FIXME: This should be moved to an appropriate category later.
What is WDS?
In IEEE 802.11 terminology a "Distribution System" is a system that interconnects so-called Basic Service Sets (BSS). A BSS is best compared to a "cell", driven by a single Access Point. So a "Distribution System" connects cells in order to build a premise wide network which allows users of mobile equipment to roam and stay connected to the available network resources.
Most home users never get in touch with Distribution Systems (DS) and the things related to it, since a single Access Point usually is enough to cover all relevant locations in a house. Things differ in common company installations where one Access Point isn't enough to provide a good coverage. This is where usually a Wired DS comes in: several Access Points (AP) which are connected to the company's ethernet network. A Wireless DS is used where a Wired DS either is not wanted or possible.
Example: Think of a company which owns two buildings on each side of a street. Each building has its own ethernet network, and the company wants to connect them both. Doing this with a cable is expensive. Either they have to rent a cable from a local Telco, or they need to bury their own wire. Using a Wireless DS (WDS) link would be much cheaper and easier to set up: configure one AP on each building for WDS, connect them to the building's ethernet, that's it.
Of course there are other scenarios where WDS might useful, refer to the Wikipedia article on WDS.
What is needed for WDS?
IEEE 802.11 (WLAN) frames have four address fields in their headers. In order to transport ethernet packets transparently over a WDS link, the IEEE 802.3 (Ethernet) frame gets encapsulated in a IEEE 802.11 (WLAN) frame. In this case all of the four address fields are used, for:
- sender of the ethernet frame
- receiver of the ethernet frame
- sender of the WLAN frame
- receiver of the WLAN frame
Sender and receiver of the ethernet frame are simply copied from the transported ethernet frame. The remaining fields allow the receiver to recognize that the frame is meant for him, and allow it to acknowledge the reception of the frame to the (WLAN) sender.
However, usually only three of the four fields are needed, so most drivers don't know about how to handle frames which make use of all four address fields. In other words: the most important ingredient for WDS is support for 4-address-headers.
WDS usually is implemented such that two or more AP are allowed to talk to each other without prior association. For this purpose each AP needs to know the MAC address of the "other side of the link" and, optionally, the keys used for WEP encryption. Every incoming Ethernet frame then is "bridged" to all known WDS partners by encapsulating it in a 4-address-header and sending it to each partner.
In case the above theory isn't enough, you should have a look at the following resources:
- Wireless Distribution System (PDF), ORiNOCO Technical Bulletin 046
- How To: Setting up WDS Bridging / Repeating, Toms Networking
Current status of WDS support in MadWifi
Note: The following situation applies to the new codebase.
We currently have support for 4-address-headers, which means that the most important ingredient for WDS support is available. Madwifi-driven stations can transparently bridge:
- in ad-hoc mode (as long as the link partner can handle 4-address-header frames from other Ad-hoc stations)
- in managed mode [also known as remote mode in the 802.11] (as long as the Access Point can handle 4-address-header frames from associated stations)
- in master mode [also known as main mode in the 802.11] (as long as the receiver is one of the stations associated to the station running in master mode)
- in repeater mode [also known as relay mode in the 802.11] (direct AP-to-AP as long as the receiver is able to accept state-less 4-address-header frames)