OSI Model and 802.11
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OSI Model and 802.11
Data Link Layer
The 802.11 Data-Link layer is divided into two sublayers:
MAC Service Data Unit
MAC Protocol Data Unit
MAC Header
Frame Body
Frame Check Sequence (FCS)
At this point, the frame is ready to be passed onto the Physical layer, which will then further prepare the frame for transmission.
Data Link Layer
The 802.11 Data-Link layer is divided into two sublayers:
- The upper portion is the IEEE 802.2 Logical Link Control (LLC) sublayer:
- identical for all 802-based networks, although it is not used by all IEEE 802 networks.
- The bottom portion of the Data-Link layer is the Media Access Control (MAC) sublayer:
- The 802.11 standard defines operations at the 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.
- Only 802.11 data frames carry an MSDU payload in the frame body.
- Maximum size of the MSDU is 2,304 bytes.
The maximum frame body size is determined by the maximum MSDU size (2,304 octets) plus any overhead from encryption. - 802.11n-2009 amendment, aggregate MSDU (A-MSDU) was introduced.
With A-MSDU, the maximum frame body size is determined by the maximum A-MSDU size of 3,839 or 7,935 octets, depending upon the STA’s capability, plus any overhead from encryption.
MAC Protocol Data Unit
- When 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
- Sequence control information
- 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.
Phyiscal Layer
PLCP Service Data Unit
PLCP Protocol Data Unit
- Similar to the way the Data-Link layer is divided into two sublayers, the Physical layer is also divided into two sublayers.
- 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,
- 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.
Reference:
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.
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