A wireless distribution system is a system enabling the wireless bridging of access points in an IEEE 802 network. It allows an extended network to be created using wireless IEEE 802.11 access points without the traditional requirement for wires 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, thus mixtures of ethernet and WDS connections can be treated as a single network.
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 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 as well as accepting wireless clients at the same time. However, with the repeater method, throughput may be 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. The effect is similar to communications between clients via an access point, and can be prevented if the node is dual band. WDS may be incompatible between different products 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.
Technical
WDS may provide two modes of access point-to-access point connectivity:
Wireless bridging, in which WDS APs communicate only with each other and don't allow wireless stations to access them
Wireless repeating, in which APs communicate with each other and with wireless STAs
In WDS mode, the two bits in Frame Control header of a wireless data frame called To DS and From DS are both set to 1. In this case, all four MAC addresses are needed to successfully deliver the frame to its final destination. The TA is the address of the transmitting AP, while the RA is the address of receiving AP and stay the same as long as the configuration does not change. Two disadvantages to using WDS are:
The maximum wireless effective throughput may be halved after the first retransmission 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 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.
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/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 there is a Wi-Fi capable device and a wireless router connected to the Internet. Assume that the Wi-Fi capable device needs to send one packet to a remote server on the Internet, and get one packet in reply. Situation 1: A Wi-Fi capable device connected to the wireless router. The packet is sent from the device over-the-air to the router, which is then sent to the remote server. The router receives a reply from the remote server, which is then forwarded to the device wirelessly. In total, 2 packets are sent over-the-air. In situations 2 and 3, another wireless router is added, with both employing WDS. The Internet-facing router acts as a main base station, and the additional router acts as a remote base station as follows: Situation 2: A Wi-Fi capable device connected to the remote base station through Wi-Fi. The device wirelessly sends one packet to the remote base station, which is wirelessly forwarded to the main base station, which in turn forwards the packet to the remote server. The main base station receives the reply, which is then forwarded wirelessly to the remote base station, then to the device. In total, 4 packets are sent over-the-air. Situation 3: A Wi-Fi capable device connected to the remote base station through wired Ethernet. The device sends one packet to the remote base station, which is then wirelessly forwarded to the main base station, which is then sent to the remote server. The main base station receives a reply, which is then forwarded wirelessly to the main base station, then to the device through wired connection. In total, 2 packets are sent over-the-air. It is observed that in situations 1 and 3, the same number of packets are sent over-the-air, with the only slowdown being the potential halving due to the half-duplex nature of the Wi-Fi. However, in situation 2, there is additional halving due to the remote base station using double air time: it re-transmits over-the-air packets that have just been received over-the-air. This is usually attributed to WDS, but does not always occur in WDS, and may occur even without WDS: it only happens when the packets are delivered through a base station over-the-air on both sides. Important Note: This "double hop" 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.
