Signal to Noise Ratio (SNR) is a measurement, usually conveyed in decibels (dB), that compares background noise to the level/strength of a signal. In most instances, the signal will be an electrical signal. SNR is a measurement that is widely used within telecommunications, and is used to measure digital and analog communication channels. SNR is used to measure the level of distortion on DSL broadband connections, and the higher the signal-to-noise ratio (dB) is, the better: this is because the signal strength is stronger in comparison to the noise distortion, and the likelihood of the router/modem producing a 'no sync' (DSL light blinking on the router, no Internet connection) is lower. The picture below, shows a graph displaying the noise margin fluctuating upon a D-Link DSL-G604T router.
Graph is produced by RouterStats Version 6.9, available at www.vwlowen.co.uk
Signal to Noise Ratio (SNR) has been likened to holding a conversation in a pub: the busier the pub the harder it is to maintain a conversation, and during 'happy hour' you may not be able to hear a single word your friend is saying. Likewise, when your router/modem is trying to hold a conversation with the network exchange across a copper telephone line, the less noise produced on the copper telephone line, the better able it is for a connection/conversation to be held. What is a major factor effecting the ability to hold a conversation in a a busy pub? how close two people are stood together; therefore, if a router is many miles from a network exchange, it is more likely that noise distortion will be picked up and the signal will degrade.
While distance (router/modem > network exchange) is a major factor effecting SNR, some other factors include: electrical noise within the home and adjacent to the home; weather conditions upon the copper-line telephone network; condition of the copper-line telephone network; quality / age of the equipment used at the network exchange; cross-talk from other other DSL/ADSL connections; and the amount of DSL/ADSL connections on the copper-line telephone network. Therefore, the SNR of a DSL broadband connection will fluctuate minute-to-minute, hour-to-hour, day-to-day, and month-to-month. SNR can be measured by programs such as RouterStats, and the software of some routers.
SNR Margin is a different measurement to SNR, and is a measurement that is often displayed by the software of routers. SNR Margin is the figure that is produced by comparing the actual SNR and the SNR required for the connection to run/sync at a specific speed. For example: Actual SNR = 15dB; SNR to sync at 2Mb = 5dB; SNR Margin = 10dB. There has been some debate about whether a high or low SNR Margin is preferable: some people say a low SNR margin is preferable as it will produce a superior speed, while other people say the SNR margin needs to be higher (over 7-8dB) to maintain a steady connection that does not frequently 'drop' / no sync. While its debatable whether a low/high SNR Margin is preferable, the same can't be said for the actual SNR: below 6dB can result in multiple disconnections (no sync) on a regular basis - although some routers are better at handling low dB - and above 60dB will result in a near flawless connection.