Subject: [mobile-ip] revised draft on WLAN and MIP

From: Masataka Ohta <[email-masked]>
Date: Fri, 5 Jul 2002 18:26:09 +0859 ()

Hi,

I have revised my draft:

             Smooth Handover over IEEE 802.11 Wireless LAN

It now describes how handover period is minimized by NOT using
link layer mobility or link layer beacon signals.

							Masataka Ohta
--

INTERNET DRAFT                                                   M. Ohta
draft-ohta-smooth-handover-wlan-01.txt     Tokyo Institute of Technology
                                                               July 2002

             Smooth Handover over IEEE 802.11 Wireless LAN

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 10 of RFC2026.

   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/1id-abstracts.html

   The list of Internet-Draft Shadow Directories can be accessed at
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   Copyright (C) The Internet Society (July/5/2002).  All Rights
   Reserved.

Abstract

   This memo describes, based on the experience of MIS (Mobile Internet
   Services, Inc.) for commercial mobile IP service over IEEE 802.11b
   based wireless LAN environment, how handover between access points is
   made small or smooth.

   The major obstacle for the smooth handover is time required to scan
   frequency channels, which is made minimal by pseudo ad-hoc mode and
   latency of mobile registration is not so much a problem, both of
   which is solved with terminals having two transceivers.

1. Introduction

   This memo describes technical detail on handover latency of the
   service of MIS (Mobile Internet Services, Inc.), the first ISP to
   have commercialized mobile IP with IEEE 802.11 wireless LAN as a

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   feedback from the real world to IETF.

   MIS uses IEEE 802.11b based wireless LAN infrastructure and puts a
   lot of wireless LAN access points to let its subscribers move around
   with IP mobility.

   MIS started public field trial in August 2001 and started commercial
   service in April 2002.

   During the trial, a possible problem of the wireless LAN became
   apparent that it takes considerable amount of time, often more than a
   second, to scan all the possible frequency channels, which is
   required upon handover to find the next access point better than the
   current.

   So, it is important to minimize handover latency, as described in
   Section 2. As a result, IP mobility with MIS wireless LAN base
   stations works, even on a car driving on highway at 100Km/h.

   For some applications such as web browsing, it is not a problem.

   However, for streaming applications such as Internet telephony,
   smooth handover with little or no service interruption is strongly
   desired.

   As discussed in Sections  3 and 4, the problem of smooth handover is
   solved by terminals having two wireless transceivers.

2. Minimizing Handover Latency of IEEE 802.11 Wireless LAN with One
   Transceiver

   IEEE 802.11 wireless LAN has two mode of operations, infrastructure
   mode and IBSS mode.  Infrastructure mode is capable of frequency
   channel scanning, link layer mobility and beacon signal processing.
   IBSS mode is capable of beacon signal processing.

   However, complex link layer capabilities are seldom useful in the
   end-to-end Internet, because such link layer is an intelligent
   intermediate entity.

   It might be misunderstood that link layer mobility combined with IP
   layer mobility reduces the latency of handover, because it responds
   quickly enough, so that IP layer handover should be used only at
   borders of link layers.

   To rely on link layer mobility, frequency channels must be scanned
   once. Then, if IP layer handover is, in addition, required, frequency
   channels must be scanned again, which unnecessarily increases the

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   latency.

   To prevent it, access points must be sought regardless of the
   boundaries of link layer, which means link layer mobility is not
   used.

   Even if wireless LAN is used with infrastructure mode, each access
   point should belong to different ESS and link layer mobility should
   be disabled.

   Another problem is beacon processing with both infrastructure and
   IBSS modes.

   In both modes, information in link layer beacon is necessary before
   starting communication.  As IEEE 802.11 does not specify anything and
   WiFi specifies beacon period between 20m second to 1 second, typical,
   or may be all the, IBSS implementations listen to a frequency channel
   for 1 second.  Then, if a terminal scans all the 14 channels, most of
   which does not have beacon signals, it takes about 14 seconds, which
   is prohibitively long.

