- Frame aggregation
The improvements in the physical layer brought about by the new 802.11n initially describe only the theoretical data throughput of the physical medium. However, the share of this theoretical bandwidth that is actually available for payload data is limited by two factors:
- in addition to the actual payload data, each data packet in a wireless LAN system contains additional information such  as a preamble and MAC address information.
- Time is lost to the management events that occur when the transmission medium is actually accessed. Thus the transmitter must negotiate access authorization with the other receivers before transmitting each data packet (frame); further delays are caused by data packet collisions and other events.
This loss, referred to as "overhead", can be reduced by combining several data packets together to form one large frame and transmitting them together. In this process, information such as the preamble are only transmitted once for all the combined data packets and delays due to negotiating access to the transmission medium only occur at longer intervals.
The use of this method, known as frame aggregation, is subject to certain restrictions:
- As information such as MAC address only needs to be transmitted once for the aggregated frame, only those data packets intended for the same address can be combined.
- All data packets that are to be combined into a single large frame must be available at the sender at the time of aggregation—as a consequence some data packets may have to wait until enough data packets for the same destination are available with which they can be combined. This aspect may represent a significant limitation for time-critical transmissions such as voice over IP.
- Block acknowledgement
Each data packet directed to a specific address (i.e. not broadcast or multicast packets) is acknowledged immediately after receipt. In this way, the transmitter is informed that the packet was received correctly and does not need to be repeated. This principle also applies to aggregated frames in 802.11n.
Two different methods are used for frame aggregation. These are not explained in detail here, but they differ in the way aggregated frames are acknowledged.
- Mac Service Data Units Aggregation (MSDUA) combines several Ethernet packets together to form one common wireless LAN packet. This packet is acknowledged only once and the acknowledgment is valid for all aggregated packets. If there is no acknowledgement the whole block is resent.
- Mac Protocol Data Units Aggregation (MPDUA) combines individual wireless LAN packets together to form one large common wireless LAN packet. In this case, each wireless LAN packet is acknowledged and the acknowledgements are combined and transmitted as a block. In contrast to MSDUA, the sender receives information about the receipt status of every single WLAN packet and can, if necessary, resend only those specific packets that were not successful.