Analysis of Distributed Resource Management in

Wireless LANs that Support Fault Tolerance

Ghassan Kbar

and Wathiq Mansoor

American University in Dubai (AUD), UAE

Zayed University, Dubai, UAE

Abstract. Deploying wireless LANs (WLAN) at large scale is mainly affected

by reliability, availability, and performance. These parameters will be a

concern for most of managers who wanted to deploy WLANs. In order to

address these concerns, new radio resource management techniques with fault

tolerance can be used in a new generation of wireless LAN equipment. These

techniques would include distributed dynamic channel assignment, sharing load

among Access Points (AP), and supporting fault tolerance. Changing from the

relatively static radio resource management techniques generally in use today

to dynamic methods has been addressed in previous research article using

centralized management, but it suffer from network availability problem. In

[10] a new distributed management for dynamic channel assignment has been

suggested. In this paper the idea has been extended to support fault tolerance,

which improves the network availability and reliability compared to centralized

management techniques. In addition, it will help in increasing network

capacities and improve its performance especially in large-scale WLANs. The

new system has been analyzed and according to binomial distribution results

showed improvement of network performance compared to static load

distribution.

1 Introduction

WLAN technology is rapidly becoming a crucial component of computer networks

that widely used in the past few years. Wireless LAN technology evolved gradually

during the 1990s, and the IEEE 802.11 standard was adopted in 1997 [1]. Companies

and organizations are investing in wireless networks at a higher rate to take advantage

of mobile, real-time access to information.

Enterprise managers want to deploy wireless networks with several important

qualities. These include; high security, highly reliable and available WLANs with

very little downtime, and high performance. The ideal wireless network is to have

reliability, availability, and performance criteria to be similar of wired enterprise

networks. In addition, it should be possible to deploy wireless networks very quickly

and without the need for extensive and time-consuming site surveys. Furthermore, the

networks should have the flexibility needed to support load balance and changes in

the radio environment. Radio resource management (RRM) forms the basis of quality

Kbar G. and Mansoor W. (2006).

Analysis of Distributed Resource Management in Wireless LANs that Support Fault Tolerance.

In Proceedings of the 5th International Workshop on Wireless Information Systems, pages 66-75

 SciTePress

of service (QoS) provisioning for wireless networks [2]. It is an intense research area

due to the wireless medium’s inherent limitations and the increasing demand for

better and cheaper services. Improving the mobility management has been addressed

in [3] based on dividing the location management into two levels, intra and inter

mobility. This will reduce the amount of signaling traffic, but still didn’t address the

problem of reliability and availability. Supporting security, reliability and QoS in

dynamic environment has been discussed in [4] using modified routing protocol

OSPF-MCDS over WLANs.

In WLANs, when AP is called on to serve a high number of users, it is likely to

become overloaded, and the resulting congestion may significantly degrade the

service received by users dependent on that AP. Overhead bits dramatically reduce

the effective data rate available as described in [1, 5, 6]. Congestion further reduces

the throughput experienced by a user because AP and the mobile computers it serves

share a single radio channel. All stations using an IEEE 802.11 AP share the same

bandwidth resource, and congestion is likely to be particularly severe in areas of high

user density [5, 7]. It is highly desirable for wireless LAN equipment to include

provisions to mitigate this problem and try to eliminate or reduce the effect of

congestion.

The design of WLAN is usually based on signal strength measurements and on

consideration of radio propagation issues. This is challenging because the building is

a three-dimensional space, and an AP located on one floor of the building may

provide signal coverage to adjacent floors of the same building and perhaps to other

buildings, as well [5, 7]. Firstly, selection of AP location should be done to provide

complete coverage of the target space without undue coverage overlap. Consideration

of the characteristics of the radio propagation environment in which the wireless

LAN is being deployed can be difficult but is important in a WLAN design [6].

