IEEE 802.11b anomaly solution

1

Yarmouk University

Hijjawi Faculty for Engineering Technology

Department of Telecommunication Engineering

Secondary Project

First Semester 2011/2012

IEEE 802.11b unfairness solution

Name Abdulhafeez Al-Zaben

Tareq Zeida

Student Number 2007973036

2007873006

Supervisor Name Dr. Haythem Bani Salameh

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Abstract

IEEE 802.11 standards were breakthrough

technologies in the jargon of broadband wireless access.

It was and still the most common used method for

internet access. Hotspots are everywhere, and everybody

has a “fair” probability for accessing it. IEEE 802.11b

has an algorithm to achieve this fairness. It provides fair

access for the medium. However, it does not provide fair

bandwidth utilization. That’s why we conducted this

project; to find an advanced algorithm to eliminate this

unfairness.

Downloadable version

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Table of Contents

Chapter1.............................................................................................................................. 4 Introduction ............................................................................................................................. 4

1.1. General description about IEEE 802.11b ......................................................... 4

1.2. Distributed Coordination Function .................................................................. 4

1.3. IEEE 802.11b Anomaly .................................................................................. 5

1.4. Related works ................................................................................................. 5

Chapter2.............................................................................................................................. 7 Theoretical background ........................................................................................................... 7

2.1 IEEE 802.11b DCF Anomaly........................................................................... 7

2.2 DCF-MB and DCF-MBA .............................................................................. 10

Chapter3............................................................................................................................ 14 Design in Details .................................................................................................................... 14

3.1 The design ..................................................................................................... 14

3.2 Advantages of DCF-MBA ............................................................................. 16

Chapter4............................................................................................................................ 18 Testing and Validation ........................................................................................................... 18

4.1 Design simulation .......................................................................................... 18

4.1.1 Network Configurations .......................................................................... 18

4.2 Simulation results and comments ................................................................... 19

Appendices............................................................................................................................. 24

Appendix A ......................................................................................................... 24

Appendix B ......................................................................................................... 25

Figure 4.1 ........................................................................................................ 25

Figure 4.2 ........................................................................................................ 30

3 nodes running at different times DCF-MB .................................................... 34

3 nodes running at different times DCF-MBA.................................................. 38

Appendix C ......................................................................................................... 42

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Chapter

1

Introduction

1.1. General description about IEEE 802.11b

IEEE 802.11b is an upgraded version of IEEE 802.11 specification that has an

extended throughput up to 11 Mbps using the same 2.4 GHz spectrum, and it’s

specified under the name of Wi-Fi.

The IEEE 802.11b is a set of IEEE standards that provide wireless networking

transmission methods in the home, office and some commercial establishments.

As we mentioned before the 802.11b has a maximum data rate up to 11 Mbps

and it is divided into several data rates; 11, 5.5, 2 and 1 Mbps assigned to the users

depending on their distance from the Access point (i.e. RF conditions), giving

11Mbps for the nodes whose RF condition is considered to be strong (i.e. high

SNR) and 1 Mbps for the stations with weak RF conditions.

1.2. Distributed Coordination Function

The use of Distributed Coordination Function ( DCF ) presents two main

drawbacks that can be summarized as the DCF algorithm is unsuitable for Quality

of service aware applications, and its throughput unfairness issues, which known

as the 802.11b anomaly which will be the goal of our study. Although these two

drawbacks, the function (DCF) algorithm, which is based on Carrier Sense

Multiple Access with Collision Avoidance (CSMA/CA) technique, the non-

centralized algorithm (DCF), strongly participates in the success of IEEE 802.11b

due to its simplicity.

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1.3. IEEE 802.11b Anomaly

Due to this anomaly the throughput of each access node dramatically reduced

once a station’s transmission bit rate decreases due to physical radio properties

(As mentioned before the IEEE 802.11b works under the band of 2.4 GHz, having

the same operating band of microwave ovens, Bluetooth devices and cordless

telephones). Specifically, a node that is relatively far from the access point is

subject to important signal fading and interference leading to repeated

unsuccessful frame transmission. As a result, the standard IEEE 802.11b reacts by

degrading the station bit rate from 11Mbps rate to 5.5, 2, 1Mbps; which will cause

a reduction in the throughput of all nodes. In other words, it means that all the

nodes are penalized due to the position of one node.

The basic CSMA/CA scheme allows a fair access to the shared channel. In this

regard, a node with a relatively low bit rate captures the channel a longer period

with respect to the remaining nodes transmitting at high bit rate. This leads to a

degradation of all access nodes’ throughput.

To solve this problem, we found a new strategy based on multiplication of

backoff window with an optimal value. We refer to this technique as the DCF-

MBA (DCF Multiple Backoff Advanced). In our scheme, access nodes are

classified into different sets according to their transmission bit rate (11, 5.5, 1

Mbps) and then each set will be characterized by each own backoff window.

1.4. Related works

In the recent years, several studies have been appeared to deal with the

analysis of the unfairness behavior of DCF protocol due to the basic CSMA/CA

algorithm, without providing a particular solution for this anomaly.

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One of these solutions based on the SNR (Signal to Noise Ratio) classes’

differentiation, by providing each SNR node class with a distinct and fixed

backoff value. This solution fixes the contention window interval size for each

SNR, and defining two SNR classes: the good and low SNR classes, with

respectively, 31 and 1023 as backoff values. In doing so, the good SNR nodes

transmit more frequently than the low SNR nodes, and the throughput of the good

SNR nodes will be not affected by the low speed transmitting nodes belonging to

the low SNR class. However, this solution is only limited to two classes of SNR,

whereas in reality much more SNR classes are found.

