Digital Signal 1 (DS1)

signaling scheme devised by Bell Labs.
A DS1 circuit is made up of 24 8-bit channel (also know as time slots or DS0). 8-KHz PCM sampling
(8000 sampling per second and 8 bit for each sample)
8 x 8000 = 64Kbps
=> (24 channel multiplexed together in a physical line)
64K x 24 = 1536Kbps
=> (plus 1 framing bit for each sample)
1536 + 8000 = 1544Kbps = 1.544kbps

http://en.wikipedia.org/wiki/Digital_Signal_1#Origin_of_Name