4 GHZ SYSTEMS VALUE “H” ANTENNA MOUNTINGAPPROXIMATE VALUE “C” WIRELE...
2.4 GHz Systems
Value “H” Antenna Mounting
Approximate Value “C”
Wireless Link-Distance
Approximate Value “F”
Height with No Obstructions
Earth Curvature
(60% Fresnal Zone at 2.4 GHz)
in Miles
17
3
1
14
36
5
31
56
43
13
10
28
81
53
15
111
61
20
50
146
68
25
78
5 GHz Systems
12
9
41
63
35
90
40
45
123
Total Distance
Antenna
Fresnel @ 60%
Height
Earth Curvature
Figure 3-10
Sample Bridge Calculator
of 0 to 60. The scale is usually represented in dBm, so the two scales would not match up.
It is also up to the vendor to determine what dBm is represented by 0 and what dBm is
represented by 100.
One tool that is used in wireless networks to give RSSI values is called Network Stumbler.
Note:
You can find the free Network Stumbler software at https://traloihay.net.
RSSI is acquired during the preamble stage of receiving an 802.11 frame. RSSI has been re-
placed with Receive Channel Power Indicator (RCPI), which is a functional measurement
covering the entire received frame with defined absolute levels of accuracy and resolution.
To gain these measurements, you can use a CB21AG card and the Aironet Desktop Utility
(ADU), which are covered in Chapter 16, “Wireless Clients.” The CB21AG card is the most
widely adopted card used by Airmagnet and OmniPeek.
Signal-to-Noise Ratio
Signal-to-noise ratio
(SNR) is the term used to describe how much stronger the signal is
compared to the surrounding noise that corrupts the signal. To understand this, suppose
you walk into a crowded park with many screaming kids and speak in a normal voice
while on the phone. The odds are that the noise is going to be so loud that the person on
the other end will not be able to distinguish your words from all the noise around you
that is also being transmitted over the phone. This is how the wireless network operates. If
the outside influences are causing too much noise, the receivers cannot understand the
transmissions.
When the software that runs your wireless card reports this measurement, it is best to
have a higher number, but this is also built on the RSSI value, so it is vendor determined.
Note:
You can explore SNR levels in the Network Stumbler application previously men-
tioned. Remember that the values are valid only for the Network Stumbler application.
Other applications might report different SNR values.
Link Budget
Link budget
is a value that accounts for all the gains and losses between sender and re-
ceiver, including attenuation, antenna gain, and other miscellaneous losses that might oc-
cur. This can be useful in determining how much power is needed to transmit a signal that
the receiving end can understand.
The following is a simple equation to factor link budget:
Received Power (dBm) = Transmitted Power (dBm) + Gains (dB) – Losses (dB)