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
Akhil Vijay said:
I’m little confused here.. the second line
“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.”
My qns are
>The maximum individual MPDU length that can be transported using A-MPDU aggregation is 4095 octets, and the maximum A-MPDU size is 65,535 octets. am I right?
>what is the maximum individual MSDU length that can be transported using A-MSDU aggregation and the maximum A-MSDU size?
Thanks in advance
Those information from 802.11-2102 standard as it is.
1. Yes , I understand it in the same way where max MPDU size within A-MPDU is 4095 bytes. Where as total length of an A-MPDU can be 64K.
2. Max A-MSDU is 7935 bytes. Individual MSDU I would say 2304 bytes.
Myunghoon Ahn said:
This is the best article about A-MSDU, A-MPDU I ever read. Thank you so much. ^^
Super article..keep it up
How is the maximum total length of A-MPDU is arrived at 64K?
In the foam of fragmentation of frames
Can I do aggregation with out QoS enabling?
Hello, I wanted to know if you have a blog which explains how to do processing of Data path layer/frames for 802.11ac ?
Siva Bandaru said:
Consider AMSDU not AMPDU and MSDU act as payload to MPDU, correct me if I am wrong.
For AMSDU, the max size is 7935 Bytes but, MPDU max size is 4095 Bytes in this case how come AMSDU fits in MPDU.
1.In 802.11n the size of MPDU Depends on the size of AMSDU
2.where in 802.11ac the size of MPDU is 11,454 bytes
3.The maximum length of an MPDU in an A-MPDU is
4095 bytes and the maximum number of MPDUs in an
A-MPDU is 64.