Updated 4 Feb 2011: Clarify throughput reduction.
Many Wi-Fi routers come equipped with wireless repeating capabilities. But properly configuring them often isn't easy. This article will review the basics of WDS-based wireless repeating and I'll then walk through setting up a few examples.
Please read references to access points (AP) as applying to either access points or wireless routers unless otherwise noted.
WDS, which stands for Wireless Distribution System, is a feature that enables single-radio APs to be wirelessly inconnected instead of using a wired Ethernet connection.
WDS connections are MAC address-based and employ a special data frame type that uses all four of the (MAC) address fields allowed in the 802.11 standard, instead of the three addresses used in normal AP <-> STA (client) traffic. (In the 802.11 frame header, address 1 is the destination address, address 2 is the source address, address 3 is the BSSID of the network and address 4 is used for WDS, to indicate the transmitter address.)
The provision for four MAC addresses in a frame is about the only thing covered by the 802.11 standards. But it was enough to allow bridging features to first be added to enterprise-grade, i.e. expensive, 802.11b products in the late 1990's. Many of these implementations were based around a medium access control (MAC) layer design originated by a long-defunct company called Choice Microsystems.
APs with wireless bridging features remained as high-priced items until fall 2002 when wireless bridging moved into consumer priced wireless products. D-Link first broke the artificially high wireless bridging price barrier by releasing a free upgrade to its DWL-900AP+ Access Point [reviewed]. This upgrade created the first consumer-priced WLAN product to support bridging and repeating. Other companies soon followed with similar upgrades, and also introduced dedicated Wireless Bridges, such as Linksys' WET11 [reviewed here].
Though these products were actually making use of the WDS feature, they didn't refer to it as such. It wasn't until products based on Broadcom's 802.11g chipset started to hit the market at the beginning of 2003 that the WDS term started to be commonly used. (Broadcom apparently included WDS support in its AP reference design code.)
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
Updated 4 Feb 2011
Two disadvantages to using WDS are:
- It's more difficult to set up than non-WDS.
- Dynamically assigned and rotated encryption keys are generally not supported in a WDS connection. This means that Wi-Fi Protected Access (WPA) and other dynamic key assignment technology may not be used. Static WEP keys only may be used in a WDS connection, including any STAs that associate to a WDS repeating AP.
Note that wireless throughput is cut approximately in half for each repeating "hop", i.e. an AP that data flows through before hitting the wired network. This is because all transmissions use the same channel and radio and must be retransmitted to reach the wired LAN. The repeating throughput reduction applies both to WDS and non-WDS repeating.
WDS isn't the only way to bridge and repeat, however. Some products don't use WDS at all and instead act like wireless client adapters. But instead of connecting via USB, Cardbus or other computery interfaces, they connect via Ethernet. So they can be used to wirelessly connect devices like media players, game systems or any other networkable device that has an Ethernet connector. I'll cover these devices in another part of this series.
As noted earlier, detailed specifications for WDS don't exist. Instead, the wireless networking industry has been pursuing mesh wireless, which is self-configuring and self-healing. There is no mesh wireless standard yet—the IEEE Mesh Networking Task Group (TGs) has been grinding away since 2004 and isn't scheduled to be done until mid 2011.
In the meantime, companies like Meraki, Proxim, Cisco and others have developed and sold wireless mesh systems. [See this review of Meraki's system.]
Figure 1: Meraki mesh APs
These systems are intended for enterprise use and have price tags to match. But if you're interested in playing with the technology, inexpensive 802.11g based mesh APs are available from Open-Mesh. Be warned, however, that throughput via 11g meshes is very low, in the single digit Mbps range. This is fine for basic connectivity for email, web browsing, etc. But not enough for high-quality video streaming.
On a practical basis, however, consumers' main method for trying out wireless repeating are routers and APs with WDS built in. And with WDS added to some 802.11n routers, it's now possible to get double-digit bandwidths through a single-hop bridge.
But WDS remains a technology, not a standard and WDS isn't tested as part of Wi-Fi Certification. As a result, manufacturers typically include weasel-words somewhere in their product documentation that say that bridging and repeating features will work only with their own products. And even if you don't find WDS interoperability disclaimers, good luck trying to get support from any vendor for a problem that involves any other vendor's product!
The gist of all this is that products from different vendors with WDS bridging / repeating features are not guaranteed to work with each other! The industry has come a long way from the early days of WDS in terms of interoperability. But, again, there are no guarantees.
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