Another solution founded for the anomaly is to specify the maximum packet

size that can be transmitted by each node according to its associated SNR class.

This solution is shown to be effective, but it counters two main limitations.

Applying this approach involves several modifications to the existing IEEE

802.11 protocol and a new layer is required at each access station to fragment the

packets according to the maximum packet size associated to each SNR class.

The last solution (DCF-MB) which almost will be like our new approach

(DCF-MBA), where the access nodes are classified into different sets according to

their transmission bit rate (11, 5.5, 1 Mbps) and then each set will be characterized

by each own backoff window. The difference between (DCF-MBA) and (DCF-

MB) is the formula being used to assign the backoff window size. Simulation in

chapter 4 shows that the collision and the throughput are better when using DCF-

MBA. So finally, we can say that each node has more stable behavior and not

being severely affected by other nodes.

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Chapter

2

Theoretical background

In this chapter we will discuss the IEEE 802.11b DCF anomaly in details,

and we will also discuss the theoretical basis of DCF-MBA depending on the

theoretical background of DCF-MB.

2.1 IEEE 802.11b DCF Anomaly

The IEEE 802.11b standard defines two access methods: the DCF

technique, which is based on the CSMA/CA protocol, and the centralized

point coordination function (PCF), which provides free collision access.

In the DCF access method which is based on the CSMA/CA principle,

when a node wishes to transmit a frame, it senses the channel’s activity until

an idle period is detected. Then, the node waits for a random backoff interval

before transmitting. The backoff time counter is decremented in term of slot

time as long as the channel is sensed free. The counter freezes once a

transmission among the channel is detected, and it resumes with the old

remaining backoff interval when the channel is sensed idle again, and it will

transmit its frame when the counter reaches zero.

If the frame is correctly received, the receiving host sends an

acknowledgement (ACK) after a short period of time called Short Inter Frame

Space (SIFS). If the sending node does not receive this ACK frame, then a

collision is assumed to be happened. In this case, the sending node attempts to

send the packet again when the channel is free, but this time with a different

backoff time interval.

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For each new transmission attempt, the backoff interval takes a random

value chosen from the range [0, CW]. During the first attempt to send, the CW

is equal to CWmin which equals to 31 time slots and the value will be chosen

randomly from the range of [0, 31]. In case of collision, the node will resend

the same packet again but now with different contention window size doubled

from the minimum size until a maximum backoff window size reached

CWmax which equals to 1023.

DCF ensure an equal access to the shared medium among all transmitting

nodes, which does not guarantee a fair medium utilization among all the

nodes. Specifically, a node located away from the access point which results

degradation in the bit rate from 11Mbps to 1Mbps. In this case, this node will

capture the channel for a period 11 times longer than a node located in front of

the access point to transmit the same packet. This situation will affect the total

throughput of the network.

To illustrate this situation, we consider three nodes situated at different

distances from the access point, which accordingly the first node will have a

bit rate of 11Mbps, the second node will have a bit rate of 5.5Mbps and the

third node will have a bit rate of 1Mbps. Moreover, the second node will be

activated after 10s and the third node after 20s, the overall network will be

tested for 30s to check the network throughput evolution. Table 1 will show

the DCF parameters used and these parameters will be mutual for DCF-MB

and DCF-MBA.

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Parameters of IEEE 802.11 b

Class bit rate 1 ; 5.5 ; 11 Mbps

DIFS 50 µs

SIFS 10 µs

Backoff-Slot Time 20 µs

CWmin 31

CWmax 1023

ACK 14 bytes

Table 1.1: Parameters of IEEE 802.11b

Figure 1.2: Throughput of each node in DCF standard case

As shown in figure 1.2, during the first 10s, only the first node is activated, its

throughput is maximal and approaches 5.92Mbps, which represents 53.82% of its

transmission bit rate (11Mbps). This difference is due to the backoff delay, SIFS and

DIFs periods left on the medium for each packet transmission.

Once the station 2 is activated, as noticed the first node throughput reduces. But,

this reduction is dramatic since the new throughput does not exceeds 2.753Mbps

which is less than the expected throughput for the second node 5.5Mbps, and the

throughput for both first and second node have the same 2.753Mbps throughput

although their different bit rates. Indeed, the throughput of first node is decelerated

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due the relatively low bit rate of the second node. In this case, node 2 occupies the

channel twice more times than the first node. As a result, node 1 is unfairly penalized

as well as the total network throughput equals to 5.506Mbps.

The anomaly of DCF will be clear when node number three activated. In this case,

the useful throughput of each station is limited to 1.691Mbps and the total throughput

becomes lower and equals to 5.073Mbps.

2.2 DCF-MB and DCF-MBA

The idea of this method is to provide each class bit rate Ci with its associated

contention window size CWmin which is equal to 31 as in standard, and then the

contention window of 5.5Mbps and 1Mbps is derived as follows

( ) ( )

(1)

According to formula (1), we get CWmin (1) = 31, CWmin (2) = 60 and CWmin

(3) = 330, which are the window sizes of classes C1 (11Mbps), C2 (5.5Mbps) and C3

(1Mbps), respectively.

Unlike the standard DCF the throughput of this method can be described as

follow. When node one activated, the throughput of this node is divided by 2 when

station activated and is divided by 3 when the third one is activated, which means that

performance of the first node only depends on the number of sharing access nodes and

no more with their relative positions with respect to the AP, which means that each

node is completely separated from other nodes.

Regarding to our approach DCF-MBA, we referred to the following equation in

order to calculate the throughput of each node; node number is activated first 10s then

node two activated after 10s and the third node activated after 20s.

To calculate the Signal to Noise Ratio (SNR) of each node,

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(2)

Where,

D = distance between the AP and the node.

n = the two ray reflection coefficient and is equal to 4.

