<p><a href="https://i.stack.imgur.com/amG1p.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/amG1p.png" alt="enter image description here"></a><br>
Multiple MSDU packets can be combined into an AMSDU. This AMSDU unit serves as one packet as passed down by higher layers to the MAC. The CRC is calculated on each of these AMSDUs. So if any single AMSDU transmission fails, the entire AMSDU has to be retransmitted. Thus the effective packet error rate (PER) for a considered bit error rate (BER) is determined by the size of the AMSDU.  </p>

<p>However, if the protocol supported only the AMSDU layer of aggregation, the benefits of aggregation achieved by aggregating multiple MAC layer units would have been limited by the achievable PER for the aggregate size. Instead, the WiFi protocol allows the sender to aggregate multiple AMSDU (also referred to as MPDUs) units into a single AMPDU while allowing CRC checks and retries for each AMSDU within an AMPDU. Thus the WiFi protocol allows us to achieve higher MAC efficiency by transmitting AMPDUs while limiting PERs and re-transmissions at the AMSDU level.  </p>

<p>Including AMSDUs as a part of AMPDUs is more efficient because this results in: </p>

<ul>
<li>Fewer CRC calculations for smaller packet sizes at sender and receiver – once 
per AMSDU as opposed to once every MSDU</li>
<li>Fewer MAC headers (MSDU headers).</li>
</ul>

<p>More information You can read here <a href="https://arxiv.org/pdf/1704.07015.pdf" rel="nofollow noreferrer">A Brief Tutorial on WiFi
Aggregation Support</a> and here <a href="https://dot11ap.wordpress.com/a-mpdu-vs-a-msdu/" rel="nofollow noreferrer">A-MPDU vs. A-MSDU</a></p>
Why two aggregation(A-MSDU&A-MPDU) needed in 11n
<p>Can someone please tell me why the need of two aggregations in the 11n.
if there is no A-MPDU in 11n, what will be the impact? </p>

<p>Note: in 11AC, only A-MPDU is there.</p>