2014 YU-ANTL Lab Seminar

Performance Analysis of the IEEE 802.11 Distributed Coordination

Function Giuseppe Bianchi

2014 YU-ANTL Lab Seminar

April 12, 2014

Yashashree JadhavAdvanced Networking Technology Lab. (YU-ANTL)

Dept. of Information & Comm. Eng, Graduate School, Yeungnam University, KOREA

(Tel : +82-53-810-3940; Fax : +82-53-810-4742http://antl.yu.ac.kr/; E-mail : [email protected])

http://antl.yu.ac.kr/

Advanced Networking Tech. Lab.Yeungnam University (YU-ANTL)

YU-ANTL Lab. SeminarYashashree Jadhav2

Outline (1) Background

MACDCFBasic Access MechanismRTS/CTS Mechanism

Main IdeaContribution

Markov ModelProbabilitiesTwo Dimensional Markov chainPacket Transmission ProbabilityThroughput



Outline (2)Basic Access Mechanism RTS/CTS Access Mechanism

Model Validation & SimulationModel ValidationMaximizing Saturation ThroughputThroughput vs Number of StationsThroughput vs Initial Window SizeThroughput vs Max. Back‐off StageThroughput vs Packet Length

Conclusion



MAC (1) IEEE802.11 is a set of standards for wireless local

area network (WLAN) This paper’s interest is in MAC layer

The MAC layer is a set of protocols which is respon-sible for maintaining order in the use of a shared medium

The MAC layer defines two different access methods The Distribution Coordination Function (DCF)

Random access scheme Based on CSMA/CA Protocol

The Point Coordination Function (PCF) Based on TDMA

Paper focus on DCF



MAC (2) WLAN MAC and PHY Layer



DCF (1) When a station wants to transmit a new packet Mon-

itor the channel activity

If senses idle for DIFS (Distributed Inter Frame Space), the sta-tion transmits

CSMA/CA

If sensed busy (immediately or during the DIFS),the station per-sists to monitor until it is measured idle for DIFS

The station generates a random back‐off interval before trans-mitting to minimize the collision probability



DCF (2) It describes two techniques to employ for packet

transmission

Basic access mechanism (two‐way handshaking) Source transmits the packet If destination receives successfully transmits a positive ACK

RTS/CTS mechanism (four‐way handshaking) Source sends RTS If destination receives RTS then sends CTS So the channel reservation is done Source then transmits the packet If destination receives successfully transmits a positive ACK



DCF (3) IEEE 802.11 DCF

At each packet transmission, the back‐off time is uniformly cho-sen in the range(0,w‐1) where w=contention window

w depends on the number of transmissions failed for the packet At first, w=CWmin (minimum contention window) At each unsuccessful, w is doubled (binary back‐off) up to a

maximum value CWmax=2mCWmin The back‐off time counter is Decremented as long as channel is

sensed idle Frozen when a transmission is detected on the channel Reactivated when the channel is sensed idle for more than a

DIFS The station transmits when the back‐off time reaches zero



Basic Access Mechanism Basic Access Mechanism

station has to wait for DIFS before sending data receiver acknowledges at once (after waiting for SIFS) if the packet was

received correctly (CRC) automatic retransmission of data packets in case of transmission errors



RTS/CTS Access Mechanism RTS/CTS Access Mechanism

station can send RTS with reservation parameter after waiting for DIFS ( reservation determines amount of time the data packet needs the medium)

acknowledgement via CTS after SIFS by receiver (if ready to receive) sender can now send data at once, acknowledgement via ACK other stations store medium reservations distributed via RTS and CTS



802.11 – Slot Time in Bianchi’s Model 802.11 – Slot Time in Bianchi’s Model



Contribution

Analytical evaluation of the saturation throughput Ideal channel conditions (no hidden terminals and capture) Fixed number of stations where each station having a packet available

for transmission Behavior of single station is studied with a Markov model The packet transmission probability (τ) of a station in randomly chosen

slot time is obtained which is independent of access mechanism The throughput of the both access mechanism is expressed as a func-

tion of τ In saturation, each station has immediately a packet available for

transmission Each packet needs to wait for a random back‐off time before transmit-

ting At each transmission attempt each packet collides with constant and in-

dependent probability (p)



