Extended Service Set (ESS) and Wireless Distribution System (WDS)

WLAN Extended Service Set Overview

WLAN Extended Service Set is WiFi formation which links a many different BSSes together with the one backbone area. Basic Service Set – BSS is the basic building block in WLAN networks. Backbone is wired Ethernet, which connects all users in the wireless network to the internet. All the clients connected to the same and different BSS can communicate among each other. Wireless Local Area Network Extended service set or just WLAN ESS.

Basic elements of the WiFi networks you can check in WLAN formation.

WLAN networks can be also called WiFi network. The name WiFi is used more often. More about the difference and similarity of WiFi and WLAN read in What is WiFi? What is WLAN? Is There a Difference Between Those Two. More about WLAN and how it works you can learn in How WLAN Works – Your Best WLAN Tutorial.

Wireless Distribution System and WLAN ESS

Extended service set

Extended service set

Properly configured wireless distribution system or WDS is the key to forming the functional ESS. WDS is responsible for tracking the locations of each client in the extended service set. Wired router in the backbone (which you see on the picture) sends a frame to client with the destination MAC address. The rest is the job of the WDS. Wireless distribution system delivers the frames to the right access point. Access point finally delivers the packet to the specific client.

More about the WDS you can read on Wireless Distribution System – How to Configure the WDS.

Inter-Access Point Protocol and Inter-Access Point Communication
Inter-access point communication is a method to manage associations among different access points. Station first associates to the AP1. After that AP1 sends packet – I “have” that station, to other access points (AP2, AP3 and AP4).

Inter-Access Point Protocol (IAPP) enables communication between access point, especially between access points from different vendors. IAPP is defined by IEEE standard 802.11F.

IAPP enables unique associations throughout the extended service set. It also ensures the secure exchange of security parameters of the client during the handoff period between the old access point and the new access point. A secure session keys between access points are distributed via a RADIUS server. The RADIUS server also provides mapping between the MAC address of the access point and IP address.

BSS Overlapping in the WLAN Extended Service Set
Wireless networks usually have the overlapping network boundaries. This fuzziness is quite desirable. If we make closer AP1 to AP3 (or increase their powers), BSS1 and BSS3 will overlap. This overlapping will ensure, that the client which moves from BSS1 to BSS3, doesn’t lose wireless connection, even if AP2 fails.

WiFi vs Ethernet
Looking above from the user’s prospective, WiFi is almost the same as Ethernet. It is up to mechanisms defined in WLAN standards, which ensure reliable wireless connection and a great user experience. This is the reason why 802.11 standard requires the numbers of additional services and more complex framing comparing to the Ethernet.

More about the extended service set read in The IEEE 802.11s Extended Service Set Mesh Networking Standard.