Translating mW, dBm, MHz and channels

Every so often I have to configure the Radio Transmit Power on an Access Point (AP) and it just happen that the wireless client transmit power is provided in milliwatt (mW) but I need to configure the APs transmit power to match that of the wireless client which need to be done in decibel-milliwatt (dBm). So let’s translate between the two.

The Radio Transmit Power setting determines the power level of the radio transmission. The default power setting is the highest transmit power allowed in your regulatory domain. Government regulations define the highest allowable power level for radio devices. This setting must conform to established standards for the country in which you use the device. The power settings may be in mW or in dBm depending on the particular radio that is being configured.

Definitions:

  • Watt (W): a unit of power equal to 1 joule per second; the power dissipated by a current of 1 ampere flowing across a resistance of 1 ohm.
  • Milliwatt (mW): a unit of power equal to one thousandth of a watt.
  • Decibel-milliwatt (dBm): an electrical power unit in decibels (dB), referenced to 1 milliwatt (mW).

Translation between mW and dBm:

 mW  dBm
1 -1
2 2
3 5
4 6
5 7
6 8
8 9
10 10
12 11
15 12
20 13
25 14
30 15
40 16
50 17
60 18
80 19
100 20
125 21
150 22
200 23
250 24

Then there is the configuration of channels for your autonomous access point and instead of providing the channel number it actually asks for the corresponding frequency in your regulatory domain.

Each 2.4-GHz channel are 22 MHz wide. The bandwidth for channels 1, 6, and 11 does not overlap, so you can set up multiple access points in the same vicinity without causing interference. Both 802.11b and 802.11g 2.4-GHz radios use the same channels and frequencies.

The 5-GHz radio operates from 5180 to 5825 MHz. Each channel covers 20 MHz, and the bandwidth for the channels overlaps slightly. For best performance, use channels that are not adjacent (44 and 46, for example) for radios that are close to each other.

Translation between channel and MHz for 802.11b/g:

Channel Identifier Center Frequency (MHz)
1 2412
2 2417
3 2422
4 2427
5 2432
6 2437
7 2442
8 2447
9 2452
10 2457
11 2462
12 2467
13 2472
14 2484

Translation between channel and MHz for 802.11a:

Channel ID Center Frequency (MHz)
5150 to 5250 MHz
34 5170
36 5180
38 5190
40 5200
42 5210
44 5220
46 5230
48 5240
5250 to 5350 MHz
52 5260
56 5280
60 5300
64 5320
5470 to 5725 MHz
100 5500
104 5520
108 5540
112 5560
116 5580
120 5600
124 5620
128 5640
132 5660
136 5680
140 5700
5725 to 5850 MHz
149 5745
153 5765
157 5785
161 5805
165 5825

Reference                                                              http://www.cisco.com/c/en/us/td/docs/wireless/access_point/12-4-25d-JA/Configuration/guide/cg_12_4_25d_JA/scg12-4-25d-JA-chap6-radio.html

 http://www.cisco.com/web/techdoc/wireless/access_points/online_help/eag/123-02.JA/1400BR/h_ap_network-if_802-11_c.html

 http://www.cisco.com/c/en/us/td/docs/wireless/access_point/channels/ios/reference/guide/atonchp2/1200_chp.html#wp1047037

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Translating mW, dBm, MHz and channels

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