Internet Engineering Task Force                        Shigeru Kashihara
Internet Draft                                          Kazuya Tsukamoto
Expires: August 30, 2006                               Youki Kadobayashi
                                                                Yuji Oie

                                                       February 26, 2006
                                   
           A simple heuristic for handover decisions in WLANs
          <draft-shigeru-simple-heuristic-wlan-handover-00.txt>


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Abstract

   This document discusses handover decision criteria for avoiding
   deterioration in communication quality during WLAN handover, in
   particular at handover initiation.  We first describe problems for
   handover decision criteria employed by existing mobility management
   technologies, such as Mobile IP and mSCTP.  We then propose the 
   number of frame retransmissions as a simple heuristic for handover
   decisions and discuss its advantages and disadvantages.







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                           Table of Contents
     
   1. Introduction....................................................2 
   2. Existing WLAN handover decision criteria........................3 
      2.1 Handover decision criteria on upper layers..................3 
      2.2 Handover decision criteria on lower layers..................3 
   3. The number of frame retransmissions.............................4 
      3.1 Frame retransmission mechanism of IEEE 802.11...............4 
      3.2 Advantages..................................................4 
      3.3 Disadvantages...............................................5 
   4. Conclusion......................................................5 
   5. Acknowledgements................................................6 
   6. References......................................................6 
   7. Author's Addresses..............................................7 

1. Introduction

   Wireless LANs (WLANs) based on the IEEE 802.11 specifications [1] 
   are spreading widely due to their low cost, simplicity of 
   installation and broadband connectivity.  WLANs are being set up not
   only in private spaces such as the home and workplace, but also in
   public spaces such as waiting areas and coffee shops as hotspots.  
   Thus, WLANs that are independently managed by different 
   organizations are starting to complementarily cover wide areas such 
   an entire city.  In the near future, WLANs will continue to spread 
   until they overlap to provide continuous coverage over a wide area, 
   and they will then be the underlying basis of ubiquitous networks.

   In ubiquitous networks consisting of WLANs, mobile nodes (MNs) can 
   access the Internet through access points (APs) at any location.  
   MNs are very likely to traverse multiple WLANs divided into 
   different IP subnets during communications, because the coverage of
   a WLAN is relatively small.  As a result, the communication is 
   disconnected due to the change in IP address of the MN required for 
   handover.

   To achieve continuous communication during handover, many mobility 
   management schemes such as Mobile IP [2,3], mobile Stream Control 
   Transmission Protocol (mSCTP) [4], and others [5,6,7] have been 
   proposed.  These schemes use various movement detection methods for 
   starting the handover process.  However, in [8], we showed that 
   these movement detection methods result in the degradation of 
   communication quality at WLAN handover initiation.  Furthermore, in 
   ubiquitous networks, as the communication quality is often degraded 
   due to both (1) the MN's movement and (2) radio interference with 
   other WLANs, proposing a handover decision criterion that can detect 
   both (1) and (2) is a critical issue.  Thus, the main focus of this 
   article is on handover decision criteria for avoiding degradation in 
   communication quality at handover initiation.  We first clarify 
   problems of handover decision criteria arising from existing 
   mobility management technologies, and then propose the number of 
   frame retransmissions on the MAC Layer (Layer 2) as a simple 
   heuristic for handover decisions to realize seamless and efficient 
   WLAN handover. 

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2. Existing WLAN handover decision criteria

   Handover decision criteria used by existing mobility management 
   technologies can be classified according to the information measured 
   on upper/lower layers.  (An upper layer is Layer 3 or above, and a 
   lower layer is Layer 2 or below.)  In [8], we investigated the 
   impact of existing handover decision criteria on communication 
   quality at handover initiation.  In this section, we clarify 
   characteristics of the existing handover decision criteria on 
   upper/lower layers.

2.1 Handover decision criteria on upper layers

   Packet loss (including data/signaling packets) and RTT are commonly 
   used as handover decision criteria in existing handover technologies 
   [2,3,6,7,9].  In [8], through simulation experiments, we showed that 
   the communication quality has already been degraded even when an MN 
   starts the handover process just after detecting the occurrence of a
   packet loss or an increase of RTT.  In a WLAN, communication quality 
   is degraded due to deterioration in the wireless link condition even 
   if packet loss does not occur or RTT does not seriously increase.  
   Therefore, these criteria on upper layers cannot detect abrupt 
   fluctuations of wireless link condition reliably and promptly.  
   To avoid degradation of communication quality at handover 
   initiation, it is essential to effectively detect deterioration in 
   the wireless link condition.

2.2 Handover decision criteria on lower layers

   Wireless signal strength is usually considered as a handover 
   decision criterion on lower layers [4,10].  Signal strength can 
   provide information about a wireless link condition directly from 
   the Physical Layer.  However, properly detecting deterioration in 
   communication quality caused by fluctuations of signal strength is 
   very difficult for an MN, because the signal strength may fluctuate 
   abruptly due to the distance from an AP and any interfering objects 
   located between the MN and the AP.

