modem to switch from half to full duplex operation. (Full duplex means transmis- sion can be taking place in both directions simultaneously. Half duplex means you can either send or receive, but not do both at the same time.) Most of today's modems have speeds that are higher than you can typically achieve on a standard telephone line. Speed limitations often have more to do with the quality of the tele- communications circuit than with the modem itself. 240 < The Telecom Handbook CHAPTER 11 When purchasing a modem, it is useful to know the volume of data you are planning to transmit. Volume is both a function of the characters per transaction and the number of transactions per day required to support your application. The volume of infor- mation not only determines the speed of transmission but whether you should use dedicated lines (enabling higher speeds) or dial up lines. Here are a few other questions to ask when you are selecting a modem: Will the modem be compatible with your devices at each end? Will new modems be able to communicate with existing ones? Will the modem be used on a dedicated line (a fixed line con- necting two points) or a dial line (a line with a dial tone requiring you to dial a telephone number to reach the distant end)? Will the modem be used in a point-to-point or multi-point configuration? What kinds of diagnostic capabilities are needed? Does the modem have network control capability to keep the network running despite problems? Multiplexers If you want multiple devices to be transmitting simultaneously or if you want to mix synchronous and asynchronous transmis- sions, consider installing multiplexers in front of the modems. The multiplexer enables two or more signals to be transmitted "simultaneously" on the same circuit. Just to be clear about the multiplexed transmissions, they are SO fast that they do appear to be simultaneous. However, the data being transmitted is still moving across the line sequentially. The The Telecom Handbook $ 241 TRANSMISSION multiplexer breaks each tr