physical layer convergence protocol (ATM, 802.11) (PLCP)

Physical Layer Convergence Protocol (PLCP) is an alternative method that was used in older T- transmission facilities to adaptdata to the transmission speed and slots on the physical transmission medium. The method is also used in ATM and WLANs.

1. In ATM networks, the PLCP protocol is used to locate the boundary for the ATM cells. PCLP is defined in IEEE 802.6 and is used for DS-3 transmission of ATM.
   

   PLCP interfaces

   The ATM cells are encapsulated in a 125-µs frame defined in PLCP for this purpose. This method has recently only been listed as an annex to the DS-3 specification of ATM and has been replaced by the HEC method.
2. In WLANs according to 802.11, the Physical Layer Convergence Procedure (PLCP) consists of the PLCP preamble, the actual PLCP header and the user data.
   

   Structure of the PLCP data frame of 802.11

   The PLCP preamble has an 80-bit long data field and a 16-bit long start frame delimiter( SFD). The PLCP preamble allows the receiver to detect the incoming OFDM signal. The sync field of the PLCP preamble consists of alternating zeros and ones that indicate to the receiving equipment that a receive signal is present. After the receiver detects the sync signal, it begins synchronization. The 16 bit long data field Start Frame Delimiter (SDF) defines the start of the frame. It is always coded in the bit combination 1111001110100000. The following PLCP header consists of the PLCP packet header, PLCP- PDU Length Word (PLW), followed by the PLCP-PDU Signalling Field (PSF) and the data field (HEC) for the cyclic block check. At the beginning of the header there is a four- bit rate field in which the data rate of the 802.11 frame is specified. in this data field, eight data rates between 6 Mbit/s and 54 Mbit/s are specified.