   Furthermore, in ubiquitous networks consisting of WLANs, degradation 
   of communication quality due to radio interference is common.  
   However, MNs cannot detect this type of degradation by assessing the 
   signal strength.  Thus, to maintain communication quality during 
   handover, an MN should be able to detect both its own movement and 
   the radio interference.

   In addition, if signal strength is used as a handover decision 
   criterion, it is very difficult to set a threshold to start the 
   handover process, because the allowable range of signal strength 
   (Received Signal Strength Indicator: RSSI) depends on each vendor, 
   e.g., Cisco chooses 100 as RSSI-max while the Atheros chipset 
   chooses 60 [10].  Therefore, monitoring signal strength is 
   insufficient to avoid degradation in communication quality at 
   handover initiation.


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3. The number of frame retransmissions

   As described in Section 2.2, to avoid degradation in communication 
   quality at WLAN handover initiation, a handover decision criterion 
   should reliably and promptly detect degradation of communication 
   quality due to both (1) MN's movement and (2) radio interference.  
   We propose the number of frame retransmissions as a simple heuristic 
   for handover decisions which satisfies these two requirements [8,12].

3.1 Frame retransmission mechanism of IEEE 802.11

   We will outline the frame retransmission mechanism of IEEE 802.11.  
   In the IEEE 802.11 specifications [1], when a data or an ACK frame 
   is lost over a WLAN, the sender (e.g., an MN) retransmits the same 
   data frame to the receiver (e.g., an AP) until the number of frame 
   retransmissions reaches a predetermined retry limit.  If RTS 
   (Request To Send)/CTS (Clear To Send) is applied, the retry limit is 
   set to four; otherwise, it is seven. (These values actually depend 
   on each vendor.)  Therefore, a data frame can be retransmitted a 
   maximum of four or seven times (the initial transmission and 
   three/six retransmissions), if necessary.  If the MN does not 
   receive an ACK frame within the retry limit, it treats the data 
   frame as a lost packet.  In addition, RTT increases due to 
   retransmissions over a WLAN.  Therefore, we can see that a data 
   frame inherently experiences retransmissions over a WLAN before the 
   occurrence of packet loss or the increase of RTT, irrespective of 
   the RTS/CTS.

3.2 Advantages

   Use of the number of frame retransmissions has the following three 
   advantages: (i) MN movement detection, (ii) radio interference 
   detection, and (iii) ease of setting of the threshold triggering the 
   handover processes.  First, when the MN moves during communication, 
   the wireless link condition is degraded due to the distance from the 
   AP and any interfering objects located between the MN and the AP.  
   As described in Section 3.1, a data frame will experience 
   retransmissions due to the degradation of the wireless link 
   condition before the occurrence of packet loss or the increase of 
   RTT.  Thus, if the number of frame retransmissions is used as a 
   handover decision criterion, the MN can detect the degradation of 
   wireless link condition with its own movement before the 
   communication quality is actually degraded.

   Next, in radio interference environments, the number of frame 
   retransmissions has another advantage that the signal strength 
   criterion can never imitate.  For instance, with signal strength, 
   the MN cannot detect the communication quality due to the radio 
   interference either, because signal strength is not influenced by 
   radio interference at all.  However, in radio interference 
   environments, frame retransmissions frequently occur due to 
   collisions between transmitted frames.  As a result, the 
   communication quality is degraded.  Therefore, using the number of 
   frame retransmissions, the MN can detect degradation of 

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   communication quality due to radio interference.

   Lastly, ease of determination of the threshold triggering the 
   handover processes should be noted here.  As mentioned earlier, 
   signal strength is measured in different ways by each vendor, so 
   that it is necessary to set a different suitable threshold for each 
   WLAN card.  The determination of an appropriate threshold, thus, 
   depends upon vendors' implementation of measures.  On the other 
   hand, as frame retransmissions can be handled in the same manner in 
   all WLAN cards, we can set the same threshold for every WLAN card, 
   unlike the signal strength.  In addition, we can simply set the 
   threshold by plain numbers (e.g., 1, 2, 3,..., n).

3.3 Disadvantages

   Although we described the advantages of the number of frame 
   retransmissions in the previous section, it also has its 
   disadvantages.  These disadvantages are as follows: (I) an MN cannot 
   detect change in wireless link condition without transmission of 
   data frames, and (II) cross-layer architecture is indispensable to 
   notify the existing mobility management technologies on upper layers 
   of the number of frame retransmissions.  First, the number of frame 
   retransmissions cannot be measured until data frames are sent.  For 
   instance, the MN cannot estimate the wireless link condition of a 
   new AP by the number of frame retransmissions before associating 
   with the new AP.  In this case, another criterion, e.g., signal 
   strength, is necessary to estimate the wireless link condition.  
   Therefore, an MN cannot detect wireless link quality without 
   transmission of data frames.  Next, to introduce this heuristic into 
   the existing mobility management technologies, as the information 
   held in each layer cannot be accessed from different layers due to 
   the concept of the traditional layered architecture, a cross-layer 
   architecture is necessary for achieving accessibility from different 
   layers.

