July 2005
doc.: IEEE 802.11-05/0710r0
IEEE P802.11
Wireless LANs
“WDS” Clarifications
Date: 2005-07-19
Author(s):
Name
Darwin
Engwer
Company
Nortel
Address
4655 Great America Pkwy,
Santa Clara, CA 95054
Phone
email
+1 408-495-7099
dengwer@nortel.com
Abstract
This document clarifies the term “WDS” as defined and described in IEEE 802.11-1999, noting that
the term is actually NOT defined and described and there is much confusion on this point. To prevent
further confusion the author recommends reducing the use of or deprecating the term in favour of the
underlying mechanism.
Notice: This document has been prepared to assist IEEE 802.11. It is offered as a basis for discussion and is not binding on the
contributing individual(s) or organization(s). The material in this document is subject to change in form and content after
further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein.
Release: The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution,
and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE
Standards publication even though it may include portions of this contribution; and at the IEEE’s sole discretion to permit
others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and
accepts that this contribution may be made public by IEEE 802.11.
Patent Policy and Procedures: The contributor is familiar with the IEEE 802 Patent Policy and Procedures <http://
ieee802.org/guides/bylaws/sb-bylaws.pdf>, including the statement "IEEE standards may include the known use of patent(s),
including patent applications, provided the IEEE receives assurance from the patent holder or applicant with respect to patents
essential for compliance with both mandatory and optional portions of the standard." Early disclosure to the Working Group of
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<stuart.kerry@philips.com> as early as possible, in written or electronic form, if patented technology (or technology under
patent application) might be incorporated into a draft standard being developed within the IEEE 802.11 Working Group. If you
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Submission
page 1
Darwin Engwer, Nortel
July 2005
doc.: IEEE 802.11-05/0710r0
Introduction
There is some confusion surrounding the IEEE 802.11-1999 term "WDS" (Wireless Distribution System).
Readers of the standard often misinterpret a WDS as:
a) a wireless DS
b) a DS that operates over a WLAN
A WDS (as defined by IEEE 802.11-1999) is neither. The misinterpretations perhaps result from an
extremely poor choice in naming the WDS capability, i.e. the inherent overloading of the term
“distribution system” and the abbreviated form “DS”. The “WDS” capability actually has nothing to do
with either of those other terms as this document will show.
Historical Analysis
The analysis begins with a review of the explicit references to the term WDS within 802.11-1999. There
are only two:
clause 4, where the acronym is defined
clause 7, where the frame control field is defined
Existing references to “WDS” in IEEE 802.11-1999:
4. Abbreviations and acronyms
“WDS wireless distribution system”
7.1.3.1 Frame Control field
Table 2—To/From DS combinations in data type frames
To/From DS values
Meaning
To DS = 0, From DS = 0
A data frame direct from one STA to another STA within the same IBSS,
as well as all management and control type frames.
To DS = 0, From DS = 1
Data frame exiting the DS.
To DS = 1, From DS = 0
Data frame destined for the DS.
To DS = 1, From DS = 1
Wireless distribution system (WDS) frame being distributed from one AP
to another AP.
Table 2 indirectly references clause 7.2.2 and Table 4:
7.2.2 Data frames
The content of the Address fields of the data frame is dependent upon the values of the To DS and From
DS bits and is defined in Table 4. Where the content of a field is shown as not applicable (N/A), the field
is omitted. Note that Address 1 always holds the receiver address of the intended receiver (or, in the case
of multicast frames, receivers), and that Address 2 always holds the address of the station that is
transmitting the frame.
To DS
0
0
1
1
Submission
From DS
0
1
0
1
Table 4 – Address field contents
Address 1
Address 2
Address 3
RA = DA
TA = SA
BSSID
RA = DA
TA = BSSID
SA
RA = BSSID
TA = SA
DA
RA
TA
DA
page 2
Address 4
N/A
N/A
N/A
SA
Darwin Engwer, Nortel
July 2005
doc.: IEEE 802.11-05/0710r0
Explanation
Here is a new, extended table that shows all those aspects together:
Table 5/710-T01: ToDs/ FromDS, Address Fields and Entities
ID Mode ToDS FromDS # of
Ext.
802.11
Flow 802.11
Address Entity Entity 1
Entity 2
Fields
 ad hoc
0
0
3
ad hoc
A Ad
hoc
STA
STA
infra mode

0
1
3
ACM_STA
B LAN
STA
access
in an AP
infra
mode
 ACM_STA
1
0
3
C LAN
STA
access
in an AP
 ??
1
1
4
yy
??
D 4A
Ext.
Entity
802.111999
clauses
5.6
xx
5.6
xx
5.6
zz
None
In case A both Entity 1 and Entity 2 are STAs operating in IBSS (aka ad hoc) mode.
Cases B and C describe an infrastructure mode STA accessing an external entity on the integrated nonIEEE 802.11 LAN through an AP and DS. The external entity is referred to as “xx”. Yes, an
infrastructure mode STA could be transferring data to another infrastructure mode STA, but note that
such traffic (from xx1 to xx2) transits the AP and also conceptually transits the DS.
Case D describes the 4 address scenario, referred to here as 4 Address (4A) mode, where the ultimate
source/ destination addresses can be something other than the Entity 1/ Entity 2 TA/RA or RA/TA
addresses.
4A mode can be used to construct a number of different types of devices, products and/ or systems.
Analysis
Hence, “WDS” is simply a mechanism for constructing 802.11 frames using the 4-address format, as
shown in the last row of Table 4 and subsequently the last row of Table 5/710-T01. The application of
the four-address addressing type and procedures for operating STAs that uses that type of addressing are
not defined or described by IEEE 802.11-1999.
