Internet Engineering Task Force Shigeru Kashihara Internet Draft Kazuya Tsukamoto Expires: September 2, 2007 Youki Kadobayashi Yuji Oie March 2, 2007 A simple heuristic for handover decisions in WLANs Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on September 2, 2007. 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. In addition, we introduce our proposed method using the number of frame retransmissions. Kashihara, et al. Expires: September 2, 2007 [Page 1] draft-shigeru-simple-heuristic-wlan-handover-02.txt March 2007 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 3.4 Consideration...............................................5 4. Handover management scheme based on the number of frame retransmissions.................................................6 4.1 Proposed method.............................................6 4.2 Consideration...............................................7 5. Conclusion......................................................8 6. Acknowledgements................................................8 7. References......................................................8 8. Author's Addresses.............................................10 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 an individual 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) reduction of signal strength due to the MN's movement and intervening objects 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 Kashihara, et al. Expires: September 2, 2007 [Page 2] draft-shigeru-simple-heuristic-wlan-handover-02.txt March 2007 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 Media Access Control (MAC) layer (layer 2) as a simple heuristic for handover decisions to realize seamless and efficient WLAN handover. 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 the reduction of signal strength and the radio interference. Kashihara, et al. Expires: September 2, 2007 [Page 3] draft-shigeru-simple-heuristic-wlan-handover-02.txt March 2007 In addition, it is very difficult for signal strength 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. 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) reduction of signal strength and (2) radio interference. We propose the number of frame retransmissions as a simple heuristic for handover decisions which satisfies these two requirements [8,12,13]. 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) detection of reduction of signal strength, (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 due to its own movement and intervening objects before the communication quality is actually degraded. Kashihara, et al. Expires: September 2, 2007 [Page 4] draft-shigeru-simple-heuristic-wlan-handover-02.txt March 2007 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 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 some 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 need to utilize signal strength to detect wireless link quality when it has no transmission data. 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. 3.4 Consideration The number of frame retransmissions has a potential to properly indicate wireless link condition influenced by reduction in signal strength or radio interference. However, it is still insufficient to achieve a seamless handover. Common WLANs employ a function of auto rate fallback (ARF). A method of ARF is not specified in [1], and depends on vendors. When frame retransmissions are occurred, a Kashihara, et al. Expires: September 2, 2007 [Page 5] draft-shigeru-simple-heuristic-wlan-handover-02.txt March 2007 sender lowers transfer speed of WLAN to maintain communication quality and thus frame retransmissions are reduced. As a result, when the number of frame retransmissions becomes large, the transfer speed has already been the lowest value, e.g., 1 Mb/s in 802.11b, at starting a handover. In non real-time applications, the degradation in communication quality, i.e., throughput, is significant problem. Therefore, to maintain the communication quality, the transfer speed controlled by ARF is also necessary to be considered with the number of frame retransmissions. In WLANs, MNs share the wireless media. When many MNs exist in a WLAN, waiting time to send frames in the WLAN becomes larger. In real-time applications, such as VoIP, an application layer passes data packets to a WLAN interface at fixed rate. At this time, when the sending speed of application is faster than waiting time to send frames, the queue length of a WLAN interface buffer increases. As a result, as RTT or jitter becomes large, communication quality may not be maintained. Therefore, the queue length of interface buffer is also useful information as an additional indicator to maintain communication quality in real-time applications. 4. Handover management scheme based on the number of frame retransmissions In [12, 14, 16], we proposed a handover management scheme based on the number of frame retransmissions. In Section 4.1, we briefly introduce our handover management mechanism as an example of handover using the number of frame retransmissions. We then describe consideration of our proposed method. 