RF Power Values

Radio frequency (RF) power levels and the most common measure, the decibel (dB).

Power Level

The dB measures the power of a signal as a function of its ratio to another standardized value. The abbreviation dB is often combined with other abbreviations in order to represent the values that are compared.

Here are two examples:

• dBm The dB value is compared to 1 mW.
• dBw The dB value is compared to 1 W.

We can calculate the power in dBs from this formula:

Power (in dB) = 10 * log10 (Signal/Reference)

This list defines the terms in the formula:

• log10 is logarithm base 10.
• Signal is the power of the signal (for example, 50 mW).
• Reference is the reference power (for example, 1 mW).

Here is an example. If we want to calculate the power in dB of 50 mW, apply the formula in order to get:

Power (in dB) = 10 * log10 (50/1) = 10 * log10 (50) = 10 * 1.7 = 17 dBm

Because decibels are ratios that compare two power levels, we can use simple math in order to manipulate the ratios for the design and assembly of networks. For example, we can apply this basic rule in order to calculate logarithms of large numbers:

log10 (A*B) = log10(A) + log10(B)

If we use the formula above, we can calculate the power of 50 mW in dBs in this way:

Power (in dB) = 10 * log10 (50) = 10 * log10 (5 * 10) = (10 * log10 (5)) +

(10 * log10(10)) = 7 + 10 = 17 dBm

These are commonly used general rules:

Increase Of	Decrease Of	Produces
3dB		Double Transmit Power
	3dB	Half Transmit Power
10dB		10 times the Transmit Power
	10dB	Divide Transmit Power by 10 times
30dB		1000 times the Transmit Power
	30dB	Divide Transmit Power by 1000 times

This table provides approximate dBm to mW values:

dBm	mW
0	1
1	1.25
2	1.56
3	2
4	2.5
5	3.12
6	4
7	5
8	6.25
9	8
10	10
11	12.5
12	16
13	20
14	25
15	32
16	40
17	50
18	64
19	80
20	100
21	128
22	160
23	200
24	256
25	320
26	400
27	512
28	640
29	800
30	1000 or 1W

Here is an example:

1. If 0 dB = 1 mW, then 14 dB = 25 mW.

2. If 0 dB = 1 mW, then 10 dB = 10 mW, and 20 dB = 100 mW.

Subtract 3 dB from 100 mW in order to drop the power by half (17 dB = 50 mW). Then, subtract 3 dB again in order to drop the power by 50 percent again (14 dB = 25 mW).

3. We can find all values with a little addition or subtraction if we use the basic rules of algorithms.

Antennas

We can also use the dB abbreviation in order to describe the power level rating of antennas:

• dBi_For use with isotropic antennas.

Isotropic antennas are theoretical antennas that transmit equal power density in all directions. They are used only as theoretical (mathematical) references. They do not exist in the real world.

• dBd_With reference to dipole antennas.

Isotropic antenna power is the ideal measurement to which antennas are compared. All FCC  calculations use this measurement (dBi). Dipole antennas are more real−world antennas. While some antennas are rated in dBd, the majority use dBi.

The power rating difference between dBd and dBi is approximately 2.2_that is, 0 dBd = 2.2 dBi. Therefore, an antenna that is rated at 3 dBd is rated by the FCC (and Cisco) as 5.2 dBi.

Effective Isotropic Radiated Power

The radiated (transmitted) power is rated in either dBm or W. Power that comes off an antenna is measured as effective isotropic radiated power (EIRP). EIRP is the value that regulatory agencies, such as the FCC or European Telecommunications Standards Institute (ETSI), use to determine and measure power limits in applications such as 2.4−GHz or 5−GHz wireless equipment. In order to calculate EIRP, add the transmitter power (in dBm) to the antenna gain (in dBi) and subtract any cable losses (in dB).

Path Loss

The distance that a signal can be transmitted depends on several factors. The primary hardware factors that are involved:

• Transmitter power
• Cable losses between the transmitter and its antenna
• Antenna gain of the transmitter
• Localization of the two antennas

This refers to how far apart the antennas are and if there are obstacles between them. Antennas that can see each other without any obstacles between them are in line of sight.

• Receiving antenna gain
• Cable losses between the receiver and its antenna
• Receiver sensitivity

Receiver sensitivity is defined as the minimum signal power level (in dBm or mW) that is necessary for the receiver to accurately decode a given signal. Because dBm is compared to 0 mW, 0 dBm is a relative point; much like 0 degrees is in temperature measurement. This table shows example values of receiver sensitivity:

dBm	mW10
10	10
3	2
0	1
-3	.5
-10	.1
-20	.01
-30	.001
-40	.0001
-50	.00001
-60	.000001
-70	.0000001

The receiver sensitivity of the radios in Aironet products is −84 dBm or 0.000000004 mW.

Estimate Outdoor Ranges

Cisco has an Outdoor Bridge Range Calculation Utility to help determine what to expect from an outdoor wireless link. Because the outputs of the calculation utility are theoretical, it is helpful to have some guidelines on how to help counteract outside factors.

• For every increase of 6 dB, the coverage distance doubles.
• For every decrease of 6 dB, the coverage distance is cut in half.

