When performing a radio frequency (RF) site survey, it’s important to define
the range boundary
of an access point based on signal-to-noise (SNR) ratio, which is the signal
level (in dBm) minus the noise level (in dBm). For example, a signal level of
-53dBm measured near an access point and typical noise level of -90dBm yields
a SNR of 37dB, a healthy value for wireless LANs. Don’t let the unit “dB” throw
you — it merely represents a difference in two logarithmic values, such as dBm.
SNR Variance Impacts Performance
The SNR of an access point signal, measured at the user device, decreases as
range to the user increases because the applicable free space loss between the
user and the access point reduces signal level. An increase in RF interference
from microwave ovens
and cordless
phones, which increases the noise level, also decreases SNR.
SNR directly impacts the performance of a wireless LAN connection. A higher
SNR value means that the signal strength is stronger in relation to the noise
levels, which allows higher data rates and fewer retransmissions — all of which
offers better throughput. Of course the opposite is also true. A lower SNR requires
wireless LAN devices to operate at lower data rates, which decreases throughput.
Real-World Values
I recently ran user-oriented tests to determine the impacts of SNR values on
the ability for a user to associate with an 802.11b/g access point and load
a particular webpage. For various SNRs, here’s what I found for the signal strength
(found in the Windows radio status), association status, and performance when
loading the http://wireless-nets.com/staff.htm webpage from a wireless laptop.
To ensure accurate comparisons, I cleared the laptop’s cache before reloading
the page:
> 40dB SNR = Excellent signal (5 bars); always associated; lightening fast.
25dB to 40dB SNR = Very good signal (3 – 4 bars); always associated; very fast.
15dB to 25dB SNR = Low signal (2 bars); always associated; usually fast.
10dB – 15dB SNR = very low signal (1 bar); mostly associated; mostly slow.
5dB to 10dB SNR = no signal; not associated; no go.
These values seem consistent with testing I’ve done in the past, as well as
what some of the vendors publish.
SNR Recommendations
Based on this testing, I recommend using around 20dB as the minimum SNR for
defining the range boundary of each access point. That ensures a constant association
with fairly good performance. Keep in mind that the corresponding level of performance
only occurs at the boundary of each access point. Users associating with access
points at closer range will have higher SNR and better performance.
When measuring SNRs, be sure to use the same radio card and antenna as the
users will have if possible. A variance in antenna gain between the survey equipment
and user device will likely result in users having different SNR (and performance)
than what you measured during the survey. Changes made in the facility, such
as the addition of walls and movement of large boxes, will affect SNR too. Thus,
it’s generally a good idea to recheck the SNR from time-to-time, even after
the network is operational.
Jim Geier provides independent consulting services and training to companies
developing and deploying wireless network solutions. He is the author of the
books Wireless LANs (Sams) and Wireless Networks – First Step (Cisco Press).
