varied. A direct current can be produced from an energy source such as a dry battery. Dibits Phase A steady direct current flowing in a circuit cannot convey infor- mation. The inclusion of a simple on-off switch enables the current to be regulated into a series of pulses. These pulses represent symbols which have an agreed meaning at the transmitting and receiving ends of the link. A series of the symbols can be concaten- ated to form a message. If the connections to the direct current source are reversed. then the direction of the current is also reversed. The current direction can therefore be considered to be positive or negative according to the way it is flowing around the circuit, and this is known as the CCITT V.29 uses 4 bits per baud which are carried using 8 polarity of the circuit. different phase changes, 45 degree apart, and two different ampli- tude changes. There are two principal methods for creating signals with direct current sources. Firstly the information may be carried by the alternate presence and absence of current or, secondly, by the switching of the direct current sources between two distinct values. Waveform and frequency The feature common to both of these methods is the variation in the amplitude of the current. There are a wide variety of waveforms that are possible with A typical use of direct current signal, using the alternate presence alternating currents. One of the simplest to produce, a sinusoidal and absence of a pulse, is between a telephone and the local waveform, shown in Figure 10.1, is created by rotating a loop of exchange. The state of the signal indicates on-hook, off-hook, dial wire in a uniform magnetic field. pulses, or the status of the connection. On-hook is indicated by an open circuit and no current flow. Off-hook is indicated by a closed Periodic time secs Periodic time secs circuit and continuous current flow. Dial pulses consist of current One cycle One cycle flow interrupted at a specified rate. There are various difficulties associated with direct curren