3Com OfficeConnect 3CRWE454A72 Reference guide

TECHNICAL BRIEF
Overview This document explains the WDS
(Wireless Distribution System)
features provided by the 3Com®
OfficeConnect®Wireless 11a/b/g
Access Point (3CRWE454A72). These
features allow you to build a com-
pletely wireless infrastructure
because the network equipment no
longer has to be connected to a wired
LAN. Also, WDS features allow you to
create large wireless networks by
linking several wireless access points
with WDS links. WDS is normally
used in large, open areas where
pulling wires is cost prohibitive,
restricted or physically impossible.
Configuring a Wireless Distribution System (WDS)
with the 3Com®OfficeConnect®Wireless 11a/b/g Access Point
Point-to-Point WDS Link
Point-to-Multipoint WDS Link

3COM®CONFIGURING A WIRELESS DISTRIBUTION SYSTEM TECHNICAL BRIEF
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Wireless Bridge and
Wireless Repeater
As shown in Figure 1, WDS can be
deployed in several configurations. In
this document, we will introduce two
basic WDS configurations: a wireless
bridge and wireless repeater.
Wireless Bridge
The wireless distribution system
shown in Figure 2 is often called a
“wireless bridge” configuration,
because it allows you to connect two
LANs at the link layer. In Figure 2, the
access point (AP) behaves as a standard
bridge that forwards traffic between
WDS links (links that connect to other
AP/wireless bridges) and an Ethernet
port. As a standard bridge, the access
point learns MAC addresses of up to
64 wireless and/or 128 total wired
and wireless network devices, which
are connected to their respective
Ethernet ports to limit the amount of
data to be forwarded. Only data des-
tined for stations which are known to
reside on the peer Ethernet link, mul-
ticast data or data with unknown
destinations need to be forwarded to
the peer AP via the WDS link.
Repeater WDS Link
Figure 2: Wireless Bridge Configuration
Figure 1: WDS Configurations
802.3 Ethernet frame
802.11 four-address format frame
WDS link

3COM®CONFIGURING A WIRELESS DISTRIBUTION SYSTEM TECHNICAL BRIEF
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Figure 3: Wireless Repeater Configuration
If, for example, an 802.3 Ethernet
frame is sent from a wired Station 1
(Sta1) to Sta3 in Figure 2, frame trans-
lations are required while the frame
forwards through the WDS link
between AP1 and AP2. When AP1
receives the 802.3 frame, the frame is
translated to a IEEE 802.11 standard
four-address format frame before it is
sent to a WDS link. In the four-
address format frame, the MAC
address of Sta1, MAC address of AP1,
MAC address of AP2 and MAC
address of Sta3 are all included in the
802.11 frame header, and the frame
data is same as the original Ethernet
frame. Based on information in this
four-address format frame, AP2 will
reconstruct the 802.3 Ethernet frame
when the frame is forwarded to
LAN2. If a security algorithm is con-
figured on the APs, AP1(AP2) will
encrypt(decrypt) this four-address
format frame before frame forward-
ing. From Sta3’s point of view, the
bridging function is transparent; i.e.,
the received frame is the same as if
Sta1 and Sta3 resided on the same LAN.
Wireless Repeater
In Figure 3, AP2 is used to extend the
range of the wireless infrastructure by
forwarding traffic between associated
wireless stations and another repeater
or AP connected to the wired LAN.
Note that the local Ethernet traffic is
not forwarded in this mode. Traffic
between Sta3 and Sta4 is not for-
warded across the WDS link, nor is
traffic between Sta5 and Sta6. As
with a wireless bridge mode, APs
operating in wireless repeater mode
need to translate frames into different
frame formats when forwarding
frames between wireless connections
and WDS links; the 802.11 three-address
frame format is used on wireless links
connected to wireless stations, while
the 802.3 four-address frame format is
used on WDS links connected to
other access points. Encryption/
decryption algorithms are also invoked
if the AP is configured to be secure.
The OfficeConnect Wireless 11a/b/g
Access Point can function as a wireless
repeater or wireless bridge if WDS
links are configured between the con-
nected AP pairs appropriately.
802.3 Ethernet frame
802.11 three-address format frame
802.11 four-address format frame
WDS link

3COM®CONFIGURING A WIRELESS DISTRIBUTION SYSTEM TECHNICAL BRIEF
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Configuring
WDS Links on
3Com OfficeConnect
Wireless 11a/b/g
Access Points
A WDS link is defined as the MAC
address pair of the connected APs.
To create a WDS link between two
OfficeConnect Wireless 11a/b/g
Access Points, enter the peer AP’s
MAC address on each AP via the
Wireless WDS web page.
In addition, make sure you configure
all WDS APs to work on the same
radio channel. Since WDS links can
operate in 2.4GHZ or 5.4 GHZ radio
channels, using Auto Channel selec-
tion is not appropriate.
Below is an example of configuring a WDS.
We will use example AP1 and AP2
with the following MAC addresses
(see these MAC addresses in the status
page):
AP1
LAN MAC Address: 00-70-46-01-23-45
11a WLAN MAC Address: 00-70-46-01-23-46
11b/g WLAN MAC Address: 00-70-46-01-23-47
AP2
LAN MAC Address: 00-70-46-01-33-62
11a WLAN MAC Address: 00-70-46-01-33-63
11b/g WLAN MAC Address: 00-70-46-01-33-64
To add a WDS link between AP1 and
AP2 in 802.11a mode with channel 60
selected:
1. On the wireless page of AP1,enable
802.11a and select channel 60.
2. On the WDS page of AP1, enable
WDS and enter AP2 WLAN MAC
address (00-70-46-01-33-63) with
802.11a mode in a WDS entry as
shown in Figure 4.

3. On the wireless page of AP2,
enable 802.11a standard and
select channel 60.
4. On the WDS page of AP2, enable
WDS and enter AP1 WLAN MAC
address (00-70-46-01-23-46) with
802.11a mode in a WDS entry as
shown in Figure 5.
3COM®CONFIGURING A WIRELESS DISTRIBUTION SYSTEM TECHNICAL BRIEF
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Figure 5: Configuring WDS
Links on AP2
Figure 4: Configuring WDS
Links on AP1

3COM®CONFIGURING A WIRELESS DISTRIBUTION SYSTEM TECHNICAL BRIEF
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Copyright © 2004 3Com Corporation. All rights reserved. 3Com, the 3Com logo, and OfficeConnect are registered trade-
marks of 3Com Corporation. Possible made practical is a trademark of 3Com Corporation. All other company and product
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accurate, 3Com does not accept liability for any errors or mistakes which may arise. Specifications and other information
in this document may be subject to change without notice.
Printed in the U.S. on recycled paper 104108-001 05/04
Security
Currently, the OfficeConnect Wireless
11a/b/g Access Point can support
secured WDS links in WEP mode. In
version 1.01, WEP keys must be the
same for both radios in the AP,
whether WEP or normal access, and
all APs in the system need to have
the same keys.
Spanning Tree Algorithm
As the OfficeConnect Wireless
11a/b/g Access Point doesn’t imple-
ment the Spanning Tree algorithm,
WDS links should be configured
appropriately to prevent loops in the
network.
Warnings
Pay special attention to avoid the
creation of network loops in the
network topology while configuring
WDS links. If a loop exists in the
network, data could be forwarded
and duplicated endlessly between
APs. This could crash networks.
Several abnormal loops likely created
in a WDS configuration are shown in
Figure 6.
Figure 6: Loops in Wireless Networks with WDS links