Synchronization in digital communications networks
DC
First Claim
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1. A communications network comprising a central station and a plurality of outstations for transmitting signals on an upstream channel to the central station using time division multiple access and receiving signals on a downstream channel from the central station using time division multiplex, in which each outstation has timing means for applying to its transmissions a timing adjustment determined by a ranging operation for that station, to compensate for differing transmission delays from the outstations to the central station, and in which the network is operable:
- (a) in an operational mode in which the outstations transmit to the central station in a frame structure having a plurality of frames containing traffic data and at least one frame reserved for ranging operations; and
(b) in a start-up mode in which ranging operations are performed, and in which outstations in respect of which the ranging operations has been performed refrain from transmitting traffic data until after the ranging operation has been performed in respect of further outstations, whereby capacity on said upstream channel is made available to permit more frequent ranging operations than is possible in said operational mode.
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Abstract
Disclosed is a digital communications network comprising a branched passive optical network connecting a head-end with a plurality of optical network units. The time taken to synchronize by ranging, all optical network units and the head-end, when the network needs to be initialized from scratch, has been minimized using the disclosed method. The synchronization time has been minimized by making all bandwidth in the upstream direction of the network available for the purpose of course ranging. Once the whole network is ranged, the bandwidth is made available, in a normal way, to combined data communications and operations and management functions.
34 Citations
12 Claims
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1. A communications network comprising a central station and a plurality of outstations for transmitting signals on an upstream channel to the central station using time division multiple access and receiving signals on a downstream channel from the central station using time division multiplex, in which each outstation has timing means for applying to its transmissions a timing adjustment determined by a ranging operation for that station, to compensate for differing transmission delays from the outstations to the central station, and in which the network is operable:
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(a) in an operational mode in which the outstations transmit to the central station in a frame structure having a plurality of frames containing traffic data and at least one frame reserved for ranging operations; and
(b) in a start-up mode in which ranging operations are performed, and in which outstations in respect of which the ranging operations has been performed refrain from transmitting traffic data until after the ranging operation has been performed in respect of further outstations, whereby capacity on said upstream channel is made available to permit more frequent ranging operations than is possible in said operational mode. - View Dependent Claims (2, 3, 4, 5)
means for transmitting a first signal to a given outstation; and
means responsive to the time of receipt of a second signal from the outstation to determine the time difference between the actual time of receipt of the second signal and a required time for receipt and for transmitting to the outstation a third signal representative of the difference, and each outstation comprises;
means responsive to the receipt of said first signal to transmit said second signal at a reference time; and
means responsive to said third signal to change said reference time such that subsequent signals from the outstation arrive at the required time.
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3. A network according to claim 2, in which the central station has a cyclic counter;
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each outstation has a cyclic counter having a controllable phase relationship with the cyclic counter at the central station and arranged to be synchronized to the counter of the central station, and at each outstation the means to change the reference time changes the phase of the cyclic counter associated with that outstation relative to the phase of the cyclic counter associated with the central station, wherein the phase change to the cyclic counter associated with the outstation is an amount determined by the third signal.
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4. A network according to claim 1, wherein the channels are provided by a passive optical network.
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5. A network according to claim 4, wherein the passive optical network comprises at least one optical fiber branch which includes optical amplification means.
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6. A method of operating a communication network comprising a central station and a plurality of outstations, including for transmitting signals on an upstream channel from the outstations to the central station using time division multiple access and transmitting signals on a downstream channel from the central station to the outstations using time division multiple, and including performing ranging operations to determine timing adjustments which are applied by the outstations to their transmissions to compensate for differing transmission delays from the outstations to the central station, wherein the method comprises:
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(a) in an operational mode, transmitting from the outstations to the central station in a frame structure having a plurality of frames containing traffic data and at least one frame reserved for ranging operations; and
(b) in a start-up mode, performing ranging operations, and refraining from transmitting traffic data from outstations in respect of which the ranging operation has been performed until after the ranging operation has been performed in respect of further outstations, whereby capacity on said upstream channel is made available to permit more frequent ranging operations than is possible in said operational mode. - View Dependent Claims (7, 8, 9, 10)
a. transmitting a first signal from the central station to a given outstation to cause the outstation to return a second signal at a reference time to the central station;
b. receiving at the central station the second signal, determining the time difference between the actual time of receipt of the second signal and a required time for receipt and generating a third signal for causing the outstation to change said reference time such that subsequent signals from the- outstation arrive at the required time, wherein said operation is repeated for each outstation in the network.
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8. A method according to claim 6, in which the central station has a cyclic counter and each outstation has a cyclic counter synchronized to the counter of the central station and the said change of the reference time for any outstation is effected by changing the phase of the cyclic counter associated with that outstation relative to phase of the cyclic counter associated with the central station.
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9. A method according to claim 6, wherein the channels are provide by a passive optical network.
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10. A method according to claim 9, wherein the passive optical network comprises at least one optical fiber branch which includes optical amplification means.
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11. A communications network comprising a central station and a plurality of outstations for transmitting signals on an upstream channel to the central station using time division multiple access and receiving signals on a downstream channel from the central station using time division multiplex, in which each outstation has timing means for applying to its transmissions a timing adjustment determined by a ranging operation for that station, to compensate for differing transmission delays from the outstations to the central station, and in which the network is operable:
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(a) in an operational mode in which the outstations transmit to the central station in a frame structure in which a plurality m of frames containing traffic data and n frames reserved for ranging operations form a multiframe of given duration, n and m being integers, m being greater than n and n being 1 or more, (b) in a start-up mode in which ranging operations are performed, and in which outstations in respect of which the ranging operation has been performed refrain from transmitting traffic data until after the ranging operation has been performed in respect of further outstations, so that in place of frames containing traffic data an increased number, p, of ranging frames occur in a multi-frame of said given duration, p being greater than n.
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12. A method of operating a communication network comprising a central station and a plurality of outstations, including for transmitting signals on an upstream channel from the outstations to the central station using time division multiple access and transmitting signals on a downstream channel from the central station to the outstations using time division multiplex, and including performing ranging operations to determine timing adjustments which are applied by the outstations to their transmission to compensate for differing transmission delays from the outstations to the central station, wherein the method comprises:
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(a) in an operational mode, in which transmission between the outstations and the central station is arranged as a sequence, m, of multi-frames, each multi-frame being of a given duration, having a plurality of frames containing traffic data and n frames reserved for ranging operations, m and n being integers, m being greater than n, and n being one or more;
(b) in a start up mode, performing ranging operations, and refraining from transmitting traffic data from outstations in respect of which the ranging operation has been performed until after the ranging operation has been performed in respect of further outstations, so that within a multi-frame of said given duration, in place of frames containing traffic data additional frames are available for ranging so that the rate of ranging during the start up mode can be increased compared to that possible in the operational mode.
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Specification
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Litigation Data
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Current AssigneePatentportfolio 2 SARL
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Original AssigneeBritish Telecommunications (BT Group Plc) (BT Group PLC)
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InventorsQuayle, John Alan
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Primary Examiner(s)Kizou, Hassan
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Assistant Examiner(s)Elallam, Ahmed
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Application NumberUS08/981,586Time in Patent Office1,296 DaysField of Search370/294, 370/301, 370/312, 370/314, 370/345, 370/347, 370/395, 370/442, 370/458, 370/475, 370/503US Class Current370/503CPC Class CodesH04J 3/0682 by delay compensation, e.g....H04J 3/1694 Allocation of channels in T...