In [8], centralized management architecture has addressed the above issues to

improve reliability, availability, performance, and deployment effectiveness in

enterprise and other large-scale wireless LANs. These improvements arise from the

radio resource management algorithms contained in the software running on the

intelligent switches that control APs. In this approach, the software controlling the

APs attempts to optimize performance without having any direct control over client

behavior, and this limits the effectiveness of the approach. The above article

discussed how radio resource management is beginning to be used to mitigate some

of the problems in enterprise wireless LANs. However, centralized management still

suffer form availability problem if the centralized node failed. Gandhi [12] has

addressed the network availability by focusing on tolerating access-points failures,

where it detects fault based on signal-to-noise ratio. A solution to network availability

has been addressed in [10] using distributed network management that has the same

benefit described in [8] (reliability, availability, performance and deployment

effectiveness in enterprise and other large-scale WLANs). In this paper the

distribution network management has extended to the one described in [10], but with

higher network availability and fault tolerance as would be described in the following

sections.

2 Distributed WLAN Network Management Architecture with

Fault Tolerance

In order to reduce the congestion at the AP and at the same time maintain high

network availability, distributed dynamic network management with fault tolerance

across multiple APs has been suggested in this paper as described in Figure 1. As

shown in Figure 1, the dotted circle indicating the coverage of a particular AP. This

coverage might overlap with adjacent AP, where interference is caused if both APs

are running at the same channel radio frequency. At the same time mobile terminal

falling within the overlapped region would have the choice to have association with

either AP. Each AP has 2 coverage areas; low coverage as shown in the dotted circle

for AP-2, Ap-4, and AP-5, and high coverage as shown in the blue solid circle of AP-

1, and in the grey solid circle of AP-7. The AP-1 and AP-7 have also low coverage

area as shown in the dark grey solid circle, and pink solid circle for AP-1 and AP-7

consecutively. The solid circle for AP-3, with dashed pattern indicates a failed AP,

where its coverage has been substituted by the high coverage area of AP-1 and AP-7.

This architecture improves the WLAN performance by reducing the interference

through assigning different channel radio frequency to adjacent as described in

section 2.1. This architecture also improves the performance in load balancing as

well as reducing the congestion by dividing The coverage region of every AP is

divided in 2 regions, left hemisphere and right hemisphere, where mobile terminal

would choose to associate with one AP according to its location in the left or right

hemisphere of the AP as described in the subsection 2.2. It also improve the WLAN

performance by providing high network availability and fault tolerance through

broadcasting 2 coverage levels by every AP to substitute for the failing AP coverage

area to allow mobile terminal falling within the failed AP to use the WLAN as

described in subsection 2.3.

Fig. 1. Distributed APs Management Techniques with Fault Tolerance.

2.1 Distributed Dynamic Channel Assignment

The performance of a network depends, in part, on the assignment of radio channels

to APs. This assignment is often done using a manual process in which the designer

attempts to assign the channels in a way that minimizes co-channel overlap. The

coverage areas, and therefore the channel assignments, are dependent on among other

things such as the radio propagation environment. Since the radio propagation

environment changes, so one cannot be sure that the channel assignments valid at the

time the network was designed will continue to be valid.

However, distributing

channel assignment between APs according to its channel frequency that minimize

the overlap between these frequencies would reduce the interference between these

channels as described in [10] where the assignment of channel frequency to adjacent

APs, are spread from minimum frequency to maximum which cause less interference

between these APs. Example Adjacent APs 1, 2, 3 are assigned channel number 1, 6,

11 consecutively, and APs 4, 5, 6 are assigned channel number 2, 7, 10 consecutively.

Distributed Dynamic Channel Assignment would add extra flexibility to assign a

proper channel number to different AP dynamically according to AP index number.

This dynamic assignment would help in replacing failed AP with another one using

the same index number. Each AP requires having an index number that can be set at

the initialization stage.

2.2 Load Balancing and Terminal Association

Since an AP and its associated clients share a limited bandwidth resource, APs can

become overloaded, leading to congestion and poor network performance. WLAN

equipment with this capability can enhance network performance considerably.

Association between a client and an AP begins with an association request that is

initiated by the client. This association request is normally preceded by the client’s

transmitting one or more probe requests on channels it selects. In each of these probe

requests, the client asks for a response from all APs operating on that channel and

able to receive the client request. This tells the client which APs are within radio

range, and the signal strengths received from the APs give an indication of which

APs will be able to provide higher-quality service. Before sending an association

request, a client should also have previously sent an authentication request that has

been granted.