No =White noise (5 * 10-2).

Pt. = transmitted power from the AP and set to be 1w.

To calculate the data rate of each node (R), using formula (2)

( ) (3)

Where;

B= 2.5 * 106 Hz.

Setting the packet length to 2 Kb, figure 2.2 shows the throughput of each node

using the DCF-MB concept and our new concept DCF-MBA.

Figure 2.2: Throughput of DCF-MB comparing with DCF-MBA

As shown in figure 2.2, during the first 10s, only the first node is activated, its

throughput is maximal and approaches 6.281Mbps for DCF-MBA and 6.12Mbps for

DCF-MB, which represents 57.1% of its transmission bit rate (11Mbps). This

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difference is due to the backoff delay, SIFS and DIFS periods left on the medium for

each packet transmission.

Once the station 2 is activated, as noticed the first node throughput reduces. But,

this reduction is slight since the new throughput is 5.934Mbps which confirms that

node one does not affected by the activation of node two, and the throughput for the

second node has the value of 4.98Mbps throughput. Indeed, the throughput of first

node is not affected by the relatively low bit rate of the second node.

The performance of DCF-MBA will be clear when node number three activated.

In this case, the useful throughput of each node is slightly affected. The throughput of

the first node will be 5.77Mbps, for the second node 4.83Mbps and for the third node

2.141Mbps.

To have the previous results, we have modified the formula used to calculate the

backoff size of each node, setting the value of optimal X to 5. When choosing this

value of X, we took in concern that we need to achieve high useful throughput for the

network, and to reduce the number of collided packets.

( ) (

( ) )

(4)

Depending on X which equals to 5, the interval of the backoff size of the first

node CW (1) will be a random value from the interval of [0, 31], the backoff size of

the second node CW (2) will be chosen randomly from the interval [0, 36] and the

third node will also took a random value from [0, 81].

Choosing a value of X=1, the backoff size for the three nodes will be reduced to

[0, 31] for the first node, [0, 32] for the second one and [0, 41] for the last one. Using

these values for the contention window size, although the total number of the

transmitted packets which indicates total useful throughput will be higher, but also the

collided number of packets will also be higher, which is not desirable.

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Choosing a value of X=10, the interval of the backoff size for the three nodes will

be expanded to [0, 31] for the first node, [0, 41] for the second one and [0, 131] for

the last one. Using these values for the contention window size, although the total

number of the collided packets will be lower, and the total transmitted number of

packets which indicates total useful throughput will also be lower, this value is not

desirable. Then the value of X=5 will be the optimal value for the DCF-MBA.

Figure 2.3: optimal X value.

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Chapter

3

Design in Details

3.1 The design

In this chapter we go through the details of our design, however, we didn’t make

any hardware. However, we will show some figures and block diagrams of the design.

In general, the IEEE 802.11b anomaly is due to the simplicity of the design, this

simplicity is achieved by making the system decentralized, which means, that each

node doesn’t take a straight-forward command from the AP to send a burst. On the

contrary, each node “senses” the medium periodically to see if it is possible to send its

queued packets.

Figure 3.1: Medium Access [3]

By using this mechanism, the probability of collision between the competing

nodes increases, and it may cause the at-the-same-time-transmitted-packets to be

discarded by the AP, which degrades the performance of the network. Our work

suggested a way to overcome the anomaly by using the MB techniques. By assigning

each node its own backoff window depending on the SNR (i.e.: data-rate class). Far

away nodes will cause the system to degrade, since they will need more channel

acquisition by re-transmitting redundant packets. Hence, by giving the low SNR

stations more backoff windows, this anomaly can be reduced.

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The design should be simple, since DCF is based on simplicity, any further

amendment in the system should take this criterion in consideration. As being said

before, the multiple backoff windows mechanism is very effective and it could reduce

the redundant packets and hence improve our system’s performance.

Figure 3.2 depicts a hypothetical system, which consists of three competing nodes.

Table 3.1 summarizes the parameters of each node. In the standard case, each node

has a random backoff window chosen from the range [0, 31], regardless of the RF

conditions of the node.

Station Backoff window

range

RF condition Data Rate

Station 1 [0,31] Excellent 11 Mbps

Station 2 [0,31] Medium 5.5 Mbps

Station 3 [0,31] Weak 1 Mbps

Table 3.1: System’s parameters

In DCF-MBA we will assign each node a backoff window depending on its RF

condition, thus, we assure that each node will get fair access to the medium.

DCF’s main concern was to provide “fair” medium access, and it really did

achieve that, but the “fair” access was not “fair” bandwidth-utilization. That’s why the

DCF algorithm needs to be adjusted to meet this limitation.

AP

Station 1 Station 2 Station 3

Figure 3.2: Hypothetical System

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As being said before, the idea of multiple backoffs is an efficient way to revoke

the limitation, the job is done and the simplicity is still there.

The AP will not being be modified, and all the IEEE 802.11b algorithm will work

just like the standardized version. However, the nodes’ MAC layer will be modified,

so that it can assign a random backoff window size to the node depending on its feed-

backed RF condition. The RF condition can be inferred by the number of re-

transmitted packets. When the number of re-transmitted packets is higher, then the

SNR has lower values.

Equation (4) is the equation that should be implemented in the nodes’ algorithms.

3.2 Advantages of DCF-MBA

Why to use multiple backoffs? This is the question that needs to be answered to

show the advantageous behavior of DCF-MBA or even DCF-MB. The anomaly exists

because, far away nodes, trying to access the medium, are given access to medium

with the same probability as near or almost-near nodes. Doing so, DCF ensures that

all the competing nodes have the chance to transmit their packets fairly, but this

fairness, is actually unfair. When far away nodes transmit packets, their SNRs are

very low; the number of corrupted bits is high. So eventually, most of the packets

arrived from the far away station (in our example station 3) at the AP will be dropped

and discarded, so, the medium was inefficiently occupied.