4. Conclusion

   In this paper, we have discussed handover decision criteria to 
   maintain communication quality during handover.  In particular, we 
   mentioned that the degradation of communication quality at handover 
   initiation becomes a critical issue.  To seamlessly move across 
   multiple WLANs divided into different IP subnets, an MN should 
   execute the handover process while reliably and promptly detecting 
   degradation of the wireless link condition before deterioration of 
   communication quality actually occurs.  Next, we argued that a 
   handover decision criterion should satisfy the following two 
   requirements: (1) MN movement detection, and (2) radio interference 
   detection.  To satisfy these two requirements, we proposed the 
   number of frame retransmissions as a simple heuristic for the 
   handover decision.  Although this heuristic has some disadvantages, 
   we consider that the number of frame retransmissions is an important 
   heuristic to maintain communication quality during WLAN handover.



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5. Acknowledgements

   This work was supported in part by a grant from the Cisco University 
   Research Program Fund at Community Foundation Silicon Valley, in 
   part by the Japan Society for the Promotion of Science, Grant-in-Aid 
   for Scientific Research(A)(15200005), in part by the Ministry of 
   Internal Affairs and Communications (MIC), Japan, and in part by 
   the 21st Century Center of Excellence (COE) Program.

6. 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]  C. Perkins (Ed.), "IP Mobility Support for IPv4, revised," 
     draft-ietf-mip4-rfc3344bis-02.txt, October 2005.
[3]  D. Johnson, C. Perkins, and J. Arkko, "Mobility Support in IPv6," 
     RFC3775, June 2004.
[4]  M. Riegel and M. Tuexen, "Mobile SCTP," 
     draft-riegel-tuexen-mobile-sctp-05.txt, July 2005.
[5]  K. Tsukamoto, Y. Hori, and Y. Oie, "Mobility Management of 
     Transport Protocol Supporting Multiple Connections," Proc. of ACM 
     MobiWac2004, pp. 83-87, October 2004.
[6]  S. Kashihara, K. Iida, H. Koga, Y. Kadobayashi, and S. Yamaguchi, 
     "Multi-Path Transmission Algorithm for End-to-End Seamless 
     Handover across Heterogeneous Wireless Access Networks," IEICE 
     Transactions on Communications, Vol. E87-B, No. 3, pp. 490-496, 
     March 2004.
[7]  S. Kashihara, T. Nishiyama, K. Iida, H. Koga, Y. Kadobayashi, and 
     S. Yamaguchi, "Path selection using active measurement in 
     multi-homed wireless networks", Proc. of IEEE/IPSJ 2004 
     International Symposium on Applications and the Internet 
     (SAINT2004), pp. 273-276, January 2004.
[8]  K. Tsukamoto, R. Ijima, S. Kashihara, and Y. Oie, "Impact of 
     Layer 2 Behavior on TCP Performance in WLAN," Proc. of IEEE 
     VTC2005 fall, in CD-ROM, September 2005.
[9]  K. El Malki (Ed.), "Low Latency Handoffs in Mobile IPv4,"
     draft-ietf-mobileip-lowlatency-handoffs-v4-11.txt, October 2005.
[10] M. Chang, M. Lee, and S. Koh, "Transport Layer Mobility Support 
     Utilizing Link Signal Strength Information," IEICE Transactions on 
     Communications, Vol. E87-B, No. 9, pp. 2548-2556, September 2004.
[11] K. Muthukrishnan, N. Meratnia, M. Lijding, G. Kopringkov, and 
     P. Havinga, "WLAN location sharing through a privacy observant 
     architecture," Proc. of First International Conference on 
     Communication System Software and Middleware (COMSWARE), January 
     2006.
[12] S. Kashihara and Y. Oie, "Handover Management based upon the 
     Number of Retries for VoIP in WLANs," Proc. of IEEE VTC2005 
     spring, in CD-ROM, May 2005.






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7. Author's Addresses

   Shigeru Kashihara
   Graduate School of Information Science,
   Nara Institute of Science and Technology (NAIST)
   8916-5 Takayama, Ikoma 630-0192, Japan.
   Tel: +81-743-72-5213, Fax: +81-743-72-5219
   E-mail: shigeru@is.naist.jp

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

   Youki Kadobayashi
   Graduate School of Information Science,
   Nara Institute of Science and Technology (NAIST)
   8916-5 Takayama, Ikoma 630-0192, Japan.
   Tel: +81-743-72-5211, Fax: +81-743-72-5219
   E-mail: youki-k@is.naist.jp
    
   Yuji Oie
   Department of Computer Science and Electronics,
   Kyushu Institute of Technology (KIT)
   Kawazu 680-4, 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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