4.1 Proposed method. Our handover mechanism takes a multihoming approach and a cross-layer approach in which the transport layer uses L2 information (i.e., the number of frame retransmissions). That is, an MN has two WLAN interfaces, and the handover manager (HM) on transport layer controls the handover process considering the number of frame retransmissions. As illustrated below, we assume that an MN with two WLAN interfaces (IF1, IF2) connects with two different WLAN carriers (AP1, AP2), and initially communicates with the CN through IF1. Kashihara, et al. Expires: September 2, 2007 [Page 6] draft-shigeru-simple-heuristic-wlan-handover-02.txt March 2007 +----+ +----- wired -----| CN | | +----+ +------------+ +--| Internet |--+ | +------------+ | | | +-----+ +-----+ | AP1 | | AP2 | +-----+ +-----+ / \ / \ / \ / \ / \ / \ | | +|--|+ | MN | - - - - - - > +----+ In our handover management scheme, an MN can flexibly select an optimal WLAN considering the wireless link condition. We outline the operation of handover management scheme. The MN associates with two APs by using two WLAN interfaces before handover in advance, i.e., the MN has two different IP addresses. When the wireless link condition of AP1 through IF1 is getting worse, the HM switches to multi-path transmission to prevent packet loss during handover and to investigate the condition of another wireless link (AP2). However, the HM should return to single-path transmission on a stable wireless link as quickly as possible, because the network load is doubled by multi-path transmission in which the same data are sent through both interfaces. Therefore, in multi-path transmission, if the condition of either wireless link is getting better, the HM immediately returns to single-path transmission. In our scheme, we employ the number of retransmissions to detect the change in a wireless link condition. The MAC layer on each interface informs the HM of the number of frame retransmissions, and the HM determines the wireless link condition for each interface from this number. In this way, the HM selects either single-path or multi-path transmission based on the wireless link condition, i.e., the number of frame retransmissions. As a result, the HM can maintain communication quality while properly switching between single-path and multi-path transmission during handover. We are now implementing TCP and UDP using the number of frame retransmissions as a handover decision criterion. In UDP (VoIP) version, we have implemented the prototype system on Linux Kernel, and presented the demonstration at Mobihoc 2006 [15]. We also think the number of frame retransmissions is applied to not only our approach but also another approaches such as MIP and mSCTP. 4.2 Consideration In [12, 14, 16], as an overlap area between two WLANs is assumed to be large enough to maintain communication quality during handover, Kashihara, et al. Expires: September 2, 2007 [Page 7] draft-shigeru-simple-heuristic-wlan-handover-02.txt March 2007 the threshold to switch between single-path and multi-path transmissions is a fixed value. However, in a real environment, an overlap area is not always large. If an overlap area is small, an MN may frequently switch between single-path and multi-path transmissions under a fixed threshold. To put it concretely, in the case of small overlap area, even if the overlap area is insufficient to maintain communication quality due to long distance to the next AP, an MN starts a handover when frame retransmissions occur in the current WLAN. As a result, the communication quality may degrade because of multiple and continuous handovers. Therefore, to achieve more sophisticated handover management, the threshold to switch between single-path and multi-path transmissions need to be dynamically changed according to the size of an overlap area. 5. Conclusion In this article, 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) detection of reduction of signal strength, 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, and introduced our proposed scheme using the number of frame retransmissions. 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. 6. Acknowledgements This work was supported in part by the Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research(S) (18000001), in part by the Ministry of Internal Affairs and Communications (MIC), Japan, and in part by the 21st Century Center of Excellence (COE) Program. 7. 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. Kashihara, et al. Expires: September 2, 2007 [Page 8] draft-shigeru-simple-heuristic-wlan-handover-02.txt March 2007 [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, September 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. [13] K. Tsukamoto, T. Yamaguchi, S. Kashihara, and Y. Oie, "Experimental Evaluation of Decision Criteria for WLAN handover: Signal Strength and Frame Retransmission," Proc. of 2006 International Symposium on Ubiquitous Computing Systems (UCS 2006); Lecture Notes in Computer Science 4239, pp. 239-253, October 2006. [14] H. Koga, S. Kashihara, Y. Fukuda, K. Iida, and Y. Oie, "A quality-aware VoWLAN architecture and its quantitative evaluations," IEEE Wireless Communications, Vol. 13, No. 1, pp.52-59, February 2006. [15] S. Kashihara, K. Tsukamoto, H. Koga, and Y. Oie, "Handover management based on the number of frame retransmissions for VoWLANs," In The 7th ACM International Symposium on Mobile Ad Hoc Networking and Computing (MobiHoc 2006), Poster and Demo Sessions (Demo), May 2006. [16] K. Tsukamoto, S. Kashihara, and Y. Oie, gHandover Management based on the Number of Frame Retransmissions for TCP over WLANs,h Proc. of the 4th Annual IEEE Consumer Communications and Networking Conference (CCNC 2007), CD-ROM, January 2007. Kashihara, et al. Expires: September 2, 2007 [Page 9] draft-shigeru-simple-heuristic-wlan-handover-02.txt March 2007 8. 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 Kashihara, et al. Expires: September 2, 2007 [Page 10] draft-shigeru-simple-heuristic-wlan-handover-02.txt March 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 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