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Internet Engineering Task Force                             Yuzo Taenaka
Internet Draft                                         Shigeru Kashihara
Expires: August 8, 2010                                 Kazuya Tsukamoto
                                                        Suguru Yamaguchi
                                                                Yuji Oie

                                                         Febrary 8, 2010


             AP selection method considering WLAN condition
       <draft-yuzo-ap-selection-considering-wlan-condition-00.txt>



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Abstract

   This document discusses AP selection method utilizing the number of
   frame retransmissions in addition to received signal strength (RSSI).
   In a real environment, WLANs are congested by lots of mobile nodes,
   and sometimes influenced by other WLAN devices. They result in
   deterioration of communication quality. In such environment,
   each mobile node has to select an AP with better performance for
   keeping its own communication stable. To enable this, we employ
   the number of frame retransmissions and RSSI as selection indexes.
   We then describe an architecture design and implementation for our
   proposed scheme.


                           Table of Contents

   1. Introduction....................................................2
   2. AP Selection Method.............................................4
      2.1 Concept of the proposed AP selection........................4
      2.2 Details of the AP Selection Method..........................5
   3. Conclusion......................................................8
   4. Acknowledgements................................................8
   5. References......................................................8
   6. Author's Addresses..............................................9


1. Introduction

   Wireless LANs (WLANs) based on the IEEE 802.11 specification [1]
   provide high data transmission and simple, low-cost installation.
   For this reason, WLANs have been spreading rapidly in both private
   and public spaces, so that these WLANs will overlap to provide
   continuous and wide coverage as the underlying basis of ubiquitous
   networks. In such ubiquitous WLANs, mobile nodes (MNs) can access
   the Internet from everywhere and at anytime.

   In WLANs, an MN requires not only permanent access to the Internet,
   but also seamless movement between access points (APs) without
   degradation of communication quality, i.e., seamless mobility is
   essential. Furthermore, in such ubiquitous WLANs, each AP will
   independently provide wireless Internet connectivity based on
   IEEE 802.11 technology. That is, the WLAN coverage consists of
   different IP subnets due to independent management by different
   organizations and operators. In such a situation, even if a mobility
   support technology applying to AP is developed and standardized,
   from now on, it will be difficult to replace all APs, which have been
   spreading, with new APs that support mobility technology. Moreover,
   since existing APs may not be able to accept such technologies
   due to restrictions such as lower CPU performance or less memory,
   update of APs' software for supporting mobility technology is also
   not realistic. Therefore, it is desirable to provide a seamless
   mobility technology without modification of APs.


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   To achieve such seamless mobility, the following two requirements
   must be satisfied: (1) selection of an AP with better performance
   from among multiple candidate APs and (2) preservation of
   communication quality during handover. For (2), we proposed
   a handover management scheme based on the number of frame
   retransmissions [2-4]. In the proposed scheme, an MN is assumed to
   move between WLANs with different IP subnets. In [2], we first
   employed multi-homing architecture to prevent a disruption period
   due to handover processes. That is, since an MN with two WLAN
   interfaces (a multi-homing MN) can connect to two APs simultaneously
   before starting handover, an MN never experiences a disruption
   period due to handover processes. Next, to properly switch the two
   associating APs based on each wireless link quality, the number of
   frame retransmissions was introduced as an indicator for detecting
   the wireless link condition. Although the Received Signal Strength
   Indicator (RSSI) is generally used as an indicator of a wireless
   link quality, we showed that the RSSI is insufficient to detect
   wireless link condition precisely because it is incapable of
   detecting the degradation of communication quality due to radio
   interference [5].  On the other hand, we showed that the number of
   frame retransmissions could promptly and reliably detect
   the performance degradation due to reduction of the RSSI and radio
   interference. Therefore, the proposed handover management method
   based on the number of frame retransmissions can preserve
   communication quality during a handover.

   Selection of an AP with better performance from among multiple
   candidate APs, however, remains unresolved. For example, in the
   method proposed in [2], even if an MN has two WLAN interfaces to
   eliminate a disruption period due to handover process, there is no
   guarantee that the MN can select an AP with better performance for
   a handover. In ubiquitous WLANs, the MN may find multiple candidate
   APs at one time and needs to select an appropriate AP from among
   them. At this time, if an AP with better performance is not selected
   appropriately, the communication quality may degrade after a
   handover. In the current general AP selection, an MN selects an AP
   with the strongest RSSI. However, since numerous APs and MNs will
   exist in high-density ubiquitous WLANs, radio interference, which
   degrades communication quality, occurs frequently due to both the
   lack of the number of channels and heavy traffic in the AP. Thus,
   an AP selection method considering not only RSSI reduction but also
   radio interference caused by other WLAN devices is essential for
   achieving seamless mobility.

