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Abstract WLANs have emerged very fast in both public and

private areas during the recent years. They provide a non trivial

replacement for the complicated and high cost wired LANs.

However, the access points (APs) that build these WLANs do not

have a very long coverage range. Consequently, many handoffs

may occur as the mobile station is moving while accessing the

network resources located at the distribution system side.

Unfortunately, these handoffs can disturb the real time

applications if they consumed a long time.

This paper investigates the reduction of the handover in

wireless network domain. We considered some mechanisms andtechniques used to reduce handoff latency in wireless protocols.

We developed a simulation tool in order to compare the data

throughput with or without the re-association and the re-

authentication phase. Experimental results show that the

reduction of the re-association and the re-authentication phases

enhances throughput and reduces the handoff latency.

Index Terms Handoff, re-association, re-authentication,

wireless LAN.

I. INTRODUCTION

LANs deals on the IEEE 802.11 standards become well

known due to their many benefits such as: easy

operation and low cost [1][2][3]. These WLANs motivated theusers because it is easy to use in such as places especially in

the hot spot areas like universities, airports and hotels. In

addition, one of the goals for these networks (Wi-Fi) is the

ability to provide wireless mobile stations with free mobility

within the coverage range of their associated access points

(AP). So stations are able to move freely while accessing the

network. Since these wireless networks (WLANs) were

designed for indoor use at the beginning, APs of these

WLANs have a limited coverage radio range. Furthermore,

the limited range of these APs enforces a station to handoff

(re-associate) from its previous AP to a new AP, whenever the

station moves beyond the coverage range of its currentlyassociated AP in order to maintain continuous connectivity.

In WLANs, fast re-association of stations from one AP to

M. Omari, associate professor, is with the laboratory of sustainable

development and computer science (LDDI) at the University of Adrar, Adrar

01000, Algeria (phone: 213-49-967571; fax: 213-49-967572; e-mail:

omari@univadrar.org).

S. Rezzougui, master student, is with the University of Adrar, Adrar 01000,

Algeria (e-mail: rezzougui_sarah@yahoo.com).

N. Talhaoui, was with the University of Adrar, Adrar 01000, Algeria (e-

mail: talhaoui_noura@yahoo.com).

another (low handoff latency) is one of these requirements

which are not supported very efficiently in the IEEE 802.11

standard. Therefore, these handoffs can cause long latencies

and packet loss which can affect the performance of such real-

time applications. Consequently, fast handoff during

mobility in WLANs is considered to be a critical issue [1].

As a result of the advancement in wireless technologies,

real-time Multimedia services such as video conferencing

have been provided by Internet Service Providers (ISPs) to the

wireless subscribers [1]. However, since the IEEE 802.11

does not support fast handoffs, the performance of theapplications that support such services can be degraded when

the station moves beyond the coverage range of its original

AP and performers a handoff to another AP. This degradation

in the performance is the result of the handoff latency which is

caused by the handoff procedure. Consequently, if the latency

of the handoff procedure is large, some packets may be lost

which can disrupt the current session and make the real-time

applications become unreachable..

The rest of this paper is organized as follows. Section 1

provides an overview of the IEEE 802.11 protocol. The IEEE

802.11 architecture and the handoff procedure and its phases

are presented in Section 2. In Section 3, we will presentrelated work in reducing handoff latency. In Section 4, we

present our simulation experiments, their parameter settings,

along with the obtained results. The conclusion is presented in

Section 5.

II. IEEE 802.11 WIRELESS LANS OVERVIEW

In Wireless Local Area Network (Wireless LAN) the user is

assisted with high bit rate connection because of wireless

(Radio) connection [1]. The range of wireless LAN is fairly

short but it support high bit rate. In Wireless LAN, IEEE

standards enumerate its different types and these IEEE

standards also include the encryption algorithm to make

Wireless LAN more secure as compare to regular LAN.

Similar with the LANs, the IEEE 802.11 based wireless

local area networks (WLANs) provide low cost and effective

way to access the internet. These WLANs have various

standards which are specified by the Institute of Electrical and

Electronics Engineers (IEEE). The famous ones are: IEEE

802.11a (Band: 5 GHz, Data rate: 54 Mbps), IEEE 802.11b

(Band: 2.4 GHz, Data rate: 11 Mbps), IEEE 802.11g (Band:

2.4 GHz, Data rate: 54 Mbps), IEEE 802.11n (Data rate: 200

Mbps), and IEEE 802.11F [1] [2].

