tion, the normal sine wave is used to repre- sent the one bit and a sine wave that is 180 degrees out of phase, the mirror image of the original wave, is used to rep- resent the zero bit. Phase modulation is most common in high speed modems. There are more complex modulation schemes that com- bine aspects of the above three and may include error-correct- ing capabilities. The following lists some features and options for modems enabling some flexibility. As with most telecommunications equipment, optional features with some manufacturers may be standard with others. Multi-port capability means that more than one outside tele- phone line can be handled by the modem at the same time. Dial back-up allows you to switch from your dedicated line to a dial line or lines in the event that the dedicated line fails. Voice and data capability allows you to alternate voice trans- mission with data transmission on the same line. The Telecom Handbook 65 239 TRANSMISSION Self-testing diagnostics perform local and remote testing to determine if there is a problem with either the line or the modem. Autocall automatically places calls for a modem on the dial network to eliminate operator intervention. Auto-answering automatically answers incoming calls to the modem. An adaptive rate system built into the modem allows it to continuously sense varying line conditions and adjust accord- ing to the highest possible speed. Echo suppression is an advanced technique that minimizes echoes and the resulting distortions for 2-wire dial line trans- missions over short and long distances at speeds. Phase roll compensation is a feature that automatically ad- justs for frequency differences between long distance carriers. A modem substitution switch is an external option that allows you to reroute your data through a spare modem that is stand- ing by powered up in the event that the other modem fails. An asynchronous-to-synchronous converter permits an asyn- chronous modem to operate with a synchronous modem. Modem strapping options enable the 