 inter- val. This is because we have to allow time for the pulse to return to the zero (voltage) condition. Bi-phase-L or Manchester coding (Figure 10.8e) is a code format that is being used ever more widely on digital systems such as on certain local area networks (LANs) (see Chapter 11). Here binary information is carried in the transition. By convention, a logic 0 is defined as a positive-going transition and a logic 1 as a negative-going transition. Note that Manchester coding has a signal transition in the middle of each unit interval (bit). Manchester coding is a form of phase coding. 10.8 DATA INTERFACE: THE PHYSICAL LAYER When we wish to transmit data over a conventional analog network, the electrical rep- resentation of the data signal is essentially direct current (i.e., has a frequency of 0 Hz). As such it is incompatible with that network that accepts information channels in the band 300-3400 Hz. A data modem is a device that brings about this compatibility. It translates the electrical data signal into a modulated frequency tone in the range of 300-3400 Hz. In most cases with modern modems that tone is 1800 Hz. Also, more often than not, the digital network (as described in Chapter 6) has extensions that are analog and require the same type(s) of modem. If the digital network is extended to a user's premise, a digital conditioning device (CSU/DSU) is required for bit rate and waveform compatibility. For this discussion of data interface, we will call the modem or digital conditioning device data communication equipment (DCE). This equipment has two interfaces, one on each side as shown in Figure 10.9. The first, which is discussed in this section, is on the user side, which is called data terminating equipment (DTE), and the applicable interface is the DTE-DCE interface. The second interface is on the line side, which is covered in Section 10.9. It should be noted that the DTE-DCE interface is well-defined. This is the conventional CCITT voice channel. DATA COMMUNICATIONS Data Communications Line Sid