Experiencing a big speed decrease using AP541 as repeater with WDS

Unanswered Question
Apr 5th, 2010

We have a WDS connection between two AP541 access points, where one AP is connected to a SA540 with cable. The AP in the other end of the WDS does just act as a repeater, broadcasting using the same SSID and channel with WPA-PSK, as the cabled AP. For the WDS channel, we have tried WPA/WEP/none encryption without any difference. Spanning tree is enabled. 802.11 mode is 'bgn'.

Sitting physically beside the remote AP (the "other" and uncabled end of WDS) with a laptop using Intel 802.11bg adapter, we get a speed in the range 5-7Mbit/sec connecting to the cabled AP (3 of 5 bars signal strength in the traybar icon on Win7). But switching to the remote AP, which is beeing feed by the WDS from the same cabled AP previously beeing connected to, we do just get a measured bandwith of 1-2Mbit/sec. I understand that some throughput is lost using WDS, but this does still look like a big decrease?

Anyone with experience using WDS with AP541, who can comment on if this is expected or not, and eventually any thing we can do to increase the speed?

- Ola

I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
Joachim Kern Wed, 06/30/2010 - 09:16

All devices on the same channel compete to send packets on this channel, but only one device can sent at a time. If one device sends, the other devices must wait. Because each packet is repeated the bandwidth will go down (50%). The PC and the AP1 need to wait until the repeater finishes sending. You basically send twice the amount of data over the Wifi.

If you maxed out your connection with one AP. Using a repeater will half the bandwidth.

This has nothing to do with which manufacturer you use.

You also add extra delay which could cause some applications to perform even slower because they wait for acknowledgements. (CIFS/SMB)

WDS repeating is good to extent access without cables. However do not expect high bandwidth.

MICHAEL JOHNSON Mon, 08/23/2010 - 23:50

Mr. Kim,

Your analysis is correct, but can you explain why throughput using WDS actually works when the APs use the 5GHz band?

You can see my prior comment for details. I can tell you, we spent a lot of time with this product in the lab, and WDS over the 2.4GHz spectrum just does not work once any below-average load is placed on it. While this might not be a big deal to you, it results in the need for additional APs for a WDS solution for a client or another product solution. In my opinion, the WDS option should not be offered until Cisco engineering can correct the throughput issue on the 2.4GHz spectrum.

In our lab, we mirrored a typical customer environment by requiring WPA across the WDS link. We tested it under several 2.4GHz scenarios, and all ended with the same poor throughput/load failure results. Once the wireless was revised to 5GHz band, the WDS performed as I would expect a Cisco WDS solution to perform.

My recommendation to Cisco is lab the units and test WDS under load with WPA so the 2.4GHz WDS can be improved.

MICHAEL JOHNSON Mon, 08/23/2010 - 22:48

I am experiencing your same pain. The throughput via the WDS link is outright horrible. Cisco shou

ld either fix it or remove the feature. During our testing, a simple standard ICMP packet run 100 times fro

m a LAN segment pinging the IP of the remote WDS unit yielded a sub 5% packet loss (bad since they

are sitting 50' apart in clear open space and average ti

me of 45ms. Once we actually added load to the WDS link via a sized ICMP packet at 1300, the packet loss increased to over 15% and average times up to 300ms.

To give these things credit, they have great throughput when in standard or cluster mode, it just appears something is missing/wrong with WDS throughput. Almost feels like a MTU issue, but without an MTU setting anywhere, that won't be fixed until a Cisco engineer sees this post and actually tests WDS on the AP541.

Joachim Kern Wed, 08/25/2010 - 03:36

I cannot explain the packet loss without testing. If you use the AP541N on 2.4ghz as an AP and as WDS and since the device has only a single radio the throughput will be not be great as previously posted. Easily it can go down 50%. This is  because the single radio needs to retransmit packets several times and only one station can send at any given time all other stations have to wait.

I suggest to deploy WDS either alone (not using the AP functionalityat tge same time and  only to interconnect two LAN segments) or only in environments where no high throughput is required.

Having said this of course the AP541N could also have a problem as well (especially when packet loss is experienced).

That would require some more testing and investigation as previously mentioned.

MICHAEL JOHNSON Wed, 08/25/2010 - 09:48

We tested 2 APS in WDS mode only- 2.4GHz and 5GHz. 2.4GHz performance was unacceptable and poor at best. 5GHz performed better than expected. The spectrum is clear since this was done in our lab and pre-spectrum discovery performed. The issue was duplicated by the person first creating this post, and I'm hopeful someone at Cisco will see these posts and take those things back to the lab for testing. If the option to bridge is available under 2.4GHz, it should work. Otherwise simply remove that option from the device and move on.

Thank you.


This Discussion

Related Content