Wireless How To

Performance

The last time I looked at WDS repeating was with 802.11g gear. Figure 12 shows the throughput that I found by associating a wireless client with the root AP (the one connected to the wired LAN) and a single-hop WDS partner.

Figure 12: 802.11g WDS throughput comparison - Base AP and one hop

The almost 16 Mbps from the root AP isn't up to snuff with the 20 - 25 Mbps that you'll get from present-day 11g products. But the main point is the 50% throughput reduction (15.8 Mbps to 7.9 Mbps) produced by the single WDS repeating hop. Once again, this is because the repeating AP has to receive, then retransmit with a single radio, which uses twice the radio's bandwidth.

The 50% throughput hit doesn't change when repeating with 802.11n products. But since you start with more bandwidth, you end up with more bandwidth from a repeated connection. Figure 13 illustrates a bit different scenario than Figure 12, comparing throughput from a client connected to the remote WDS partner with Ethernet and wireless connections.

Figure 13: 802.11n throughput comparison- wired and repeating connections

Since the EnGeniuses default to using Auto 20/40 MHz bandwidth mode (because they are not Wi-Fi Certified), the performance shown is a bit higher than you'd see using a Wi-Fi compliant 20 MHz channel bandwidth. But the relative performance would be similar.

The upper trace ("bridge" trace) shows almost 87 Mbps throughput between a wired client on the LAN, through a 2.4 GHz WDS connection initiated by an ESR7750, to a test client connected to one of the remote ESR9850's Ethernet ports. The lower trace ("repeat" trace) uses the same setup, but with the client connected via 802.11n wireless to the ESR9850.

You can see once again, ~50% throughput reduction from the repeating connection (86.6 Mbps to 38.9 Mbps). But where we got around 8 Mbps from 11g repeating, the 11n repeater yields almost 40 Mbps!

But what if you don't want the 50% penalty from repeating, but still want a wireless connection at the remote end? With a two-radio AP at the remote end of the connection, you can set up a WDS "backhaul" link and then use the second radio for a normal, non-repeated wireless LAN.

Figure 14 shows that this method ("backhaul" trace), produces throughput somewhere in between the two other methods at 54 Mbps. To get this connection, I swapped the positions of the single-band ESR9850 and the simultaneous dual-band ESR7750, making the ESR7750 the remote WDS partner.

Figure 14: 802.11n throughput comparison- wired repeating and backhaul connections

The downside of using a dual-band simultaneous router is that you can't set both radios to the same band. So you need to choose where you want the better range, in the backhaul link, or far-end WLAN. I'd tend toward using 2.4 GHz for backhaul and 5 GHz for the remote WLAN. But that only works if you have dual-band capable clients.

Closing Thoughts

WDS can come in very handy for extending wireless LANs or connecting wired LANs where a cable isn't practical. But the technique is difficult to configure and has compatibility problems due to its use of non-standard technology. And since it is limited to using WEP encryption, you're forced to sacrifice throughput when using WDS with 802.11n gear.

But there is a better and easier way to extend your wireless LAN. Next time we'll explore bridges and repeaters that don't need no steenking WDS to work.