Q20Mobile Computing

<h3>IEEE 802.11 Architecture</h3><p>The IEEE 802.11 architecture consists of several interacting components designed to provide wireless local area network connectivity.</p><ul><li><strong>Basic Service Set (BSS):</strong> The fundamental building block, consisting of a group of stations communicating with each other. A BSS can be an Independent BSS (IBSS or ad-hoc mode) without an access point, or an Infrastructure BSS with an Access Point (AP).</li><li><strong>Access Point (AP):</strong> A station that provides access to the distribution services via the wireless medium for associated stations.</li><li><strong>Distribution System (DS):</strong> The mechanism by which multiple APs communicate to form a larger network, connecting different BSSs.</li><li><strong>Extended Service Set (ESS):</strong> A set of one or more interconnected BSSs and a DS that appears as a single logical network to the Logical Link Control (LLC) level.</li></ul>

<h3>MAC Layer and CSMA/CA</h3><p>The MAC layer in 802.11 must manage access to the shared wireless medium. Since collision detection (CSMA/CD) is difficult in wireless environments, it uses Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) and the Distributed Coordination Function (DCF).</p><ul><li><strong>Carrier Sensing:</strong> A station senses the medium before transmitting. If idle for a duration called Distributed Inter-Frame Space (DIFS), it transmits.</li><li><strong>Random Backoff:</strong> If the medium is busy, the station waits for it to become idle, then defers transmission for a random backoff period to avoid collisions with other waiting stations.</li><li><strong>RTS/CTS:</strong> To combat the hidden terminal problem, a station can optionally send a Request to Send (RTS) frame. The AP responds with a Clear to Send (CTS) frame. This reserves the medium for the duration of the transmission.</li><li><strong>Acknowledgments (ACK):</strong> After successful data reception, the receiver sends an ACK frame after a Short Inter-Frame Space (SIFS). If no ACK is received, the sender assumes collision or error and retransmits.</li></ul>