Wireless distribution system

A wireless distribution system (WDS) is a system enabling the wireless interconnection of access points in an IEEE 802.11 network. It allows a wireless network to be expanded using multiple access points without the traditional requirement for a wired backbone to link them. The notable advantage of WDS over other solutions is that it preserves the MAC addresses of client frames across links between access points.[1]

An access point can be either a main, relay, or remote base station.

All base stations in a wireless distribution system must be configured to use the same radio channel, method of encryption (none, WEP, WPA or WPA2) and the same encryption keys. They may be configured to different service set identifiers (SSIDs). WDS also requires every base station to be configured to forward to others in the system.

WDS may also be considered a repeater mode because it appears to bridge and accept wireless clients at the same time (unlike traditional bridging). However, with the repeater method, throughput is halved for all clients connected wirelessly. This is because wifi is an inherently half duplex medium and therefore any wifi device functioning as a repeater must use the Store and forward method of communication.

WDS may be incompatible between different products (even occasionally from the same vendor) since the IEEE 802.11-1999 standard does not define how to construct any such implementations or how stations interact to arrange for exchanging frames of this format. The IEEE 802.11-1999 standard merely defines the 4-address frame format that makes it possible.[2]

Technical

WDS may provide two modes of access point-to-access point (AP-to-AP) connectivity:

Two disadvantages to using WDS are:

OpenWRT, a universal third party router firmware, supports WDS with WPA-PSK, WPA2-PSK, WPA-PSK/WPA2-PSK Mixed-Mode encryption modes. Recent Apple base stations allow WDS with WPA, though in some cases firmware updates are required. Firmware for the Renasis SAP36g super access point and most third party firmware for the Linksys WRT54G(S)/GL support AES encryption using WPA2-PSK mixed-mode security, and TKIP encryption using WPA-PSK, while operating in WDS mode. However, this mode may not be compatible with other units running stock or alternate firmware.

Example

Suppose you have a WiFi-capable game console. This device needs to send one packet to a WAN host, and get one packet in reply.

Network 1: A wireless base station acting as a simple (non-WDS) wireless router. The packet leaves the game console, goes over-the-air to the router, which then transmits it across the WAN. One packet comes back, through the router, which transmits it wirelessly to the game console. Total packets sent over-the-air: 2.

Network 2: Two wireless base stations employing WDS: WAN connects to the master base station, that connects over-the-air to the remote base station, which talks over-the-air to the game console. The game console sends one packet over-the-air to the remote, which forwards it over-the-air to the master, which sends it to the WAN. Reply comes from the WAN to the master base station, over-the-air to the remote, and then over-the-air again to the game console. Total packets sent over-the-air: 4.

Network 3: Two wireless base stations employing WDS, but this time the game console connects by Ethernet cable to the remote base station. One packet goes from the game console over cable to the remote, from there by air to the master, and on to the WAN. Reply comes from WAN to master, over-the-air to remote, over cable to game console. Total packets sent over-the-air: 2.

Notice that network 1 (non-WDS) and network 3 (WDS) send the same number of packets over-the-air. The only slowdown is the potential halving due to the half-duplex nature of wifi.[3]

But network 2 gets an additional halving because the remote base station uses double the air time because it's retransmitting over-the-air packets that it has just received over-the-air. This is the halving that is usually attributed to WDS, but that halving only happens when the route through a base station uses over-the-air links on both sides of it. That does not always happen in a WDS, and can happen in non-WDS.

Important Note: This "double hop" (one wireless hop from the main station to the remote station, and a second hop from the remote station to the wireless client [game console]) is not necessarily twice as slow. End to end latency introduced here is in the "store and forward" delay associated with the remote station forwarding packets. In order to accurately identify the true latency contribution of relaying through a wireless remote station vs. simply increasing the broadcast power of the main station, more comprehensive tests specific to the environment would be required.

See also

References

  1. Wireless Distribution System Linked Router Network DD-WRT Wiki. Retrieved December 31, 2006.
  2. IEEE: 4-address-format.doc

External links

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