ADSL Explained

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.

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’s data network when it eventually reaches IP network.

Terms

  • Attenuation – Line attenuation is in relation to the “loop loss” on your line. The lower this figure the better, and the better chance you have of getting the faster speeds.
  • SNR – Signal to Noise Ratio. This is a measurement in decibels of the Signal strength to the level of Noise on the line. The higher your SNR is, the better, as there is less background Noise.   
  • SNRm Target – This is the SNR Margin target. This fixed margin assigned by the DSLAM for which your modem uses as a window before it re-syncs.
  • SNRm – This is the SNR Margin. This figure will not be fixed and can fluctuate in either direction within your the SNR Target Margin dependent of changes (noise) to your line.
  • BRAS – Broadband Remote Access Server – This routes the traffic to and from the DSLAM and has the ability to inject policy management.
  • BRAS Profile – The BRAS profile is responsible for regulating the maximum throughput (data rate) you will receive on your broadband service.
  • DSLAM – A DSLAM collects data traffic from multiple subscribers into a centralized point so that it can be uploaded to the router over a Frame Relay, ATM, or Ethernet connection.

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.

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.

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.

Margins

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.

Example:
If you’re 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) .
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.

Note: ADSL Routers / Modems always refer to SNRm as the SNR.

Interleaving

Interleaving resolves issues where interferences to the line/frequencies resulted in damaged or corrupted packets being received at either end.
It works but chopping up the packets into a number of smaller chunks. It then reorders and sends them. If any of these smaller chunks are damaged the complete packet is not lost but just one of the smaller chunks.
Though this feature can stabilize lines which are prone to small bursts of noise or lines which are far from the exchange the re-ordering and error control adds latency to your traffic.

Rick Donato
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