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PHY Layer



The OSI model is must-know knowledge for everybody that even thinks about toughing the network. Wireless acts at Layer 1 and Layer 2. In this blog, I want to look into the PHY layer (Layer 1), but first an overview.

Layer 7 – Application
Layer 6 – Presentation
Layer 5 – Session
Layer 4 – Transport
Layer 3 – Network
Layer 2 – Data Link
Layer 1 – Physical

Layer 2 is divided in two sublayers
- Logical Link Control (LLC)
- Media Access Control (MAC)
  o MAC Service Data Units (MSDU)
  o MAC Protocol Data Unit (MPDU)
Layer 1 is also divided in two sublayers
- Physical Layer Convergence Procedure (PLCP)
  o PLCP Service Data Units (PSDU)
  o PLCP Protocol Data Unit (PPDU)
- Physical Medium Dependent (PMD) that communicates with bits.

Layer 2
The MSDU is received from the upper layers and goes through the LLC so it can be managed and transmitted by the lower layers. Another word for the MSDU is the LLC Protocol Data Unit (LPDU). The MAC layer adds a header and a footer, so it extends the frame. The MSDU is now encapsulated in the MPDU.

The MPDU is delivered to Layer 1. So, it receives the MSDU from the upper layers and prepares the MSDU so it is ready for the lower layers (called the MPDU).

Layer 1
The MPDU is the same as the PSDU. The PSDU is what is received from layer 2 to layer 1. The PSDU adds information and creates the PPDU that is ready to deliver to the PMD.

The PMD is responsible for the modulation (QAM), the actual transmitting of the data. This is where the bits are.

The information about layer 2 will be written down in another blog post, as this one about the first layer.

PPDU & PSDU
As said, the PSDU is encapsulated with the PPDU. The PPDU has three blocks:
- PLCP Preamble
- PLCP Header
- PSDU

PPDU

The PLCP Preamble has two fields, the Sync field and the Start Frame Delimiter (SFD).

The Sync bit is not data and is a pattern of 0s and 1s. This pattern will not be buffered by the receiver and is an indication that a frame is coming. Preparing the client for listening to the data. After the Sync field the SFD is delimiter that the frame starts.

There is a long PPDU and a Short PPDU. The size differences are only in the preamble. In the long preamble, the Sync field is 128 bits and the SFD is 16 bits. In the short preamble, the Sync field is 56 bits and the SFD is the same (16 bits). The SFD for the long preamble is 1111 0011 1010 0000 and the SFD in the short preamble is 0000 0101 1100 1111.

In the PLCP Header there are 4 fields. The Signal (8 bits) the Service (8 bits) the Length (16 bits) and the CRC (16 bits).

Signal:
This is the indication which modulation is used (BPSK, QPSK or QAM).
Service:
This indicates the data modulation (Barker, CCK, PBCC), and the data length
Length:
The number of microseconds that it takes to transmit the PSDU. This is not the same as the NAV that you see in the MAC Header.
CRC:
This field is for protection the other fields.