The method by which a client decides with which AP to request

association is not specified in the IEEE 802.11 standard.

However, the association between mobile terminals in distributed dynamic

management can be controlled not only according to signal strength which cause

network congestion at some AP, but also according to its location in the right or left

hemisphere of the AP [9]. This management technique works according to

distributing the load between adjacent APs where mobile terminal fall within the

overlapped region would be associated only with one AP located to its right

hemisphere. If mobile terminal located within the overlapped region, the signal

strength would not be used to determine to which AP it will associate with. In fact,

the mobile terminal location would be used to distributed load to different AP, where

it will associate with AP that is located at the right hemisphere of itself. As shown in

Figure 1, if mobile terminal located anywhere within the region of AP-1 without

overlapping with other AP, it always associate with this AP-1. However, if it locates

within the overlapped region of AP-1, and AP-2, it will associate with AP-2 where

the terminal is located at the left hemisphere of AP-2.

2.3 WLAN with Fault Tolerance

WLAN with multiple AP suffer from network unavailability when one or more AP

failed. This will cause some mobile terminals falling within the area of the failed AP

to lose connection and become unable to use the network. The distributed architecture

suggested previously in section 2 has the advantage of providing fault tolerance. This

is done by increasing the broadcasting power of the adjacent APs of the failed one to

high coverage to cover the area of the failed AP. In order for this architecture to

work, each AP should operate in 2 level of coverage power. Each AP would also

detect the failed neighbor AP through continuous broadcast management signal that

occurs at specific interval of time, where each AP will broadcast its available network

table for all adjacent APs that have sensitive antenna to detect it. The available

network table would be updated by each AP when it receive broadcast from

neighboring APs, where after few broadcast the table would contain a list of all

operating AP that belong to the same network as explained in the following sequence

diagram. Assume in this diagram that AP-1 has joined the WLAN and broadcasted its

own availability network table (that contains list of known AP {1}) to neighboring

APs (AP-2, and AP-3). AP-1 also receives broadcast from AP-2 and AP-3. The

management signal received from AP-2 would contain a list of all APs (2, 4*, and 5*)

that are neighbor to AP-2, where 4* is the list of APs for AP-4 that had been received

previously at AP-2 from AP-4. The list 4* indicating another list of AP-4 that

contains the list of APs that are neighbor of AP-4 which are AP-5 and list of AP-7

(7*). Also list from AP-5 (5*) would contain the list of AP-5 that are neighbor of AP-

5 which are (3, 4, 6, 7, 8). At the same time the list received from AP-7 would contain

the list of APs that are neighbor to AP-7 which are (4, 5, 8, 9). Eventually, the

availability table at AP-1 would contain a list of all APs that are belonging to the

same network. The other APs would also update their availability table which also

would include the list of all APs of the same network. If one of the AP failed

(example Ap-3) as shown in figure 1, all of its neighbor APs (AP-1, and AP-7) would

not receive a regular broadcast from it. If AP-1 and AP-7 didn’t receive broadcast

management signal from its neighbor AP-3, they will assume a failure on AP-3 and

adjust their coverage power to operate at high level. This will allow AP-1, and AP-7

to cover the area of the failed AP-3 as shown in figure 1 (blue solid circle, and grey

solid circle).

Fig. 2. Sequence of broadcast management at AP.

3 Analysis Results

The binomial distribution gives the discrete probability distribution )/( NnP

obtaining exactly n successes (terminal falling only within a particular AP range eg.

AP-5) out of N Bernoulli trials or terminals falling within the coverage of a particular

AP and its neighbors APs (where the result of each Bernoulli trial is true with

probability p and false with probability

−

). The binomial distribution is

therefore given by:

(1)

(2)

where

⎟

⎠

⎞

⎜

⎝

⎛

is a binomial coefficient.

Assume that the desired probability of terminals falling within one Access Point (eg.

