WDS: Wireless distribution system

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fuente: http://en.wikipedia.org/wiki/Wireless_Distribution_System

Wireless distribution system

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 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. 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 stations or to main stations. Connections between «clients» are made using MAC addresses.

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 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 this method, throughput is halved for all clients connected wirelessly.

Implementations

Wds may be incompatible between different products (even ocasionally from the same vendor) since it is not certified by the WI-FI Alliance.

Technical

WDS may 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 may be halved after the first retransmission (hop) being made. For example, in the case of two APs connected via WDS, and communication is made between a computer which is plugged into the Ethernet port of AP A and a laptop which is connected wirelessly to AP B. The throughput is halved, because AP B has to retransmit the information during the communication of the two sides. However, in the case of communications between a computer which is plugged into the Ethernet port of AP A and a computer which is plugged into the Ethernet port of AP B, the throughput is not halved since there is no need to retransmit the information. Dual band/radio APs may avoid this problem, by connecting to clients on one band/radio, and making a WDS network link with the other.
  • 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 cannot 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

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 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.[2]

But network 2 gets an additional halving because 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.

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.

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