 and the hard- ware to make the medium useful, what else is there to think about relating to transmission? Bandwidth Two things to consider when sending communications sig- nals is how much information can be sent at the same time, and how fast can it get there. Bandwidth refers to the capacity of a circuit for carrying communications signals. Bandwidth has come to mean not width, but rather the speed at which information travels over a circuit. For example, you may hear someone say that the bandwidth of a circuit is 56 Kbps (kilobits per second). True bandwidth refers to the range or speed of frequencies that can be carried by a circuit. The hardware at each end and in the middle determines the number of separate frequencies which can be transmitted simultaneously on one circuit. The capacity of a particular circuit and the speed with which communications signals can travel over that circuit are a func- tion of both the bandwidth of the circuit and the capability of the hardware used at both ends and in the middle. Just to clarify, if a circuit has a higher speed (higher band- width) the signals do not actually move faster. Its just that more information can travel over the circuit within a given time interval. Simplex VS. Duplex Another basic concept relating to circuits is whether they are simplex, half duplex or full duplex. On a simplex circuit, 244 35 The Telecom Handbook CHAPTER 11 information travels in one direction only, such as a radio broad- cast. On a half duplex circuit, the information travels in both directions, but only one way at a time such as with a CB radio. You can either talk or listen. A full duplex circuit enables simul- taneous two way transmission such as with a telephone conversation where you may talk and listen at the same time. Protocols Protocols are the rules, procedures or conventions relating to the format and timing of communications transmissions between two devices. Both devices must use and accept the same proto- cols in order to understand each other. Analog VS. Digital Let