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