l most closely matches and decodes that state to provide the digital bit stream. Reference line Coding line Direction of coding Figure 60.3 Modified READ coding. (See Table 60.6) G3 facsimile equipment 60/9 Table 60.7 Comparison of G3 modulation methods Bit rate (kbit/s) Carrier (Hz) Modulation Baud rate Bits/symbol States in (symbols/s) constellation V.21 channel 2 FM 100Hz V.27 ter (Trellis coded) To establish the symbol timing, and allow the receive modem to means that only certain decoded sequences are valid. The receiver set equaliser conditions to compensate for the network impairments, knows from the recent signal history when an unlikely condition an initial training signal is sent from the transmit modem. In G3 this occurs and can make an educated guess at the correct data. It is this is followed by a 1.5s period all '0' TCF, so that the machine can difference in operation which gives a 3dB signal/noise improve- decide if it can correctly receive at this rate. If not the 300bit/s ment in Gaussian noise tests, and noticeable end to end improve- handshake reply initiates a new training sequence, which the trans- ment over QAM in actual use. mitter may choose to send at a lower rate. With ECM it is often found desirable to operate at the higher data rates, accepting that a limited number of errors which can be rapidly corrected may pro- 60.4.4 Error correction mode vide an overall increase in transmission efficiency. In CCITT ECM the transmitted signal is separated into HDLC frames so that on receive the FCS bytes allow the data accuracy to 60.4.3.3 14.4kbit/s option be checked. G3 transmission is half duplex and when the trans- mission stops the receiver replies giving the condition for each of The CCITT option at 14.4kbit/s uses V.17 TCM (Trellis Coded the frames. The transmitter then selectively repeats the frames Modulation). The symbol rate is the same as V.29 9.6kbit/s which which the receiver advised were in error. appears optimum for the telephone network, but the bits per symbol increase not to 6 a