Excavating
Patent
1981-01-15
1983-06-28
Smith, Jerry
Excavating
371 53, G06F 1110
Patent
active
043909892
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a method and an arrangement when transmitting data between a transmitting computer, which sends signal words containing start-stop information, message words and address- and control signal information, and a receiving computer, to provide separate, supervision, of faults occurring on the transmission lines and of such errors which are caused by faults in the signalling equipment associated with the computer on the receiver side.
DISCUSSION OF THE PRIOR ART
There is a method described in the INTERNATIONAL STANDARD/ISO 3309-1976 (E) for monitoring transmission lines with the help of a so-called frame checking sequence (FCS) or outer check sum. The majority of known modern systems within this field of technology apply monitoring by means of check sums according to the standardized method, or some form of parity check. parity check and check sum can also be combined.
A disadvantage as well as a problem with the known and standardized method is that it only monitors the transmission line (including repeaters). Apart from start-stop bits, the format for a complete message also includes the message transmission portion or header, message words and check sums. The header contains addresses and control signals, e.g. acknowledgement signals. In each node or terminal in a transmission path between two computers, the contents of the header is altered. The information in the header will thus be somewhat different on the input from what it is on the output of a terminal. The outer check sum is calculated with regard to the contents in the header and the message words, from which it is apparent that the check sum is recalculated in each terminal before a new transmission. If a fault occurs in the terminal equipment, giving rise to an error in the data message, an incorrect check sum is obtained from the terminal output. The receiving station, however, accepts the message as correct since it makes the calculation on the same data as the transmitting station. The receiving station can thus accept an incorrect message since the method does not allow supervision of the terminal equipment itself.
SUMMARY OF THE INVENTION
The method and the arrangement in accordance with the invention, which solves the problem constitutes an arrangement for generating and checking two check sums which complement, each other: an inner check sum S for monitoring the hardware included in the computers and intermediate stations on a transmission path, and an outer check sum YS for monitoring the transmission lines. The inner check sum S is formed by adding all the data words in a message. The information in the message header is not included in the inner check sum S. In every terminal or node between a transmitting computer and a receiving computer a check is made of the correctness of the check sum S, and at each point where an error is detected, retransmission or change-over to a reserve link can be initiated. Since the check sum S is only checked and not recalculated in the terminals, i.e. since the same check sum accompanies the message the whole way between transmitting and receiving computer, detection of the errors caused by faults in the terminal equipment is also enabled. The outer check sum YS is formed by adding all the data words in a message, the information in the header and the inner check sum S.
By utilizing two check sums there is obtained a combined monitoring effect, enabling the circuits for generating and checking the outer check sum YS to be made just as simple as the circuits for generating and checking the inner check sum S, without deterioration of the error detection probability on the transmission lines.
The advantage with the arrangement in accordance with the invention in relation to the state of the art is thus that by combination of two check sums there is obtained supervision which is not limited to the transmission lines, but all the hardware in a transmission path between two computers is completely monitored in a mode which is economically advantageous and t
REFERENCES:
patent: 3573726 (1971-04-01), Towell et al.
patent: 3753225 (1973-08-01), Liddell
patent: 3967250 (1976-06-01), Senda et al.
patent: 4112414 (1978-09-01), Iscol et al.
patent: 4208650 (1980-06-01), Horn
Smith Jerry
Telefonaktiebolaget L M Ericsson
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