The Federal Standard entitled Time and Frequency Information in Telecommunication Systems establishes the requirements for telecommunications facilities and systems of the Federal government for obtaining Coordinated Universal Time (UTC) coordinated time and frequency reference information based on national time scales maintained by the National Institute of Standards and Technology (NIST) and the US naval Observatory (USNO). The time scales of NIST and USNO are steered to the international UTC time scale maintained by the Bureau International des Poids et Mesures (BIPM) in Paris, France. NIST and USNO also maintain atomic clocks which contribute to the UTC average. The values of UTC promulgated by NIST and USNO are used by the United States Federal Government to obtain standard values of time.>

The CCITT (Consultive Committee for International Telephone and Telegraph) is developing recommendations for a new generation of facsimile equipment which is designed as Group 4. This class of equipment will transmit an ISO A4-sized page overcommunications networks having error control. Most commercial packet switched and circuit data networks have been designed primarily for the communications of short bursty messages (typically 1000-2000 bits/message between computers and data terminals. The length of a G4 message is forecast to be very long-typically 500 000 bits. There is serious concern that data networks may not handle facsimile traffic very efficiently. This paper projects the near term characteristics of data networks and Group 4 facsimile systems, and estimates the efficiency with which Group 4 messages will be transmitted over three types of data networkspacket switching (PSDN), circuit switched (CSDN), and the public switched telephone network (PSTN). Throughput has been measured by the amount of time required to send a single facsimile page of 500 000 coded bits. In all cases the overhead, as a percentage of the basic facsimile transmission time, is in the range of 50-60 percent. For each network there is a different factor is the halts are forced is transmission as a result of the network window. For the CSDN, the process if converting from voice to data mode is the dominating factor. For the PSTN, packet retransmission due to transmission errors is the dominating factor. The paper includes the assumptions and some of the analytical details of the throughput analysis. Conclusions are drawn regarding the relative transmission efficints through the three types of networks.

The CCITT (Consultative Committee for International Telephone and Telegraph) has defined Group 4 facsimile apparatus as that which transmits an ISO A4 (210 × 297 mm) document over public data networks including Packet Switched Data Networks (PSDN) and Circuit Switched Data Networks (CSDN). Data compression is employed in Group 4 systems to minimize the number of bits required for transmission of a message. CCITT Recommendation T.6 defines the basic Group 4 coding scheme to be a variation of the modified READ code (MRC II) which is the Group 3 coding option. This paper measures the compression of the MRC II algorithm for three different documents at resolutions of 200, 240, 300, 400, and 480 picture elements per inch. The compression measurement was accomplished by means of computer simulation. It was determined that the bits per page measure increases linearly with resolution. Results are presented and conclusions drawn. The Group 4 CCITT Recommendations include a mode of operation known as Mixed Mode where a page is divided into two parts: character-coded information and facsimile-coded information. This paper describes four alternative mixed-mode algorithms and measures the compression of each coding technique for CCITT document number 1. The compression measurements were performed by means of computer simulation. Conclusions are drawn regarding the relative advantages of the four algorithms.

The CCITT has defined Group 3 facsimile apparatus as that which digitally transmits an ISO A4 document over a switched telephone circuit in approximately one minute. Data compression is employed to achieve the reduced transmission time. Study Group XIV of the CCITT has drafted Recommendation T.4 to achieve compatibility between Group 3 facsimile devices. The standard data compression technique specified by T.4 is a one-dimensional coding scheme in which runlengths are encoded using a modified Huffman code (MHC). The recommendation also includes an optional twodimensional compression technique known as the modified READ code (MRC). It is recognized that the switched telephone network is prone to error when transmitting digital data at the standard T.4 data rate of 4800 bits/s. This paper evaluates the error sensitivity of the MHC and MRC when operating over a typical telephone circuit. The error sensitivity analysis is accomplished by means of computer simulation. The error performance of the two coding techniques is analyzed both quantitatively and qualitatively. The quantitative analysis is accomplished using the error sensitivity factor which represents the average number of incorrect pels in the output document caused by a transmission error. The qualitative analysis is based upon viewing actual error-contaminated images generated in the simulation process. Two separate analyses have been performed. First, error sensitivity data for both the MHC and MRC are developed under identical operational conditions, and their relative performance is discussed. In the second part four different techniques for processing the received facsimile signal (MRC), to minimize the subjective effect of transmission errors, are analyzed.