ZyXEL Elite 2864 User Guide - Page 35

Distinctive Ring This is a service available from some phone companies in some areas of the USA and elsewhere. You can have several phone numbers on the same subscriber phone line. The phone company sends a different ring signal for each phone number assigned to the same line. You can have your phone equipment answer only a certain type of ring corresponding to a certain phone number. ZyXEL modems can be set to answer on one or a combination of four types of ring signals. The problem is that every country may have a different specification for different ring types. Digitized Voice Modems with digitized voice capability can digitize an incoming voice message with the computer storing it as a file. The modem can also playback a recorded digital voice message either locally or to the line as an announcement. Digital voice messages need speech compression to reduce the storage requirements. This speech compression is done in real time by the modem and it does the decompression during playback. A simple voice capability using an ADPCM (Adaptive Differential Pulse Code Modulation) algorithm can reduce the speech data rate to 1/2 or 1/3 of the original rate. An advanced algorithm is needed to reduce the speech data rate even further. Cellular Modem A cellular modem is a data modem that can be used on the analog cellular mobile phone system. With the rising popularity of notebook computers this is of increasing importance for people on the move. There are two kinds of phenomena in the cellular mobile phone system that are particularly bad for modem data communications. One is called cell hand-off, meaning that a cellular phone is instructed to change the radio link from one cell site to another when travelling across a site boundary. The radio link will be temporarily interrupted and most modems will retrain or even hang up. When a cellular phone is instructed by the cell site to change transmitting power, it will cause similar problems. The other particularly annoying phenomenon is multipath fading. Here, a moving cellular phone will experience periodical fading or drop-out of the radio signal. This causes high data error rates and low throughput. ZyXEL developed cellular modes using its proprietary data pump that handles the cell hand-off and multipath problems. The V.42 link layer protocol with selective reject greatly improves data file transfer throughput in high error situations. 2-9