 is called a T-1 circuit. The T-1 has the capability for 24 separate outside lines, all delivered on two pairs of wires (sometimes called a four-wire circuit). In order to work, the T-1 needs a multiplexer at either end to separately identify the 24 conversations, break them 218 The Telecom Handbook CHAPTER 10 down for sending and put them back together at the other end. Sometimes the multiplexer is external to the telephone system. The most common type of external multiplexer for voice commu- nications is called a channel bank. Most PBXs also have the capability for a T-1 circuit board which is a multiplexer inside the PBX. If you have a T-1 circuit board, there is no need for a channel bank. Use of a channel bank is becoming less common, although many are still in use. You may have different types of trunks combined on a T-1. For example you can have 12 combination trunks and 12 DID trunks. Digital services like T-1 have evolved based upon the digital signal known as DSO (digital signal zero) which is 64 Kbps. This is the bandwidth for a voice grade circuit. DS1 (also known as a T-1) is 1.544 Mbps, which is twenty-four 64 Kbps channels. There is also a T-3, which is 44.73 Mbps (the equivalent of 28 T-1s). On a T-1, signals are multiplexed SO that two pairs of wire can carry 24 separate voice conversations. The same two pairs of wires would carry only two voice conversations using analog technology. With each conversation using 64 Kbps and another 8 Kbps being used for control signals, the total T-1 capacity is 1.544 Mbps (million bits per second). (24 X 64kbps = 1536kbps + 8kbps = 1544kbps or 1.544Mbps). You may also hear the term Fractional T-1 which means that you are getting a circuit with some fraction or increment of the total T-1 capacity. For example, the circuit may provide only twelve 64 Kbps channels. Actually, the entire T-1 is in place, but some of the channels are turned off in the multiplexers. T-1 Circuit Applications for PBXs T-1 to the Local Telephone Company Central Office (Figure 10.6A & 10.6