60.4.6 G3 handshake protocol

CCITT G3 uses signals generated in an HDLC frame structure exchanged in half duplex mode. A preamble of a series of flags, starting each transmission ensures that valuable data is preserved when truncation occurs at echo suppressor turnaround. The flag octet 01111110 is a unique signal that is preserved by a' 10' insertion after a fifth ' 1' occurring in the data. A 2-byte FCS is used to allow the receiver to check data validity.

Figure 60.6 gives the T.30 G3 handshake signal format showing DIS.

60.4.6.1 G3 handshake basic operation

This is usually started by a call originated from the transmit ma­chine. However, the significant signal flow is initiated when the receiver answers the ringing current by sending a DIS to indicate its capabilities. These includes the paper size, coding schemes and modem rates it has available.

The transmit machine selects appropriate conditions in conjunc­tion with document size and operator selections, commands the receiver using DCS and sends the modem training signal and TCF at the high speed data rate.

The receiver attempts to train, checks the performance on TCF and replies either CFR or FTT. The transmitter then either starts the image transmission or sends DCS and training again.

Figure 60.7 shows a typical G3 signal flow chart, and Table 60.8 gives popular G3 handshake signals.

When the transmitter indicates the end of page the receiver replies with MCF for a correctly received page, or one of the retrain signals indicating that it requires retraining for any further pages.

If the transmitter signalled MPS, advising more pages to be sent, it continues with the following pages, finishing with EOP and DCN in response to MCF before releasing the connection.

60.4.6.2 G3 machine identification

Identification of terminals is a useful feature in G3 equipment with several popular methods implemented.

CCITT CSI/CIG/TSI signals

These are optional T.30 signals completely defined to allow inter-operation between different manufacturers' machines. They are intended by CCITT to contain a '+' sign followed by up to 20 numeric digits giving the subscriber's international telephone num­ber. In most implementations the user can insert/update the number freely. The way the machine receiving the number uses it varies with manufacturer; it may be reported on the machine display, listed in an activity report or printed by the receive machine on the received copy.

Messages on the transmitted facsimile page

Numeric, alphanumeric or graphical messages held in a transmit memory and sent as part of the facsimile transmission. They are not sent in T.30 format. Because they are facsimile encoded the receive machine remains ignorant of the content and cannot record them in the reports. The information may contain:

1. Document number, time and date of sending.

2. Sender's name or TSI.

3. Urgent or priority comment.

Although the implementation may be proprietary to the manufac­ture it is printed on all receivers because it is included in the facsimile message. It is often transmitted outside the regular A4 page increasing the received page length.

CCITT NSF signals

Under the non standard facilities procedure a manufacturer may implement any proprietary identification form desired. When trans­mitting to another machine of the same manufacturer it may include alpha as well as numeric characters. This can enable the name of the communicating machine to be shown on the display or recorded in the activity report. It is not recognised between machines of differ­ent manufacturers.

NSF covers a wide range of proprietary features but in the ident­ification area it also allows the transmission of passwords for confidential operation.

Received copy with typical identification

In addition to the text at the top (date, time and sender identification) transmitted as part of the facsimile image, other information, com­piled at the receiver from the transmitted TSI/NSS and from locally available information in the receiver, may be added.

The word 'END' is sometimes inserted at the bottom of the facsimile image to confirm that this is the last page of the document.