The time needed to transmit station 3’s packets was wasted and the other good

conditioned nodes’ too. This time could have been well utilized if the channel access

was given to station 1 or even 2. In DCF-MB/A, this is what really happens. The good

stations are given less backoff window size, so that they can transmit their packets

more frequent than other low-SNR stations. Doing so, not only enhance the

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performance of the stations, it also improves the performance of the whole system

(i.e. total network throughput).

But what is the difference between the DCF-MB and DCF-MBA’s performances.

Both of them use the multiple backoff window sizes, in the case of MB, the backoff

window is given by equation (1), where it really, improves the good-SNR stations’

performance by unfair medium access of far-away nodes (low SNRs),then the idea

behind DCF ( fair access) is abused.

The DCF-MBA, improves the performance of the good-SNR stations, and slightly

delays the far-away nodes from accessing the medium. But this delay is negligible

compared to the MB.

The delay doesn’t affect the close-to-the-AP nodes. However, the performance of

the far-away nodes is much way better in MBA than MB. This is due to changing the

formula and using an optimum value of X.

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Chapter

4

Testing and Validation

4.1 Design simulation

In this chapter, we will talk about our simulation and the results that we gained by

running our different network configurations.

4.1.1 Network Configurations

Different network configurations mean that, by using different system parameters

and fixing them, we obtained results for a specific variant parameter.

The next section will show the simulation results and comments on those results,

but here we will talk about the network configurations in general that gave us the

desired results.

First of all, we created a system, consisting of 3 nodes, their parameters are the

same as Table 3.1 (in the case of the standard DCF), the node with the strongest RF

condition is working for the first 10 seconds, after that, node 2 (i.e. Station 2) joins the

network. Finally, at the 30th

second, Station 3 joins. The parameters being monitored

are the number of packets collided, the total throughput of the network and the

throughput per node. This system was simulated by the DCF-MB and DCF-MBA

techniques, to compare which one is more efficient.

We also simulated the standard case of the IEEE 802.11b, along with the DCF-

MB and DCF-MBA techniques.

In the DCF-MBA technique, there is parameter X that should be chosen carefully,

so in order to find the optimum X, we ran the system with different values of X ( we

talk about it in the next section), but for now, X equals 5 is our choice.

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And finally, we will simulate a system, with 36 nodes, distributed across three

different ranges of coverage (150, 300 and 450 m width x height).

4.2 Simulation results and comments

At this section, we show the results of the network configurations. At first, we will

show the standard case we talked about in 4.1.1.

Figure 4.1: Standard IEEE 802.11b

Figure 4.1 depicts the case of the three-nodes-system while using the standardized

version of IEEE 802.11b. We can see that, the node with the strong RF condition (11

Mbps data rate) is affected by the entrance of the second node and severely affected

by the entrance of the third node.

Next, we consider the optimum range of a network, with 36 competing nodes, and

three ranges of coverage. By range of coverage we mean that all the nodes are

distributed randomly across that coverage area. Figure 4.2 depicts that the range 350

m is the optimal.

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Figure 4.2: Optimum Range

This is due to the fact that, at small coverage areas, all the nodes, takes values of

backoff windows in the range of [0, 31] so the probability of collision increases.

Hence, the total throughput of the network degrades. However, if the coverage area is

too large, then most of the nodes will get medium and low conditions, hence, low

data-rates (i.e. poor performance of the network).

After that, we are concerned about the X value in the DCF-MBA technique, in

order to find the optimum value of X; we need to run the system multiple times, with

variant number of X.

We ran the system, with 10 different values of X, ranging from 1 to 10. And we

plotted the throughout in terms of packets and the collided packets.

Figure 4.3 below depicts the simulated system. We can see that at X equals to 1,

the total throughput of the network is maximized. However, the total number of

collided packets is also maximized as well. And at X equals to 10, we see that the

throughput here is minimum as well as the number of collided packets is minimized.

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So, in order to set the value of X, we have two choices, either by finding optimum

value, or by trade-offs.

Figure 4.3: Optimum X

By trade-offs, we mean that, you either choose the X equals to 1, where the total

throughput is higher, but also with higher values of collided packets. Or when X is

equal to 10 or greater, then throughput is minimized, by with less number of collided

packets.

In this project, we took the first approach. We wanted to find the optimum value

of X which was 5. At X equals to 5, we can see that, the number of packets being

transmitted is “good” and the number of collided packets is “good” too.

By now, we have found our parameters for an optimum DCF-MBA algorithm; we

need to compare this algorithm with DCF-MB and the standard DCF algorithm to

show our advantageous design.

Figure 4.4 depicts that our algorithm is better than the DCF-MB in general, and

specifically for the weak RF conditioned nodes. It’s like a hybrid between the

standard case and the MB. It’s design to make a “fair” access and utilization of the

broadband spectrum in IEEE 802.11b.

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Figure 4.4: DCF-Mb vs. DCF-MBA (Throughput per node)

As you can see, in station 1’s case, the performance enhancement is negligible,

however, it’s significant in the case of station 2 and 3 (medium and low RF conditions

respectively).

The previous figure shows us the enhancement in terms of throughput per node,

also, enhancement per node, means enhancement over the total throughput of the

network. This is shown in figure 4.5, where the total throughput of the network is

shown for both DCF-MB and DCF-MBA. In terms of bandwidth efficiency and

throughput enhancement, the DCF-MBA is way better than the DCF-MB and the

DCF standard. Nevertheless, we need to see the number of collided packets, for each

technique; just to make it clearer that DCF-MBA is better.

