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 recommended modem for transmitting data at 9600 bits per second on leased circuits are as follows: a) fallback rates of 7200 and 4800 bits per second; b) capable of operating in a duplex or half-duplex mode with continuous or controlled carrier; c) combined amplitude and phase modulation with synchronous mode of operation; d) inclusion of an automatic adaptive equalizer; e) optional inclusion of a multiplexer for combining data rates of 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 network@ should be points for further study. The types of special quality circuits@ e.g. M.1020 [1] or M.1025 [2] should be studied. Note 2 - The values of some circuit characteristics@ for example@ noise and nonlinear distortion@ are subject to further study. Note 3 - When the multiplexer option is installed@ provisions in ??2 may supersede provisions given in other sections. Note 4 - 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.