Internet DRAFT - draft-song-80211-seamless-handoff
draft-song-80211-seamless-handoff
CAPWAP Working Group Song Yubo
Internet Draft Li Ying
Expires: May 2006 Yang Xiaohui
Southest University
November 2005
Virtual AP for 802.11 Seamless Handoff
draft-song-80211-seamless-handoff-00.txt
Status of this Memo
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Copyright (C) The Internet Society (2005).
Abstract
This memo documents the concept of virtual AP which refers to using
multi physical APs to simulate only one virtual AP, method and
process used for seamless handoff between APs upon IEEE802.11 network.
It defines the stages to actualize smooth handoff, mentions the
concepts, theories and requirements in these stages.
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [i].
Table of Contents
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1. Introduction...................................................2
2. Terminology....................................................2
3. The realization of virtual AP..................................3
3.1 The concept of virtual AP..................................3
3.2 The issues of virtual AP...................................4
4. The procedure of 802.11 seamless handoff.......................4
4.1 Association phase..........................................4
4.2 Handoff detection phase....................................4
4.3 Handoff execution phase....................................5
5. Formal Syntax..................................................5
6. Security Considerations........................................5
7. References.....................................................5
Author's Addresses................................................6
Full Copyright Statement..........................................6
1.
Introduction
The handoff duration could affect the quality of packet transition in
the wireless channel between AP and station. When a user is using
wireless terminal to receive voice stream by connecting with AP
through wireless channel and moving from one AP’s coverage region to
another, handoff may take place in the overlay region. We must seek
an effect method to narrow down the duration existing in the handoff
process to maintain the high quality of corresponding.
As most of the duration is existing in the detection and search
stages, we use virtual AP to reduce the most part in these stages.
IEEE802.11f protocol also works in this method for solving the main
problems of exclusive channel collision and corresponding between APs.
2.
Terminology
access point (AP)
Any entity that has station functionality and provides access to the
distribution services, via the wireless medium (WM) for associated
stations [1].
association
The service used to establish access point/station (AP/STA) mapping
and enable STA invocation of the distribution system services (DSSs).
disassociation
The service that removes an existing association.
distribution system (DS)
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A system used to interconnect a set of basic service sets (BSSs) and
integrated local area networks (LANs) to create an extended service
set (ESS).
handoff
The handoff refers to the process of the association exchanged by a
mobile station from one AP to another.
handoff duraion
The handoff duration, which is often referred to as the handover
latency or handover delay, is the period when the STA in unable to
exchange data traffic via its old and new AP.
mobile station
A type of station that uses network communications while in motion.
station (STA)
Any device that contains an IEEE 802.11 conformant medium access
control (MAC)and physical layer (PHY) interface to the wireless
medium (WM).
virtual Access Point(virtual AP)
A virtual AP is a logical entity that to a STA is indistinguishable
from a physical AP residing within the same enclosure.
wireless medium (WM)
The medium used to implement the transfer of protocol data units
(PDUs) between peer physical layer (PHY) entities of a wireless local
area network (LAN).
3.
The realization of virtual AP
3.1
The concept of virtual AP
A virtual AP is a logical entity that to a STA is indistinguishable
from a physical AP residing within the same enclosure [2]. As with
all idealizations, a virtual AP implementation may approximate the
ideal behavior to a greater or lesser degree.
Although there may be many different physical APs in one distributed
system, STAs which is under the coverage area will consider itself
only associates with one AP. So, when a STA is moving from the
coverage area of one AP to another, it will not detect the loss of
the association.
In order to provide STAs with the illusion of only one physical AP
within the same enclosure, it is necessary for virtual APs to emulate
the same operation at the MAC layer.
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Virtual APs emulate the MAC layer behavior of physical APs by
operating with the same BSSIDs, SSIDs, channel, mac address,
capability advertisements and default key sets.
3.2
The issues of virtual AP
1) multiple association
In the 802.11 standard [1], the station MUST have only one single
association at a given time. But when the sta move into the overlap
region, it may connect to not only one AP at the same time because
all the MAC layer parameters of APs are the same.