In order to make these adjustments, choose antennas with higher (or lower) gain. Or use longer (or shorter) antenna cables.

• If we change to 100−foot cables instead of 50−foot (which adds 3 dB of loss on each end), the range drops to 9 miles.
• If we change the antenna to 13.5−dBi yagis instead of the dishes (which reduces gain by 14 dBi overall), the range drops to less than 4 miles.

Estimate Indoor Ranges

There is no antenna calculation utility for indoor links. Indoor RF propagation is different than outdoor propagation. However, there are some quick calculations that we can do in order to estimate performance.

• For every increase of 9 dB, the coverage area doubles.
• For every decrease of 9 dB, the coverage area is cut in half.

Radio Resource Management

This feature is very importent to understand; Resource Management (RRM) to continuously monitor the RF environment. The controller uses the information from the access points (AP) and makes any changes to AP channels and power levels to try to mitigate such things as non-802.11 signal (noise), interference from other 802.11 devices, coverage gaps, and co-channel interference caused by the network.

Here is links to understand the RRM process:

Radio Resource Management Part 1
Radio Resource Management Part 2

The RRM feature, also known as Auto-RF, uses the RF information gathered by the APs to make decisions on whether channel assignment or power levels need to be adjusted.

RRM enables controller to monitor their associated AP for the following information.

RRM Analyze	RRM Performs
Traffic Load Interference Noise Coverage Others AP	Radio resource monitoring Transmit power level Dynamic channel assignment Coverage hole detection and correction

RRM Key Figures

RRM Neighbor messages is sent

• At lowest mandatory speed, max power(standard according to country regulations)
• Every 60 seconds by default
• On all serviced channels

Important points when dealing with RRM

• The controllers elect the RF group leader.
• The RF group leader is responsible for dynamic channel assignment (DCA) and transmits power control (TPC).
• An individual controller handles coverage hole detection and correction.
• RF groups and mobility groups are independent functions.
• RF grouping is per radio. The RF group leader for the 802.11b/g network might not be the same RF group leader for the 802.11a network.
• With code Release 4.2.99 or higher, RRM supports up to 20 controllers and 1000 APs in a single RF group.
• RF fluctuations can cause the RF group leader to change.
• By default, the RF group leader polls the other controllers in the RF group for AP statistics and neighbor messages.
• The transmit power threshold setting should be the same between all controllers in the same RF group, because you do not want an entire network to start fluctuating because of a group leader change.
• Each AP maintain a list of up to 34 RRM neighbors per radio
• Controller keeps the best 24 (per radio)
• Controller forwards the list of 24 to the RF-Leader
• Neighbor AP must be -80dBm or better to be on the list
• An AP on the list gets dropped if its signal falls below -85dBm
• Group leader tries to maintain -70dBm threshold between APs

Basic workflow:

1. The controllers and their APs use the configured RF group name to determine if other APs they hear are part of their RF group.
2. The APs use neighbor messages (sent every 60 seconds) that are authenticated by other APs that hear them. The neighbor messages include information about the AP, the controller, and the configured RF group name.
3. The APs that hear the neighbor message of another AP authenticate that message using the RF group name and pass it to their respective controller.
4. The controllers use this information to determine what other controllers should be in their RF group, and then form logical groups to share the RF information from their respective APs, and elect an RF group leader.
5. The RF group leader runs the RRM algorithm against the RF information from all the APs in the RF group. Depending on the outcome, a power level or channel change for an AP or group of APs might take place.

RRM Inter-controller communication

• Controller must be part of the same mobility group
• RF group is a subset of mobility group
• Max limits 20 controllers or 1000 AP per RF-Groups
• Group Leader elected automatically based on controller id value and highest MAC address
• Updates group member every 600 seconds for channels and power values
• For 802.11b/g ports 12124 and 12134 are used
• For 802.11a ports 12125 and 12135 are used.

RRM message contains:

• Radio ID: If the AP had multiple radios, this field identifies the radio used to transmit the message.
• Group ID:  The 16-bit value and controller MAC address.
• Management IP address of the controller (if OATP enabled): RF group leader’s management IP address.
• Channel Count (unused)
• Antenna Pattern: The antenna pattern currently in use.
• Measurement Interval
• Key
• Channel: The native channel that the AP uses to service clients.
• Power (Unused)

When controllers learn of another controller from the AP neighbor messages, they communicate directly with one another to form a system-wide RF group. After the system-wide RF group is formed, the controllers elect an RF group leader.

The group leader is the controller with the highest group leader priority. The group leader priority is based on the group identifier (group ID) information element (IE) contained in the AP neighbor messages. Every controller maintains a 16-bit counter that starts at 0 and increments following events like adding or leaving an RF group or the controller being rebooted. This counter value and the MAC address of the controller make up the group ID IE. Every controller in the RF group selects one controller, or itself, that has the highest group ID value and compares this to the selected controller from the other controllers in the group. The single controller with the highest group ID is elected the RF group leader.

If the RF group leader goes offline, the entire RF group breaks up and the RF grouping process and election of an RF group leader starts over.

For an RF group to form, it takes only one AP on one controller to hear another AP on a different controller.