The IEEE 802.11 Distributed Coordination Function (DCF) describes contention-based medium access used in legacy Wi-Fi operation. Because multiple stations share radio airtime and cannot reliably detect a collision while transmitting, they use carrier sensing, waiting intervals and randomized backoff to reduce\u2014not eliminate\u2014the chance of transmitting simultaneously.<\/p>\n
The access point is also a station for this contention purpose. It does not centrally grant every uplink frame transmission in ordinary DCF operation. Understanding that fact helps diagnose hidden-node collisions, contention, latency and the effect of later QoS mechanisms.<\/p>\n
Before transmitting, a station examines whether the medium appears busy from physical carrier sensing and may consider virtual reservation information recorded in a Network Allocation Vector (NAV). If the channel remains busy, the station defers. A station may hear an access point but not another client that is transmitting to that AP; this hidden-node relationship can produce collisions even though both clients obey their local carrier-sense rules.<\/p>\n
Clear Channel Assessment has radio-specific thresholds and behavior. A \u201cbusy\u201d indication is not the same as successfully decoding another device's payload, and a quiet indication at one receiver does not prove silence across the entire cell.<\/p>\n
Under legacy distributed coordination (DCF), a station normally waits for the relevant DCF interframe space (DIFS) before counting eligible idle slots from a randomized contention window. It freezes the remaining counter when the channel becomes busy and resumes under the applicable idle conditions; transmission starts when the counter reaches zero and the medium-access rules permit it. Enhanced distributed channel access (EDCA) uses access-category-specific arbitration interframe spaces (AIFS) and contention parameters, so a DIFS example should not be presented as the complete timing rule for every modern QoS transmission.<\/p>\n
Slot times, interframe spacing and contention windows depend on the PHY and QoS mechanism. Numbers drawn from 802.11b examples cannot be copied as universal timing constants for 802.11ax or 802.11be. The concept remains the combination of deferral and randomization under specified parameters.<\/p>\n
Successful reception may be signaled by an 802.11 acknowledgment after a short interframe space (SIFS). If no expected ACK arrives, a station may increase its contention window and retry according to limits. That absence can result from a collision, weak signal, interference, roaming, a busy receiver or ACK loss; it does not prove another station transmitted at the same instant.<\/p>\n
Measure frame retries, PHY errors, airtime utilization and client SNR rather than interpreting high retries as one specific failure mode. A radio can show healthy RSSI while an interferer or oversubscribed channel makes application latency unacceptable.<\/p>\n
Request-to-Send\/Clear-to-Send exchanges can reserve airtime through duration information and reduce certain hidden-node collisions. They add overhead and do not solve every topology, interference or load problem. Their value must be tested at realistic packet sizes and network conditions, not enabled as a universal default remedy.<\/p>\n
An access point may coordinate selected functions under newer features, but RTS\/CTS remains part of a defined channel-access process. It does not make all ordinary 802.11 DCF access centrally scheduled by the AP.<\/p>\n