Indicators of speed of data transfer over telecommunications lines, where:

DS0- A single 64 Kb/s channel.
DS1- A T1 line (1.544 Mb/s), or 24 DS0s.
DS3- A T3 line (45 Mb/s), or 28 DS1s.

The errors (if you do the math) are due to clocking information and overhead in managing the multiplexing of the channels on the fiber.