Finding Cisco Tx Power Levels

Tx Power Level is an important variable that, in combination with antenna gain, influences coverage patterns. Cisco wireless controller dynamically adjusts transmit power levels according to current RF conditions, access point’s capabilities and local regulatory domain recommendations, that may vary per band and/or channel. We, as an engineers, should be capable to confirm current Tx Power level, list of all supported Tx Power levels and antenna gain (if applicable). No coverage or heatmap approximations can be made without this information. With this post I will provide a number of different useful CLI commands that may be used to quickly find Tx Power levels information per-AP and in bulk.

Different regulatory domains vary in their recommendations as to what channels can be used by 802.11 standard devices and maximum EIRP (Effective Isotropic Radiation Power) levels, which consists of maximum Tx Power and max antenna gain. The following table show FCC limitations in 2.4GHz range only for your reference (applicable to USA, Canada and some South America countries).

2.4GHz Point-to-Multipoint
Max Tx Power Max Antenna Gain (dBi) Max EIRP (dBm) Max EIGRP (Watts)
30 dBm or 1 W 6 36 4
27 dBm or 500 mW 9 36 4
24 dBm or 250 mW 12 36 4
21 dBm or 125 mW 15 36 4
18 dBm or 62 mW 18 36 4
15 dBm or 31 mW 21 36 4
12 dBm or 16 mW 24 36 4

Note: It is possible to increase antenna gain at expense of Tx Power. For each dBm subtracted from Tx Power, one dBi can be added to antenna gain.

 

2.4GHz Point-to-Point
Max Tx Power Max Antenna Gain (dBi) Max EIRP (dBm) Max EIGRP (Watts)
30 dBm or 1 W 6 36 4
29 dBm or 800 mW 9 38 6.3
28 dBm or 630 mW 12 40 10
27 dBm or 500 mW 15 42 16
26 dBm or 400 mW 18 44 25
25 dBm or 316 mW 21 46 39.8
24 dBm or 250 mW 24 48 63
23 dBm or 200 mW 27 50 100
22 dBm or 160 mW 30 52 158

Note: As opposed to the Point-to-Multipoint rules, for each dBm subtracted from Tx Power, three (3) dBi can be added to the antenna gain.

Surprisingly, Cisco does not refer to dBm values on its controllers and access points, instead a number of transmit power levels represent different amount of actual transmit power, where Level 1 stands for max possible Tx Power. The following output examples show supported 802.11a Tx Power levels and corresponding Tx Power amount in dBm for different access point models and regulatory domains.

Europe / Cisco 1242AG-E

Tx Power
 Num Of Supported Power Levels ............. 8
 Tx Power Level 1 .......................... 17 dBm
 Tx Power Level 2 .......................... 15 dBm
 Tx Power Level 3 .......................... 14 dBm
 Tx Power Level 4 .......................... 11 dBm
 Tx Power Level 5 .......................... 8 dBm
 Tx Power Level 6 .......................... 5 dBm
 Tx Power Level 7 .......................... 2 dBm
 Tx Power Level 8 .......................... -1 dBm
 Current Tx Power Level .................... 1
 External Antenna Gain (in .5 dBi units).... 7
Europe / Cisco 1142N-E

Tx Power
 Num Of Supported Power Levels ............. 7
 Tx Power Level 1 .......................... 17 dBm
 Tx Power Level 2 .......................... 14 dBm
 Tx Power Level 3 .......................... 11 dBm
 Tx Power Level 4 .......................... 8 dBm
 Tx Power Level 5 .......................... 5 dBm
 Tx Power Level 6 .......................... 2 dBm
 Tx Power Level 7 .......................... -1 dBm
 Current Tx Power Level .................... 1
 Internal Antenna Gain (in .5 dBi units).... 8
Ukraine / Cisco 2602I-E

Tx Power
 Num Of Supported Power Levels ............. 5
 Tx Power Level 1 .......................... 18 dBm
 Tx Power Level 2 .......................... 15 dBm
 Tx Power Level 3 .......................... 12 dBm
 Tx Power Level 4 .......................... 9 dBm
 Tx Power Level 5 .......................... 6 dBm
 Current Tx Power Level .................... 1
 Internal Antenna Gain (in .5 dBi units).... 8
Australia / Cisco 1142N-N

Tx Power
 Num Of Supported Power Levels ............. 6
 Tx Power Level 1 .......................... 14 dBm
 Tx Power Level 2 .......................... 11 dBm
 Tx Power Level 3 .......................... 8 dBm
 Tx Power Level 4 .......................... 5 dBm
 Tx Power Level 5 .......................... 2 dBm
 Tx Power Level 6 .......................... -1 dBm
 Current Tx Power Level .................... 1
 Internal Antenna Gain (in .5 dBi units).... 8

