th one of the bits used for signaling. Figure 20-4 shows a single-channel sample, with one bit used for signaling. This is called in-band signaling. One channel sample = 8 bits 8,000 samples/sec = 64 kbps 11111111 One bit is taken for signaling Sample now =7 bits 8,000 samples/sec = 56 kbps Signaling bit Figure 20-4. One-channel sample Chapter 20: T1 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 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. D4/Superframe In standard voice framing, called D4 or superframe, each 8-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. With the addi- tion of the framing bit, we have 193 bits. These 193-bit chunks are called frames. Each set of 12 frames is called a superframe. The framing scheme is outlined in Figure 20-5. The T1 devices keep track of the frames by inserting the pattern 110111001000 into the framing bits over the span of a superframe. 11111111 One channel sample = 8 bits Phone audio is 8khz 8,000 samples/second Channel 1 Channel 2 Channel 3 Channel 4 Channel 5 Channel 23 Channel 24 1111111111111111 Framing bit 24 channels 192 bits Frame (24 8-bit channels + 1 framing bit)= 193 bits Frame 1 Frame 2 Fr