Simulation of Reducing Re-association and Re-

authentication Phases for Low Handoff Latency

Mohammed Omari, Sarah Rezzougui, and Nora Talhaoui

W

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A. Wireless LANs Architecture

In the IEEE 802.11 architecture, there are several

components and services which interact and work together in

order to provide WLAN functionality [1]:

1- Station: A station is the basic component of the wireless

network. It is any device which provides 802.11functionality

by implementing the 802.11 functions in Medium Access

Control (MAC) layer as well as physical (PHY) layer. A

station could be a laptop, PDA or an AP.

2- Access point (AP): An AP is any device that has

802.11functionality and allows the associated mobile stations

to access the services of the distributed system (DS) via the

wireless medium. Each frame on an 802.11 network must be

converted to another type of frame in order to be delivered to

the wired network by an AP which performs this wireless-to-

wired bridging function.

3- Wireless medium: The wireless medium is used by the

802.11 standards in order to move frames from one station to

another.

4- Distribution System (DS): A distribution system (DS) is

also called the backbone network. It is used to forward framesbetween several connected access points (APs), which form a

large coverage area, so they can communicate with each other

in order to track the movements of mobile stations (stations).

[1]

B. Wireless topologies

The 802.11 standard supports the following three

topologies [1]:

1- Independent Basic Service Set (IBSS) networks: IBSS

networks are commonly referred to as Ad Hoc Networks. In

IBSS networks, all mobile stations must be within direct

communication range in order to communicate with each

other. In addition, these networks consist of a small number ofstations which are set up for a specific purpose and for a short

period of time, for instance a single meeting in a conference

room.

2- Infrastructure Basic Service Set (BSS) networks: BSS

networks are differentiated by the use of an AP. In BSS

networks, APs are used for all communications, including

communication between mobile stations in the same service

area. When stations need to communicate with each others,

they communicate by transferring each frame to the AP,

which forwards them to their destination.

3- Extended Service Set (ESS) networks: The ESS consists

of different BSSs networks which are combined together inorder to form a large network. Each BSS has a single AP that

acts as bridge between the wireless link and any other

connections. The AP in each BSS is connected to a

distribution system that is usually an Ethernet backbone. In

general, the ESS is the union of the multiple BSSs that are

connected with each other through a DS.

C. Handoff Procedure

The handoff procedure occurs whenever the mobile station

moves farther than the radio coverage range of its currently

associated AP. When the received signal strength identifier

(RSSI) value becomes less than a predefined handoff

threshold (HT) the station initiates the handoff procedure and

decides to associate to a new AP which has a better RSSI

value in order to maintain its wireless connectivity [1].

During the handoff procedure, there are sequence of

messages (management frames) that are exchanged between

the APs and the station. As a result of these messages

exchanging, the current state information is transferred fromone AP to another with respect to the station.

The entire handoff procedure can be classified into three

logical phases namely: Discovery phase, Re-authentication

phase and Re-association phase.

D. Discovery Phase

In the discovery phase, two sub-phases are involved: the

handoff initiation sub-phase and the scanning sub-phase.

When a station is moving farther than the radio covered area

of its currently associated AP, the signal strength and the

signal-to-noise ratio SNR of the signal from the currently

associated AP decreases until it becomes less than HT. This

causes the station to initiate a handoff procedure. On the other

hand, before the station disconnects the connection to the

currently associated AP, the station needs to find new APs and

selects the best one among them in order to connect itself

with. This is achieved by the Medium Access Control (MAC)

layer scanning function [1].

E. Re-authentication phase

The re-authentication is a process in which the identity of

the station is either accepted or rejected by the AP. The re-

authentication process starts by the station sending a re-

authentication request frame from the station to the selected

AP. This frame informs the selected AP of the station identity.Upon receiving the re-authentication request frame, the

selected AP responds with a re-authentication response frame.

This frame indicates the acceptance or rejection of the selected

AP. Once the station has been successfully authenticated, then

it can send a re-association request frame to the selected AP

[4].

There are two authentication services or methods that have

been defined in the IEEE 802.11 standard: The open system

authentication and the shared key authentication.