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Figure 4.5: DCF-MB vs. DCF-MBA (Total throughput)

Figure 4.6 depicts that, when node 2 and 3 joins the network, collisions occurs,

and it shows also that, the collision in the DCF-MB is way better than the case of

standard DCF algorithm, however, the collided packet in the case of DCF-MB is

logically less than DCF-MBA, where the backoff intervals ranges are much larger

than DCF-MB, hence, collision will be less. No need to simulate DCF-MB’s collided

packets. However, in telecommunications, tradeoffs occur, and we must seek for

what’s best for our applications. Here the enhanced throughput is needed more than

the slightly increased collided packets.

Finally, it can be seen that DCF-MBA can be seen as a hybrid of DCF-MB and

standard DCF, and it’s the mean to achieve the real meaning of fairness, which was

the main purpose while developing the IEEE 802.11b.

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Appendices

Appendix A

References

1 http://en.wikipedia.org/wiki/IEEE_802.11b-1999

2 http://paper.ijcsns.org/07_book/200701/200701B14.pdf

3 Yassine Chetoui, Nizar Bouabdallah, “Adjustment mechanism for the IEEE 802.11 contention window: An efficient bandwidth sharing scheme”,p. 3

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Appendix B

Matlab codes

Figure 4.1

clc clear all clf close all transmitting_nodes = [];% this array will be used to indicate if

there's a transmitting node & it will be used to caclulate # packets transmitting_nodes1=0; lop= 2*1024; % Length of Packet % SIM=input('Enter the simulation time: '); timeslot=20e-6; % timeslot duration DIFS = 50e-6 / timeslot; SIFS = 10e-6 / timeslot; ACK = 14 * 8 ; % # bits in an ACK collided = zeros(1,30); Noise=0.05; n=4; tx=1; B=2.5*(10e6); % transmitting_nodes = zeros(1,3); % this array will be used to

indicate if there's a transmitting node & it will be used to

caclulate # packets datarate=[]; packet=[]; % CW=[31 62 341]; tx1 = 0; tx2 = 0; tx3 = 0; collided2=0; % packet=zeros(1,3); ; packet1=zeros(1,30); packet2=zeros(1,30); packet3=zeros(1,30); datarate=[11e6 5.5e6 1e6]; x=0; for k=1:3 time(k)=lop/datarate(k); CW(k)=31+(((11e6)/datarate(k))-1).*x; end counter=0; counter2=0; for time2=1:30 for ts=1:(1/timeslot) if time2<10 transmitting_nodes=false; for z=1:1 if CW(z)== 0 transmitting_nodes(z) = 1; % assign '1' to a transmitting

node new_cw1 = randi(30)+1; if new_cw1 == 0 new_cw1 = 31;

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end CW(1) = new_cw1;

current_ts = fix(time(z) / timeslot); % stop 'current_ts'

timeslots before decreasing the non-transmitting nodes i = i + current_ts + DIFS + SIFS + fix((ACK/datarate(z))

/ timeslot); else transmitting_nodes(z) = 0;

end

if sum(transmitting_nodes) == 1 % if there's no colliding

nodes then add a packet to the transmitting node. packet1(time2) = packet1(time2) + 1; elseif sum(transmitting_nodes) > 1 collided(time2) = collided(time2) + 1; end end CW = CW - [1 0 0]; end if time2>=10 && time2<20 transmitting_nodes=zeros(1,2);

% packet=zeros(1,2); for z=1:2

if CW(z)== 0

transmitting_nodes(z) = true; % assign '1' to a

transmitting node if z == 1 tx1 = 1; % new_cw1 =31; % if new_cw1 == 0 % new_cw1 = 31; % end

end if z ==2 tx2 = 1; % new_cw2 = 31; % if new_cw2 ==0 % new_cw2 = 31; % end % CW(2) = new_cw2; end

current_ts = fix(time(1) / timeslot); % stop 'current_ts'

timeslots before decreasing the non-transmitting nodes i = i + current_ts + DIFS + SIFS + fix((ACK/datarate(1)) /

timeslot); else % tx1 = 0; % tx2 = 0; transmitting_nodes(z)= false;

27

end end


nodes then add a packet to the transmitting node. if tx1 == 1 && tx2 ==0 packet1(time2) = packet1(time2) + 1; CW(1) = randi(30)+1; elseif tx1==0 && tx2==1 packet2(time2) = packet2(time2) + 1; CW(2) = randi(30)+1; end

elseif sum(transmitting_nodes) > 1 counter=counter+1; if counter==1 CW(1)=randi(61)+1; CW(2)=randi(61)+1; elseif counter>1 && counter<=8 CW(1)= randi(counter*61)+1; CW(2)= randi(counter*61)+1; elseif counter>=9 CW(1)= randi(1023)+1; CW(2)= randi(1023)+1; end

% if CW(1)>1023 % CW(1)= randi(30)+1; % % CW(2)=randi(30)+1; % end

collided(time2) = collided(time2) + 1;

end tx1 = 0; tx2 = 0; CW = CW - [1 1 0]; end

if time2>=20

transmitting_nodes=zeros(1,3); % packet=zeros(1,3); for z=1:3

if CW(z)== 0 transmitting_nodes(z) = true; % assign '1' to a

transmitting node if z==1 tx1 = 1; % new_cw1 = 31; % if new_cw1 == 0 % new_cw1 = 31; % end % CW(1) = new_cw1;

