Effect of message aggregation on 802.11n performance

effect of message aggregation on 802.11n performance

The 2009 802.11n Protocol was formally standardized, and once again the physical rate was increased, and the maximum physical rate reached 300Mbps. Moreover, the 802.11n A-MPDU (message aggregation) function fully improves the channel utilization of the space medium, and brings the channel load performance of the WLAN network multiplied. Below we will mainly analyze the effect of A-MPDU (message aggregation) function on 802.11n performance.

802.11 protocol definition Each sending a message is bound to channel competition, all need to add the physical beginning according to the pattern, for the single-broadcast text also need to wait for the physical layer ACK confirmation, and so on, and so on, and so on the actual transmission of the message needs to consume channel resources.

The following table assumes the continuous sending of a single broadcast of 1538bytes size, comparing the single-channel theoretical performance with the physical rate of 54Mbps, 130Mbps, and 300Mbps, regardless of retransmission or packet error:

Physical send Rate (Mbps)	54	130	300
Time consumed by sending a message (US)	217	136	88
Time wasted in sending a message (US)	144	150	150
Number of messages sent per second (PPS)	2768	3497	4202
Performance (Mbps)	32.5	40.0	48.1

As can be seen from the data in the table, from 54Mbps to 300Mbps, although the physical rate achieved nearly 6 times times the increase, but the actual performance did not rise. 802.11N protocol in order to realize the synchronous elevation of WLAN network performance and provide sufficient performance for WLAN network application, the message aggregation (A-MPDU) technology is proposed.

802.11 of any one of the messages in the physical transmission will be sent as a MDPU, each transmission will necessarily need to channel competition and avoidance, thereby consuming channel resources. and message aggregation A-MPDU by aggregating multiple MPDU into a physical beginning, only one channel competition or avoidance is needed to complete the simultaneous transmission of n MPDU, thus reducing the channel resource consumption caused by sending N-1 MPDU messages. Through the message aggregation feature, the utilization rate of channel resources is improved, and the performance of 802.11 network is greatly improved.

Figure 1 is a structure diagram of A-MPDU, where MPDU delimiter is specifically defined for A-MPDU. In addition, A-MPDU technology only converges the contract for a client's MPDU:

Figure 1 A-MPDU message structure

The A-MPDU message aggregation feature provides strong support for block ACK functionality. In a typical 802.11 network, any single broadcast will require an ACK acknowledgement from the destination device, and each ACK is a 802.11 message that consumes channel resources. The block ACK can be combined with the A-MDPU feature, which requires only one block ACK message for all 802.11 messages in the entire A-MDPU, which reduces the consumption of the channel resources.

The following is a theoretical analysis of 802.11n performance in the case of 20MHz and 40MHz information respectively. By Figure 2, Figure 3, Figure 4, Figure 5 you can see that 802.11 of the A-MPDU message aggregation for the WLAN network brings a great performance improvement (the following analysis assumes that each 802.11 message is 1534bytes):

Figure 2 20MHZ Aggregate message count and performance relationship

Figure 3 20MHZ Aggregation message count and channel utilization relationship

Figure 4 40MHZ Aggregate message count and performance relationship

Figure 5 40MHZ aggregation message count and channel utilization relationship

So far we can see that While the physical rate of 802.11n achieves a nearly 6 times-fold increase, it does not bring a big boost to WLAN performance, but improves WLAN performance by up to 6 times times through A-MPDU message aggregation, and finally realizes the synchronous promotion of the physical transfer rate and performance of the WLAN network, bringing the WLAN application to a high-speed access era.

Effect of message aggregation on 802.11n performance