Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Patent
1992-05-26
1994-10-04
Chilcot, Jr., Richard E.
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
359189, 359193, 307540, 307549, 307552, 307558, 375 76, H04J 1408, H04B 1006
Patent
active
053531434
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a passive optical network and in particular to the design of an optical receiver for a head-end station used in such a network.
2. Related Art
The present applicants have developed a bit transport system (BTS) for use in a TPON (telephony on a passive optical network) network. In this network downstream data is broadcast in a TDMA multiplexed frame from a head-end station to a number of terminations connected to the head-end station via a passive optical fibre network. In the return or upstream direction, each termination transmits data in a predetermined time slot so that at the head-end station the data from the different terminations are interleaved to form an upstream TDMA frame of predetermined format. The optical receiver in the head-end station therefore has to cope with interleaved pulses arriving from a number of terminations. The precise number of these pulses will be indeterminate at any given instant, varying with the amount of traffic in the upstream direction. It follows that the head-end station in normal operation has to handle a wide range of mean power levels. In addition, certain fault conditions can result in a laser in one of the terminations transmitting continuously, causing an arbitrary shift in the mean levels received at the head-end station. It is important that the receiver should be capable of continuing to function without such faults significantly disrupting traffic.
SUMMARY OF THE INVENTION
According to the present invention, there is provided a network head-end station arranged to receive an upstream TDMA frame comprising interleaved optical signals from a plurality of terminations connected to the head-end station by a fibre network, the station including an opto-electronic receiver and a DC clamping circuit for the opto-electronic receiver, characterised in that: the terminations are arranged such that none of the terminations transmit in a quiet phase of the upstream TDMA frame; the DC clamping circuit is arranged, when operated, to clamp signals received during the remainder of the TDMA frame to a DC reference level received during the quiet phase; and the station further including a clamping pulse generator arranged to generate a pulse at a predetermined time falling within the quiet phase to operate the DC clamping circuit.
By providing a DC clamping circuit in the head-end opto-electronic receiver and timing the operation of that clamping circuit to fall within a predetermined quiet phase in the upstream TDMA frame, the present invention avoids the need for tracking the widely varying DC levels at the head-end receiver and so considerably simplifies the design of that receiver. The timing of the transmissions from the terminations is arranged so that no correctly functioning termination transmits signals timed to arrive during the quiet phase. If however a faulty termination includes a laser which is continuously ON then power from that termination will be received during the quiet phase and is effectively cancelled by the DC clamping process. A failure of this sort can therefore disrupt the upstream traffic for no longer than the period from the first occurrence of the fault to the next clamping pulse.
Preferably the head-end station is arranged to receive TDMA frames including phase-1 ranging pulses, phase-2 ranging pulses and immediately following the phase-2 ranging pulses a plurality of data frames and the clamping pulse generator is arranged to generate a pulse timed to fall in a quiet phase between the phase-1 ranging pulses and the phase-2 ranging pulses.
In order to synchronise the operation of the terminations it is necessary to compensate for the different path lengths between the head-end station and different terminations. The calculation of the delay to the nearest clock cycle and the setting of a complementary delay in the timing of the transmissions from the termination concerned is termed phase-1 ranging and in the preferred aspect of the present invention the
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patent: 3315033 (1967-04-01), Sennhenn et al.
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patent: 4648133 (1987-03-01), Vilnrotter
patent: 4723237 (1988-02-01), Andrew et al.
Proceedings of the IEEE 1981 National Aerospace and Electronics Conference, Dayton Convention Center, May 19-21, 1981, vol. 3, IEEE, (New York, US), D. R. Porter et al.: "Optical data bus receiver for MIL-STD-1553B." pp. 1319-1323.
Bacares R.
British Telecommunications public limited company
Chilcot Jr. Richard E.
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