Wireless Distribution System (WDS)

A Wireless Distribution System (WDS) is a system that enables 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 need for a wired backbone to link them, as is traditionally required. The notable advantage of WDS over other solutions is that it preserves the MAC addresses of client frames across links between access points.

An access point can be either a main, relay or remote base station. A main base station is typically connected to the wired Ethernet. A relay base station relays data between remote base stations, wireless clients or other relay stations to either a main or another relay base station. A remote base station accepts connections from wireless clients and passes them on to relay or main stations. Connections between "clients" are made using MAC addresses rather than by specifying IP assignments.

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

WDS may also be referred to as repeater mode because it appears to bridge and accept wireless clients at the same time (unlike traditional bridging). However, with this method, throughput is halved for all clients connected wirelessly.

 Implementations

WDS may be incompatible between different products (even occasionally from the same vendor) as it is not certified by the Wi-Fi Alliance.

Technical

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

Two disadvantages to using WDS are:

The maximum wireless effective throughput is halved after the first retransmission (hop) that is made. For example, in the case of two routers connected via WDS, and communication is made between a computer that is plugged into router A and a laptop that is connected wirelessly using router B's access point, the throughput is halved, because router B has to retransmit the information during the communication of the two sides. However, in the case of communications between a computer that is plugged into router A and a computer that is plugged into router B, the throughput is not halved since there is no need to retransmit the information.

Dynamically assigned and rotated encryption keys are usually not supported in a WDS connection. This means that dynamic Wi-Fi Protected Access (WPA) and other dynamic key assignment technology in most cases can not be used, though WPA using pre-shared keys is possible. This is due to the lack of standardization in this field, which may be resolved with the upcoming 802.11s standard. As a result only static WEP or WPA keys may be used in a WDS connection, including any STAs that associate to a WDS repeating AP.

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[2]

Suppose you have an airport-capable xbox. Suppose it needs to send one packet to a WAN host, and get one packet in reply.

Network 1: An AirPort base station acting as a simple (non-WDS) wireless router. One packet leaves the xbox, goes over the air to the router, from there into WAN-land. One packet comes back, through the router, from there over the air to the xbox. Total packets sent over the air: 2.

Network 2: Two AirPort base stations acting as a 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 xbox. Xbox 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 xbox. Total packets sent over the air: 4.

Network 3: Two AirPort base stations acting as a WDS, but this time the xbox connects by ethernet cable to the remote base station. One packet goes from the xbox over cable to the remote, from there by air to the master, and on to the WAN. Reply comes from WAN to master, over air to remote, over cable to xbox. 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.

But network 2 gets an additional halving due to the fact that the remote base station uses double the air time because it's retransmitting over air packets that it just received over the air. That's the halving that's 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.