In order to solve this problem, we use the 802.11f [3] standard to
provide the enforcement of the restriction of ISO/IEC 802-11:1999[1]
that a STA may have only a single association at any given time.
2) channel collision
As the working channels of all APs are set identically, the channel
collision probabilities are becoming larger than before. To make it
worse, serious collision can make the packet transition rate much
lower, or even obstruct this transition. The capability of the DS
using virtual AP will obviously becomes smaller than the normal
situation. But we think it is acceptable for users.
3) security problem
4.
The procedure of 802.11 seamless handoff
4.1
Association phase
When a STA is trying to associates with one AP using an 802.11
association request frame in the coverage area, AP which receives the
association request will send an IAPP-notify packet to inform other
APs in the same DS. There may be more than one AP receiving the
association request of the STA, which often happens when the STA is
in the overlap region. To solve this problem, we add a timestamp in
IAPP-notify frame. By comparing the timestamp of each IAPP-notify
frame sent by the APs, we can conclude which AP to associate with the
STA. Once the association is established, a Layer 2 Update Frame will
be sent to cause the forwarding tables of layer 2 internetworking
devices, e.g. bridges and switches, to be updated.
4.2
Handoff detection phase
The actions during the detection phase vary depending on which entity
initiated the handover. However, the most common handover is the one
initiated by the station, in which stations have to detect the lack
of radio connectivity based on failed frame transmissions. Most STAs
will explicitly probe the link by sending probe requests after a
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series of unsuccessful transmissions [4]. Different cards showed
different detection times depending on the number of failed frames
allowed and the number of probes sent. Commonly, this will cause
obvious handoff delay. In Virtual AP circumstance, the handoff is
network initiated. The AP determines when the handoff takes place by
detecting the radio signal strength. This will avoid the handoff
delay due to the handoff initiated by the station.
4.3
Handoff execution phase
Once other AP received data frames from the move station that this
station has move into the AP coverage area, it will attempt to send
an IAPP MOVE-notify packet to the AP with which the move STA was
previously associated to notify that AP of the STA’s association.
5.
Formal Syntax
The following syntax specification uses the augmented Backus-Naur
Form (BNF) as described in RFC-2234 [ii].
6.
Security Considerations
The current version of this document does not change the underlying
security issues inherent in the existing 802.11 standard. In the
future versions, new security requirements may be added.
7.
References
7.1 Normative reference
i Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997
ii Crocker, D. and Overell, P.(Editors), "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, Internet Mail Consortium and
Demon Internet Ltd., November 1997
7.2 Informative reference
[1] IEEE Std 802.11, 1999 Edition (R2003) (ISO/IEC 8802-11: 1999),
IEEE Standard for Information Technology—Telecommunications and
Information Exchange between Systems—Local and Metropolitan Area
Network—Specific Requirements—Part 11: Wireless LAN Medium Access
Control (MAC) and Physical Layer (PHY) Specifications.
[2] Bernard Aboba, “Virtual Access Points”, 11-03-154r1-I-Virtual-
Access-Points.doc, May 22, 2003
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[3] IEEE Std 802.11F, 2003, IEEE Trial-Use Recommended Practice for
Multi-Vendor Access Point Interoperability via an Inter-Access
Point Protocol Across Distribution Systems Supporting IEEE 802.11
Operation.
[4] Héctor Velayos and Gunnar Karlsson, “Techniques to Reduce IEEE
802.11b MAC Layer Handover Time”, www.it.kth.se/~hvelayos/papers/,
Apirl 2003.
Author's Addresses
Song Yubo
Southeast University
2# Sipailou, Nanjing, Jiangsu Province, China
Email: songyubo@mail.edu.cn
Li Ying
Southeast University
2# Sipailou, Nanjing, Jiangsu Province, China
Email: lyly110513@163.com
Yang Xiaohui
Southeast University
2# Sipailou, Nanjing, Jiangsu Province, China
Email: xhyang@seu.edu.cn
Full Copyright Statement
Copyright (C) The Internet Society (2005). 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 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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