CTS 156: 802.11 Service Sets
802.11 has different topologies to be aware of. They describe how 802.11 radios may be used to communicate with each other. Many are familiar with the most common ones such as BSS and ESS. But additionally, a client can operate in an IBSS, PBSS, and MBSS.
- IBSS: Independent Service Set
- BSS: Basic Service Set
- ESS: Extended Service Set
- PBSS: Personal Service Set
- MBSS: Mesh Basic Service Set
- QBSS: QoS Basic Service Set
What is a Basic Service Set?
The BSS is the foundation of a WLAN. It is the most common 802.11 topology. The BSS is 1 AP with one or more client stations associated. The BSA (Basic Service Area) is the coverage area of an AP.
SSIDs have a unique BSSID. The BSSID is a MAC address of the AP’s radio of a particular SSID. The SSID is advertised in beacons (here’s a BSS!)
The 802.11 standard references BSS in various ways
- HT BSS
- VHT BSS
- HEW BSS (802.11ax)
Clients move between BSAs as they roam, negotiating rates dynamically as RSSI and SNR changes. There is no sharp cutoff of a BSA as it depends on external factors.
What is an ESS
It is the Extended Service Set. Or multiple BSSs with the same SSID (ESSID) connected to the same infrastructure. An example would be running a WLAN for an entire campus or running WLAN for a large office. Clients roam from one BSS to another within the same ESS
Overlap of BSA is for roaming. Hopefully, efficiently done with Fast BSS (802.11r)
IBSS
The Independent BSS. Sometimes, but not often, you aren’t connecting to a ESS. It is a basic WLAN consisting of only two clients. An ad hoc network created by, for example, a laptop.
Another client, say a laptop or tablet, communicates directly with the other client. It consists of clients that are directly connected which, technically, there is only one BSS.
There is no AP, just client radios. The BSSID is randomly generated by the first station which setup the IBSS
PBSS
The Personal BSS. Similar to IBSS but for DMG (Directional multi-gigabit) – 60GHz and clients communicate directly with each other. One client will be a PBSS control point (PCP).
PBSS is established through DMG clients. DMG is 802.11ad (directional multi-gigabit), commonly in mmWave
MBSS
The Mesh BSS. All clients in an MBSS establish links with neighboring clients. They determine hop capabilities.
An AP can both provide connectivity to clients and be a client of another AP to provide a mesh backhaul connection. There are a lot of consumer APs using mesh.
QBSS
The Quality of Service Basic Service Set. Simply a BSS that implements QoS.
Any enterprise AP manufactured in the past 10 years supports QoS, therefore, each BSS in most enterprise deployments is considered an QBSS.
