 G3 is a half duplex transmission which uses 2 types of modula- 010011011 0000001100101 tion/demodulation; the first of 300bit/s for the T.30 protocol and 000000000001 000000000001 handshaking between terminals. This 300bit/s V.21 modem is a robust FSK transmission. It is easily implemented and does not require training. It is used for the relatively low volume of data establishing machine parameters and providing feedback from re- Table 60.6 Modified READ coding. (See Figure 60.3) ceiver to transmitter. The second is the high speed modem used for the compressed image data transfer. In G3 there is a basic V.27 mode of 4.8kbit/s with 2.4kbit/s fallback where the lines are poor, and optional modes Points up to 9.6kbit/s and up to 14.4kbit/s for use where terminals and Vertical T, & T. 2 Up to +3 7 code words networks permit. It is generally acknowledged that over 80% of (VO)(V1) changing allocated transmissions used these higher rates. elements (1 to 7 bits) Table 60.7 compares the G3 modulation methods, where QAM offset coded refers to quadrature amplitude modulation. otherwise use horizontal 60.4.3.2 Modem operation Pass (P) T 4 & T5 When no Single code accompanying allocated (0001) The data rate modem takes a number of bits of the incoming data changes on (e.g. 4 bits for V.29 9.6kbit/s), and for the symbol time (1/2400 coding line second for V.29 9.6kbit/s) generates the relevant modulation condi- tion for these bits (16 conditions for V.29 9.6kbit/s). Horizontal T6 & T7 When no Single identifying The modulation condition may be generated by a controlled reference for code (001) combination of sine and cosine modulated carriers which are then the change on followed by normal filtered before application to the network to limit damaging out of coding line MH black and band signals. The receiving modem tracks the carrier to correct for white run pair network frequency shift and recovers the symbol timing. It then studies the signal during this symbol period to decide which of the possible states the received signa