Moxa ANT-WSB-ANM-05 Manuel D’Utilisation
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17
Understanding Industrial WLAN – IEEE 802.11
Bandwidth, Data Rate, and Throughput
Usually when “bandwidth” is mentioned, it means one of two things:
1. The actual width of a frequency band measured in Hz (Hertz); the effective bandwidth would be the
frequency band that is actually carrying data.
frequency band that is actually carrying data.
2. The maximum data rate available (bits per second) in a communication link.
The former is the technically correct definition of bandwidth. For example, the 802.11b/g standards operate
between 2.4 GHz and 2.4835 GHz, giving a total effective bandwidth of 83.5 MHz with a channel bandwidth of
22 MHz.
between 2.4 GHz and 2.4835 GHz, giving a total effective bandwidth of 83.5 MHz with a channel bandwidth of
22 MHz.
The data rate of a particular wireless standard is the maximum data transfer speed (bit per second) the
communication link can achieve, such as 54 Mbps for 802.11g. Please note that this is the specified transfer
rate for raw data. The WLAN protocol packages the user data with layers of headers and trailers with
inter-packet gaps in between the packets. For example, TCP communication requires the receiving end to
acknowledge the received data by sending ACK packets back to the receiver. Therefore, the actual user
data rate will be lower than the specified data rate because user data is only a portion of the raw data being
transmitted via the wireless media. The actual user data rate is called the “throughput” of the wireless link.
Typically, we can expect the throughput to be about half of the specified data rate (i.e., throughput = 25 Mbps
when data rate = 54 Mbps).
communication link can achieve, such as 54 Mbps for 802.11g. Please note that this is the specified transfer
rate for raw data. The WLAN protocol packages the user data with layers of headers and trailers with
inter-packet gaps in between the packets. For example, TCP communication requires the receiving end to
acknowledge the received data by sending ACK packets back to the receiver. Therefore, the actual user
data rate will be lower than the specified data rate because user data is only a portion of the raw data being
transmitted via the wireless media. The actual user data rate is called the “throughput” of the wireless link.
Typically, we can expect the throughput to be about half of the specified data rate (i.e., throughput = 25 Mbps
when data rate = 54 Mbps).
The following figure is an example of throughput measurements as signal attenuation increases (curves
correspond to different noise immunity settings):
correspond to different noise immunity settings):
As you can see, when the signal is too strong (low attenuation) or too weak (high attenuation), the overall
throughput dips bellow the optimum value.
throughput dips bellow the optimum value.
Throughput can be measured with various throughput measuring tools. One of the free throughput measuring
tools available is Jperf, downloadable here:
tools available is Jperf, downloadable here:
http://sourceforge.net/projects/iperf
The receiver’s sensitivity is the minimum power level the receiver can accept to process the received data. The
specified sensitivity is not the power detected by the receiving antenna but the power present as the receiver
module. An important point to note from the above equation is that as frequency increases, the effective
distance decreases. Therefore, the 802.11a (5 GHz) standard will yield a shorter communication distance than
802.11b/g (2.4 GHz). Users who wish to communicate long distances should therefore select 802.11b/g as
their operating standard.
specified sensitivity is not the power detected by the receiving antenna but the power present as the receiver
module. An important point to note from the above equation is that as frequency increases, the effective
distance decreases. Therefore, the 802.11a (5 GHz) standard will yield a shorter communication distance than
802.11b/g (2.4 GHz). Users who wish to communicate long distances should therefore select 802.11b/g as
their operating standard.