Controller Based WLANs

What is the relationship between data rate, SNR, and RSSI?

Product and Software: This article applies to all Aruba controllers and ArubaOS versions.

 

Some common devices in facilities require the wireless engineer to target a minimum Received Signal Strength Indication (RSSI) or signal-to-noise ratio (SNR) at the edge of each cell, instead of a specific data rate. Voice over Wi-Fi handsets are the most common devices that use this criterion, but some handheld data terminals also recommend uniform SNR as a best practice. A minimum RSSI value of -65 dBm (equivalent to SNR of 20 dB with a noise floor of -85 dBm) is typical for such devices.

 

In reality, these are two different ways of saying exactly the same thing. The 802.11 standard establishes minimum SNR values required to properly decode each data rate. The minimum SNR values for specific modulations (data rates) are shown in this table. A minimum of about 4 dB SNR (± 2 dB depending on the design) is required for any reliable 802.11 communication (at 1 Mb/s or 6 Mb/s).

 

DSSS Rates

OFDM Rates

 

Rate (Mb/s)

1

2

5.5

11

6

9

12

18

24

36

48

54

SNR (dB)

4

6

8

10

4

5

7

9

12

16

20

21

Signal level (dBm)

-81

-79

-77

-75

-81

-80

-78

-76

-73

-69

-65

-64

 

 

Using this table, we can easily convert minimum SNR criteria into bandwidth. This allows the wireless designer to minimize the number of units that are being worked with. Aruba recommends converting each device whose manufacturer stipulates minimum performance criteria in terms of RSSI or SNR, into the equivalent data rate. This conversion provides for maximum convenience and consistency during the planning process.

 

The equivalence between data rate and SNR can be visualized in the following figure. The actual distance, or radius, from the AP to each data rate/SNR boundary will vary with the EIRP of the AP, the receive sensitivity of the client, and the radio frequency being used.

 

1634_image001.jpg

 

 

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Last update:
‎07-02-2014 06:22 AM
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