Upload
truongkhanh
View
226
Download
3
Embed Size (px)
Citation preview
Helsinki University of TechnologyHelsinki University of TechnologyS-72.333 Postgraduate Seminar on Radio Communications
Multiple Access MethodsMultiple Access Methods
Er Liu
[email protected]@cc.hut.fiCommunications LaboratoryCommunications Laboratory
16.11.200416.11.2004
Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu ([email protected]@cc.hut.fi))
Page Page 22
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 ([email protected]@cc.hut.fi))
Page Page 33
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 ([email protected]@cc.hut.fi))
Page Page 44
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 ([email protected]@cc.hut.fi))
Page Page 55
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 ([email protected]@cc.hut.fi))
Page Page 66
FDMA FDMA PerformancePerformanceSystem model:
M independent queuesM/G/1 queueing system
Throughput-Delay
Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu ([email protected]@cc.hut.fi))
Page Page 77
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 ([email protected]@cc.hut.fi))
Page Page 88
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 ([email protected]@cc.hut.fi))
Page Page 99
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 ([email protected]@cc.hut.fi))
Page Page 1010
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 ([email protected]@cc.hut.fi))
Page Page 1111
CDMA CDMA ------ SpreadingSpreading
Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu ([email protected]@cc.hut.fi))
Page Page 1212
CDMA CDMA ------ DeDe--spreadingspreading
Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu ([email protected]@cc.hut.fi))
Page Page 1313
DynamicDynamic AllocationAllocation ProtocolsProtocols
Dynamic Allocation = On demandRealized via reservation schemesBasic access methods:
PollingToken passing
OthersMSAP
MiniSlotted Alternative PriorityBRAM
Broadcast Recognition Access Method
Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu ([email protected]@cc.hut.fi))
Page Page 1414
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 ([email protected]@cc.hut.fi))
Page Page 1515
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 ([email protected]@cc.hut.fi))
Page Page 1616
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 ([email protected]@cc.hut.fi))
Page Page 1717
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 ([email protected]@cc.hut.fi))
Page Page 1818
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
Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu ([email protected]@cc.hut.fi))
Page Page 1919
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!
Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu ([email protected]@cc.hut.fi))
Page Page 2020
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)
Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu ([email protected]@cc.hut.fi))
Page Page 2121
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 ([email protected]@cc.hut.fi))
Page Page 2222
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
Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu ([email protected]@cc.hut.fi))
Page Page 2323
NonpresistentNonpresistent (NP) CSMA(NP) CSMA
Impolite -> ”listen before talk”
Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu ([email protected]@cc.hut.fi))
Page Page 2424
11--persistent CSMApersistent CSMA
Transmit as soon as when itdetect the channel is idle
Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu ([email protected]@cc.hut.fi))
Page Page 2525
SlottedSlotted CarrierCarrier SensingSensing ProtocolsProtocols
Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu ([email protected]@cc.hut.fi))
Page Page 2626
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 ([email protected]@cc.hut.fi))
Page Page 2727
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
Multiple Access MethodsMultiple Access Methods Er Liu (Er Liu ([email protected]@cc.hut.fi))
Page Page 2828
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 ([email protected]@cc.hut.fi))
Page Page 2929
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 ([email protected]@cc.hut.fi))
Page Page 3030
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 ([email protected]@cc.hut.fi))
Page Page 3131
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!