A Multichain Backoff Mechanism A Multichain Backoff Mechanism for IEEE 802.11 WLANsfor IEEE 802.11 WLANs

Alkesh Patel & Hemant PatelAlkesh Patel & Hemant PatelECE 695 – Leading DiscussionECE 695 – Leading Discussion

By : Shiang- Rung Ye and Yu-Chee TsengBy : Shiang- Rung Ye and Yu-Chee Tseng

BackgroundBackground

WLAN is emerging as a promising technology. WLAN is emerging as a promising technology.

MAC plays an important role on efficient and fair MAC plays an important role on efficient and fair use of the wireless medium. use of the wireless medium.

Multiple Access Scheme (CSMA) require station Multiple Access Scheme (CSMA) require station to sense carries on the wireless channel before to sense carries on the wireless channel before transmittingtransmitting

Related WorkRelated Work

MILD increases the contention window by 1.5 times MILD increases the contention window by 1.5 times when collision occurs & decreases the contention when collision occurs & decreases the contention window by 1 when transmission succeedwindow by 1 when transmission succeed

In high traffic load this increases collusion probability In high traffic load this increases collusion probability and decreases throughputand decreases throughput

DCF of IEEE 802.11 is a variant of persistent CSMADCF of IEEE 802.11 is a variant of persistent CSMA If medium is busy, transmission defer until the medium If medium is busy, transmission defer until the medium

becomes idlebecomes idle Physical Carrier Sense Physical Carrier Sense Physical layer Physical layer Virtual Carriers Sense Virtual Carriers Sense MAC MAC

Concept/IdeaConcept/Idea

MCB – Multi Chain BackoffMCB – Multi Chain Backoff

Enables station to adapt to different congestion levelEnables station to adapt to different congestion level

No restriction on number of contending stationsNo restriction on number of contending stations

High throughput and fair channel accessHigh throughput and fair channel access

MCB AlgorithmMCB Algorithm

During back off period station shall detect any During back off period station shall detect any collision event by other stationcollision event by other station

Collision flag fCollision flag fcolcol is used to record whether frame is used to record whether frame collision occurs on the wireless channelcollision occurs on the wireless channel ffcolcol set to 1 if a station itself experiences a collision or set to 1 if a station itself experiences a collision or

medium is busy – Collision/Transmission occurring medium is busy – Collision/Transmission occurring with other stationwith other station

ffcolcol set to 0 after successful transmission. Backoff set to 0 after successful transmission. Backoff counter reaches 0, the station transmit data.counter reaches 0, the station transmit data.

MCB AlgorithmMCB Algorithm

Figure 1: The transition diagram of MCB.

The j- th backoff stage of chain i is denoted by (i, j) in the figure

MCB AlgorithmMCB Algorithm

wwii: The minimum contention window of chain : The minimum contention window of chain ii

mmii: The maximum back off stage of chain : The maximum back off stage of chain ii

uuii: The transition probability from chain : The transition probability from chain i i to chain to chain i+1i+1

vvii: The transition probability from chain: The transition probability from chain i i to chain to chain i -1 i -1

Performance AnalysisPerformance Analysis

Saturation Throughput SSaturation Throughput S (i, j, k) The station is in stage j of chain i & has a (i, j, k) The station is in stage j of chain i & has a

backoff value k. backoff value k. T T Probability that station will transmit in a randomly Probability that station will transmit in a randomly

chosen backoff slotchosen backoff slot Xi Xi Probability that a station will detect at least one Probability that a station will detect at least one

collision during backoff periodcollision during backoff period

Performance AnalysisPerformance AnalysisExpressing collision probability can be expressed by Expressing collision probability can be expressed by TT

Now the saturation throughput can be obtained byNow the saturation throughput can be obtained by

PPtrtr Probability that transmission occurs randomly Probability that transmission occurs randomly PPss Probability that transmission succeeds in a backoff Probability that transmission succeeds in a backoff

slotslot TTss Time require for frame exchange Time require for frame exchange TTcc Length of a colliding duration Length of a colliding duration

Performance EvaluationPerformance Evaluation

Performance opposed to MILD, DCF, GDCF, EIEDPerformance opposed to MILD, DCF, GDCF, EIED

Comparison of saturation throughput & fairness indexComparison of saturation throughput & fairness index

FI is bounded in the interval [0,1]FI is bounded in the interval [0,1]

Algorithm is fair as its FI is close to 1Algorithm is fair as its FI is close to 1

Optimal value of u and v when they are the sameOptimal value of u and v when they are the same

Fig 3: The optimal u and v with frame size 1024Fig 3: The optimal u and v with frame size 1024

Fig 4: The ratio of optimal u and v under different n and cFig 4: The ratio of optimal u and v under different n and c

Fig 5: Saturation throughput under different u and v with a frame size of 1024 bytesFig 5: Saturation throughput under different u and v with a frame size of 1024 bytes

Fig 6: Saturation throughput under different u and v with a frame size of 128 bytesFig 6: Saturation throughput under different u and v with a frame size of 128 bytes

Fig 7: Saturation throughput versus frame sizesFig 7: Saturation throughput versus frame sizes

Fig 8: Throughput of MCB with u and v which are chosen for n=6 and n=46Fig 8: Throughput of MCB with u and v which are chosen for n=6 and n=46

Fig 9: Fairness IndexFig 9: Fairness Index

Fig 10: Saturation throughput of MCB and GDCF with frame size 128 bytesFig 10: Saturation throughput of MCB and GDCF with frame size 128 bytes

Fig 11: Saturation throughput of MCB and GDCF with frame size 1024 bytesFig 11: Saturation throughput of MCB and GDCF with frame size 1024 bytes

Fig 12: Fairness index of MCB and GDCF with frame size 1024 bytesFig 12: Fairness index of MCB and GDCF with frame size 1024 bytes

Fig 13: Saturation throughput of MCB, IEEE 802.11, and MILDFig 13: Saturation throughput of MCB, IEEE 802.11, and MILD

Fig 14: Saturation throughput of MCB and EIED(x, y) with fixed y=2Fig 14: Saturation throughput of MCB and EIED(x, y) with fixed y=2

Fig 15: Throughput of MCB and EIED(x, y) with fixed x =2Fig 15: Throughput of MCB and EIED(x, y) with fixed x =2

Fig: 14

Fig: 15

Fig 15: Throughput of MCB and EIED(x, y) with fixed x=2Fig 15: Throughput of MCB and EIED(x, y) with fixed x=2

Fig 16: Fairness index of MCB and EIED (x, y) with fixed x = 2Fig 16: Fairness index of MCB and EIED (x, y) with fixed x = 2

ConclusionConclusion

MCB algorithm explores the possibility of using MCB algorithm explores the possibility of using multiple backoff chainsmultiple backoff chains

Considering collision event offersConsidering collision event offers To choose a proper chain for transmissionTo choose a proper chain for transmission Capability of switching to different backoff chainCapability of switching to different backoff chain High throughput then any existing algorithmsHigh throughput then any existing algorithms Fair access to the wireless channelFair access to the wireless channel

How to apply Multichain concept to an error-How to apply Multichain concept to an error-prone wireless channel to resolve the issue prone wireless channel to resolve the issue would be the next step in future developmentwould be the next step in future development