Almost time for a vacation? Jim Hamm wants us to know about connecting to wifi and SNR (signal to noise ratio). He informs us, "When traveling, I'm trying to connect to wifi access points in RV parks and motels with my computer, often with poor success. I may show a good RSSI (Received Signal Strength Indicator) but yet have a poor connection to the net. In other words, a strong wifi signal, but perhaps unable to connect to the internet. The problem may be too much 'noise' interfering with the signal. To determine this, you need to calculate the signal to noise ratio (SNR)." Jim goes on to explain, "In the RV park I'm presently in I have a very good SNR. The signal strength is -49 dBm and the noise level is -95 dBm -- a spread of 46, which is good. The minimum spread show be at least 20 dB. I'm using a wifi booster, the Engenius EOC 1650, which gives a lot of detail about all the various wifi signals that show up in the park. Even with this, the download speed is slow (that's all the vendor provides), but I can connect and use the internet okay. "Here is a review of the Engenius EOC 1650. It's available on Amazon. "There are other similar wifi boosters available, and some plug directly into a USB port on your computer. The Engenius booster requires electricity and plugs into your computer via an ethernet cable. "Following is an article explaining more about this. At the end are some links you might look at to help you get the SNR ratio,".Jim suggests. 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. 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. http://www.metageek.net/products/inssider/ http://istumbler.net/ (for Mac) http://www.softpedia.com/get/Network-Tools/Network-Monitoring/NetStumbler.shtml (for PC)