Markov Model (1) s(t) : stochastic process of back‐off stage of a station

at time t b(t): stochastic process of back‐off time counter for a

station Defines W=CWmin m=maximum back‐off stage such that CWmax=2mW Wi= 2iW where i Є(0,m) is the back‐off stage It is possible to model the bi‐dimensional process {s(t),b(t)} with the

discrete‐timeMarkov chain



Markov Model (2) Probabilities P{i, k |i, k+1}=1 k Є (0,Wi ‐2) and i Є (0, m)

At the beginning of each slot time the back‐off time is decremented P{0, k |i, 0}=(1-p)/W0 k Є (0,W0 ‐1) and i Є (0, m)

New packet following a successful transmission (probability=1‐p) and starts with back‐off stage 0.The back‐off is initially chosen between (0, W0‐1)

P{i, k |i-1, 0}=p/Wi k Є (0,Wi ‐1) and i Є (1, m) Unsuccessful transmission (probability=p) occurs at back‐off stage i-

1,The new back‐ off is uniformly chosen between (0, W i) P{m, k |m, 0}=p/Wm k Є (0,Wm ‐1)

Once the back‐off stage reaches the value m, it is not increased in sub-sequent packet transmission



Markov Model (3) Two Dimensional Markov chain



Markov Model (4) Packet Transmission Probability bi, k= lim t-> ∞ P{s (t)=i, b(t)=k} , k Є (0,Wi ‐1) and

i Є(0,m) Stationary distribution of the chain Closed‐form solution is needed

All the bi, k values can be expressed as functions of the values b0,0 and p

τ = probability that a station transmits in a randomly chosen slot time

transmission occurs when back‐off counter=0 regardless of the back‐off stage



Markov Model (5) Packet Transmission Probability

When m=0 (no exponential back‐off)

One station transmits, collision occurs when at least one of the other n‐1station transmits

Using the two equations it can be derived that

τ (p) Can be shown to be a monotone decreasing function that Starts from ,reduces up to



Throughput (1) S=Normalized system throughput [fraction of time

the channel is used to successfully transmit pay-load bits]

Ptr=probability that there is at least one transmis-sion in the considered slot time=p=1‐(1‐ τ)n

Ps=probability that a transmission in the channel is successful =



Throughput (2) E[P]=average packet payload size PtrPs=probability of successful transmission in a slot

time 1-Ptr=probability of the empty slot time Ptr (1-Ps)=probability of collision Ts =average time the channel is busy due to success-

ful transmission Tc =average time the channel is busy during a collision σ=duration of an empty slot time

S depends mainly on Ts and Tc



Basic Access Mechanism H=packet header=PHYhdr + MAChdr

δ=propagation delay

E[P* ]=Average length of the longest packet payload involved in a collision



RTS/CTS Access Mechanism H=packet header=PHYhdr + MAChdr δ=propagation delay



Model Validation & Simulation (1) Used event‐driven custom simulation program in C+

+ It closely follows all the 802.11 protocol details for

each in dependent transmitting station The analytic model is extremely accurate The analytic results (lines) practically coincide with

the simulation results (symbols) in both basic and RTS/CTS access



Model Validation & Simulation (2) Model Validation



Model Validation & Simulation (3) Maximizing Saturation Throughput

Max throughput achievable by Basic is very close to by RTS/CTS Throughput of RTS/CTS is less sensitive on τ RTS/CTS throughput has a much lower dependence on the system

engineering parameters



Model Validation & Simulation (4) Throughput vs Number of Stations

The greater the network size, the lower is the throughput [Except W=32]

For Basic Access it varies with the values of n For RTS/CTS it is almost independent of n



Model Validation & Simulation (5) Throughput vs Initial Window Size

For both Basic Access and RTS/CTS , a high value of W de-pends on the n



Model Validation & Simulation (6) Throughput vs Max. Back‐off Stage

For both Basic Access and RTS/CTS , with W=32 and n=10 –50

Choice of m doesn’t practically affect the system through-put as long as is m is greater than 4 or 5



Model Validation & Simulation (7) Throughput vs Packet Length

RTS/CTS mechanism is effective when packet size increases



Conclusion Simple but extremely accurate analytical model to

study 802.11 DCF Covers both Basic Access and RTS/CTS mechanism as

well as the hybrid one Provides good simulation results with comparison The best analytical model so far for DCF Finite number of terminals No hidden terminal Fixed Data Rate

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2014 YU-ANTL Lab Seminar