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I'd like to know if the distribution system allowing roaming between access points is considered as being a part of the ESS (extended service set).

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To directly answer your question, I will provide a quote directly from the 802.11-2012 standard:

An ESS is the union of the infrastructure BSSs with the same SSID connected by a DS. The ESS does not include the DS.

This makes it very clear that while the DS is essential to the establishment of the ESS, the DS is not part of the ESS.

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A group of access points (APs) connected via a distribution system (DS) all sharing the same SSID would be considered Extended Service Set (ESS). The DS is usually a wired network (like Ethernet) but could be wireless as in a "wireless backhaul" or a mesh of APs. In "standard" 802.11 infrastructure implementations (and leaving out other topics such as AP "forced" load balancing) the mobile station (MS) is ultimately responsible for handoff between APs, the DS is used by the AP to forward any packets already buffered at the previous AP to the new destination AP. The protocol on the DS for inter-AP communication is vendor proprietary so for an "optimized" DS you don't want to mix APs from different vendors on an ESS.

Another way to look at this (with respect to infrastructure networks):

  • A BSSID is the MAC Address of the AP.
  • The SSID is the name of the wireless network that the MS uses connect to that network. Think of the SSID when you think of one AP with one or more MSs.
  • A ESS is when multiples APs look like one big distributed wireless network all connected to each other by the DS. The name of this distributed wireless network is termed the ESSID. One or more MSs can connect to this network and roam between APs.

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