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end if z==2 tx2 = 1; % new_cw2 =31; % if new_cw2 ==0 % new_cw2 = 31; % end % CW(2) = new_cw2; end if z==3 tx3 = 1; % new_cw3 =31; % if new_cw3 == 0 % new_cw3 = 31; % end % CW(3) = new_cw3; end


timeslots before decreasing the non-transmitting nodes i = i + current_ts + DIFS + SIFS + fix((ACK/datarate(1))

/ timeslot); else transmitting_nodes(z) = false;

end end


nodes then add a packet to the transmitting node. if tx1 ==1 && tx2 ==0 && tx3==0 CW(1) = randi(30)+1; packet1(time2) = packet1(time2) + 1; elseif tx1 ==0 && tx2 ==1 && tx3==0 CW(2) = randi(30)+1; packet2(time2) = packet2(time2) + 1; elseif tx1 ==0 && tx2 ==0 && tx3==1 CW(3) = randi(30)+1; packet3(time2) = packet3(time2) + 1; end elseif sum(transmitting_nodes) > 1 counter2=counter2+1; if counter2==1 CW(1)=randi(61)+1; CW(2)=randi(61)+1; CW(3)=randi(61)+1; elseif counter2>1 && counter2<=8 CW(1)= randi(counter*61)+1; CW(2)= randi(counter*61)+1; CW(3)= randi(counter*61)+1; elseif counter2>=9 CW(1)= randi(1023)+1; CW(2)= randi(1023)+1; CW(3)= randi(1023)+1; end collided(time2) = collided(time2) + 1; end

29

tx1 = 0; tx2 = 0; tx3 = 0; CW = CW - 1; transmitting_nodes = zeros(1,3); end % CW = CW -1 ; % ts=ts+i;

end end

% packet_total=[0,packet1,packet2,packet3]

th1=(packet1.*lop)./1024.^2; th2=(packet2.*lop)./1024.^2; th3=(packet3.*lop)./1024.^2; total = packet1 + packet2+packet3; % hold on plot(1:30,smooth(total),'-k'); % plot(1:30,smooth(th2(:,1:30)),'--k'); % plot(1:30,smooth(th3(:,1:30)),':k'); % legend ('11Mbps node','5.5Mbps node','1Mbps node') % xlabel('Time') % ylabel('Throughput') % grid on

31

Figure 4.2

clc clear all clf close all transmitting_nodes = []; % this array will be used to indicate if

there's a transmitting node & it will be used to caclulate # packets lop= 576; % Length of Packet SIM=input('Enter the simulation time: '); timeslot=20e-6; % timeslot duration DIFS = 50e-6 / timeslot; SIFS = 10e-6 / timeslot; ACK = 14 * 8 ; % # bits in an ACK collided = 0; z_new = []; f_thr=[]; time_axis = 1:SIM; f_thr_axis = zeros(size(time_axis)); Noise=0.05; n=4; tx=1; B=2.5*(10e6); for L=150:150:450

l_new(L/150) = L; transmitting_nodes = []; % this array will be used to indicate if

there's a transmitting node & it will be used to caclulate # packets

datarate=[]; packet=[]; CW=[]; time_axis = []; f_thr_axis = []; % l_new=[]; z=36; packet=zeros(1,z); datarate=zeros(1,z); distance=[]; if(L/150)==1 nn=1; for i=1:6

netXloc = [25 50 75 100 125 150]; for j=1:6 netYloc = [25 50 75 100 125 150] ; distance(nn)= sqrt((netXloc(i) - (L/2))^2 + (netYloc(j) - (L/2))^2); nn=nn+1; end

end elseif (L/150)==2 nn=1; for i=1:6

netXloc = [37 75 112 186 223 260]; for j=1:6

31

netYloc = [37 75 112 186 223 260] ; distance(nn)= sqrt((netXloc(i) - (L/2))^2 + (netYloc(j) - (L/2))^2); nn=nn+1; end

end elseif (L/150)==3 nn=1; for i=1:6

netXloc = 40:75:450; for j=1:6 netYloc = 40:75:450 ; distance(nn)= sqrt((netXloc(i) - (L/2))^2 + (netYloc(j) - (L/2))^2); nn=nn+1; end

end end for i=1:z

SNR=(1/((distance(i)^n).*Noise)).*tx; datarate(i)=B*log2(1+SNR); if datarate(i)<=1 datarate(i)=1; elseif 1<datarate(i) && datarate(i)<=2 datarate(i)=2; elseif 2<datarate(i) && datarate(i)<=5.5 datarate(i)=5.5; elseif 5.5<datarate(i) datarate(i)=11; end datarate(i)=datarate(i)*(10^6); % calculation of the datarate

to assign a CW according to it and to calculate transmission time time(i)=lop/datarate(i); % transmission time per packet at a

given datarate CW(i)=(11000000/datarate(i))*31; % we assign a general CW per

packet then we randomize the value

if CW(i)==31 CW(i)=randint(1,1,31); elseif CW(i)==62 CW(i)=randint(1,1,62); % CW ~ [0 , 62] elseif CW(i)==170.5 CW(i)=randint(1,1,171); % CW ~ [0 , 170.5] elseif CW(i)==341 CW(i)=randint(1,1,341); % CW ~ [0 , 341] end % CW= fix(CW); % rounding the CW value end nn=1;

for sim=1:SIM

packet=zeros(1,36);

for i=1:(1/timeslot) for j=1:z % per node check whether the CW equals to zero if CW(j)== 0

