What Are the Physical Rate, Theoretical Rate, and Actual STA Rate in the 802.11 Standard?
The WLAN physical rate is the physical layer (PHY) rate of the air interface, that is, the PHY rate at which the air interface keeps sending data. For example, a common physical rate supported by 802.11ax is 9.6 Gbit/s.
What is the relationship between the theoretical rate and physical rate? The physical rate indicates only the performance of the air interface, but users only care about available bandwidth and rate for them.
Bandwidth loss from the PHY to the service layer includes time consumption of the process required by 802.11 MAC layer communication, and header and trailer overheads of data frames at different layers.
Specifically, bandwidth provided by the communications system can be calculated using the number of bits transmitted per unit time, that is, number of bits/time.
- Number of valid bits transmitted
In the preceding figure, A-MSDU Subfrm1 marked in dashed lines is the packet sent from the Ethernet, where the MSDU field is the service packet sent from the upper layer (packet described in the test result of Chariot). The entire 802.11 packet also has other headers including PLCP Preamble, PLCP Header, and 802.11 Header.
- Time for transmitting the preceding bits
The following figure shows the protocol process through which each data packet transmitted on the 802.11 air interface goes.
The transmitter sends a data frame after Distributed Interframe Space (DIFS) times out and waits for the backoff time. After receiving the data frame, the receiver waits until Short Interframe Space (SIFS) times out and sends an ACK frame to the transmitter. It can be considered that a data packet is successfully sent only after the entire process is correctly completed. In this data packet, only the Frame Body field in 802.11 DATA carries valid data, and other fields (such as DIFS, Backoff, SIFS, and ACK) are overheads for ensuring valid transmission. In multi-user (MU) scenarios, the backoff window is doubled due to collisions, which further reduces the transmission efficiency.
To sum up, the actual rate of a STA is affected by the STA capabilities (such as the Wi-Fi protocol and number of spatial streams supported by the STA), the signal strength, frequency bandwidth, and air interface environment of the AP associated with the STA, and the overhead during data transmission. In an ideal case, the actual rate of a single STA is about 60% to 70% of the negotiated PHY rate. In a multi-STA scenario, the actual test result is much lower than that in a single-STA scenario because multiple STAs share bandwidth resources. This is because more concurrent users consume more resources.