Framing

The data of the ADSL transport is organized into frames and superframes. One superframe is built of 68 ADSL frames and one sync frame. The sync frame is used to provide superframe synchronization, identifying the start of the superframe.

Because the ADSL lines can vary and are asymmetrical, the frame sizes may vary also. Therefore, there are no absolute frame sizes for the superframe.

Each superframe must be sent every 17 ms and each frame must be sent every 250 microseconds.


Each frame contains:

  1. Fast Byte - used for special superframe-related processing functions.
  2. Fast Data - transmits time sensitive information and it can vary in length.
  3. Forward Error Correction (FEC) - is used to ensure the accuracy of the fast data.
  4. Interleaved Data  - is the user data that the ADSL interface transmits, typically internet data, and it can be vary in length.


ADSL DMT freame rate synchronized to the 4 kHz, and multiplexed into two data buffers fast and interleaved. FEC is applied to each of the buffers.

 

Superframe
Figure1. ADSL superframe

 

Next: ADSL and ISDN

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