Bridge vs Repeater

Unanswered Question
Oct 14th, 2009

Hello,

I read on the net about wds that

WDS can be used to provide two modes of wireless AP-to-AP connectivity:

• Wireless Bridging in which WDS APs communicate only with each other and don't allow wireless clients or Stations (STA) to access them

• Wireless Repeating in which APs communicate with each other and with wireless STAs

I also read about the throughput drop when connecting in the repeater mode, but i did not understand why the throughput drops?

Suppose there is a daisy chain of 4 repeaters, how much will be the throughput drop? will it be half ? or more?

what abt WDS in Wireless Bridging mode, is there also a throughput drop?

Another query I had was,

Suppose we have 4 WDS connections, all in Wireless Repeating mode.

Can we achieve a roaming wifi client succesfully?

What would be the thoughput drop?

What would be the distance between two WDS(reapter) devices? Will it have to be twice that of the client distance/range, so as to connect to one the the Repeaters?

Please help!!

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jeff.kish Thu, 10/15/2009 - 04:47

First of all, understand that WDS is a Cisco protocol that allows APs to communicate with each other to improve client roaming, among other improvements. It has nothing to do with the AP's mode of operation. So try to separate the concept of WDS from the concepts of Repeater and Bridge. It will help you understand this a lot better, because this stuff is complicated enough as it is!

The primary role of a repeater is to expand a signal from an AP to a place where you cannot pull a cable. If you can get a cable there, then simply install a new AP in Root mode, which is the normal mode for an access point. Repeaters are only meant to be used if a cable cannot be pulled because it creates a very complicated RF situation, and throughput drops significantly (as you're aware).

Understand two things about wireless signals. For one, only one wireless radio (AP or client) can talk at once in a given area. This is called half-duplex communication. The other thing to consider is that wireless devices on the same channel cause interference with each other.

Repeaters exacerbate both of these problems, because they must be on the same channel as the root AP and they can only talk if the root AP (or subsequent repeaters) aren't talking. You're geographically expanding your collision and interference domain, allowing a lot more clients to connect back to the same AP while speaking on the same channel.

So would 4 repeaters work in a daisy-chain? Maybe, for Internet surfing at least. Your throughput will be a tiny fraction of the 20Mbps or so you'd be connecting at if directly speaking to a root AP.

As for bridging, this is when APs form a link between each other and pass data. This is generally used when connecting two buildings. While Cisco allows you to configure bridges to accept clients as well, it's highly recommended that you do not allow this unless absolutely required. Bridges should be specialized for bridging traffic, not for client connectivity.

I hope that makes sense. Please let me know if you have any questions!

Jeff

Queen245_goa Sun, 10/18/2009 - 23:10

Hi jeff,

Thanks for your reply...

Regarding wds relation with Bridge and Repeater.. here's what i read.

Check this site:

http://www.smallnetbuilder.com/content/view/24449/99/1/1/

and this one too

http://www.smallnetbuilder.com/wireless/wireless-howto/24308-howtowdsbridge?start=1#

http://en.wikipedia.org/wiki/Wireless_Distribution_System

Hmmm regarding throughput drop in repeaters, Is this drop bec of collisions and interference since the same channel is used? how exactly? You mean once it will talk to the rootAP and the other time to the wifi client or another repeater...

In the scenario below,

Ethernet Client -- AP1 -- Repeater -- Wifi Client

It is said that throughput drops to half? is it bec of the same reason ?

They say that its due to retransmissions of wifi on both sides, but wifi moves all around rite?

jeff.kish Wed, 11/11/2009 - 06:19

Ah, sorry man! I never saw your response.

I see the discrepancy now - we're talking about two different WDSs. The WDS you're referencing is not Cisco's WDS. Cisco's WDS = Wireless Domain Services, and it is completed unrelated to Wireless Distribution System bridging. In fact, Cisco doesn't support this latter form of WDS.

The slowdown is certainly partially caused by interference, but the primary reason is that only one device can speak at a time. The repeater is a client to the root AP, so not only does a client need to wait and speak to the repeater, but then the repeater needs to wait and speak to the root AP.

So repeaters do get data through, but it experiences more delay than usual. It is not ideal for anything other than low-bandwidth data.

I hope that helps. Sorry again for not responding for so long :) I'll try to do better this time if you have any more questions!

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