ition to IP as a protocol for sending the voice signals from one point to an- other in a digital form. The same transition is being made as the voice is sent out over the telecommunications network to the person at the other end as telecommunications service provides change over to IP for transmitting both voice and data. Your Voice Moving through the PBX Once the digitized voice arrives from the telephone into the PBX through the cable within the walls, it remains in the digital form as it moves through the PBX on what is called the digital The Telecom Handbook * 31 THE PBX Pulse Code Modulation (PCM) This is the most common method of encoding an ana- log voice signal into a digital form. First, the amplitude of the voice conversation is sampled. This is called Pulse Amplitude Modulation. This PAM is then coded into a binary number (combination of ones and zeroes). The voice signal can then be transmitted, switched and stored digitally. There are three main benefits to PCM: (1) it is less expensive to transmit a digital signal than an analog signal, (2) by turning an analog signal into a digital sig- nal, it can be interleaved with other digital transmissions such as computer and fax transmissions and (3) a voice signal that is transmitted end to end in a digital form comes through more clearly with less noise than one which is transmitted and switched in the analog form. PCM refers to a technique of digitizing voice and is not a universally accepted standard. The most common PCM method is to sample a voice conversation 8,000 times per second. The theory behind this is if the sam- pling is at least twice the highest frequency on the channel then the result sounds okay. PCM is used to transmit 24 voice conversations over a T-1 circuit. It places 24 voice conversations on two pairs of copper wires. It contains 8,000 frames (logical units of data also called packets) each of 8 bits of 24 voice channels plus one framing bit which equals 1.544 megabits per second, the speed of a T-1. <8000 X (8x24+1) = 1.544 megabits