Define terminology related to the MAC & PHY
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Define terminology related to the MAC & PHY

MAC

• The data link layer is divided into 2 sub-layers
• The upper portion is the IEEE 802.2 Logical Link Control (LLC) sublayer
• The bottom portion of the Data-Link layer is the Media Access Control (MAC) sublayer

 

• MAC Service Data Unit
  • When the Network layer (layer 3) sends data to the Data-Link layer,
  • that data is handed off to the LLC and becomes known as the MAC
  • Service Data Unit (MSDU).
  • The MSDU contains data from the LLC and layers 3–7.
  • A simple definition of the MSDU is that it is the data payload that contains the IP packet plus some LLC data.
  • The 802.11-2012 standard states that the maximum size of the MSDU is 2,304 bytes.
• MAC Protocol Data Unit
  • The LLC sublayer sends the MSDU to the MAC sublayer, the MAC header information is added to the MSDU to identify it.
  • The MSDU is now encapsulated in a MAC Protocol Data Unit (MPDU).
  • A simple definition of an 802.11 MPDU is that it is an 802.11 frame.
  • As shown in Figure 9.1, an 802.11 MPDU consists of the following three basic components:
• MAC Header
  • Frame control information, duration information, MAC addressing, and sequence control information are all found in the MAC header.
  • Furthermore, QoS data frames contain specific QoS control information.
• Frame Body
  • The frame body component can be variable in size and contains information that is different depending on the frame type and frame subtype.
  • The MSDU upper layer payload is encapsulated in the frame body.
  • The MSDU layer 3–7 payload is protected when using encryption.
• Frame Check Sequence (FCS)
  • The FCS comprises a 32-bit cyclic-redundancy check (CRC) that is used to validate the integrity of received frames.
  • At this point, the frame is ready to be passed onto the Physical layer, which will then further prepare the frame for transmission.
    

MPDU

PHY

• The Physical layer is divided into 2 sub-layers
• The upper portion of the Physical layer is known as the Physical Layer Convergence Procedure (PLCP) sublayer
• The lower portion is known as the Physical Medium Dependent (PMD) sublayer.
• The PLCP prepares the frame for transmission by taking the frame from the MAC sublayer and creating the PLCP Protocol Data Unit (PPDU).
• The PMD sublayer then modulates and transmits the data as bits.

 

• PLCP Service Data Unit
  • The PLCP Service Data Unit (PSDU) is a view of the MPDU from the
  • Physical layer.
  • The MAC layer refers to the frame as the MPDU, while the Physical layer refers to this same frame as the PSDU.
  • The only difference is from which layer of the OSI model you are looking at the frame.
• PLCP Protocol Data Unit
  • When the PLCP receives the PSDU, it then prepares the PSDU to be
  • transmitted and creates the PLCP Protocol Data Unit (PPDU).
  • The PLCP adds a preamble and PHY header to the PSDU.
  • The preamble is used for synchronization between transmitting and receiving 802.11 radios.
  • When the PPDU is created, the PMD sublayer takes the PPDU and modulates
  • the data bits and begins transmitting.
• Figure 9.2 depicts a flowchart that shows the upper-layer information moving between the Data-Link and Physical layers.
  

Figure 9.2 Data link and Physical layers

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Reference:
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.