32

transmitting_nodes(j) = true; % assign '1' to a

transmitting node CW(j)=(11e6/datarate(j))*31; % assign new CW current_ts = fix(time(j) / timeslot); % stop 'current_ts'

timeslots before decreasing the non-transmitting nodes i = i + current_ts + DIFS + SIFS + fix((ACK/datarate(j)) /

timeslot); if CW(j)==31 CW(j)=randint(1,1,31); if CW(j) == 0 if sum(transmitting_nodes) == 1 CW(j) = 1; else CW(j) = 15; end end elseif CW(j)==62 CW(j)=randint(1,1,62); elseif CW(j)==170.5 CW(j)=randint(1,1,171); elseif CW(j)==341 CW(j)=randint(1,1,341); end % CW(j)= fix(CW(j)); else transmitting_nodes(j) = false; end end if sum(transmitting_nodes) == 1 % if there's no colliding

nodes then add a packet to the transmitting node. packet = packet + transmitting_nodes; elseif sum(transmitting_nodes) > 1 collided = collided + 1; end CW = CW -1 ; transmitting_nodes = zeros(1,z);

end

f_thr= sum(packet) * lop / 1024^2; time_axis(round( sim)) = sim; f_thr_axis(round( sim)) = f_thr; sim end

f_thr1(L)=f_thr xxx=[f_thr1(150) f_thr1(300) f_thr1(450)]; plot(150:150:450,smooth(xxx)); axis([0 500 5 9])

figure(1) hold on xlabel('Time (s)'); ylabel('Throughput (Mbps)') title('Throupht with different ranges') t_ref = f_thr_axis(1); time_axis = [0 0.2 0.4 0.6 0.8 time_axis]; f_thr_axis = [0 0.2*t_ref 0.4*t_ref 0.6*t_ref 0.8*t_ref f_thr_axis]; if (L/150)==1

33

plot(smooth(time_axis),smooth(f_thr_axis),'--

r','LineWidth',1,'MarkerEdgeColor','k','MarkerFaceColor','g','MarkerS

ize',7) elseif (L/150)==2 plot(smooth(time_axis),smooth(f_thr_axis),'--

g','LineWidth',1,'MarkerEdgeColor','k','MarkerFaceColor','g','MarkerS

ize',7) elseif (L/150)==3 plot(smooth(time_axis),smooth(f_thr_axis),'--

k','LineWidth',1,'MarkerEdgeColor','k','MarkerFaceColor','g','MarkerS

ize',7) end axis([0 SIM 0 11]) legend('Range = 150m','Range = 300m','Range = 450m',4) grid on end

34

3 nodes running at different times DCF-MB


there's a transmitting node & it will be used to caclulate # packets transmitting_nodes1=0; lop= 2048; % Length of Packet % SIM=input('Enter the simulation time: '); timeslot=20e-6; % timeslot duration DIFS = 50e-6 / timeslot; SIFS = 10e-6 / timeslot; ACK = 14 * 8 ; % # bits in an ACK collided = 0; Noise=0.05; n=4; tx=1; B=2.5*(10e6); % transmitting_nodes = zeros(1,3); % this array will be used to


caclulate # packets datarate=[]; packet=[]; CW=[31 62 341]; tx1 = 0; tx2 = 0; tx3 = 0; % packet=zeros(1,3); ; packet1=zeros(1,30); packet2=zeros(1,30); packet3=zeros(1,30); datarate=[11e6 5.5e6 1e6]; for k=1:3 time(k)=lop/datarate(k); end for time2=1:30 for ts=1:(1/timeslot) if time2<10 transmitting_nodes=false; for z=1:1 if CW(z)== 0 transmitting_nodes(z) = 1; % assign '1' to a transmitting

node new_cw1 = randi(31,1,1); if new_cw1 == 0 new_cw1 = 15; end CW(1) = new_cw1;




end

35


nodes then add a packet to the transmitting node. packet1(time2) = packet1(time2) + 1; elseif sum(transmitting_nodes) > 1 collided1 = collided1 + 1; % packet1(time2) = packet1(time2) - 1; end end CW = CW - [1 0 0]; end if time2>=10 && time2<20 transmitting_nodes=zeros(1,2);


if CW(z)== 0


transmitting node if z == 1 tx1 = 1; new_cw1 = randi(31,1,1); if new_cw1 == 0 new_cw1 = 15; end CW(1) = new_cw1; end if z ==2 tx2 = 1; new_cw2 =randi(62,1,1); if new_cw2 ==0 new_cw2 = 45; end CW(2) = new_cw2; end




end end


nodes then add a packet to the transmitting node. if tx1 == 1 && tx2 ==0 packet1(time2) = packet1(time2) + 1; elseif tx1==0 && tx2==1 packet2(time2) = packet2(time2) + 1; end

36

elseif sum(transmitting_nodes) > 1 collided = collided + 1; if tx1 == 1 && tx2 ==0 % packet1(time2) = packet1(time2) - 1; elseif tx1==0 && tx2==1 % packet2(time2) = packet2(time2) - 1; end end tx1 = 0; tx2 = 0; CW = CW - [1 1 0]; end

if time2>=20



transmitting node if z==1 tx1 = 1; new_cw1 = randi(31,1,1); if new_cw1 == 0 new_cw1 = 15; end CW(1) = new_cw1; end if z==2 tx2 = 1; new_cw2 = randi(62,1,1); if new_cw2 ==0 new_cw2 = 45; end CW(2) = new_cw2; end if z==3 tx3 = 1; new_cw3 = randi(341,1,1); if new_cw3 == 0 new_cw3 = 250; end CW(3) = new_cw3; end




