3Com OfficeConnect 3CRWE454A72 Guida di riferimento

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

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

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

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

Figure 5: Configuring WDS

Links on AP2

Figure 4: Configuring WDS

Links on AP1

3COM®CONFIGURING A WIRELESS DISTRIBUTION SYSTEM TECHNICAL BRIEF

3Com Corporation, Corporate Headquarters, 350 Campus Drive, Marlborough, MA 01752-3064

To learn more about 3Com solutions, visit www.3com.com. 3Com is publicly traded on NASDAQ under the symbol COMS.

marks of 3Com Corporation. Possible made practical is a trademark of 3Com Corporation. All other company and product

names may be trademarks of their respective companies. While every effort is made to ensure the information given is

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

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