Scrambling in a digital communications network

Claim 2

A central station for a digital communications network, arranged to transmit data to outstations in the form of a stream of frames, each comprising a synchronisation portion containing a synchronisation signal in the form of a predetermined pattern of bits, the central station comprising a scrambling means for scrambling the frames in accordance with a predetermined binary sequence, means for inverting successive nth bits in at least a part of the scrambled frames adjacent to the scrambled synchronisation portions thereof, but not inverting successive nth bits in the scrambled synchronisation portions themselves, thereby preventing false recognition of synchronisation due to an identical pattern of bits in said part of the frames adjacent to the synchronisation portion.

Claim 3

A central station as claimed in Claim 1 wherein each frame comprises a first portion containing housekeeping data for the outstations and said synchronisation portion, and also comprises a second portion containing message data for the outstations, and the inverting means is arranged to invert successive nth bits of only the housekeeping data in the first portion of each frame.

Claim 4

A central station as claimed in Claim 2 wherein the scrambling means comprises a first scrambler arranged to scramble the contents of the first portions of the frames in accordance with a first predetermined binary sequence, and a second scrambler arranged to scramble the second portions of the frames in accordance with a second predetermined binary sequence.

Claim 5

A central station as claimed in Claim 3 wherein the first and second scramblers of the scrambling means share a single predetermined binary sequence generator and utilize respective taps of a shift register of the generator.

Claim 6

A central station as claimed in any one of the preceding Claims wherein n is sixteen.

Claim 7

An outstation for a digital communications network, arranged to receive frames from a central station as claimed in Claim 1, the outstation comprising descrambling means for descrambling the frames in accordance with the predetermined binary sequence, means for identifying the synchronisation signal in the output of the descrambling means, and means arranged to correct the inverted nth bits thereby to reconstitute the original form of said at least a part of the scrambled frames.

Claim 8

An outstation as claimed in Claim 6 and arranged to receive frames from a central station as claimed in Claim 3, wherein the descrambling means comprises a self-syndronising descrambler coupled to said identifying means; another descrambler for descrambling the contents of the first portions of the frames in accordance with said first predetermined binary sequence, and a further descrambler for descrambling the contents of the second portions of the frames in accordance with said second predetermined binary sequence.

Claim 9

An outstation as claimed in Claim 7, wherein said another and said further descramblers share a single binary sequence generator and utilize respective taps of a shift register of the generator.

Claim 10

A digital communications network comprising a central station as claimed in Claim 1, a plurality of outstations as claimed in Claim 6 and a branch arrangement between the central station and the outstations which carries in use multiplexed signals for the outstations in the form of a stream of broadcast frames each including a synchronisation signal, said network being adapted for return signals from the outstations to be multiplexed passively in a stream of return frames onto said transmission medium, or onto a similar transmission medium specifically for said return signals; wherein for establishing synchronisation of signals returning to the central station from the outstations the central station comprises means for transmitting a first signal to the outstations, means responsive to the time of receipt of a respective second signal from an outstation to calculate a respective delay for the respective second signal and to transmit a respective third signal to the outstation representative of the respective delay; and each outstation comprises means responsive to receipt of said first signal to transmit said second signal in a predetermined relationship with a received synchronisation signal and means responsive to said third signal to delay the transmission of said second signal by the appropriate amount whereby the second signals from all transmitting outstations are received simultaneously at the central station and effectively form a single synchronising signal for the return multiplexed signals.