CWAP-405 Sample Questions & Answers

BSS Coloring is a spatial reuse technique introduced in 802.11ax. A station can detect a frame from an OBSS and, if the signal is below a certain threshold, ignore it and transmit simultaneously, even if its carrier sense mechanism would normally defer. The BSS color is a value from 1-63 that identifies frames belonging to a particular BSS, allowing STAs to make this differentiation and improving medium efficiency in dense environments.

The RSN (Robust Security Network) Information Element is used to advertise the security capabilities of the BSS. For WPA3-Personal, the AKM suite selector within the RSN IE would be set to indicate support for SAE (Simultaneous Authentication of Equals).

The RM (Radio Measurement) Enabled Capabilities element (part of the 802.11k amendment) is used to indicate that the AP supports radio measurement features, such as providing Neighbor Reports to clients, which is a key part of 802.11k assisted roaming.

This statement is true. DCF is designed to provide fair access to the medium. To prevent a single station from monopolizing the channel, every station must contend for each transmission opportunity. After a successful transmission (frame and ACK), the station must wait for the medium to be idle for a DIFS period and then begin a new random backoff procedure before it can transmit again.

This is a classic example of Adjacent Channel Interference (ACI). The AP on channel 7 is 'adjacent' but also 'overlapping' with channel 6. Unlike CCI, where devices on the same channel can understand each other's transmissions and defer properly, ACI introduces raw RF energy (noise) into the adjacent channel. This raises the noise floor and can corrupt frames, preventing clear reception and decoding, which leads to retransmissions and poor performance. CCI is interference from a device on the same channel.

The Block Ack mechanism allows for selective retransmission. The bitmap in the Block Ack frame informs the transmitter exactly which frames in the block were received successfully. The AP will use this information to retransmit only the missing frames, which in this case are those with sequence numbers 100 and 103. This is much more efficient than retransmitting the entire block.

Packet slicing, often configured via the 'snapshot length' or 'snaplen' parameter, instructs the capture engine to save only the first N bytes of each frame. Setting a snaplen of around 256 bytes is typically sufficient to capture the full MAC header and the entire body of most management and control frames (which are small), while truncating the large data payloads of Data frames. This dramatically reduces the capture file size without losing the required header and management frame information.

The RTS/CTS mechanism is designed to combat the hidden node problem. Any station that hears either the RTS or the CTS will update its Network Allocation Vector (NAV) based on the Duration/ID value in the frame. This is known as virtual carrier sense. Station B, having heard the RTS, will set its NAV for 1500 µs and will not attempt to access the medium until this timer expires, even though it cannot hear the AP's CTS or the subsequent data transmission.

The Trigger frame is the core of uplink multi-user (OFDMA and MU-MIMO) coordination in 802.11ax. The AP sends a single Trigger frame which contains multiple 'User Info' fields. Each User Info field is addressed to a specific client (by its Association ID) and contains instructions for that client, including which Resource Unit (RU) to transmit on, the target power level, and the modulation and coding scheme (MCS) to use.

The RSN IE advertises the security capabilities. The Pairwise Cipher Suite (CCMP) defines the encryption used for unicast traffic. The Authentication and Key Management (AKM) Suite defines the authentication method. Advertising both 802.1X and PSK in the AKM suite list means the AP is configured to allow clients to connect to this single SSID using either enterprise credentials (username/password/certificate) or a pre-shared key. This is a common feature on enterprise-grade equipment.

The scenario describes Over-the-Air Fast BSS Transition. The key indicator is the direct exchange of Authentication frames (containing FT Action details) between the client and the target AP over the wireless medium. This happens before the Reassociation Request. This method is extremely fast because it bypasses the need to route FT Action frames through the wired distribution system (as Over-the-DS FT does), making it ideal for latency-sensitive applications like VoIP.