Phone audio is sampled 8,000 times per second (i.e., at 8 kHz). Each sample is converted to an eight-bit value, with one of the bits used for signaling.
Figure 21-4 shows a single-channel sample, with one bit used for signaling. This is called in-band signaling.
Figure 21-4. One-channel sample
When a T1 is configured as a PRI, all eight bits in each channel may be used for data, because one entire channel is reserved for signaling (as opposed to pulling one bit from each channel). This reduces the number of usable channels from 24 to 23, and is called out-of-band signaling.
T1s use time-division multiplexing, which means that each channel is actually a group of serial binary values. The channels are relayed in order, but the receiving equipment needs to know when the first channel starts and when the last channel ends. The way this is done is called framing.
In standard voice framing, called D4 or superframe, each eight-bit sample is relayed from each channel in order. One sample from channel one is relayed, then one sample from channel two is relayed, and so on, until all 24 channels have relayed one sample. The process then repeats.
For the receiving end to understand which channel is which, framing bits are added after each of the 24 channels has relayed one sample. Because each sample is 8 bits, and there are 24 channels, one iteration for all channels is 192 bits. ...