With the ratification of 802.11n amendment, two types of frame aggregation were added to 802.11
1. Aggregate MAC Service Data Unit (A-MSDU)
2. Aggregate MAC Protocol Data Unit (A-MPDU)
Frame aggregation allows multiple smaller MSDU or MPDUs to be grouped together into a single frame, reducing the amount of overhead that would have been necessary for each individual frame.
An A-MSDU is a sequence of A-MSDU subframes as shown below. Each A-MSDU subframe consists of an A-MSDU subframe header followed by an MSDU and 0 to 3 octets of padding. Each A-MSDU subframe (except the last) is padded so that its length is a multiple of 4 octets. The last A-MSDU subframe has no padding.
- If encryption is enabled, then all MSDU are encrypted together as a single payload.
- An A-MSDU shall contain only MSDUs whose DA and SA parameter values map to the same RA and TA values.
- The constituent MSDUs of an A-MSDU shall all have the same priority parameter value.
- An A-MSDU shall be carried, without fragmentation, within a single QoS data MPDU.
- The Address 1 field of an MPDU carrying an A-MSDU shall be set to an individual address.
- The channel access rules for a QoS data MPDU carrying an A-MSDU are the same as a data MPDU carrying an MSDU (or fragment thereof) of the same TID.
Below diagram (CWAP Study Guide – Page 225) show the A-MSDU Aggregation
The maximum MPDU length that can be transported using A-MPDU aggregation is 4095 octets. An A-MSDU cannot be fragmented. Therefore, an A-MSDU of a length that exceeds 4065 octets (4095 minus the QoS data MPDU overhead) cannot be transported in an A-MPDU.
- If encryption enabled, then each MPDU is encrypted individually.
- The individual MPDU within an A-MPDU must all have same receiver address.
- The individual MPDU must all be of the same 802.11e QoS access category.
- A-MPDU also require the use of Block Ack.
- The Duration/ID fields in the MAC headers of all MPDUs in an A-MPDU carry the same value.
- All protected MPDUs within an A-MPDU have the same Key ID.
1. CWAP Official Study Guide – Chapter 6
2. IEEE 802.11-2012 standard