IEEE 802.11 Data Exchange Interfaces

In MPDUs, the necessary LLC sub-layer protocol header consumes three octets of payload capacity within the IEEE 802.11 MPDU's frame body. The LLC sub-layer protocol header and whatever follows it is part of the MSDU, although strictly speaking, the MSDU is the data structure that is passed from LLC to the MAC. The MSDU is encapsulated in an MPDU header, and the MPDU is exchanged between MAC entities. The MSDU is essentially what is passed between MAC sub-layer entities (see Figure 4-14).

Figure 4-14. MLME interfaces for sending and receiving data

The MPDU header contains various control information that is important to ensure that the receiver can properly interpret the frame, and to facilitate the proper operation of the IEEE 802.11 MAC sub-layer protocol's arbitration techniques.

The MSDU

The MSDU is a data object that is handed from the LLC to the MAC layer through the IEEE 802.11 "MA-UNITDATA.request" interface primitive. Similar interface primitives^[17] have been defined by other IEEE MAC sub-layer protocols (the names differ slightly among the standards, but tend to be quite similar).

^[17] For example, IEEE 802.3 has equivalent interfaces, which are referred to as "MA-DATA.request" and "MA-DATA.indication."

MA-UNITDATA.request (
                     source address,
                     destination address,
                     routing information,
                     data,^[18]
                     priority,
                     service class
                     )

^[18] This includes the LLC header plus whatever higher layer protocol data is encapsulated within LLC. It is also possible that the "data" argument will be null, which is allowed in IEEE 802.11, and might be used by a STA to tell an AP that it is entering power-save mode. It is also possible that a non-null data frame can be used for the same purpose.

The routing information field is not employed by IEEE 802.11. The priority field is used to indicate Contention (DCF) or Contention-Free (PCF) operation (DCF is the default), and the service class is used to indicate Reorderable Multicast or Strictly Ordered service (Reorderable Multicast is the default). Note that only the addressing and this other control information must be passed from the LLC layer to the MAC layer across the MA-UNITDATA.request interface. The MAC sub-layer protocol knows all the remaining information to enable it to construct the rest of the IEEE 802.11 MAC sub-layer protocol header.

The MPDU and PSDU

The MPDU is effectively analogous to the PLCP PSDU, in that the PSDU is what is exchanged by PHY-layer entities (as the MSDU was what was exchanged by MAC-layer entities). The PSDU is encapsulated in a PPDU header, just as the MPDU header encapsulated the MSDU.

The PPDU

At the lowest layer, the PLCP PPDU is what is actually transferred between two STAs. The PPDU consists of a PLCP header and the PSDU.