AP-5) only is

8.0=p . Therefore the probability of failure is 2.01 =

−

pq . The

AP-1

AP-2 AP-3

2, 4*, 5*

3, 5*, 6

4* =

5, 7*

5*=

3, 4, 6, 7*, 8

7* =

4, 5, 8, 9

total number of terminals (trials) is 50

N . According to binomial distribution of

formula 2, the probability of at least up to i terminals is falling within only one AP

(eg. AP-5) is:

knk

kkn

ixp

−

=≤

∑

)1(

!)!(

)(

(3) (3)

The probability that more than i terminals are falling within the same AP is given by:

knk

kkn

ixpixp

−

−=≤−=>

∑

)1(

!)!(

1)(1)(

(4) (4)

Using formula 4, we can obtain the probability for more than i terminals are falling

within AP-5 according to different value of i {i=20 to 48} as shown in Figure 3. As

shown in this figure, the probability would reach one when desired probability is 0.8

if the number of terminals is 32. The number of terminals would increase to 42 if the

desired probability increases to 0.95.

According to distributed management and using binomial distribution, the total

number of terminals that are only falling within AP-5 can be obtained by the

following formula as shown in figure 4:

)( ixpiT >×=

(5)

Using signal strength terminal management the total number of terminals that are

falling within AP-5 is N. Hence the advantage of using distributed management is

reducing the number of terminals associated with a particular AP (eg. AP-5) by:

(

)

ixpiNTN >×−=−

(6)

If AP failed and its load distributed equally among 3 neighbor APs then extra load

would be added to its neighbor AP in addition to formula 4 as follow:

3)1(

!)!(

213)()(

⎟

⎠

⎞

⎜

⎝

⎛

−

−=≤+>

−

∑

knk

kkn

ixpixp

(7)

As shown in figure 4, the maximum total number of terminals depends on the desired

probability. When the desired probability is 0.95, the maximum number is 43

terminals. This number drops to 32 when the desired probability is reduced to 0,8.

Using signal strength terminal management the total number of terminals that are

falling within AP-5 is N.

As shown in figure 5, the total number of terminals falling within the same access

points using signal strength is N (50 terminals) independent of the desired

probability. The total number of terminals using distributed management depends on

the desired probability, where the total number would be 32 at desired probability of

0.8, and it increases to 43 when the desired probability increases to 0.95. Hence, the

total number of terminals falling within the same AP using distributed management

drops by 1-32/50 (35%). This would allow terminals to have better bandwidth

allocation using the distributed management scheme.

Using

formula 7, figure 6 shows that at desired probability of 0.8, the probability of

40 terminals joins one AP when there is a failed AP is 0.6 higher than 0.4 when there

is no AP failure. This increases of terminals associating with one AP is caused by the

failure of neighbor AP where its terminals re-associate with other AP that fallen

within its range after increasing its power coverage. Figure 7 shows similar effect of

adding extra load to AP caused by terminals were associated with the failure AP and

re-associated with neighbor AP after the failure.

Probability distribution p(x>i)

0.2

0.4

0.6

0.8

1.2

48 46 44 42 40 38 36 34 32 30 28 26

Number of terminals (i)

probability

desired p=0.8

desired p=0.85

desired p=0.9

desired p=095

Fig. 3. Binomial Probability Distribution.

Total terminals p(x>i)

48 46 44 42 40 38 36 34 32 30 28 26

Number of terminals (i)

maximum number of

term inals

Total Terminals,

desired p=0.8

Total Terminals,

desired p=0.85

Total Terminals,

desired p=0.9

Total Terminals,

desired p=0.95

Fig. 4. Total number of terminals.

4 Conclusion

The architecture that has been described has the potential to improve performance,

and deployment effectiveness in enterprise and other large-scale wireless LANs, and

at the same time maintain a high network availability and reliability through network

fault tolerance that is based on dynamic power coverage assignment. These

improvements arise from the distributed dynamic resource management deployed in

mobile terminals and APs.

The suggested technique assists the APs to optimize its performance by

dynamically allocating different frequency to adjacent AP in order to reduce the

signal interference. In addition, the association between mobile terminals and AP are

distributed to different APs according to their location in the left or right hemisphere

of the AP. Furthermore, fault tolerance has been provided through the dynamic power

coverage assignment. The effect of fault tolerance improves the network availability,

but it increases loads on APs. This increase of load would be temporarily until the

failed AP has been replaced by working one.

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