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FAQ-What Are the Physical Rate, Theoretical Rate, and Actual Rate in the 802.11 Standard

Publication Date:  2015-06-16 Views:  651 Downloads:  0

Issue Description

What Are the Physical Rate, Theoretical Rate, and Actual Rate in the 802.11 Standard?


1, The WLAN physical rate is the physical layer rate of a radio interface, that is, the physical layer rate at which a radio interface keeps sending data. For example, the 802.11b physical rate is 11 Mbit/s and the 802.11g physical rate is 54 Mbit/s.

2, What is the relationship between the user theoretical rate and physical rate? The physical rate indicates only the performance of a radio interface, but users only care about how much bandwidth and rate they can use. The following uses the 802.11b standard as an example and assumes that a user packet is 1500 bytes. After a 32-byte header is prepended to the packet, the packet is longer than an Ethernet data frame. The checksum bits in 802.11b and Ethernet are both 4 bytes. The longest data frames (1536 bytes) are transmitted at the rate of 11 Mbit/s. The transmission time is [1536 (bytes) x 8 (bit)]/11 Mbit/s = 1117 microseconds.

On the WLAN, a link code and PLCP header (exclusively used by WLAN) are prepended to a data frame. The transmission time of the link code and PLCP header is 192 microseconds. In addition to the interframe gap, a random period (delay offset) is required during the transmission of data frames on WLANs. In 802.11b, the average delay offset is 360 microseconds.

On the WLAN, an ACK frame is received from the remote end each time a data frame is sent to confirm successful communication. The next data frame is sent only after the ACK frame is received. The total transmission time is 213 microseconds.

The transmission time of a 1500-byte data frame includes the waiting time and ACK transmission time, equaling 1882 microseconds.

1117 + 192 + 360 + 213 = 1882

In this case, the theoretical maximum UDP throughput for 1500-byte data frames is 7.1 Mbit/s.

3, The preceding calculation result is based on the UDP model and 1500-byte frames. The actual usage scenarios are much more complex than the preceding scenario. Additionally, the number of STAs also greatly affects AP performance. Therefore, the actual user rate is usually tested. In most cases, the actual rate of 802.11b can reach about 4.7 Mbit/s.