Sniffer® Portable Professional User's Guide - NetScout

Chapter 5
The Dashboard Gauge Tab
76 Sniffer Portable Professional
The Gauge tab is displayed by default when you start the Dashboard.
When capturing from wireless networks, the Dashboard’s Gauge tab
provides a Throughput gauge. This gauge provides a real-time
measurement of the data rate (in bits per second) observed by Sniffer
Portable Professional. When calculating throughput, Sniffer Portable
Professional only counts data frames. Management and control frames
are not part of this calculation. However, the throughput measurement
does include the header portions of data frames (see How Wireless
Utilization is Calculated on page 76 for details).
How Wireless Utilization is Calculated
The Dashboard provides network utilization percentage measurements
on both the Gauge and Detail tabs. Sniffer Portable Professional
calculates network utilization by storing the airtime (in microseconds)
for each observed frame in a buffer. Every second, the value in this
buffer is divided by 1,000,000 microseconds (that is, a second) to obtain
a percentage utilization measurement.
The airtime for each frame is calculated as follows:
1 First, the duration of the frame’s PLCP header is stored. PLCP
headers can be either:
192 microseconds. This is the Long header format specified
in IEEE 802.11b/g for 1 and 2 Mbps transmission speeds.
96 microseconds. This is the Short header format specified
in IEEE 802.11b/g for 5.5 and 11 Mbps transmission speeds.
NOTE: The calculations for 802.11a are performed similarly
except that they use the duration of the PLCP header specified
for different 802.11a rates.
2 Each frame’s PLCP header includes a field indicating the length of
the data portion of the frame in microseconds. Sniffer Portable
Professional adds this value to the duration of the PLCP header
observed in the previous step and stores the sum in a buffer.
3 Each second, the value in the buffer is divided by 1,000,000
microseconds to obtain a percentage utilization measurement.