Hi Romain,
We often say that DS1 line is composed of 24 DS0 (which are 8-bit channels) but are they really composed of 24 little cables physically or is it just a normal cable which is logically cut?
It is a normal cable just "logically cut", as you call it. With DS1, we speak about time division multiplexing or TDM - each of these channels occupies only a predefined timeslot on the cable. In fact, DS1 uses bit interleaving, meaning that you take the 24 DS0 channels, and one by one, you take a single bit from all DS0 channels, one by one, and put them on the wire. Then you take another single bit from all 24 DS0 channels, again put them on the wire, and so on, until you take 8 bits from each DS0 channel. This way, you will put 24x8 = 192 bit on the cable. You will then put yet one more special bit which is used to carry signalling messages on the cable. This 192 voice bits plus 1 signalling bit forms a single DS1 frame comprising 193 bits. Then the whole process repeats itself.
This entire DS1 frame is sent 8000 times in a second (remember, one such frame contains 1 entire byte from each DS0 channel, and because each DS0 channel supplies an 8-bit sample each 125 us, there are 8000 such DS1 frames necessary to keep up with the amount of data coming from each individual DS0 channel). If you now compute the amount of data consumed by the flow of DS1 frames, you get 8000 * 193 bits = 1,544,000 bps = 1544 Kbps.
Best regards,
Peter
