IEEE 802.11 PHY Layer
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Questions and Answers

What is the purpose of the SIGNAL field in the PLCP header?

  • To provide error correction for the PSDU
  • To specify the data rate of the transmission
  • To indicate which modulation method will be used for PSDU/MPDU (correct)
  • To resolve data length field ambiguities for ERP-PBCC

    • What is the purpose of bit 3 in the SERVICE field?

  • To supplement the Length field for CCK 11Mbps
  • To indicate the modulation method (CCK or PBCC) (correct)
  • To resolve data length field ambiguities for ERP-PBCC
  • To indicate the transmit frequency and symbol clock

    • How many bits are used to indicate the data rate in the OFDM SIGNAL field?

  • 4 bits (correct)
  • 6 bits
  • 12 bits
  • 8 bits

    • What is the purpose of the CRC field in the PLCP header?

    <p>To provide protection for the SIGNAL, SERVICE, and LENGTH fields</p> Signup and view all the answers

    What is the purpose of the LENGTH field in the PLCP header?

    <p>To indicate the number of microseconds required to transmit the PSDU</p> Signup and view all the answers

    What is the purpose of bit 17 in the OFDM SIGNAL field?

    <p>To act as a parity bit for the previous 16 bits</p> Signup and view all the answers

    What is the main function of the PLCP sublayer?

    <p>To prepare the frame for transmission by taking the frame from the MAC sublayer</p> Signup and view all the answers

    What is the term used to describe the frame when it is handed down to the physical layer?

    <p>PSDU</p> Signup and view all the answers

    What are the three parts of the PPDU?

    <p>PLCP Preamble, PLCP Header, and PSDU</p> Signup and view all the answers

    What is the function of the PLCP Preamble?

    <p>To alert the Rx STA of transmission</p> Signup and view all the answers

    How many subcarriers does a short OFDM training symbol consist of?

    <p>12</p> Signup and view all the answers

    What is the total training length of the PLCP Preamble?

    <p>16μS</p> Signup and view all the answers

    How many different preambles are defined in IEEE 80211.2007?

    <p>3</p> Signup and view all the answers

    What is added to the PSDU by the PLCP sublayer?

    <p>A preamble and a PHY header</p> Signup and view all the answers

    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.

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