ZyXEL Elite 2864 User Guide - Page 150

A cellular phone may be instructed to change its transmission power depending on its distance from the cell site station. The radio link will be interrupted for about 0.2 second. An effect similar to cell hand-off will occur. A particularly difficult cellular impairment for data communication is called multipath fading. A cellular phone receives the cell site station's radio signal through many indirectly reflected paths. Because of the phase difference of the arriving signals caused by the signals taking different paths the combined signal, depending on location, may be strong, weak or totally faded. A moving cellular phone will experience a periodical signal weakness and this is called multipath fading. Fading will cause data errors because the carrier is lost due to fading. The analog cellular phone system was designed for voice communications. It employs companding (concatenated from compressing and expanding; it makes small signals better at the sacrifice of large signals) and pre-emphasis which are good for voice, but are not suitable for a modem signal. Modem signals, if too strong, may saturate the cellular link and cause distortion. Modem signal power that is too weak will result in a lower signal-to-noise ratio at the receiving end. There is an optimum transmission power in cellular data communications, but it is different in each case. Cellular Modems and ZyCellular Technology A truely cellular modem must handle the cellular impairments and maintain a reliable data link with reasonable data throughput through a cellular channel. Some modems today claiming to be "cellular" lack most of these capabilities. A link-layer-only protocol does not do anything to enhance modem data pump performance or physical communication robustness. It does not help much to handle cellular impairments. ZyXEL developed special technologies for cellular data communications to provide reliable, high throughput data links over a cellular channel. We called these technologies ZyCELL technologies. A ZyXEL modem utilizing ZyCELL technologies is called a ZyCellular modem. ZyCELL technologies specifically improve the modem data pump's performance with respect to cellular impairments. 14-2