Cryptography – Key management – Having particular key generator
Patent
1992-12-03
1994-07-26
Cangialosi, Salvatore
Cryptography
Key management
Having particular key generator
380 50, H04L 900
Patent
active
053331974
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a communications system, and in particular, though not exclusively, to a communication system incorporating a satellite link.
2. Related Art
In communications systems such as those having a satellite link, data is sent in a synchronous block mode to keep down the overhead of control and signalling information. Such a synchronous link is maintained in the absence of user data by transmitting a null character e.g. ASCII value 00H. A problem arises because the user data may contain the null character, and unless action is taken the receiving part of the synchronous link will discard a null in the user data as if it were not data. To indicate such characters to the receiving part of the link it is known to send "Escape" control sequences of characters but these increase the control overhead of a link sharply and are liable to cause significant errors if one of the characters of a sequence is lost in transmission. This is a particular example of the more general problem of transmitting data, over whatever medium, when that data includes so called reserved characters or characters which in the circumstances the receiving part of a link will not recognise as proper data.
SUMMARY OF THE INVENTION
According to the invention there is provided a communications system for handling a stream of data characters comprising:
a transmitting part having: predetermined set of characters, means to repeatedly encode an incoming data character until an encoded character occurs which is not one of the predetermined set of characters, and character is one of the predetermined set,
a character from the predetermined set; and
a receiving part having: predetermined set of characters, provided from the receiver when the corresponding received character is determined not to be one of the set of characters.
According to a second aspect of the invention there is provided a data transmission system in which one or more characters are reserved for system use comprising an encoder, a transmitter, a receiver and a decoder; a data stream entering the system is coded by the encoder a character at a time and a test for the reserved character is performed on each encoded character, if this is positive the character is encoded again, all encoded characters are transmitted by the transmitter. After the transmitted characters have been received in the receiver the test for the reserved character is performed on each received character, if this is negative the received character is decoded by the decoder and passed out of the system as data.
According to a third aspect of the invention there is provided a communication system in which data characters are each repeatedly encoded in an encoder until an encoded character occurs which is not one of a predetermined set of characters; each encoded character, whether or not that encoded character is one of the predetermined set is transmitted by a transmitter to a receiver and a decoded output character provided from a decoder when the corresponding received character is determined not to be the one of the set of characters.
According to a fourth aspect of the invention there is provided a communication system for transmitting data from a discontinuous stream of data over a data link requiring a continuous stream of data comprising:
a transmitting part having: character basis, which encoding varies between successive characters of the continuous stream, predetermined set of characters, encoding means to repeatedly encode incoming data until an encoded character occurs which is not one of the predetermined set of characters, stream, in the absence of incoming data one of the predetermined set of characters, or when incoming data is present, each encoded character, unless an encoded character is one of the predetermined set when one of the predetermined set of characters is transmitted;
a receiving part having: in synchronism with the transmitting part; predetermined set of characters; provided from the receiver when t
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patent: 4079188 (1978-03-01), Kinck, Jr. et al.
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patent: 4606056 (1986-08-01), Perloff
patent: 4627074 (1986-12-01), Murkwitz et al.
patent: 4771463 (1988-09-01), Beeman
patent: 4811357 (1989-03-01), Betts et al.
patent: 5012489 (1991-04-01), Burton et al.
PCT Search Report Considered for 91/00579
British Telecommunications public limited company
Cangialosi Salvatore
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