The Open System authentication is considered a null

authentication algorithm which means that all the requesting

mobile stations (stations) can be authenticated by the recipient

AP. The authentication algorithm at the recipient AP is set to

Open System authentication. In addition, this method required

the exchange of only two frames between the station and the

new AP, an authentication request frame and an authentication

response frame [1].

In the shared key authentication method, the mobile station

initiates an authentication process by sending an

authentication request frame to the new selected AP. Upon

receiving the authentication request frame, the new AP utilizes

a Wired Equivalent Privacy (WEP) key in order to generate a

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challenge text for the station. After the challenge text has been

generated, the new AP attaches the challenge text into an

authentication response frame and sends the frame as a replay.

When the station receives the challenge text, it encrypts this

text with the correct shared WEP key and returns an

authentication request, which contains the encrypted challenge

text, to the new AP [1].

Once the authentication request frame received

successfully, the new AP gets the encrypted text from thereceived frame and decrypts it using the shared WEP key.

Then, the new AP compares the decrypted and the original

challenge texts. If these two texts match, the new AP sends an

authentication response frame back to the station in order to

confirm a successful authentication. This method requires four

exchange messages between the station and the new AP.

F. Re-association phase

The re-association phase can be defined as the process in

which the association of a mobile station is transferred from

one AP to another. After the station has been successfully

authenticated with the new selected AP, the re-association

process begins [1].

III. REDUCING HANDOFF LATENCY IN WIRELESS PROTOCOLS

Reducing handoff delay can be divided into four

subcategories. The first category is to reduce the probe delay

during the scanning phase where the focus is mainly to reduce

the number of channels to be scanned. The second category is

to reduce the re-authentication delay where the focus is

mainly on pre-authentication before station joins the new

network. The third category is to reduce the re-association

delay and the forth category is to reduce the overall handoff

delay which covers the previous three delays. Next, we will

discuss some mechanisms and techniques that are currently

used to reduce handoff delay.

A. Reducing MAC Layer Handoff Latency in IEEE

802.11Wireless LANs

The probe delay constitutes the biggest part (over 90%) of

the handoff latency. For this reason, Shin and Forte [6]

focused on minimizing this delay by improving the scanning

procedure, using a selective scanning algorithm. Furthermore,

Shin and Forte had to minimize the number of times the

previous scanning procedure was needed. This second point

was achieved with the use of a caching mechanism that is

described below.

In the selective scanning procedure, when a station scans

APs, a channel mask is built. In the next handoff, during the

scanning process, this channel mask will be used. In doing so,

only a well-selected subset of channels will be scanned,

reducing the probe delay.

The selective scanning procedure reduced the handoff

latency between 30 to 60% [6]. For seamless VoIP, it is

recommended that overall latency does not exceed 50 ms.

This further improvement was achieved by using an AP cache.

The AP cache consists of a table which uses the MAC address

of the current AP as the key. Corresponding to each key entry

in the cache is a list of MAC addresses of APs adjacent to

current one which were discovered during scanning. This list

is automatically created while roaming. The cache has a size

of ten, meaning that it could store up to ten keys and a width

of two, meaning that for each key, it can store up to two

adjacent APs in the list.

B. Reducing Layer Two Handoff Latency in WLANs UsingAdvanced Context Distribution (ACD)

This mechanism aims to reduce the re-association delay

which is caused by the transferring of the mobile station

context information from the old AP to the new AP using

the Inter Access Point Protocol (IAPP). Context information

of a station is the stations security information that may allow

faster re-authentication of a station on re-association. Using

IAPP for transferring stations context can increase the re-

association delay (up to 40 ms) due to its four additional

messages during re-association phase. transferring of stations

context from the old AP only if it can satisfy a specific

condition which is called context threshold. Only new AP

with RSSI value bigger than the context threshold value (CT)

can request stations context to be transferred from the old

AP.

C. Eliminating handoff latencies in 802.11 WLANs using

Multiple Radios

In the multi-radio scenario, a node is assumed to have two

interfaces: the primary interface and the secondary interface

[7]. Suppose that the primary interface is associated with APoldand is used for communication, while the secondary interface

is available to perform other tasks. Clearly, such multi-radio

node will have an advantage since it will be able to

communicate normally and perform management operationssimultaneously. In a naive approach, the secondary interface

could perform the scanning stage (which is the most time

consuming stage of a handoff), while the primary interface is

communicating normally with its AP. Once the secondary

interface determines an AP to which the node needs to

connect next, the primary interface could start the handoff

process skipping the scanning stage. This optimized handoff

can be performed in less than 5 ms. Besides the delay due to

the last two stages of handoff, just switching the card to a

different channel may require as much 20 ms, depending on

chipset, which is significant for real-time applications. This

naive approach vastly reduces latency due to handoff and is

absolutely safe, since from the AP infrastructures point ofview, the node does not do anything unexpected, it simply

appears as if the node knows which AP to connect to without

a scan [7].

