60.5 64Kbit/s facsimile equipment

The availability of switched digital networks with 64kbit/s channels continues to increase with basic rate and primary rate ISDN. The I Series recommendations developed by CCITT have established international standards for 64kbit/s intercommunication.

The G3 transmissions use the voice speech band which, when converted to PCM digital signals for transmission within the public network, use the complete 64kbit/s channel. This means 9.6kbit/s or 14.4kbit/s maximum rate on a 64kbit/s channel.

Clearly where 64kbit/s digital access is available, as through the ISDN connection, a direct digital input at 64kbit/s machine data rate is desirable giving an improvement of four to six times in speed and reduction in transmission cost. To take advantage of this a 64kbit/s data rate facsimile machine is required.

60.5.1 G4 equipment

CCITT has produced recommendations for G4 digital equipment in 3 classes based on the ISO 7 layer model for interoperation with open systems communication standards.

Class 1 is for the transmission of facsimile documents only and is a higher performance equivalent to G3. Class 2 has the transmission capabilities of Class 1 and the receive capabilities of class 3. It is similar in operation to class 1 but can take advantage of the in­creased efficiency of character transmission where available.

Class 3 provides for the transmission of characters and facsimile data, either separately or combined in a mixed mode format. This is a more complex terminal giving the possibility of keyboard and scanner input with the document architecture to support interoper­ation with ISO hierarchical models.

The requirements of Classes 2 and 3 will continue to develop as necessary to track the evolution of document architecture standards. Class 1 machines are already being implemented to provide higher performance facsimile with direct digital access to the ISDN net­work, 3 seconds/page transmission and higher resolutions operation to 400 x 400 pels/inch.

60.5.2 64Kbit/s g3 type equipment

With the tremendous growth in G3 an enhancement is being dis­cussed in CCITT to provide a similar operation at the faster digital interface rate of 64kbit/s, to that recommended on the analogue network interface at between 14.4kbit/s and 2.4kbit/s.

This will simplify implementation and provide the user with current G3 facilities at the faster and more efficient digital interface rate. G3-64kbit/s is proposed as an option to the basic G3 mode which is essential for interworking with the existing G3 machine population. In this machine the basic G3 operation is likely to be provided on the I series port by the implementation of the equivalent of a codec converting the G3 modem signals to the ISDN speech equivalent.

1 Learn the words & word combinations:

driver	привод, рычаг
film	пленка
plain	простой
fluid	жидкий
capability	возможность
rapidly	быстро
a series of	ряд …
attempt	пытаться
intend	намереваться
contain	содержать
remain	оставаться