   So far, there have been numerous discussions on AP selection for
   improving the communication performance of MNs in WLANs[6-10].
   In ubiquitous WLANs, since an MN must be able to freely connect
   with all APs for an inter-domain handover, it is desirable that
   an AP is not modified in order to maintain compatibility with
   existing APs. Almost all existing AP selection methods [6-9],
   however, necessitate some modifications of APs, e.g., additional
   information should be inserted in the beacon frame transmitted from
   the AP. Moreover, the modification needs to be implemented in both
   an AP and an MN. On the other hand, [10] explained that the effects
   from hidden mobile nodes affect throughput degradation.

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   In the method proposed herein, although modification of an AP is
   unnecessary, the method is applied to only IEEE 802.11e, which has
   not yet become widespread.  Therefore, in this article, we focus
   a new proactive AP selection method based on RSSI and frame
   retransmissions. Since we have showed that the number of frame
   retransmissions is useful to detect radio interference in [5],
   this article describes how to utilize the number of
   frame retransmissions to AP selection method achieved in end-to-end.
   The proposed AP selection method enables an MN to select an AP with
   better performance taking radio interference into consideration by
   exploiting the number of frame retransmissions in addition to
   the RSSI.


2. AP Selection Method

   Here, we describe the proposed AP selection method. We first describe
   the concept of the proposed method in Section 2.1. In Section 2.2,
   we describe the proposed method with flowcharts.


2.1 Concept of the proposed AP selection

   This article focuses on a proactive AP selection for a radio
   interference environment. In the handover method, we proposed in a
   previous study [11], a multi-homed MN appropriately switches WLAN
   interfaces (WIFs) according to wireless link condition.
   Figure 1 shows an overview of operation between a handover and an
   AP selection on an MN. An AP selection is performed on the WIF2
   during communicating through the WIF1. On the other hand, after
   a handover, since the communication switched to the WIF2,
   the AP selection is next executed on the WIF1. That is, the AP
   selection is always executed on an idle interface.



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      +-------------------------------------------------------+
      |                      Application                      |
      +-------------------------------------------------------+
        |                 +---------------+
      +-|-----------+     | AP selection  |     +-------------+
      | | IP Layer  |     +---------------+     | IP Layer    |
      +-|-----------+                   |       +-------------+
      | | MAC Layer |                   +------>| MAC Layer   |
      +-|-----------+                  control  +-------------+
      | | PHY Layer |                           | PHY Layer   |
      +-|-----------+                           +-------------+
        | communication
        V                         A
                                  |
                                  | handover
                                  |
                                  V

      +-------------------------------------------------------+
      |                      Application                      |
      +-------------------------------------------------------+
                          +---------------+                 |
      +-------------+     | AP selection  |     +-----------|-+
      |   IP Layer  |     +---------------+     | IP Layer  | |
      +-------------+       |                   +-----------|-+
      |   MAC Layer |<------+                   | MAC Layer | |
      +-------------+  control                  +-----------|-+
      |   PHY Layer |                           | PHY Layer | |
      +-------------+                           +-----------|-+
                                              communication |
                                                            V

             Fig.1: A handover and an AP selection on an MN


2.2 Details of the AP Selection Method

   In this section, we describe the proposed AP selection method.
   The AP selection method is divided into two main parts: the AP
   selection procedure and the AP search procedure, as shown in
   Figures 2 and 3, respectively. In both figures, words in CAPITAL
   denote the system parameters in the proposed AP selection method.
   In the AP selection procedure, an MN periodically investigates
   the wireless link condition of an associating AP and decides whether
   the wireless link has sufficient wireless link quality. On the other
   hand, in the AP search procedure, an MN scans candidate APs and
   selects an AP with better performance from among them.
   Hence, MN starts the AP search procedure only when it detects the
   degradation of the communication quality on the current interface
   by exploiting the number of frame retries. That is, MN does not
   start the AP search procedure even when the RSSI degrades unless
   the frame retries increases.


