PERFORMANCE ANALYSIS ON 802.11Soonmok Kwon

2007-05

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REFERENCES

[1] Giuseppe Bianchi, “Performance Analysis of the IEEE 802.11 Distributed Coordination Function”, IEEE Journal on Selected Areas in Communications, vol. 18, no. 3, 2000

[2] Omesh Tickoo and Biplab Sikdar, “Queueing Analysis and Delay Mitigation in IEEE 802.11 Random Access MAC based Wireless Networks”, IEEE INFOCOM, 2004

[3] Hongqiang Zhai, Xiang Chen and Yuguang Fang, “How well can the IEEE 802.11 wireless LAN support quality of service?”, IEEE Transactions on Wireless Communications, vol. 4, no. 6, pp. 3084-3094, 2005

[4] H. Zhai and Y. Fang, “Performance of wireless LANs based on IEEE 802.11 MAC protocols”, in Proc. IEEE Personal Indoor and Mobile Radio Communications (PIMRC), pp. 2586-2590, 2003

[5] H. Zhai, Y. Kwon, and Y. Fang, “Performance analysis of IEEE 802.11 MAC protocols in wireless LANs”, Wiley Wireless Commun. Mobile Comput., Special Issue on Emerging WLAN Technologies and Applications, vol. 4, n. 8, pp. 917-931, December, 2004

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INDEX

Introduction

Throughput

Normalized throughput

Saturation throughput Bianchi‟s analysis

Fang‟s analysis

Delay

MAC layer service time Generalized state transition diagram

System time

Notes on 802.11 Behaviors

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INTRODUCTION

802.11

Wireless MAC standard, widely used

Backoff mechanism is adopted by most of wireless protocols

System overview

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AP

MH

MH MH

MH

Queueing model

1 transmission at a time

Queue: sum of each node‟s queue

Arrival: sum of data arrival at each node

Service time: MAC service time

Hidden terminal

Original system: occurs

RTS/CTS: not occursn=5

THROUGHPUT – TIME SLOT

Throughput can be derived with or without Queueing Model

We use simpler one: without queueing model

Tx prob. for each node in a slot :

Here, a slot means

Idle (Empty) slot time :

A period with successful tx :

A period with collision :

With probability for each:

5n : node number

starting point

THROUGHPUT - NORMALIZED THROUGHPUT

Channel performance metrics

Channel idleness ratio:

Channel busyness ratio:

Channel utilization:

Normalized throughput (goodput):

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THROUGHPUT – CONDITIONAL COLLISION PROBABILITY

Conditional collision probability :

A collision prob. seen by txed packet

P( collision | tx )

A cond. coll. prob. with maximum throughput:

By plotting, we know the throughput envelope is convex

A cond. call. prob. with saturation throughput :

For specific n, there exist a value of p at which the network operates in the saturated status.

This is maximum achievable p for given n.

Therefore, the cond. coll. prob. with MAX throughput is 7

THROUGHPUT – SATURATION CONCEPT

Definition of saturation

We say a network is saturated (with given # of contending nodes) when the conditional collision probability is maximized.

If n nodes are maintaining their tx queue non-empty, they will reach saturation condition.

For this reason, we sometimes say a node is saturated is it maintains its tx queue non-empty.

Traffic load and performance degradation by saturation

Low load (no inefficiency)

# of simultaneous access „n‟ is small and MAX(p) < proot

High load (inefficient due to high contention (p higher than proot))

# of simultaneous access „n‟ is large and proot < MAX(p)

Throughput can be enhanced

See slide 19 8

THROUGHPUT – SATURATION THROUGHPUT (1)

Bianchi‟s model [1]

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Empty queue prob.

Tx prob. at saturation

No limit onretransmission

, if m=0 (no exponential back-off)

THROUGHPUT – SATURATION THROUGHPUT (2)

Fang‟s model [3, 4, 5]

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1

Delay factor: covered later...

THROUGHPUT – SATURATION THROUGHPUT (3)

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Bianchi‟s result

# nodes increase -> MAX(p) increases -> Throughput decreases after proot

MAC layer service time as a function of n saturated nodes.

MAX(p) : Coll. prob at saturation t-put

DELAY - OVERVIEW

MAC layer service time can be

Modeled with generalized state transition diagram and

Solved with the Mason formula

Example

“Infinite retransmission with any backoff mechanism”

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Arrival : Poisson or Deterministic

Service : MAC layer service time

P(time | success)=Tsuc

P(time | collision) = Tcoll

Mason formula

DELAY – GENERALIZED STATE TRANSITION DIAGRAM (1)

Successful Tx:

Tx but collision occurs:

Decreasing backoff timer by 1:

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1 * Z^[time which is not related with tx process]

*Z^0

DELAY – GENERALIZED STATE TRANSITION DIAGRAM (1)

Successful Tx:

Tx but collision occurs:

Decreasing backoff timer by 1:

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Channel busy among (n-1) stations

Successful Tx occurs among (n-1) stations

Collision occurs among (n-1) stations

Mason formula

DELAY – GENERALIZED STATE TRANSITION DIAGRAM (1)

MAC layer service time in total

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Mason formula

DELAY – SYSTEM TIME

M/G/1

G/G/1

?/G/1

M/M/1

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NOTES ON 802.11 MAC BEHAVIORS

Throughput and Delay17

THROUGHPUT - COLLISION PROBABILITY

18Throughput is mostly the function of p But n controls the maximum achievable p and, thus, controls the saturation throughput

THROUGHPUT – OPTIMAL AND SATURATED CASE

19Maximum throughput is achieved in the non-saturated case rather than in the saturated case when n > 5

collision

MAC LAYER SERVICE TIME – COLLISION PROB.

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Ts directly depends on p. However, n determines maximum p and, thus, performance at saturated state.

Thus, we can define following function. It‟s very useful!M(n) : MAC layer service time with given n contenders (saturated nodes)

MAC LAYER SERVICE TIME – APPROXIMATION (1)

21Lognormal distribution provides a good approximation.Exponential distribution is reasonably good except with very low p.

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MAC LAYER SERVICE TIME – APPROXIMATION (2)

Markov chain model over-estimate service time compared to the simulation results from ns-2. So, the model gives

Lower bound for throughputUpper bound for delay

CONCLUSION

WLAN Performance Analysis

Throughput and Delay are formulated with p, n

For non-saturated and saturated case

To understand the throughput, understand the concept of saturation and conditional collision probability

To understand the delay, most analysis on CSMA MAC targets the MAC layer service time with n contenders.

Using two equations:

First,

Second, the equation derived from Markov model for saturation situation

It determines relationship between tau and p at given protocol parameters.

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