Wireless Protocols

Data Frame

Q: How does a data frame differ from a packet?

A packet is the Layer 3 (network layer) protocol data unit, typically an IP packet with a source and destination IP address. A data frame is the Layer 2 (data link layer) unit that encapsulates one or more packets for transmission over a specific physical medium. The frame adds the addressing and control that the medium needs: a preamble for receiver synchronization, MAC source and destination addresses, sequence control, and a frame check sequence for error detection. In an 802.11 system, an IP packet of 1500 bytes becomes the payload inside a MAC frame, which the PHY then prepends with a preamble and PLCP header before modulating it onto OFDM subcarriers. The same packet may be fragmented across several frames if it exceeds the medium maximum transmission unit, and frames are stripped of their headers and FCS as the data climbs back up the protocol stack at the receiver.

Q: What happens when a data frame fails its FCS check?

The receiver computes a cyclic redundancy check (CRC-32 in 802.11 and Ethernet) over the received frame and compares it against the 4-byte frame check sequence appended by the transmitter. If the values disagree, at least one bit was corrupted in transit and the entire frame is discarded; the receiver does not pass partial or damaged data up the stack. In 802.11 the receiver withholds the acknowledgment, and the transmitter retransmits after a timeout, backing off and often dropping to a more robust modulation and coding scheme. A 4-byte CRC-32 detects all single-bit errors, all double-bit errors, all odd numbers of errors, and any error burst up to 32 bits long, with an undetected error probability near 2^-32 for longer bursts. Persistent FCS failures indicate low SNR, interference, or multipath beyond what the guard interval can absorb.

Q: How long does a typical Wi-Fi data frame take to transmit?

Frame duration depends on payload length and the modulation and coding scheme. In legacy 802.11a/g the preamble and SIGNAL field consume a fixed 20 microseconds, independent of payload; HT and VHT preambles are longer because they add training fields for each spatial stream. The data portion duration equals the payload bits divided by the data rate, rounded up to whole OFDM symbols of 4 microseconds each (3.6 microseconds with the short guard interval). A 1500-byte frame at MCS 7 (64-QAM, rate 5/6) on an 80 MHz channel with one spatial stream needs only about eleven symbols, so its data portion is near 44 microseconds and the whole frame including preamble is under 100 microseconds, while the same frame at the most robust MCS 0 (BPSK, rate 1/2) on a 20 MHz channel takes about 1.9 milliseconds. Adding the SIFS interval, the acknowledgment frame, and contention overhead, the protocol efficiency for a single unaggregated frame is often below 60 percent, which is why frame aggregation is used to amortize the fixed overhead across many payloads.

/DAY-tuh fraym/

The structured Layer 2 unit of transmission that bounds a discrete block of information for delivery over a wireless channel. A data frame begins with a preamble that lets the receiver achieve symbol timing and frequency lock, followed by physical-layer and MAC headers that describe the modulation, length, and addressing, then a variable-length payload carrying the user data, and finally a frame check sequence that lets the receiver detect corruption. The framed bitstream is mapped onto OFDM subcarriers and modulated onto the carrier; at the receiver the headers steer demodulation and the FCS gates whether the payload is accepted or discarded. Framing is what turns a continuous over-the-air waveform into addressable, error-checkable, retransmittable units.

Category: Wireless Protocols

Protocol Layer: Data Link (L2)

802.11 MTU: 2,304 bytes payload

Anatomy of a Wireless Data Frame

Every wireless protocol that shares a medium must delimit where one transmission ends and the next begins, identify who sent the data and who should receive it, and give the receiver a way to know whether what arrived matches what was sent. The data frame solves all three at the data link layer. It is the protocol data unit of Layer 2, sitting directly above the physical layer and directly below the network layer, and it is the level at which medium access control, addressing, and error detection live. When an IP packet is handed down for transmission, it is encapsulated as the payload of a frame; the receiving station strips the framing and hands the packet back up only after the integrity check passes.

A frame is built in nested layers. The MAC sublayer assembles a MAC protocol data unit containing the frame control field, duration, source and destination addresses, sequence control, the payload, and a 4-byte frame check sequence computed as a CRC-32 over the frame body. The physical layer then wraps that MPDU in a PLCP preamble and header, which together form the physical-layer protocol data unit that is actually modulated onto the carrier. The preamble is not user data; it is a known training sequence the receiver correlates against to recover timing, estimate the channel, and correct carrier frequency offset before a single payload bit is decoded.

