx

Rx

5

Tx

23

4

67

8

1

Tx

Tx

Tx

Rx

Rx

Rx

Rx

F1 F2 F1' F2'

Amplitude

Frequency

5

Tx

23

4

67

8

5

Rx Rx1

Tx

23

4

67

8

23

4

67

8

23

4

67

8

5 5

1 1 1

Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu (liuer@cc.hut.filiuer@cc.hut.fi))

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CodeCode--Division Multiple AccessDivision Multiple Access

Hybrids combination of FDMA and TDMACharacteristics

Multiple usersSimultaneously operatingEntire bandwidth of time-frequency domainSeparated by distinct user-signal codes (Spread spectrum)

Two common CDMADirect sequence (DS) CDMAFrequency Hopping (FH) CDMA

Spreading codeML (Maximum length) codeGold codeWalsh-Hadamard code

Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu (liuer@cc.hut.filiuer@cc.hut.fi))

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CDMA CDMA ------ SpreadingSpreading

Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu (liuer@cc.hut.filiuer@cc.hut.fi))

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CDMA CDMA ------ DeDe--spreadingspreading

Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu (liuer@cc.hut.filiuer@cc.hut.fi))

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DynamicDynamic AllocationAllocation ProtocolsProtocols

Dynamic Allocation = On demandRealized via reservation schemesBasic access methods:

PollingToken passing

OthersMSAP

MiniSlotted Alternative PriorityBRAM

Broadcast Recognition Access Method

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Polling TechniquesPolling Techniques (1)(1)

Centralized controlOne station is equipped as a controllerPeriodically polling all the other stations

ClassificationHub pollingRoll polling

Polling procedureHub polling

From furthest stationPolled station starts sending if it has something to transmitIf not, a negative response is detected by the controllerThe polled station transmits the poll mesage to its neighbor in upstream(control)Control message finally is regained by the controller

Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu (liuer@cc.hut.filiuer@cc.hut.fi))

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Polling TechniquesPolling Techniques (2)(2)Roll polling

Controller keeps a polling list, giving the order in which the terminalsare polledPolled station starts sending if it has something to transmitIf not, a negitve reply is detected by the controllerController then polls the next terminals in the sequenceInitial exchange of short messages required(between a station and the controler )

Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu (liuer@cc.hut.filiuer@cc.hut.fi))

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CharactertisticsCharactertistics and and performanceperformance

Polling protocols are efficient in systemsPropagtion delay is smallOverhead is lowNumber of stations shouldn’t be large (proportional to overhead)

Polling protocols are inefficientLightly loadedPart of stations have data to transmitSubdivide stations into subsets (variations)

Hub polling overhead is much smaller than that of roll pollingApplications:

Widely used in dedicated telephone networks for data communicationsGenerally not been adopted in existing mobile data network or WLAN

Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu (liuer@cc.hut.filiuer@cc.hut.fi))

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TokenToken PassingPassing ProtocolProtocol (1)(1)Two logical topologies

BusRing

Token ring is originally developed by IBM, specified in IEEE 802.5Token ring protocol operation

Networks move a small frame, called a token, around the networkPossession of the token grants the right to transmit.If the node, receiving the token, has no information to send, it passes the token to the next station.If the node, possessing the token does have information to transmit

Seize the tokenAlter 1 bit of the tokenAppend the information to be transmitted, and send to the next station in the ring

The intended destination station flips the recognized address and frame-copied bits in frame status field in the frame, and sends the modified frameback out to the ring

Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu (liuer@cc.hut.filiuer@cc.hut.fi))

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TokenToken PassingPassing ProtocolProtocol (2)(2)When information reaches the sending station again, it examines and removed the frame from the ringThe source station then transmits a new token

Phycally ”star” topology, logically ”ring” topology

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TokenToken Ring Ring CharacteristicsCharacteristicsToken passing netowrks are deterministic, so the maximum propagationtime is possibly calculated, more predictable than Ethernet

Priority schemes can be deployed to improve the efficiencyUser-designated, high priority station can use network more frequently

PriorityStation >= Prioritytoken can capture the token

Several mechanisms for detecting and compensating for network fault

One station is selected as activeactive monitormonitor

It provides centralized source of timing information for other stations

Ring-maintenance functionRemoval of continously circlating framesGeneration of the new token

No collisions occur, contention-free!

