802.11 Network Architecture
Topologies
A BSS is one AP plus its clients, identified by the BSSID (the AP radio's MAC address). An ESS links BSSs through a distribution system under one SSID. An IBSS (ad hoc) has no AP. Mesh uses 802.11s concepts with a mesh portal. The SSID is a logical name and is not unique; the BSSID is.
Joining the network
Clients discover APs passively (beacons, default 100 TU) or actively (probe request/response), authenticate (open system is the norm; shared-key WEP authentication is weak), then associate. Reassociation supports roaming between APs in the same ESS. Frame families: management (beacon, probe, authentication, association, action), control (ACK, RTS/CTS, Block ACK, PS-Poll) and data. Three MAC addresses appear in normal AP traffic; a fourth is used in WDS/bridge links.
Medium access
802.11 uses CSMA/CA with DCF: sense the medium, random backoff, transmit, await ACK. RTS/CTS mitigates hidden nodes at the cost of overhead; fragmentation mitigates interference. Power save uses the TIM in beacons and DTIM intervals to signal buffered traffic.
QoS and roaming
WMM defines access categories (voice, video, best effort, background) with different contention parameters. Roaming decisions are client-driven; the infrastructure can only assist via 802.11k/v/r.
Traps
Treating the SSID as a unique identifier; assuming 802.11 authenticates the client the same way in Personal and Enterprise; and thinking RTS/CTS solves all interference.
Rule of thumb
A 100 TU beacon interval with DTIM 2–3 is typical; a lower DTIM wakes sleeping clients more often and drains batteries faster.