Because the medium is shared and lossy, framing is tightly coupled to retransmission. In 802.11, a correctly received and FCS-validated unicast frame triggers an acknowledgment after a short interframe space; a frame that fails its FCS, or whose acknowledgment is lost, is retransmitted, often after the rate-control algorithm steps down to a more robust modulation and coding scheme. The fixed per-frame overhead of preamble, headers, and interframe spacing is the reason modern standards aggregate many subframes into a single A-MPDU, amortizing that overhead across a much larger payload.

How Frame Bits Map to OFDM Symbols

After the MAC builds the frame and the PHY adds its header, the bitstream is scrambled, forward-error-correction coded, interleaved, and grouped into constellation points. Those points are loaded onto the data subcarriers of each OFDM symbol. The number of OFDM symbols a frame occupies, and therefore its air time, follows directly from the payload length, the bits per subcarrier set by the modulation order, the code rate, and the number of usable data subcarriers in the channel bandwidth. A cyclic prefix guard interval is prepended to each symbol to absorb multipath delay spread, trading a few percent of throughput for immunity to inter-symbol interference.

Frame Timing and Throughput

Frame air time (PHY):
T_frame = T_preamble + N_sym × T_sym

OFDM symbols required:
N_sym = ⌈ (8 × L_payload + N_service + N_tail) / N_DBPS ⌉

Data bits per symbol:
N_DBPS = N_SD × N_BPSC × R × N_SS

Where L_payload = payload bytes, N_SD = data subcarriers (52 on a 20 MHz HT channel), N_BPSC = bits per subcarrier (6 for 64-QAM), R = code rate (5/6 for MCS 7), N_SS = spatial streams, T_sym ≈ 4 μs (3.6 μs short GI). Example: a 1500-byte frame at MCS 7, 1 stream, 20 MHz → N_DBPS ≈ 260, N_sym ≈ 47 symbols ≈ 188 μs of data.

Frame Types and Field Structure

Frame field / type	Typical size	Layer	Function	Notes
PLCP preamble	8 to 20 μs	PHY	Timing, AGC, channel estimate	Known training sequence
PHY / PLCP header	3 to 4 bytes	PHY	Rate, length signaling	SIGNAL field, BPSK
MAC header	24 to 36 bytes	MAC (L2)	Addresses, frame control, seq	Up to 4 address fields
Payload (frame body)	0 to 2,304 bytes	MAC (L2)	Encapsulated upper-layer data	Holds the IP packet
Frame check sequence	4 bytes	MAC (L2)	CRC-32 error detection	Discard on mismatch
Control frame (ACK)	14 bytes	MAC (L2)	Confirms receipt	No payload

Common Questions

Frequently Asked Questions

How does a data frame differ from a packet?

A packet is the Layer 3 network unit (an IP packet with source and destination IP addresses); a data frame is the Layer 2 unit that encapsulates one or more packets for a specific medium. The frame adds the preamble, MAC addressing, sequence control, and FCS the medium needs. A 1500-byte IP packet becomes the payload inside a MAC frame, which the PHY prepends with preamble and header before modulating onto OFDM subcarriers. Large packets may be fragmented across frames if they exceed the medium MTU.

What happens when a data frame fails its FCS check?

The receiver computes CRC-32 over the received frame and compares it to the 4-byte FCS; on mismatch the entire frame is discarded and no partial data is passed up. In 802.11 the receiver withholds its acknowledgment, so the transmitter retransmits after a timeout, usually dropping to a more robust MCS. A 4-byte CRC-32 catches all single, double, and odd-count bit errors and any burst up to 32 bits, with undetected-error probability near 2⁻³². Persistent failures point to low SNR, interference, or multipath beyond the guard interval.

How long does a typical Wi-Fi data frame take to transmit?

Air time is the fixed preamble plus the data portion. The legacy 802.11a/g preamble and SIGNAL field take about 20 μs regardless of payload (HT and VHT preambles run longer); the data portion equals payload bits divided by the data rate, rounded up to whole 4 μs OFDM symbols (3.6 μs with short GI). A 1500-byte frame at MCS 7 on an 80 MHz channel with one stream needs only about eleven symbols, so its data portion is near 44 μs and the whole frame stays under 100 μs, while the same frame at MCS 0 on 20 MHz runs about 1.9 ms. With SIFS, the ACK, and contention, single-frame efficiency often falls below 60 percent, which motivates aggregation.

Related Terms

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