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                                start
                                  |
                                  V
              send PPC probe packets at PPI ms interval
                                  |
                                  V
               obtain retry count at each probe packet
                                  |
                                  V                           no
           # of packets with more than frame retries >= RCT ---+
                                  | yes                        |
                                  V                            |
                   execute procedure of AP search              |
                                  |                            |
                                  V                            |
                     update network configuration              |
                                  |                            |
                                  V                            |
                                 end <-------------------------+

             Fig.2 Flowchart of AP selection procedure


                                start
                                  |
                                  V
                           make an AP list
                                  |
                                  V
               remove the APs that were associated with
                   immediately preceding WIF1/WIF2
                                  |
          no                      V
        +--- is there one or more available AP in the list? <---+
        |                         | yes                         |
        |                         V                             |
        |   associate with an AP in order of strong RSSI        |
        |                         |                             |
        |                         V                             |
        |    send PPC probe packets at PPI ms interval          |
        |                         |                             |
        |                         V                           no|
        | # of packets with more than ERC frame retries < RCT --+
        |                         | yes
        |                         V
        +----------------------> end

              Fig.3 Flowchart of the AP search procedure


   In this section, in order to clarify the proposed AP selection
   method, we assume that an MN with two WIFs (the WIF1 and the WIF2)
   first communicates through the WIF1, and the WIF2 associates with
   an AP with better performance at this time. Hence, the AP selection


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   procedure is executed on the WIF2. Initially, the MN starts a timer
   to control the AP selection procedure, i.e., the AP selection
   procedure is periodically executed at each AP Selection Execution
   Interval of APSEI seconds based on the timer. This is because an MN
   needs to periodically investigate the wireless link condition of
   the associating AP in order to detect changes in the wireless link
   quality due to the movement of the MN and radio interference.
   If the AP selection method is not periodically executed, an MN
   unfortunately maintains the association with the AP until its
   wireless link quality clearly degrades, similarly to an AP selection
   based on the RSSI. This causes severe degradation of
   the communication quality at handover. When the AP selection
   procedure is executed, the WIF2 sends probe packets to the AP
   at constant intervals. The Probe Packet Count (PPC) denotes
   the number of probe packets for one AP, and the Probe Packet Interval
   (PPI) indicates the sending interval of probe packets. After sending
   all of the probe packets, the MN obtains the number of frame
   retransmissions for each probe packet. If the number of probe packets
   experiencing more than Experienced Retransmission Count (ERC)
   retransmissions is less than the Retransmission Count
   Threshold (RCT), the MN decides that the AP has good wireless
   link condition and maintains the connection with the AP.
   After selecting an AP with better performance, the procedure
   terminates and then the MN executes the AP selection procedure
   every APSEI seconds.

   In contrast, if the number of probe packets that experience
   more than ERC frame retransmissions exceeds RCT, i.e., the AP
   associated with the WIF2 is not an AP with better performance,
   an AP search procedure is executed. As shown in Fig. 3, in the AP
   search procedure, the MN first makes a list of all candidate APs
   detected by the WIF2, and removes the APs currently associated by
   the WIF1/2 from the list. In our approach, to efficiently find
   an AP with better performance, our proposed method first checks
   the RSSI of candidate APs because the RSSI can be easily and
   passively obtained than the number of frame retransmissions.
   That is, it does not need both establishment of the association
   with APs and the packet transmission of probe packets.
   Therefore in our proposed scheme, after temporary sorting
   the candidate APs based on the strength of the RSSI, the MN
   investigates the APs by exploiting the number of
   frame retransmissions in order of high RSSI. If not sorting,
   the MN may transmit unnecessary packets to an AP with
   low performance (low RSSI), not to an AP with good performance
   (high RSSI). Hence, creation of an AP list based on RSSI prevents
   the unnecessary packet transmissions and shortens the detection time
   as much as possible. In the flowchart, if the number of packets
   experiencing more than ERC-times frame retries exceeds RCT,
   our proposed scheme detects the degradation in the wireless condition
   of the selected AP degrades and then the investigation is repeated
   in order of high RSSI until finding an AP with better performance.
   If an AP with better quality cannot be found, the MN retries the AP
   search at the next AP selection, i.e., after APSEI seconds.

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   After finishing these procedures, the MN can select an AP with low
   radio interference and strong RSSI.


3. Conclusion
   In this article, in order to select an AP with better performance
   from among multiple candidate APs in ubiquitous WLANs, we discussed
   a proactive AP selection method based on frame retransmissions and
   the RSSI. In an AP selection based on only the RSSI, an MN cannot
   always select an AP with better performance in ubiquitous WLANs,
   because the RSSI alone cannot detect the degradation of wireless link
   quality due to radio interference. We therefore used the number of
   frame retransmissions as an index for detecting radio interference
   in addition to the RSSI and then proposed an AP selection method for
   the proposed handover management system.


4. Acknowledgements

   This work was supported in part by the Kinki Mobile Radio Center
   Inc., the Japan Society for the Promotion of Science, Grant-in-Aid
   for Scientific Research(B)(No. 20700064), and the NEC C&C Foundation,
   Japan.


