PHY Layer – CWAP#5
This chapter accounts for 10% of the Knowledge Domain in the CWAP exam. Approx. 6/60 questions!
Exam Moment from the Book : It is not important, for the CWAP exam, that you know all the details of the variations of the PHY preambles; however, you should know that the preamble adds extra overhead to the communications and that older devices may introduce a preamble that reduces performance overall and forces all devices in the BSS to communicate based on that long preamble.
Carrier Sense > State of STA where it is ready to transmit or receive packets/signals
Clear Channel Assessment > Identify whether the channel is unused and available prior to the packet transmission
Transmit (Tx) > Upon checking if the wireless medium is available the STA needs to transmit a frame which is enabled by CS/CCA process. Unlike ethernet the wireless frames cannot transmit and receive the frames at the same time.
Receive (Rx) > The transmitting STA will precede the data portion of the frame with a preamble. It contains a binary strings that the receiving station can identify and synchronise with , essentially alerting the receiving station to the transmission. The preamble also includes a Start Frame Delimiter field, which the receiving station uses to identify the beginning of the frame. An ACK frame Is sent with the entire frame is received.
PMD > transmits the data as RF modulated 1s and 0s. When receiving , the PMD listens to the RF and passes the received data up to the PLCP sublayer.
PLCP Protocol Data Unit> When PLCP receives PSDU, it then prepares PPDU. PLCP adds a preamble + PHY header to the PSDU.
PLCP Preamble > String of 0/1 bits that are used to synchronise incoming transmissions. IEEE 802.11-2007 standard defines 3 different PPDUs.
Long PPDU > 144 bit PLCP Preamble, 128 bit Sync field + 16 bit Start of Frame Delimiter (SFD).
Short PPDU > 72 bit PLCP Preamble, 56 bit Sync field and 16 bit SFD
OFDM PLCP Preamble >10 short symbols + 2 long symbols
PLCP Header > Long & Short PLCP Headers are both 48 bits log and contain 4 fields (Signal(8) + Service(8) + Length(16) + CRC(16).
Non-HT Legacy PPDU
- Consists of Preamble(Short/Long symbols)
- Mandatory for 802.11n radios and transmissions can occur in only 20MHz channels.
- Effectively same format used by legacy 802.11a/g radios.
- 802.11n amendment
- Likely be most commonly used format as it supports HT + Legacy 802.11a/g.
- Transmission can occur in both 20MHz and 40MHz channels
- 2nd of the two new PPDU formats defined by 802.11n.
- Not compatible with legacy 802.11 radios, only the HT Radios can communicate with this format.
- Can transmit using 20MHz and 40MHz fields.
The data field portion of PPDU is the PSDU. In easy terms, the data field is the 802.11 MAC frame.