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ContentionContention ProtocolsProtocols

No guarantee to be successfulLarge users ->Contention-free schemes are impracticalResolution schemes are needed

Static resolutionProtocol actual behavior is not influenced by the dynamics of the systemExamples:

Aloha familyCSMA family

Dynamic resolutionTracking and taking the advantage of the system changesExample:

Binary-Tree CRP (collision Resolution Protocol)

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Pure Pure AlohaAloha

It is the simplest contention protocolWhenever packet needs transmission:

Send without waiting

If collision occurs, then wait for a random time and resend, untilsuccessful

Highly inefficient at large loads. Maximum utilization of 18% at a mean load of 0.5

Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu (liuer@cc.hut.filiuer@cc.hut.fi))

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SlottedSlotted AlohaAloha

C C CE E ES S S

3 3 3

1

2 2 2

1 1 1Node 1

Node 2

Node 3

Time is divided into equal size slots (= packet Tx time)Node with new arriving packet: transmit at beginning of next slotIf collision: retransmit packet in future slots with probability p, until successful.Maximum utilization of 36% at a mean load of 1 transmission/slot

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NonpresistentNonpresistent (NP) CSMA(NP) CSMA

Impolite -> ”listen before talk”

Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu (liuer@cc.hut.filiuer@cc.hut.fi))

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11--persistent CSMApersistent CSMA

Transmit as soon as when itdetect the channel is idle

Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu (liuer@cc.hut.filiuer@cc.hut.fi))

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SlottedSlotted CarrierCarrier SensingSensing ProtocolsProtocols

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CSMA/CDCSMA/CDCarrier sense multiple access with collision detectionSame as CSMA except a collision is detected

Ternimate transmission immediatelyTime slot and non/1-persistent concepts can also be applied

Slotted nonpersistent CSMA/CD Slotted 1-persistent CSMA/CD

Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu (liuer@cc.hut.filiuer@cc.hut.fi))

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CollisionCollision ResolutionResolution ProtocolProtocol (CRP)(CRP)

Concentrate on Resolving Collisions as soon as they occurs

Exploit feedback information to control the retransmission

System model:Similar to slotted Aloha

Typical protocol:Binary-Tree CRP

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BinaryBinary--TreeTree ProtocolProtocol (CRP)(CRP)

Binary-Tree CRP

Enhanced Binary-Tree CRP

• When collision occurs, the users are divided in two subsets

• Divided method can be likeflipping coin

• One subset should wait untilthe other set has finishedtransmission

• Within one subset, if collisionoccurs again, performace in the similar way again

• Enhanced method:”flipping beforehand”

Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu (liuer@cc.hut.filiuer@cc.hut.fi))

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ConclusionConclusion

Multiple Access Protocols

Conflict FreeContention

ALOHAALOHA

Carrier sensingCarrier sensingBinary treeBinary tree

Dynamic Allocation

Token passingToken passingPollingPolling

Static Allocation

FDMAFDMATDMATDMA

CDMACDMA

Static Resolution Dynamic Resolution

Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu (liuer@cc.hut.filiuer@cc.hut.fi))

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ReferenceReference[1] Raphael Rom, Moshe Sidi, “ Multiple Access protocols: Performance

and Analysis”www-comnet.technion.ac.il/rom/PDF/MAP.pdf

[2] Simon Haykin, Michael Moher, “Modern Wireless Communications”ISBN 0-13-124697-6, Prentice Hall 2005

Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu (liuer@cc.hut.filiuer@cc.hut.fi))

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HomeworkHomework

Please explain what is bit-map protocol? What’s the advantage and disadvange of this protocol?

What is FDDI, how it works. Please give a brief descriptionon its MAC protocol structure and operation.

Helsinki University of TechnologyHelsinki University of TechnologyS-72.333 Postgraduate Seminar on Radio Communications

AnyAny questionsquestions??

Thanks!