These outputs were taken directly from the controllers by using the following very useful command:

show ap config 802.11a ap-name

show ap config 802.11b ap-name

This command also displays a lot of different information about a particular access point, such as supported and current channels, model, software version, supported rates, mode and other. Unfortunately, old versions of AireOS do not support output filtering, like in Cisco IOS (include, exclude). You have to scroll quite a few pages before Tx Power information appears on the screen. Recent versions (starting from 7.5) have grep feature implemented. It can be used at our advantage to minimize the displayed output to only show Tx Power related information, such as shown in my outputs. The full modified command is the following

grep include “Tx Power|Gain” “show ap config 802.11a ap-name

To show summary information that includes the whole range of access points that are registered on the controller and their current channels and Tx power levels the following commands may be useful.

show advanced 802.11a txpower

show advanced 802.11b txpower

The output of this command is AireOS version dependent. Earlier versions of AireOS only provide Tx Power Control (TPC) algorithm configuration, such as shown below:

(Cisco Controller) >show advanced 802.11a txpower

 Automatic Transmit Power Assignment
 Transmit Power Assignment Mode................. AUTO
 Transmit Power Update Interval................. 600 seconds
 Transmit Power Threshold....................... -70 dBm
 Transmit Power Neighbor Count.................. 3 APs
 Min Transmit Power............................. -10 dBm
 Max Transmit Power............................. 30 dBm
 Transmit Power Update Contribution............. SNI..
 Transmit Power Assignment Leader............... CONTROLLER-1 (10.10.10.10)
 Last Run....................................... 201 seconds ago

Recent versions provide significantly more information, that includes the list of all access points, their supported Tx Power Levels (in dBm) and current channel and Tx Power level configuration:

(Cisco Controller) >show advanced 802.11a txpower

Leader Automatic Transmit Power Assignment
 Transmit Power Assignment Mode................. AUTO
 Transmit Power Update Interval................. 600 seconds
 Transmit Power Threshold....................... -70 dBm
 Transmit Power Neighbor Count.................. 3 APs
 Min Transmit Power............................. -10 dBm
 Max Transmit Power............................. 30 dBm
 Update Contribution
 Noise........................................ Enable
 Interference................................. Enable
 Load......................................... Disable
 Device Aware................................. Disable
 Transmit Power Assignment Leader............... CONTROLLER-2 (10.10.10.20)
 Last Run....................................... 414 seconds ago
 Last Run Time.................................. 0 seconds
 TPC Mode....................................... Version 1
 TPCv2 Target RSSI.............................. -67 dBm
 TPCv2 VoWLAN Guide RSSI........................ -67.0 dBm
 TPCv2 SOP...................................... -85.0 dBm
 TPCv2 Default Client Ant Gain.................. 0.0 dBi
 TPCv2 Path Loss Decay Factor................... 3.6
 TPCv2 Search Intensity......................... 10 Iterations

AP Name Channel TxPower Allowed Power Levels
-------------------------------- ---------- ------------- ------------------------
AP-1        *40 *1/5 (18 dBm) [18/15/12/9/6/6/6/6]
AP-2        *36 *1/5 (18 dBm) [18/15/12/9/6/6/6/6]
AP-3        *44 *2/5 (15 dBm) [18/15/12/9/6/6/6/6]

Note: 1/5 TxPower means that current Tx Power level is 1 out of 5 total. In turn, Allowed Power Levels column lists all possible Tx Power levels in dBm, where Level 1 is represented by the leftmost value, or 18 dBm in our example. Even though 8 different values are shown, only 5 leftmost are significant (that is, access points support only 5 different levels). The rest must be ignored.

Another command that may be useful finding Tx Power information for all access points in one go is

show advanced 802.11a summary

show advanced 802.11b summary

It shows brief information about all connected access points and their operational state for the specific band, including channel in use and current Tx Power level. Apart from Tx Power Level, recent versions of AireOS also show a real Tx Power amount in dBm. Older versions of AireOS show Tx Power Level only and no any values in dBm. Therefore engineer must be aware what dBm values correspond to shown levels.

WLC 4402 using 7.0.250.0 code:

(Cisco Controller) >show advanced 802.11a summary

AP Name  MAC Address      Admin State Operation State Channel TxPower
-------------------------------- ------------------ ------------ -----
Kyiv-1  6c:9c:ed:b5:a4:d0     ENABLED             UP      64*    1(*)

WLC 5508 using 7.5.102.0 code:

(Cisco Controller) >show advanced 802.11a summary
Member RRM Information
AP Name MAC Address Slot Admin Oper Channel TxPower
-------------------------------- ----------------- ---- -------- 
AP-1 f4:1f:c2:e7:a1:80 1 ENABLED UP    40*  *1/5 (18 dBm)
AP-2 0c:27:24:0f:7f:30 1 ENABLED UP    36*  *1/5 (18 dBm)
AP-3 f4:1f:c2:5b:a4:b0 1 ENABLED UP    44*  *2/5 (15 dBm)

Hope you find this useful.

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