802.11a PHYs ============== .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **What is the PHY in IEEE 802.11a?** The PHY (Physical Layer) is responsible for transmitting and receiving raw data bits over the wireless medium using radio signals. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **What modulation does 802.11a use?** IEEE 802.11a uses Orthogonal Frequency Division Multiplexing (OFDM) with multiple modulation types such as BPSK, QPSK, 16-QAM, and 64-QAM. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **What is OFDM in 802.11a?** OFDM is a modulation scheme that splits a high-speed signal into multiple lower-speed signals, transmitted simultaneously over different frequencies. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **How many subcarriers are used in 802.11a OFDM?** 802.11a uses 52 subcarriers: 48 for data and 4 as pilot subcarriers for synchronization and tracking. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **What are pilot subcarriers in OFDM?** Pilot subcarriers are known reference signals inserted into OFDM symbols for phase tracking and channel estimation. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **What coding technique is used in 802.11a PHY?** 802.11a uses convolutional coding with coding rates of 1/2, 2/3, or 3/4 to provide forward error correction. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **What is convolutional coding?** Convolutional coding is an error-correction technique that encodes data using a shift register and generator polynomials to increase transmission reliability. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **What is interleaving in 802.11a?** Interleaving rearranges bits before transmission to protect against burst errors by spreading errors over multiple codewords. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **What data rates are supported by 802.11a PHY?** 802.11a supports data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbps. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **Which modulation corresponds to the 6 Mbps rate?** 6 Mbps uses BPSK modulation with a 1/2 coding rate in the 802.11a PHY. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **Which data rate uses 64-QAM?** 54 Mbps in 802.11a uses 64-QAM with a 3/4 coding rate for high throughput. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **What is the OFDM symbol duration in 802.11a?** Each OFDM symbol is 4 microseconds long, including a 0.8 microsecond guard interval. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **What is a guard interval?** The guard interval is a brief time added between OFDM symbols to reduce inter-symbol interference caused by multipath delay. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **What is the FFT size used in 802.11a?** 802.11a uses a 64-point Fast Fourier Transform (FFT) to modulate and demodulate OFDM signals. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **How wide is the 802.11a channel in MHz?** The channel width in 802.11a is 20 MHz, accommodating the OFDM spectrum. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **Why is OFDM suitable for 5 GHz PHY?** OFDM handles multipath interference well, which is common in indoor 5 GHz environments, enhancing robustness and throughput. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **Is the 802.11a PHY half-duplex or full-duplex?** 802.11a PHY operates in half-duplex mode—devices cannot transmit and receive simultaneously on the same channel. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **Does 802.11a PHY support MIMO?** No, MIMO (Multiple Input, Multiple Output) is introduced in later standards like 802.11n. 802.11a uses single-antenna systems. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **What is the function of the PHY preamble in 802.11a?** The PHY preamble includes training sequences used for synchronization, channel estimation, and signal detection by the receiver. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **What is the SIGNAL field in the PHY header?** It contains information such as modulation type, data rate, and length of the payload, allowing the receiver to decode the frame properly. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **What is the service field in 802.11a PHY?** The service field includes initialization bits for scramblers and reserved bits for future use. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **What is the tail field in PHY?** The tail field is used to reset the convolutional decoder to a known state, improving decoding accuracy. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **Can PHY parameters change dynamically in 802.11a?** Yes, devices can dynamically switch modulation and coding schemes based on channel quality using rate adaptation algorithms. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **How does 802.11a handle multipath effects?** OFDM and guard intervals help minimize the impact of multipath delay spread in the 5 GHz band. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **What limits the range of 802.11a PHY?** The higher frequency (5 GHz) has less wall penetration and shorter range compared to lower-frequency protocols like 802.11b/g. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **Does PHY impact latency in 802.11a?** Yes, PHY layer efficiency and symbol timing can affect transmission latency, especially at lower data rates. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **What is spectral efficiency in PHY terms?** Spectral efficiency refers to how efficiently data is transmitted per unit of bandwidth. Higher modulations like 64-QAM improve it. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **How does PHY relate to MAC in 802.11a?** PHY transmits the data bits over the air, while MAC handles framing, addressing, and access control—both layers work together. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **Is the 802.11a PHY backward compatible?** No, 802.11a is not backward compatible with 802.11b/g due to operating on different frequency bands. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **Is 802.11a PHY still used today?** While newer standards have replaced it, the 802.11a PHY remains foundational and is still used in backward compatibility modes of modern devices. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow Topics in this section, * :ref:`Reference links ` .. _phy_a_step17: .. tab-set:: .. tab-item:: Reference links * Reference links