CCO Gems: Cisco Aironet Antennas and Accessories Reference Guide

In an attempt to wrap my head around a lot of the Layer 1 details in the Wireless LAN world, I came across the following doc on Cisco’s website under their wireless antennas product info:

Cisco Aironet Antennas and Accessories Reference Guide

One of the most important units of measure to understand when working with RF is the decibel (dB). While this unit is used in many other fields such as sound and audio, this doc presents it in a clear and concise manner as it relates to WLAN’s:

Decibels
The decibel (dB) scale is a logarithmic scale used to denote the ratio of one power value to another.
For example:
X1`dB = 10 log10 (Power A/Power B)
An increase of 3 dB indicates a doubling (2x) of power. An increase of 6 dB indicates a quadrupling (4x) of power. Conversely, a decrease of 3 dB reduces power by one half, and a decrease of 6 dB results in a one fourth of the power.

Increase

Factor

Decrease

Factor

0 dB

1 x (same)

0 dB

1 x (same)

1 dB

1.25 x

-1 dB

0.8 x

3 dB

2 x

-3 dB

0.5 x

6 dB

4 x

-6 dB

0.25 x

10 dB

10 x

-10 dB

0.10 x

12 dB

16 x

-12 dB

0.06 x

20 dB

100 x

-20 dB

0.01 x

30 dB

1000 x

-30 dB

0.001 x

40 dB

10,000 x

-40 dB

0.0001 x

If you’re a newbie like me, this makes a lot more sense than some of the other documentation I’ve read on the subject, especially with the table above that clearly shows how dB increases logarithmically.

Now that we know a decibel is a relative value, we need to know what values are being measured and referenced. In the wireless world, those two values are Received Signal Strength Indicator or RSSI, and Signal-To-Noise ratio (SNR).

RSSI is a vendor-specific grading of received signal strength. Because it is vendor-specific and not a standard measure, these values cannot be used to compare between vendors. It is usually measured in decibel milliwatt (dBm), which is basically the ratio of power referenced to 1 milliwatt. For example, 3 dBm is 2mW of power. Again, the Cisco Aironet Antenna reference guide provides a good chart for common dBm values to give you an idea of how dBm scales in reference to wattage.

Table 2. Common mW Values to dBm Values

dBm

mW

dBm

mW

0 dBm

1 mW

0 dBm

1 mW

1 dBm

1.25 mW

-1 dBm

0.8 mW

3 dBm

2 mW

-3 dBm

0.5 mW

6 dBm

4 mW

-6 dBm

0.25 mW

7 dBm

5 mW

-7 dBm

0.20 mW

10 dBm

10 mW

-10 dBm

0.10 mW

12 dBm

16 mW

-12 dBm

0.06 mW

13 dBm

20 mW

-13 dBm

0.05 mW

15 dBm

32 mW

-15 dBm

0.03 mW

17 dBm

50 mW

-17 dBm

0.02 mw

20 dBm

100 mW

-20 dBm

0.01 mW

30 dBm

1000 mW (1 W)

-30 dBm

0.001 mW

40 dBm

10,000 mW (10 W)

-40 dBm

0.0001 mW

Signal-To-Noise ratio is defined as the power ratio between a signal, such as a WLAN waveform, and the background noise. Again, we’re measuring in decibels since it is the ratio of the RSSI and the surrounding garbage RF noise.

Since I’m coming from a wired world where everything is essentially plug-and-play, I’m working from the ground up at Layer 1 to hone my WLAN skills.
Anyways, the Cisco Aironet document here gives a very succinct overview of various L1 WLAN concepts such as 802.11 modulation techniques, antennas ratings and specs, understanding RF power levels, as well as a slew of other little details. The first 1/3 of the document gives a nice overview of these concepts, while the other 2/3’s list all of Cisco’s Aironet antenna products.

This is one of those gems on Cisco’s site that I’m surely packing the PDF away for future reference (especially for my Cisco wireless studies).