ITU-T V.33 PDF

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.