5. References

   [1] "Wireless LAN Medium Access Control (MAC) and Physical Layer
       (PHY) Specifications", ANSI/IEEE Std 802.11, 1999 Edition,
       Available at
       http://standards.ieee.org/getieee802/download/802.11-1999.pdf

   [2] S. Kashihara, K. Tsukamoto, and Y. Oie. Service-oriented mobility
       management architecture for seamless handover in ubiquitous
       networks. IEEE Wireless Communications, 14(2):28-34, April 2007.

   [3] K. Tsukamoto, S. Kashihara, and Y. Oie. Unified Handover
       Management Scheme Based on Frame Retransmissions for TCP over
       WLANs. IEICE Transactions on Communications, E91-B(4):1034-1046,
       April 2008.

   [4] S. Kashihara and Y. Oie. Handover Management based on the Number
       of Data Frame Retransmissions for VoWLANs. Elsevier Computer
       Communications, 30(17):3257-3269, November 2007.

   [5] K. Tsukamoto, T. Yamaguchi, S. Kashihara, and Y. Oie. Experimental
       Evaluation of Decision Criteria for WLAN handover: Signal Strength
       and Frame Retransmis- sion. IEICE Transactions on Communications,
       E90-B(12):3579-3590, December 2007.

   [6] M. Abusubaih, J. Gross, S. Wiethoelter, and A. Wolisz. On Access
       Point Selection in IEEE 802.11 Wireless Local Area Networks.
       In proceedings of the sixth International workshop on Wireless
       Local Networks (WLN2006), November 2006.

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   [7] K. Sundaresan and K. Papagiannaki. The Need for Cross-Layer
       Information in Access Point Selection Algorithm. In Proceedings of
       the 6th ACM SIGCOMM conference on Internet Measurement (IMC'06),
       pages 257-262, October 2006.

   [8] Y. Fukuda, J. Fukuda, and Y. Oie. Decentralized Access Point
       Architecture for Wireless LANs. In Proceedings of IEEE Vehicular
       Technology Conference 2004-fall (VTC 2004-Fall), September 2004.

   [9] Y.Fukuda,M.Honjo,andY.Oie. Development of Access Point Selection
       Architecture with Avoiding Interference for WLANs. In Proceedings
       of the 17th international symposium on Personal, Indoor and Mobile
       Radio Communications (PIMRC' 06), pages 1-5, September 2006.

   [10] L. Du, Y. Bai, and L. Chen. Access Point Selection Strategy for
        Large-Scale Wireless Local Area Networks. In Proceedings of
        IEEE Wireless Communications and Networking Conference
        (WCNC 2007), pages 2161-2166, March 2007.

   [11] Y. Taenaka, S. Kashihara, K. Tshukamoto, Y. Kadobayashi, , and
        Y. Oie. Design and Implementation of Cross-layer Architecture
        for Seamless VoIP Handover. In Proceedings of The Third IEEE
        International Workshop on Heterogeneous Multi-Hop Wireless and
        Mobile Networks 2007 (IEEE MHWMN' 07), October 2007.


6. Author's Addresses


   Yuzo Taenaka
   Graduate School of Information Science,
   Nara Institute of Science and Technology (NAIST)
   8916-5 Takayama, Ikoma, 630-0192, Japan
   Phone: +81-743-72-5216
   Email: yuzo-t@is.naist.jp


   Shigeru Kashihara
   Graduate School of Information Science,
   Nara Institute of Science and Technology (NAIST)
   8916-5 Takayama, Ikoma, 630-0192, Japan
   Phone: +81-743-72-5213
   Email: shigeru@is.naist.jp


   Kazuya Tsukamoto
   Department of Computer Science and Electronics,
   Kyushu Institute of Technology (KIT)
   680-4 Kawazu, Iizuka, 820-8502, Japan.
   Tel: +81-948-29-7687, Fax: +81-948-29-7652
   E-mail: tsukamoto@cse.kyutech.ac.jp


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   Suguru Yamaguchi
   Graduate School of Information Science,
   Nara Institute of Science and Technology (NAIST)
   8916-5 Takayama, Ikoma, 630-0192, Japan
   Phone: +81-743-72-5216
   Email: suguru@is.naist.jp


   Yuji Oie
   Department of Computer Science and Electronics,
   Kyushu Institute of Technology (KIT)
   680-4 Kawazu, Iizuka, 820-8502, Japan.
   Tel: +81-948-29-7687, Fax: +81-948-29-7652
   E-mail: oie@cse.kyutech.ac.jp


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