Boosting Wireless Efficiency with Frame Aggregation- A-MSDU and A-MPDU



In the evolution of wireless networking protocols, one of the key advancements that has significantly improved efficiency and the throughput of the wireless client is the concept of frame aggregation. Traditional wireless frame transmission methods often incurred considerable overhead, with each frame requiring its own MAC Header and FCS. Frame aggregation addresses this challenge by allowing multiple data frames to be combined into a single, larger transmission.

The Basics of Frame Aggregation Frame aggregation is a technique introduced in the 802.11n wireless standard and further enhanced in later versions like 802.11ac and 802.11ax. The underlying principle is to combine multiple MAC Protocol Data Units (MPDUs) into a single Physical Protocol Data Unit (PPDU) for transmission.


Frame Aggregation Methods

There are two primary types of frame aggregation:

Aggregated-MSDU (A-MSDU): This method combines multiple MAC Service Data Units (MSDUs) into a single MPDU, reducing the overhead of per-frame MAC headers.
 
Aggregated-MPDU (A-MPDU): Here, multiple MPDUs are aggregated into a single, larger PPDU transmission. This approach saves significant time by only requiring a single PHY layer header.

As seen in the following image, an A-MSDU combines Multiple higher-layer packets (e.g., IP packets) destined for the same receiver are combined into one MPDU at the MAC layer. Each MSDU within the A-MSDU retains its own header for identification but shares a common MAC header. 
The QOS Control Field Specifies if the Payload Type is MSDU/AMSDU :






Benefits :
This  fewer MAC headers and acknowledgments reduces overhead as each MAC header is about 40bytes. Combining Multiple MSDU's together save multiple bytes of overhead per transmission. 

Limitations: While A-MSDU improves performance, it works best in the networks with less delays between packets and low error rates. Now remember that A-MSDU uses single MAC Header for each MSDU,  if a single frame in the transmission is corrupted or lost, the entire AMSDU is required to be retransmitted to the receiver.

The A-MPDU technique is particularly beneficial and overcome the challenges of A-MSDU, as it allows wireless devices to transmit multiple data frames with a single PHY layer header, rather than requiring individual headers for each frame. At the PHY layer, multiple MPDUs are bundled, transmitted together, and acknowledged in a single block(Block-ACK). 


Benefit:

Each MPDU  within A-MDPU has its own MAC Header and in case of frame lost or corruption, the individual MPDU is retransmitted. This reduces overhead and improves the overall efficiency of the wireless link.

Limitations:

The maximum size of an A-MPDU frame is defined by the wireless standard, for 802.11n the max size of A-MPDU supported is 65535 bytes and with 802.11ac its 1,048,575 bytes. The HT/VHT Capabilities Information shows the A-MPDU Parameters:


In an environments with a high density of VoIP clients or other devices transmitting large A-MPDUs, reducing the maximum size can help mitigate the impact on voice quality. This is because large A-MPDUs can consume significant airtime, leading to increased latency and jitter for time-sensitive applications like voice and video.

By adjusting the A-MPDU size, network administrators can find the right balance between maximizing throughput and ensuring optimal performance for latency-sensitive traffic. If supported by the Wireless Access Point, this level of fine-tuning is a key advantage of frame aggregation over legacy wireless transmission methods.

Benefits of Frame Aggregation 

The primary benefits of frame aggregation in wireless networks include:
 
Increased Throughput: By reducing the overhead associated with individual frame transmissions, frame aggregation can significantly boost the overall throughput.
Reduced Latency: The reduced airtime consumption of aggregated frames can lead to lower latency, particularly for time-sensitive applications like real-time communications.
Better Noise Immunity: Transmitting larger frames with fewer headers makes the signal less susceptible to interference and noise, improving the overall reliability of the wireless connection.

Conclusion 
Frame aggregation, by combining multiple data frames into a single, larger transmission, addresses the inefficiencies of legacy wireless protocols and delivers significant performance improvements. Most Enterprise level Access Points intelligently choose between AMSDU and AMPDU based on the network conditions for the faster and efficient transmission.



Ref: https://www.cwnp.com/certifications/cwap
 

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