Which two MAC modes are described as coexisting in 802.11 WLANs?

• A. PCF-mode and DCF-mode
• B. CSMA/CA-mode and TDMA-mode
• C. OFDM-mode and DSSS-mode
• D. MAC-layer mode and PHY-layer mode

Answer: (A)

Two MAC access methods used in 802.11 WLANs can coexist: DCF and PCF. DCF, based on CSMA/CA, lets stations contend for the medium and handles the bulk of traffic under normal operation. PCF provides a contention-free period where the access point polls stations to grant transmission rights, offering more predictable timing for time-sensitive data. When both are supported, the network can alternate: the AP manages a contention-free period with PCF while the rest of the time uses DCF for regular, contention-based access. This coexistence is the reason these two modes are described together. Note that PCF is optional and not always implemented, and the other choices mix up MAC versus PHY concepts (like TDMA, OFDM, DSSS) or describe modulation schemes rather than MAC access methods.