G3 handshake protocol

InkJet

Ink is a simple way to print on plain paper. The problem has been to keep the ink fluid and flowing until printed and then for it to dry rapidly. This technique is being used in facsimile equipment with a replaceable bubble jet ink cartridge. The cartridge is moved across the paper to print several facsimile lines (typically 16 fine mode lines) at once and the ink bubbled appropriately through a series of apertures, one for each facsimile line. The hardware is not compli­cated or bulky and a range of low cost plain paper machines may emerge for use at about 30 seconds for an A4 page.

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.

Дата добавления: 2015-03-31