ADSL (Asymmetric Digital Subscriber Line) is a technology (method) for the transmission of data across a standard phone line. It does this by using frequencies that are not used by standard telephone calls. This is where the term “Broadband” comes from as a broad band of frequencies are used to transmit high speed data and voice calls via your telephone exchange.<\/p>\n
At the end of the telephone exchange the line generally terminates at a DSLAM (Digital Subscriber Line Access Multiplexer) where another frequency splitter separates the voice band signal for the conventional phone network. The data is then normally routed over the Telco\u2019s data network when it eventually reaches IP network.<\/p>\n
Terms<\/strong><\/p>\n One of the major features of ADSL is its ability to adjust to various line qualities and lengths. Lines are prone to noise interference and length both affects the quality and strength of the “signal” (frequencies) sent across the line.<\/p>\n To adapt to this the line will initially sync. This means that each end sends a range of signals and either end reports back what is can hear.<\/p>\n There is an automatic system to establish that a line is re-syncing a lot and adjust the margin to be higher in future. This is called Dynamic Line Management (DLM) and runs all of the time. It can take a few days for a new line to get the right margins for stable operation when first install, but in practice it is rarely more than the first few hours.<\/p>\n Margins<\/strong><\/p>\n To prevent the line from re-syncing every time there was a change in its SNR, SNR margins are used. This provides a buffer for which the lines SNR margin can alter without causing your line to resynchronize. This re-sync which would cause a small outage along with your line re-syncing at a lower speed.<\/p>\n Example:<\/em>\n
If you\u2019re SNR was 40dB. And your SNR margin target 6dB (this is typical) then at the point your line syncs it would sync your speed to 34dB (40dB – 6dB)<\/em><\/span> .
Your line would then have a SNRm of 6db. Your line would then only resync once you had reached the bottom end of your SNRm. Most DSLAMs are configured to resynchronize at around 3dB.
The higher your SNRm the more scope the line has before resynchronizing but the lower your sync speeds will be. But the lower your SNRm the faster the sync speeds but the less room for changes to your SNR before the line resynchronize.<\/p>\n