- 3.1 Understand and describe the functions and the PLCP and PMD sublayers
- 3.2 Apply the understanding of PHY technologies (including PHY headers, preambles, training fields, frame aggregation and data rates) to captured data
- 3.3 Identify and use PHY information provided in pseudo-headers within protocol analyzers
- 3.4 Recognize the limits of protocol analyzers in capturing PHY information including NULL data packets and PHY headers
- 3.5 Use appropriate capture devices based on an understanding of PHY types
3.1
PLCP
- When a MPDU is received the PLCP Preamble and PLCP Header are added to the now PSDU to create the PPDU
- The MAC layer communicates to PLCP using Primitives though a service access point (SAP)
- When the MAC layer instructs to do so PLCP will prepare the MPDUs for transmission
- PCLP minimizes the dependence of the MAC layer on the PMD sublayer by mapping the MPDUs into a format which can be transmitted by the PMD
- Passes the frames from the PMD up to the MAC layer upon reception from PMD
- 802.11-2016 defines PPDU as the unit of data exchanged between two peer PHY entities to provide the PHY data service
- CS/CCA used to determine if the medium is BUSY or IDLE
- Preamble contains the sync field and start frame delimiter (SFD)
- The sync field alerts the receiver of a potential receivable signal
- Important that the STA sync prior to the SFD
- SFD denotes the start of the frame
PMD
- Responsible for transmitting bits on and off the medium
- Data is converted to 1’s and 0’s that are represented by RF wave changes
- Determines the modulation techniques capable to the stations
- Responsible for modulation/demodulation and encoding/decoding
3.2
3.2.1 DSSS
- 802.11 prime
- Barker code with DBPSK for 1 Mbps and QPSK for 2Mbps
- 10 microsecond SIFS
- 20 microsecond Slot Time
- Uses a long preamble
3.2.2 HR/DSSS
- 802.11b
- Added 5.5 and 11Mbps
- Complementary code keying
- Uses a short preamble
- Preamble and header still done at 1Mbps
3.2.3 OFDM
- 802.11a is the OFDM PHY
- Split into smaller signals carried on the sub-carriers
- 52 sub-carriers per channel
- 20MHz wide
- Data rates 6, 9, 12, 18, 24, 36 ,48 54
- Introduced QAM
- QAM is considered a mixture of amp and phase modulation
3.2.4 ERP
- 802.11g
- Introduced OFDM int 2.4Ghz
3.2.5 HT
- 802.11n
- Added block ack or BA
- Uses SGI short guard interval
- MIMO
- Modes
- 0 – Greenfield
- 1-non-member protection – a client or AP not part of the BSS
- 2-when using HT and 1 STA is using 20MHz
- 3- mixed mode
3.2.6 VHT
- 802.11ac
- MU-MIMO
3.3
3.3.1 Pseudo-header formats
- Radiotap
- Per Packet Information (PPI)
- Developed to provide HT information but can be used for other information
3.3.2 Signal strength
- Signal strength and noise can be found under the radiotap header
3.3.3 Data rate and MCS index
3.3.4 Length information
3.3.5 Channel center frequency or received channel
3.3.6 Channel properties
3.3.7 Noise
3.4
3.5
3.5.1 Supported PHYs
3.5.2 Supported spatial streams
3.5.3 Short Guard Interval (SGI)