The 4-address format enables the implementation of devices that move MPDUs between various points,
i.e. a relay type devices.
For example, the 4-address mechanism can be used to build some special products, like:
a) a point-to-point (or bldg-to-bldg) bridge,
b) a wireless Access Unit (AU), aka repeater AU (i.e. an AU, which includes an AP, with a wireless
backhaul connection to a real AU which in turn has a direct connection to the uplink network),
c) an infrastructure mode STA with bridging capabilities,
d) an 802.11 DS where the DSM is constructed from 802.11 STAs (the elusive, true, “wireless
distribution system”),
e) and so on.
The IEEE 802.11-1999 standard does not define how to construct any such implementations or how
stations interact to arrange for exchanging frames of this format, it merely defines the 4-address frame
format that makes it possible. Neither does the standard define all the other supporting operations or
protocols that would be required to build such devices.
Submission
page 3
Darwin Engwer, Nortel
July 2005
doc.: IEEE 802.11-05/0710r0
Example Instantiations
The collection of diagrams below help to illustrate some examples of the uses of the various addressing
modes.
1.
2.
3.
4.
5.
6.
7.
802.11 ad hoc LAN
802.11 Access Unit (connecting to non-IEEE 802.11 LAN)
large scale 802.11 LAN (with multiple APs connecting to non-IEEE 802.11 LAN)
point-to-point bridge (constructed from 802.11 4A-mode STAs)
802.11 DS (constructed from 802.11 4A-mode STAs)
large scale 802.11 LAN with 4A-mode STAs with bridging
large scale 802.11 LAN with fully wireless Access Units
July 2005
doc.: IEEE 802.11-05/0710r0
802.11 ad hoc LAN
A
ad hoc
STA
ad hoc
STA
A
A
ad hoc
STA
Submission
Submission
Slide 4
Darwin Engwer, Nortel
page 4
Darwin Engwer, Nortel
July 2005
doc.: IEEE 802.11-05/0710r0
July 2005
doc.: IEEE 802.11-05/0710r0
802.11 Access Unit
connecting to a non 802.11 LAN
non 802.11 LAN
Portal
Access
Unit
DS
AP
ACM_STA
B/C
B/C
Infra mode
STA
Infra mode
STA
Submission
Slide 5
July 2005
Darwin Engwer, Nortel
doc.: IEEE 802.11-05/0710r0
Large scale 802.11 access LAN
with multiple APs connecting to a non 802.11 LAN
non 802.11 LAN
Portal
DS
AP
ACM_STA
B/C
Infra mode
STA
Submission
Submission
AP
ACM_STA
B/C
B/C
Infra mode
STA
Infra mode
STA
Slide 6
Darwin Engwer, Nortel
page 5
Darwin Engwer, Nortel
July 2005
doc.: IEEE 802.11-05/0710r0
July 2005
doc.: IEEE 802.11-05/0710r0
Point-to-point bridges
constructed from 802.11 4A mode STAs
non 802.11 LAN “A”
Bridge
4A STA
Point-to-point
bridge
D
4A STA
Bridge
Point-to-point
bridge
non 802.11 LAN “B”
Submission
Slide 7
Darwin Engwer, Nortel
July 2005
doc.: IEEE 802.11-05/0710r0
802.11-based DS
constructed from 802.11 4A mode STAs
non 802.11 LAN
Portal
DS
4A STA
D
D
4A STA
4A STA
D
AP
ACM_STA
Submission
Submission
AP
ACM_STA
B/C
B/C
Infra mode
STA
Infra mode
STA
Slide 8
Darwin Engwer, Nortel
page 6
Darwin Engwer, Nortel
July 2005
doc.: IEEE 802.11-05/0710r0
July 2005
doc.: IEEE 802.11-05/0710r0
Large scale 802.11 access LAN
with 4A mode STA with bridging
non 802.11 LAN
Portal
DS
AP
ACM_STA
B/C
AP
ACM_STA
D
Infra mode
STA
B/C
4A STA
Infra mode
STA
non-802.11
STA
Submission
Bridge
4A mode
STA with
bridging
non-802.11
STA
non-802.11
STA
Slide 9
July 2005
Darwin Engwer, Nortel
doc.: IEEE 802.11-05/0710r0
Large scale 802.11 access LAN
with a wireless Access Unit
non 802.11 LAN
Portal
DS
AP
ACM_STA
B/C
Infra mode
STA
AP
ACM_STA
D
B/C
4A STA
Infra mode
STA
AP
ACM_STA
Wireless
Access
Unit
B/C
Infra mode
STA
Submission
Submission
Slide 10
Darwin Engwer, Nortel
page 7
Darwin Engwer, Nortel
July 2005
doc.: IEEE 802.11-05/0710r0
Summary
The 4-address frame format does not itself constitute a wireless distribution system or a wireless DS, it is
simply a frame addressing mechanism that allows creation of a multitude of specialized implementations,
one of which could be a wireless distribution system.
Conclusion
References to the term WDS should be reduced, or perhaps the term could even be considered for
deprecation. The material with respect to the 4-address addressing mechanism is valid, valuable and
should be maintained. At such time as new task groups define some specific uses for the 4-address
addressing mechanism that corresponding new material should be added to the appropriate clauses of the
standard to define the use of the 4 address mechanism for that specific purpose, while still allowing for
the addition of subsequent uses of the 4 address mechanism for other applications.
Submission
page 8
Darwin Engwer, Nortel
July 2005
doc.: IEEE 802.11-05/0710r0
References:
1. IEEE 802.11-1999
2. 11-05-0710-00-000m-wds-clarifications.ppt
-end-
Submission
page 9
Darwin Engwer, Nortel