Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Patent
1989-02-02
1992-10-06
Coles, Sr., Edward L.
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
359135, 359167, H04J 1408
Patent
active
051537648
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method of control of an optical system a signal control system and a transceiver for use in such a control system and in particular for use with multiple access optical networks.
The use of a multiple access network allows for potentially highly efficient use of network resources. More expensive equipment for network management, for example, may be located at a central node, or exchange and its facilities shared by a large number of individual network outstations. In such circumstances the exchange may be connected via a single main line with branch feeder lines linking the network outstations to the main line. In an optical network the branching may conveniently be effected by passive optical splitters. It is therefore feasible to produce a multiple access optical network with no active components other than at the exchange and at the outstations themselves. The advantages of such a passive network include easier maintenance and reduced overall cost.
In a multiple access network it is necessary to ensure that signals intended for, or originating from, a particular outstation are correctly identifiable. The technique of Wavelength Division Multiplexing (WDM) or Time Division Multiplexing (TDM), for example, enable this identification to be achieved.
Where TDM is used, each outstation is allocated an individual time slot or channel specifically for reception or transmission of its own signals. When a composite TDM signal is broadcast from the exchange each outstation will receive the whole of that signal from which it selects and decodes its own allocated channel. However, in the reverse direction, each outstation contributes its own signal alone and the network itself must effectively assemble all the separately originating channels into a composite TDM signal for passing back to the exchange in the correct order. The outstations are generally irregularly spaced at different distances from the exchange. It is therefore essential to provide some method of ensuring that transmissions from each outstation occupy a designated time slot in the assembled return signal irrespective of the physical position of the outstation relative to the exchange, both at the time of connection to the network and at all subsequent times.
To deal with this problem a ranging protocol may be employed. Ranging protocols are known for use in radio networks and will not be further discussed here. However, it has been found that provision of a conventional ranging protocol is not necessarily sufficient to ensure the accurate assembly of the return TDM signal in a passive optical network as described above. Localised variations can occur in the transmission path, for example, such that the words or bits in a signal originating from an outstation may not be exactly within the boundaries of the allocated time slot in the return TDM signal. The signal from one outstation may then overlap the signal from another to the detriment of the network efficiency.
A further problem arises because the amplitude of the signals from each outstation depends on the power of the transmitter associated with the outstation and on the attenuation of the optical path from that outstation. Both these factors will vary from one outstation to another. Thus, unless compensating measures are taken, the composite return TDM signal may have signal levels which differ from one time slot to the next, according to whichever outstation each slot is allocated. Such a variable signal is difficult to demultiplex.
It is an object of the present invention to provide a method and apparatus controlling the signal transmission from network outstations in a multiple access passive optical network such that the aforementioned problems are substantially overcome or at least mitigated.
According to the present invention a method of controlling a signal from an outstation in a multiple access passive optical network including a central node and a plurality of outstations comprises, at the central node, monitoring the value of a parameter of the si
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Faulkner David W.
Fordham David I.
Henning Ian D.
Hutcheon Alistair
Russ Dianne M.
British Telecommunications public limited company
Coles Sr. Edward L.
Negash Kinfe-Michael
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