end

37

end


nodes then add a packet to the transmitting node. if tx1 ==1 && tx2 ==0 && tx3==0 packet1(time2) = packet1(time2) + 1; elseif tx1 ==0 && tx2 ==1 && tx3==0 packet2(time2) = packet2(time2) + 1; elseif tx1 ==0 && tx2 ==0 && tx3==1 packet3(time2) = packet3(time2) + 1; end elseif sum(transmitting_nodes) > 1 if tx1 ==1 && tx2 ==0 && tx3==0 % packet1(time2) = packet1(time2) - 1; elseif tx1 ==0 && tx2 ==1 && tx3==0 % packet2(time2) = packet2(time2) - 1; elseif tx1 ==0 && tx2 ==0 && tx3==1 % packet3(time2) = packet3(time2) - 1; end collided = collided + 1; end

tx1 = 0; tx2 = 0; tx3 = 0; CW = CW - 1; transmitting_nodes = zeros(1,3); end % CW = CW -1 ; ts=ts+i;

end end


th1=(packet1.*lop)./1024.^2; th2=(packet2.*lop)./1024.^2; th3=(packet3.*lop)./1024.^2; total_thr = packet1 + packet2 + packet3; save ('tun_approach.mat','total_thr') % hold on % plot(1:30,smooth(th1,'-k')); % plot(10:30,smooth(th2(:,10:30)),'--k'); % plot(20:30,smooth(th3(:,20:30)),':k'); % legend ('11Mbps node','5.5Mbps node','1Mbps node') % xlabel('Time') % ylabel('Throughput') % grid on

38

3 nodes running at different times DCF-MBA


there's a transmitting node & it will be used to caclulate # packets transmitting_nodes1=0; lop= 2*1024; % Length of Packet % SIM=input('Enter the simulation time: '); timeslot=20e-6; % timeslot duration DIFS = 50e-6 / timeslot; SIFS = 10e-6 / timeslot; ACK = 14 * 8 ; % # bits in an ACK collided = zeros(1,30); Noise=0.05; n=4; tx=1; B=2.5*(10e6); % transmitting_nodes = zeros(1,3); % this array will be used to


caclulate # packets datarate=[]; packet=[]; % CW=[31 62 341]; tx1 = 0; tx2 = 0; tx3 = 0; % packet=zeros(1,3); ; packet1=zeros(1,30); packet2=zeros(1,30); packet3=zeros(1,30); datarate=[11e6 5.5e6 1e6]; x=5; for k=1:3 time(k)=lop/datarate(k); CW(k)=31+(((11e6)/datarate(k))-1).*x; end

for time2=1:30 for ts=1:(1/timeslot) if time2<10 transmitting_nodes=false; for z=1:1 if CW(z)== 0 transmitting_nodes(z) = 1; % assign '1' to a transmitting

node new_cw1 = randint(1,1,31); if new_cw1 == 0 new_cw1 = 15; end CW(1) = new_cw1;




39

end


nodes then add a packet to the transmitting node. packet1(time2) = packet1(time2) + 1; elseif sum(transmitting_nodes) > 1 collided(time2) = collided(time2) + 1; end end CW = CW - [1 0 0]; end if time2>=10 && time2<20 transmitting_nodes=zeros(1,2);


if CW(z)== 0


transmitting node if z == 1 tx1 = 1; new_cw1 = randint(1,1,31); if new_cw1 == 0 new_cw1 = 15; end CW(1) = new_cw1; end if z ==2 tx2 = 1; new_cw2 = randint(1,1,36); if new_cw2 ==0 new_cw2 = 34; end CW(2) = new_cw2; end




end end


nodes then add a packet to the transmitting node. if tx1 == 1 && tx2 ==0 packet1(time2) = packet1(time2) + 1; elseif tx1==0 && tx2==1

41

packet2(time2) = packet2(time2) + 1; end elseif sum(transmitting_nodes) > 1 collided(time2) = collided(time2) + 1; end tx1 = 0; tx2 = 0; CW = CW - [1 1 0]; end

if time2>=20



transmitting node if z==1 tx1 = 1; new_cw1 = randint(1,1,31); if new_cw1 == 0 new_cw1 = 15; end CW(1) = new_cw1; end if z==2 tx2 = 1; new_cw2 = randint(1,1,36); if new_cw2 ==0 new_cw2 = 34; end CW(2) = new_cw2; end if z==3 tx3 = 1; new_cw3 =randint(1,1,81); if new_cw3 == 0 new_cw3 = 40; end CW(3) = new_cw3; end




end end

41


nodes then add a packet to the transmitting node. if tx1 ==1 && tx2 ==0 && tx3==0 packet1(time2) = packet1(time2) + 1; elseif tx1 ==0 && tx2 ==1 && tx3==0 packet2(time2) = packet2(time2) + 1; elseif tx1 ==0 && tx2 ==0 && tx3==1 packet3(time2) = packet3(time2) + 1; end elseif sum(transmitting_nodes) > 1 collided(time2) = collided(time2) + 1; end

tx1 = 0; tx2 = 0; tx3 = 0; CW = CW - 1; transmitting_nodes = zeros(1,3); end % CW = CW -1 ; ts=ts+i;

end end


th1=(packet1.*lop)./1024.^2; th2=(packet2.*lop)./1024.^2; th3=(packet3.*lop)./1024.^2; our_thr = packet1 + packet2 + packet3; save('our_approach.mat','our_thr') % hold on % plot(1:30,smooth(th1),'-k'); % plot(10:30,smooth(th2(:,10:30)),'--k'); % plot(20:30,smooth(th3(:,20:30)),':k'); % legend ('11Mbps node','5.5Mbps node','1Mbps node') % xlabel('Time') % ylabel('Throughput') % grid on %

42

Appendix C

Abbreviations

AP Access Point

DCF-MB Distribuated Coordinated Function - Multiple Backoff

DCF-MBA Distribuated Coordinated Function - Multiple Backoff Advanced

IEEE Institute of Electrical and Electronics Engineers

QoS Quality of Service

Documents

IEEE 802.11b anomaly solution