D. Reducing Re-authentication Delay

Pack et. al. have proposed a fast predictive handoff scheme,

which is based on mobility prediction, for reducing the re-

authentication delay [4]. In this proposed scheme, the

reduction of the re-authentication delay was achieved by

enabling a mobile station to perform re-authentication

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procedures for multiple access points (APs) rather than just

the current AP whenever it enters the coverage radio range of

a new AP and when the initial registration is performed. The

authentication information, related to the station, is

proactively propagated to multiple APs (neighbour APs)

depending on the stations mobility. In addition, a prediction

method called the Frequent Handoff Region (FHR) selection

algorithm, which is introduced in this scheme, was used for

the selection of these multiple APs. This algorithm takes intoaccount the stations mobility patterns and service classes.

E. Reducing Re-association Delay

To further reduce the re-association delay during handoff

procedures, Funn has proposed a layer two handoff

mechanism called Selective Pro-active Context Caching

(SPCC). The main idea of SPCC mechanism is that, when a

mobile station first initiates a handoff process, it starts the

scanning phase. During this phase, the station sends a

modified probe request frame to all APs discovered in the

scanning phase except the old AP. The modified probe request

frame contains the old APs MAC address. After that, all the

APs that receive the modified probe request frame will send

the station-AP link quality information to the old AP (base on

the old AP MAC address received in the probe request frame)

using Link Quality Info management packet which is

introduced for the potential next APs selection process.

Consequently, a list of potential next APs to which a station is

likely to associate with will be created in the old AP [1].

IV. DEPLOYMENT OF SDES SECURE MECHANISM IN

WIRELESS DOMAIN

A. Authentication and Association

Soliman and Omari [8][9][10] developed a new securitymechanism based on stream ciphers. The Synchronous

Dynamic Encryption System (SDES) performs encryption,

integrity, and authentication. SDES is characterized by its

efficiency while maintaining higher security through dynamic

keys. At network initialization stage, all APs go through a

registration process authenticating themselves with the

authentication server (AS) (once in their life cycle). Then,

every AP is authenticated with its neighboring APs via the AS

that generates and transmit a private secret shared key SSK to

each pair of authenticated APs. When a mobile station joins

the network, with the pair (MAC address, secret

authentication key SAK) installed in its wireless card, it sends

a first authentication request to its local AP. The AP forwards

the station's request to the AS in order to authenticate the

station, and transfer its newly generated SAK back to the AP.

Fig. 1 explains in detail the protocol sequence of the

stations initial authentication.

Notice that the station authentication with the AS is done

only once; subsequent authentications are performed directly

with the associated AP. Only in case a station remains out of

rang with its AP for long time, would it need to re-

authenticate with the AS again.

B. Handover

When the communication signal between the mobilestation and its currently associated AP (say AP1) get weak, the

station roams for another AP (say AP2) of stronger signal.

Then, the station sends a handover request to AP1 including

AP2's info. Usually, AP1 and AP2 are adjacent and wired;

therefore, they are already pre-authenticated to each other via

the AS. Following the rule of a trusted by a trusted is

trusted, AP1 sends a secure handover request to AP2including the station's authentication information. This AP-AP

communication is secured via their private shared SSK. Then,

AP1 sends a secure integrity check message to the station in

order to check the previously received data integrity.

V. SIMULATION, RESULTS AND ANALYSIS

Our simulator was developed based on a framework

initially created by Soliman and Omari [8] [9] [10] to simulate

security protocols in ESS networks. Every station performs

tasks independently; so the simulator needs to implement

concurrent programming. Java has packages to facilitate

multiprogramming, namely threading. Next is an example of

some code that is part of base station (AP) simulation:baseSt ati onThr eads = newBaseSt at i onThr ead[ si mul at i onPar amet ers. get NumberOf BaseSt ati ons( ) ] ;f or ( i nt i = 0; i