ITU-T - International Telecommunication Union/ITU Telcommunication Sector
适用范围
Introduction This modem is intended to be used primarily on special quality leased circuits@ e.g. Recommendation M.1020 [1] or M.1025 [2] circuits but this does not preclude the use of this modem over circuits of lower quality at the discretion of the administration concerned (see Notes 1 and 2). On leased circuits@ considering that there exist and will come into being many modems with features designed to meet the requirements of the administrations and users@ this Recommendation in no way restricts the use of any other modems. The principal characteristics of this modem are as follows: a) fallback rate of 12 000 bits per second; b) capable of operating in a duplex mode with continuous carrier; c) combined amplitude and phase modulation with synchronous mode of operation; d) inclusion of an eight state trellis coded modulation; e) optional inclusion of a multiplexer for combining data rates of 12 000@ 9600@ 7200@ 4800 and 2400 bits per second (see Note 3). Note 1 - The principal use of this recommended modem is on 4-wire leased circuits. Other applications@ such as stand-by operation on the switched telephone network@ half duplex or multipoint operation are for further study. Circuits should be of the special quality type@ e.g. M.1020 [1] or M.1025 [2]. However@ administrations and users may wish to note that modems conforming to this Recommendation@ even assuming proper implementations@ will not necessarily operate satisfactorily on all circuits conforming to M.1020 and M.1025; particularly where noise is at or near the specified limiting magnitude. Note 2 - Attention should be given to the selection of appropriate equalization techniques in the modem implementation@ if acceptable performance on circuits conforming to Recommendation M.1025 is desired. Note 3 - When the multiplexer option is installed@ provisions in section 10 may supersede provisions given in other sections.