Questions and Answers
What is the purpose of the SIGNAL field in the PLCP header?
What is the purpose of bit 3 in the SERVICE field?
How many bits are used to indicate the data rate in the OFDM SIGNAL field?
What is the purpose of the CRC field in the PLCP header?
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What is the purpose of the LENGTH field in the PLCP header?
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What is the purpose of bit 17 in the OFDM SIGNAL field?
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What is the main function of the PLCP sublayer?
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What is the term used to describe the frame when it is handed down to the physical layer?
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What are the three parts of the PPDU?
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What is the function of the PLCP Preamble?
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How many subcarriers does a short OFDM training symbol consist of?
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What is the total training length of the PLCP Preamble?
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How many different preambles are defined in IEEE 80211.2007?
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What is added to the PSDU by the PLCP sublayer?
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Study Notes
802.11 PHY Layer
- Divided into two sublayers: PLCP (Physical Layer Convergence Procedure) and PMD (Physical Medium Dependent)
- PLCP prepares the frame for transmission by taking the frame from the MAC sublayer and creating PLCP Protocol Data Unit (PPDU)
- PMD sublayer modulates and transmits the data as bits
PPDU Structure
- Consists of 3 parts: PLCP Preamble, PLCP Header, and PSDU
- PLCP Preamble alerts the Rx STA of transmission by sending it at the beginning of transmission
- IEEE 802.11-2007 defines 3 different preambles: Long PLCP Preamble, Short PLCP Preamble, and OFDM PLCP Preamble
- 802.11n amendment further defines 3 additional preambles: non-HT legacy PPDU, HT-mixed PPDU, and HT-Greenfield PPDU
PLCP Preamble
- Also known as OFDM training structure
- Consists of 10 short symbols (t1-t10) and 2 long symbols (T1-T2)
- GI2 is a long guard interval
- Total training length is 16μS
- Short OFDM training symbol consists of 12 subcarriers
- Long OFDM training symbol consists of 53 subcarriers
PLCP Header
- Long and Short PLCP Header contain 4 fields: Signal, Service, Length, and CRC
- Signal field (8 bits) indicates which modulation method will be used for PSDU/MPDU
- Service field (8 bits) is used for various purposes such as indicating modulation method, transmit frequency, and symbol clock
- Length field (16 bits) indicates the number of microseconds required to transmit the PSDU
- CRC field (16 bits) provides protection for other 3 fields
OFDM Transmission
- SIGNAL field is 24 bits long
- First 4 bits (0-3) indicate the data rate (6, 9, 12, 18, 24, 36, 48, 54)
- Next bit (bit 4) is reserved for future use
- Next 12 bits (bit 5-16) make up the PLCP Length field
- Bit 17 is a parity bit for 0-16 bits
- Last 6 bits (bit 18-23) make up the SIGNAL TAIL with all 6 bits set to 0
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Description
Explore the 802.11 PHY layer, its sublayers, and the PPDU structure. Learn about PLCP and PMD, and how they prepare and transmit data.
