switches or PBXs, but it is uneco- analog-to-digital conversion, bonding and grounding for nomical when the number of connections exceeds six to safety, power, and a variety of other functions when eight circuits. At that point, it is usually more efficient to needed. Analog voice interfaces have evolved over the use digital trunks. In North America, the T1 years to provide for these basic functions while (1.544 Mbps) trunk speed is used, consisting of 24 digi- addressing specific applications. Because basic POTS tized analog voice conversations. In other parts of the two-wire analog interfaces operate in a master/slave world, El (2.048 Mbps) is used to carry 30 voice channels. model, data equipment must emulate two basic types of (Engineers refer to the adoption of El and T1 in- analog interfaces: the user side and the network side. The ternationally as "the baseball ule"-there is a strong user side (telephone) expects to receive power from the correlation of countries that play baseball to the use of T1. network as well as supervision. Therefore, the United States, Canada, and Japan have the A foreign exchange service (FXS) interface is used to largest T1 networks, while other countries use E1.) connect an analog telephone, fax machine, modem, or The first step in conversion to digital is sampling. The any other device that would be connected to a phone Nyquist theorem states that the sampling frequency line. It outputs 48 vdc power, ringing, and SO on, and it should be twice the rate of the highest desired fre- accepts dialed digits. The opposite of an FXS interface is quency. Early telephony engineers decided that a range a foreign exchange office (FXO) interface. It is used to of 4000 Hz would be sufficient to capture human voices connect to a switching system providing services and (which matches the performance of long analog loops). supervision and, at the other end, it expects the switch to Therefore, voice channels are sampled at a rate of 8000 provide supervision and other elements. (Why "foreign"? tim