MSDU, MPDU, PSDU and PPDU – How 802.11 communications works?
First we need to understand that in the 802.11 communications the data that flows from one layer to another will be encapsulated from one sub layer to another. Yes each Layer has sub-layers.
Encapsulation is the process of enclosing upper-layer information into the current layer’s delivery format. For example, IP packets encapsulate TCP datagrams or messages and 802.11 frames encapsulate IP packets.
Which Layers and Sub-Layers the 802.11 standard includes?
There are 2 Layers and 4 sub-layers in the 802.11 standard:
- Layer 1 with PLCP and PMD as sub-layers plus PSDU and PPDU as encapsulation units
- Layer 2 with LLC and MAC as sub-layers plus MSDU and MPDU as encapsulation units
How the Data flow through the Layers and Sub-Layers?
The Layer 2 receive all the data from the upper layers (3 to 7) in the LLC sub-layer and encapsulate it in a MSDU (MAC Service Data Unit) then when the flow continues to the other Layer 2 sub-layer which is the MAC the MPDU is created encapsulating the MSDU from the LLC and adding the MAC header. Now at this time you should understand the MPDU nature right? so why the MPDU is equal to PSDU? the reason is simple and is because the both includes the same information but the name (MPDU or PSDU) depends on what Layer (2 or 1) your are analyzing or considering the information so if you talk from the Layer 2 sub-layer MAC perspective, then is called MPDU but if you talk from the Layer 1 sub-layer PLCP perspective this information unit is called PSDU. So in other words the PSDU do not add any information to the MPDU but when the PSDU (in the Layer 1) move down the PLCP sub-layer then have to add the PLCP header and the PLCP preamble and then you will have as a result the PPDU which will be transmitted in bits (0’s and 1’s) through the PMD sub-layer.
Within Layers 1 and 2, the service data units (SDUs) go by specific names, which you should be familiar with: The MAC Service Data Unit (MSDU) and MAC Protocol Data Unit (MPDU) exist at Layer 2 (specifically, in the MAC sublayer of the Data Link Layer). The PLCP Service Data Unit (PSDU) and PLCP Protocol Data Unit (PPDU) exist at Layer 1 (specifically, in the PLCP sublayer of the Physical Layer). The terms service data unit (SDU) and protocol data unit.
The terms service data unit (SDU) and protocol data unit (PDU) come directly from the OSI reference model document, ISO-IEC 7498-1, corrected edition June 16, 1996. This document defines the SDU and PDU as follows:
▪ Service Data Unit (SDU): an amount of information whose identity is preserved when transferred between peer-(N+1)-entities (layer peers) and which is not interpreted by the supporting (N)-entities (lower layers).
▪ Protocol Data Unit (PDU): a unit of data specified in an (N)-protocol and consisting of (N)-protocol-control-information and possible (N)-user-data. In these definitions, (N) is a placeholder for a layer.
For example, if you were considering the Network Layer, where IP operates, the PDU definition would be, “a unit of data specified in a Network Layer protocol (IP) and consisting of IP control information and possible IP user data (IP packets).” Beginning with this definition, it is easier to understand the SDUs and PDUs in the context of 802.11 communications. All OSI model layers have SDUs and PDUs. Sublayers, such as the MAC sublayer of the Data Link Layer, can also have SDUs and PDUs. The MSDU and MPDU are indeed the SDU and PDU of the MAC sublayer for 802.11. The LSDU and LPDU, which are the LLC sublayer SDU and PDU, are defined in the 802.2 standard and are used whenever the LLC is used with any 802 specifications. Figure 1.10 shows the location of the SDUs and PDUs in 802.11 implementations.
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