   Even if we can somehow shorten the listening period, there is still a
   problem with link layer beacon signals.  While IEEE 802.11 frequency
   channels are scanned based on limited information in link layer
   beacon signals, real world scanning needs upper layer information.
   For example, whether an access point supports IPv4 and/or IPv6 is
   important upper layer information for scanning. Another important
   information is the number of remaining IP addresses pooled at an
   access point (with FA/MN collocation model, which MIS assumes to
   remove an intermediate intelligent entity of FA from the network).

   That is, for efficient frequency channel scanning, upper layer
   software of access points should generate its own beacon signals
   containing capabilities of the upper layer. To minimize handover
   latency, it should be as frequent as link layer beacon signals. As
   inter-frame gaps of IEEE 802.11 is considerably large, separately
   generating link and upper layer beacons reduces the tolerable beacon
   frequency by half.

   Another option is for terminals to actively query upper layer
   information to access points.  However, with many candidate access
   points, communication with which may involve packet drop and
   retransmission delay, considerable amount of time is required.

   That is, link layer beacon signals are not useful.

   Fortunately, NICs from most, if not all, vendors have so called
   "pseudo ad-hoc" mode, with which there is no link layer intelligence

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   such as half hearted mobility or beacon signals.

   MIS relies on the pseudo ad-hoc mode to minimize the latency for
   frequency channel scanning. Access points of MIS generate beacon
   signals 30 time a second and it takes about a second to scan all the
   14 frequency channels.

   It is confirmed that IP mobility take over between access points
   works even with terminals on cars running at 100Km/h.

3. Smooth Handover with Two Transceivers

   To prevent the service interruption, it is necessary that a terminal,
   which is expected to run service-interruption-sensitive applications,
   should have two wireless LAN transceivers, one for keeping connection
   to the the current access point and another for scanning frequency
   channels to search alternative ones.

4. Smoother Handover with Two Transceivers

   With the elimination of service interruption for frequency channel
   scanning, there still is a smaller amount of service interruption for
   mobility registration, which can also be eliminated.

   With two wireless LAN transceivers, just after one transceiver find a
   new access point better than the current, a new connection to the new
   access point should be established through the transceiver.

   Then, mobility registration for the new access point should be
   initiated.

   Still, another transceiver should keep connecting to the current
   access point.

   A while after the terminal confirms a successful mobility
   registration, the terminal should terminate the connection to the old
   access point and, as described in section 2, start scanning newer
   access points.

   During mobile registration, packets from CH comes from either of the
   access points. As the terminal receives packets from both access
   points, there is no packet loss caused by the latency of the
   registration.

5. Applications of the Technique

   The technique described in sections 3 and 4 was deployed in recent
   PHS (personal handy phone, 32Kbps mobile telephone system available

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   in Japan and other countries) service, only after which, PHS can
   support stable and smooth handover.

   In general, the technique requires two sets of transceivers, which
   increases the cost of terminals, which is welcome to wireless LAN
   chip vendors, it also reduce the cost of network by eliminating
   intelligent intermediate entities for half-hearted smooth mobility.

   Note that a CDMA based wireless transceiver can simultaneously use
   two access points with different code without additional RF modules.

   The technique is L1 or L2 independent and is applicable to smooth
   handover between different media, such as between PHS and WLAN or
   Ethernet and WLAN.

6. Security Considerations

   To prevent anonymous and/or unpaid access to the Internet, access
   points of MIS have packet-wise cryptographical authentication
   mechanism to disallow unauthorized access to the Internet.

   To prevent subscribers share a single subscriber ID with single
   payment, the mechanism disallows multiple terminals with a single
   subscriber ID simultaneously use access points.

   As a side effect, the mechanism, basically, disallows a terminal
   simultaneously use multiple access points.

   However, to allow for the smooth handover, a terminal is allowed to
   simultaneously use access points with overlapping service area.

7. Author's Address

   Masataka Ohta
   Graduate School of Information Science and Engineering,
   Tokyo Institute of Technology 2-12-1, O-okayama, Meguro-ku
   Tokyo 152-8552, JAPAN

   Phone: +81-3-5734-3299
   Fax: +81-3-5734-3299
   EMail: [email-masked]

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8. Full Copyright Statement

   "Copyright (C) The Internet Society (July/1/2002).  All Rights
   Reserved.

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