Wireless Distribution System
Encyclopedia
A wireless distribution system (WDS) is a system enabling the wireless interconnection of access point
s 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.
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 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 identifier
s. 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.
.
Two disadvantages to using WDS are:
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.
. This device 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. The packet leaves the Xbox, 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 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 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 [xbox]) 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.
Wireless access point
In computer networking, a wireless access point is a device that allows wireless devices to connect to a wired network using Wi-Fi, Bluetooth or related standards...
s in an IEEE 802.11
IEEE 802.11
IEEE 802.11 is a set of standards for implementing wireless local area network computer communication in the 2.4, 3.6 and 5 GHz frequency bands. They are created and maintained by the IEEE LAN/MAN Standards Committee . The base version of the standard IEEE 802.11-2007 has had subsequent...
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.
An access point
Wireless access point
In computer networking, a wireless access point is a device that allows wireless devices to connect to a wired network using Wi-Fi, Bluetooth or related standards...
can be either a main, relay, or remote base station
Base station
The term base station can be used in the context of land surveying and wireless communications.- Land surveying :In the context of external land surveying, a base station is a GPS receiver at an accurately-known fixed location which is used to derive correction information for nearby portable GPS...
. A main base station is typically connected to the (wired) Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
. 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 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
Wired Equivalent Privacy
Wired Equivalent Privacy is a weak security algorithm for IEEE 802.11 wireless networks. Introduced as part of the original 802.11 standard ratified in September 1999, its intention was to provide data confidentiality comparable to that of a traditional wired network...
, or WPA
Wi-Fi Protected Access
Wi-Fi Protected Access and Wi-Fi Protected Access II are two security protocols and security certification programs developed by the Wi-Fi Alliance to secure wireless computer networks...
) and the same encryption keys. They may be configured to different service set identifier
Service set identifier
A service set is all the devices associated with a local or enterprise IEEE 802.11 wireless local area network .-Service set identifier :...
s. 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
Wireless bridge
A wireless bridge is a hardware component used to connect two or more network segments which are physically and logically separated. It does not necessarily always need to be a hardware device, as some operating systems provide software to bridge different protocols...
). 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) since it is not certified by the Wi-Fi AllianceWi-Fi Alliance
The Wi-Fi Alliance is a trade association that promotes Wireless LAN technology and certifies products if they conform to certain standards of interoperability. Not every IEEE 802.11-compliant device is submitted for certification to the Wi-Fi Alliance, sometimes because of costs associated with...
.
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 repeatingWireless RepeaterWhen two or more hosts ought to be connected with one another over the IEEE 802.11 protocol and the distance is too long for a direct connection to be established, a wireless repeater is used to bridge the gap. It can be a specialized stand alone computer networking device. Also, some WNICs...
, 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) being made. For example, in the case of two routers connected via WDS, and communication is made between a computer which is plugged into router A and a laptop which 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 which is plugged into router A and a computer which 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 AccessWi-Fi Protected AccessWi-Fi Protected Access and Wi-Fi Protected Access II are two security protocols and security certification programs developed by the Wi-Fi Alliance to secure wireless computer networks...
(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 an WiFi-capable XboxXbox
The Xbox is a sixth-generation video game console manufactured by Microsoft. It was released on November 15, 2001 in North America, February 22, 2002 in Japan, and March 14, 2002 in Australia and Europe and is the predecessor to the Xbox 360. It was Microsoft's first foray into the gaming console...
. This device needs to send one packet to a WAN host, and get one packet in reply.
Network 1: An AirPort
AirPort
AirPort and AirPort Extreme are local area wireless networking products from Apple Inc. based on the IEEE 802.11 standard ....
base station acting as a simple (non-WDS) wireless router. The packet leaves the Xbox, 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 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 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 [xbox]) 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
- Ad hoc wireless networkAd hoc wireless networkAn ad hoc network typically refers to any set of networks where all devices have equal status on a network and are free to associate with any other ad hoc network devices in link range...
- Wireless mesh networkWireless mesh networkA wireless mesh network is a communications network made up of radio nodes organized in a mesh topology. Wireless mesh networks often consist of mesh clients, mesh routers and gateways.The mesh clients are often laptops, cell phones and other wireless devices while the mesh routers forward traffic...
- Wireless intrusion detection system
- Bridging (networking)Bridging (networking)Bridging is a forwarding technique used in packet-switched computer networks. Unlike routing, bridging makes no assumptions about where in a network a particular address is located. Instead, it depends on flooding and examination of source addresses in received packet headers to locate unknown...
External links
- Swallow-Wifi Wiki (WDS Network dashboard for DD-WRT devices)
- Alternative Wireless Signal-repeating Scheme with DD-WRT and AutoAP
- What is Third Generation Mesh? Review of three generation of mesh networking architectures.