Physical Layer Operations
Each station in one of 3 states: waiting for data, transmitting data or listening for clear channel to transmit
Carrier Sense/Clear Channel Assessment
CS/CCA is used to determine the state of the wireless medium.
Busy
idle
The procedure is executed while the receiver is switched on and the station is not receiving or transmitting data. The Carrier sense part listens to detect the beginning of the signal to receive (Carrier Sense). It also determines if the channel is available prior to transmission of a packet (Clear Channel Assessment)
Transmit
If the is channel clear (using CS/CCA), station will transmit frame & return to receive mode. The station cannot detect any collisions like you can on wired ethernet. So it must receive ACK from the receiving station to verify delivery. If it does not receive and ACK in a timely fashion it will try and send the packet again.
Receive
If the medium is busy (via CS/CCA). The station needs to understand if this is due to an incoming frame to the station. Transmitting station sends pre-amble (a string of 1's and 0's) to alert & sync receiver. The preamble also includes the start of frame delimiter (SFD) to indicate the beginning of the frame. After pre-amble, length field indicates show long frame is. The receiver sends ACK when frame successfully received. Physical Layer
Two sub-layers:
PLCP - Physical Layer Convergence procedure (upper layer). Takes MPDU (called PSDU at this layer) and prepares for transmission and creates PPDU
PMD - Physical Media Dependent (lower layer). Modulates & transmits PPDU as bits
PLCP Service Data Unit (PSDU)
PSDU is the MAC layer MPDU, but a different name at the physical layer.
PLCP Protocol Data Unit (PPDU) PLCP layer adds preamble & PHY header to PSDU (MPDU). The preamble provides sync between stations.
Physical Medium Dependent (PMD)
Responsible for transmitting and receiving PPDU at the physical layer modulates/demodulates binary data into/from RF signals
PLCP Protocol Data Unit
Three parts which combine to form PPDU:
- PLCP Preamble
- PLCP Header
- PSDU
PLCP Preamble
The preamble is a string of 1's and 0's to sync to the incoming transmission. 802.11-2007 defines 3 preambles:
- Long PPDU format
- Short PPDU format
-OFDM PLCP preamble
802.11n amendment defines 3 additional:
- non-HT legacy PPDU
- HT-mixed PPDU
- HT-Greenfield PPDU
Long PLCP Preamble
- 144-bit preamble
- 128-bit sync field + 16-bit SFD (start of frame delimiter)
- Sync between Tx & RX must occur before SFD field
- SFD indicates PLCP header coming next
- Long preamble & header sent using DBPSK 1mbps modulation
- Modulation of PSDU not necessarily sent at the same rate as preamble & header
Short PLCP Preamble
- 72-bit preamble
- 56-bit sync field + 16-bit SFD
- Half the overhead of long preamble
- Short preamble sent using DBPSK 1mbps, Header sent using DQPSK 2mbps (both fixed)
- Modulation of PSDU not necessarily sent at the same rate as preamble & header
OFDM PLCP Preamble
- Also known as OFDM training structure.
- 10 short symbols, 2 long symbols.
- No SFD, the signal field of header follows preamble.
- The total training length 16uS short training symbol: 12 sub-scarriers
- long training symbol 53 sub-carriers
PLCP Header
- PLCP Header for long & short headers both 48 bits long, made up of 4 fields:
+ Signal (8 bits) - indicates modulation method for PSDU
- short header, PSDU maybe 1, 2 5.5 or 11mbps
- Long header, PSDU may be 2,5.5, 11 Mbps
+ Service (8 bits) - bit 3 indicates modulation method used CCK/PBCC
+ Length (16 bits) - # of micro-secs required to transmit PSDU
+ CRC (16 bits) - protects jist the signal service & length fields
- Clause 17 OFDM transmission, only have signal field, 24 bits long:
+ bits 0-3 indicate data rate (6 - 54 Mbps)
+ bits 5-16 form PCLP length field
+ bit 17 parity bit bits 18023 are signal tail (all zeros)
802.11n PPDUs
802.11n ammendment defines 3 additional:
- non-HT legacy PPDU
- legacy format structured as clause 17 (OFDM) and clause 18 (ERP)
- The preamble is 10 short & 2 long symbols
- support mandatory for 802.11n radios
- 20MHz support only
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