This visualiser was generated by Claude (Anthropic) as part of the Wireless 101 course materials. The field offsets and their sum, the whole Frame Control decode, and the CRC-32 over the real frame bytes are computed live from the rules printed at the foot of the page — the frame is assembled byte by byte and the trailer is a genuine CRC-32, never a number typed in. Three kinds of number appear, each labelled where it is used: computed (offsets, the decode, the CRC, the FCS check, the MCS rate), published figure (the 802.11 MAC header layout, the Frame Control bit map, the CRC-32 polynomial, and the 802.11n 20 MHz PHY constants), and illustrative (the specific addresses, Duration, Sequence Control and body bytes, chosen so there is a concrete frame to run the CRC over). It does not capture any traffic and it does not call out to a server.
802.11 MAC frame: Frame Control (2) | Duration/ID (2) | Address 1/2/3 (6 each) | Sequence Control (2) | [Address 4 (6)] | Frame Body (0–n) | FCS (4). FCS is a CRC-32 with reflected polynomial 0xEDB88320 (IEEE 802.3); CRC-32("123456789") = 0xCBF43926. Sources: IEEE 802.11-2020 §9.2 (frame format), IEEE 802.3 (FCS), course M11-L1 WiFi.
Take an 802.11 MAC frame apart field by field, decode the Frame Control word bit by bit, and check the FCS trailer is a real CRC-32 over the frame. Change the Frame Control hex to see the type and flags change; stretch the frame body and watch the offsets and the total length move; flip a byte and the CRC no longer matches — the frame reads corrupt. Every offset, bit and checksum is computed from the bytes, never sketched.
A receiver takes the raw bytes, walks the fixed field layout to find each field, decodes the Frame Control word into type/subtype and flags, and verifies the FCS is a CRC-32 over everything before it. Each thumbnail is drawn from the same computation as the panels below — nothing here is an icon. Hover a block to highlight the panel and the knobs that feed it.
Each cell is one field, with its byte offset and length. The offsets are contiguous — each field starts exactly where the previous one ends — so the lengths sum to the total frame length. Add Address 4 (only present when both ToDS and FromDS are set) or stretch the body and every offset after it shifts.
The 2-byte Frame Control word carries the protocol version (bits 0–1), the type (bits 2–3), the subtype (bits 4–7) and eight flags (bits 8–15). Type 0 is Management, 1 is Control, 2 is Data. Type the hex and watch the bits and the decode change.
The transmitter computes a CRC-32 over the MAC header and body and appends it as the 4-byte FCS. The receiver recomputes the CRC over the bytes it got and compares. Flip a byte and the recomputed CRC diverges from the transmitted trailer — that is how a corrupt frame is caught.
The rate is not looked up — it is computed from the physical parameters: rate = N_SD · N_BPSCS · R · N_SS / T_SYM, with the 802.11n 20 MHz constants N_SD = 52 and T_SYM = 4.0 µs (or 3.6 µs with the short guard interval). The MCS index only names the modulation and coding rate.