Author Topic: Reducing Layer 2 Handoff Latency in WLANs using Advanced Context Distribution  (Read 3749 times)

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Author : Laeth A. Al-Rawi, Rosli Salleh, Ghaith A. Al-Rawi, Hassan A. Al-Rawi, H.Keshavarz
International Journal of Scientific & Engineering Research Volume 2, Issue 11, November-2011
ISSN 2229-5518
Download Full Paper : PDF

Abstractó WLANs have experienced very fast deployment in both public and private areas over recent years. They provide nontrivial replacement for the complicated and high cost wired LANs. However, the Access Points that WLANs are build from do not have very wide coverage range (usually under 100m indoors). Consequently, many handoffs occur as the mobile host moves while accessing the network resources located at the distribution system. Unfortunately, these handoffs can disturb real time applications if they take too long (more than 50ms). In order to resolve this problem, this paper introduces a new mechanism for reducing handoff delay called Advanced Context Distribution (ACD). ACD is able to reduce re-association phase delay by eliminating Inter Access Point Protocol (IAPP) excess time consumed for transferring station context information from the old access point to the new associated access point (up to 40ms delay).

Index Termsó IEEE 802.11, WLANs, IAPP, Hand-off Latency.

Recently, WLANs have become very popular as they provide user mobility, high data rates (up to 54Mbps) and low cost [3]. By deploying them, users can access LAN
resources without the annoying wires by using an entity called Access Point (AP) that can bridge user traffic from and to the distribution system (LAN). However, this AP does not have a very long coverage range (usually under 100m indoors) which may restrict user mobility. Consequently, many APs are needed to  expand WLAN boundaries and  so  the  user  can reach longer distances and still have accessibility to the LAN (something not possible by using only one AP). Although this is a good improvement, the user still has to change association from the old AP to a new AP while moving, and the received signal from the old associated AP gets weaker (handoff). This association changing process is called a handoff, during which no data is sent or received by the user. This process can affect real time applications like voice and video over Internet Protocol (VOIP) because these applications need real-time traffic speed possibly affected by long delays (over 50ms) [6].
The rest of this paper is organized as follows. First, the basics of IEEE 802.11 handoff procedure are reviewed. Next, the IAPP is briefly explained. The last sections review one of the mechanisms for reducing re-association delay. Finally, we will conclude by explaining in detail our new mechanism (ACD) and simulation results.

The total handoff process can be divided into two logical phases, namely, Discovery phase and Re-authentication phase, in which management frames are exchanged between the Mobile station (MS) and the Access Point (AP) to move MS association from the old to the new AP [7]. This process also involves  some  cooperation  between  the  APs  in  order  to transfer MS context information from the old to the new AP (e.g. using IAPP) (Figure 1).

2.1   The Discovery Phase
Whenever the Received Signal strength (RSS) from the associated Access Point (AP) gets weaker, the Mobile Station (MS) starts the discovery phase by switching to each channel defined by the standard used (11 channels in IEEE802.11b/g and 32 channels in IEEE802.11a) and scans for any available APs [2]. The MS does this because it does not have any information about the surrounding APs so it must discover them itself. There are two kinds of scanning defined by the IEEE802.11 standard, active and passive scanning. In active scanning, the MS scans each channel by sending probe request frames and waits for responses from all available APs on that channel. This scanning type can take a long time, up to 400ms, as the MS must wait for the MinChannelTime (minimum channel timer is used by the MS to specify the minimum time needed for receiving responses from APs) on each channel while it is being scanned [4]. On the other hand, by using passive scanning, the MS only switches on each channel and waits for beacons sent by the APs located on that channel. Although this type looks easier done by MS as it does not consume a lot of power or bandwidth, it takes longer than active scanning because MS has to wait for at least a one- beacon interval on each channel (normally 100ms) introducing big delays (~1s) which are not acceptable in real time applications.

2.2   The Re-authentication phase
When the Mobile Station (MS) finishes scanning all available channels and discovers the surrounding Access Points (APs), it tries to (re)associate to the AP with the best Received Signal Strength Identifier (RSSI). However, this association or re- association process is not accepted by the new found AP without the MS being (re)authenticated (authentication phase consists of two processes: re-authentication and re-association) [7]. Consequently, the MS starts by sending an authentication request frame to that AP and waits for a response which indicates AP acceptance or rejection. After the MS is successfully (re)authenticated by the AP, it (re)associates to this AP by sending an (re)association request frame and waits for a response. Whenever the AP receives the (re)association frame, it sends its response depending on some required features like supported rates by the MS. Finally, the MS associates with the new AP and all traffic destined to the MS will be sent through that AP.
The re-association process involves cooperation between the old and new APs in order to transfer MS-related context information to  the  new  AP.  This  cooperation can  be  done using  Inter  Access  Point  Protocol  (IAPP).  The  IAPP  can transfer MS credentials between APs from different vendors (context transferring process will be explained in the next section). This context transfer introduces extra delay to the total handoff latency because of the extra IAPP packets exchanged between the APs for the transfer of context information [1].

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