Wavelength-division multiplexing in passive optical networks

Details Wavelength-division multiplexing in passive optical networks Wavelength-division multiplexing in passive optical networks

1998-03-04

2002-06-25

Pascal, Leslie (Department: 2633)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200, C359S199200, C359S199200, C359S199200

Reexamination Certificate

active

06411410

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to wavelength-division multiplexing in passive optical networks.
2. Description of the Prior Art
FIG. 1
shows a block diagram of parts of a conventional communications network employing a passive optical network (PON). The communications network
1
has an optical line termination unit (OLT)
2
and a plurality of optical network units (ONUs)
4
1
to
4
4
. The ONUs
4
1
to
4
4
are connected to the OLT
2
by a passive optical network
6
which consists of optical fibre links
8
and optical splitters
10
. The OLT
2
is located at the so-called “head end” of the PON
6
and serves to connect the PON to a core network. Customers or subscribers are connected to the ONUs.
The communications network
1
shown in
FIG. 1
may be employed as part of an asynchronous transfer mode (ATM) communications network. In this case, the so-called “downstream traffic”, i.e. the data (ATM cells) to be transmitted from the OLT
2
to the ONUs
4
1
to
4
4
, is broadcast at a single optical wavelength &lgr;
1
to all of the ONUs and each ONU then selects the appropriate ATM cells destined for it and ignores any other cells.
In the upstream direction, from the ONUs to the OLT, the individual signals from the ONUs
4
are interleaved in a predetermined time-division multiple-access (TDMA) format. For example, in the TDMA format shown in
FIG. 1
itself, each ONU
4
i
is allocated its own time slot TS
i
within a frame FR
UP
. All upstream traffic is at a single wavelength &lgr;x which may be the same as the downstream wavelength &lgr;
1
or may be different from &lgr;
1
. The upstream traffic from the ONUs to the OLT will generally be of a much lower data rate than that of the downstream traffic. The maximum capacity of the PON
6
is therefore required to correspond to the maximum data rate of the downstream traffic.
The PON
6
may be of the two-fibre type which is effectively two passive optical networks (two sets of fibre links
8
and optical splitters
10
) used in parallel, one for the downstream traffic and the other for the upstream traffic. The capacity of the upstream-traffic PON can, if desired, be lower than that of the downstream-traffic PON.
Alternatively, the PON
6
shown in
FIG. 1
may be of the single-fibre type which uses just one set of fibre links
8
and optical splitters
10
to connect the OLT to the ONUs; in this case a return path from the ONUs to the OLT is provided by time-division multiplexing the downstream and upstream traffic over the single-fibre PON. Again, depending on the time-division format used, the effective capacity available to the upstream traffic may be made lower than the effective capacity available to the downstream traffic.
For simplicity, the embodiments described specifically in the present application will make use of the two-fibre type PON but as will be readily apparent the present invention can also be used with single-fibre type PONs.
In order to increase the maximum capacity of a passive optical network it is possible to employ wavelength-division multiplexing. If, for example, the downstream traffic capacity of the PON
6
is f
max
when the downstream traffic is broadcast on a single wavelength, the capacity of the PON
6
is increased to N x f
max
when N optical signals at different respective wavelengths are employed to broadcast the downstream traffic.
Using this technique it would be possible to pre-assign each ONU with its own unique wavelength on which to receive data from the OLT
2
. However, such an approach is unsatisfactory for two reasons. Firstly, even with state-of-the-art technology a maximum of 32 different wavelengths is presently possible, whereas it may be desired to support over 100 ONUs from the same OLT. Secondly, the downstream traffic requirements for the different ONUs are not fixed over time, so that at any given time the amount of downstream traffic can vary greatly from one ONU to the next. At certain times, some of the ONUs may have no downstream traffic at all. Preassigning all ONUs with an equal or fixed amount of capacity is therefore potentially wasteful of the overall downstream traffic capacity of the PON.
BRIEF SUMMARY OF THE INVENTION
According to a first aspect of the present invention there is provided a communications network including: an optical transmitter for generating a plurality of optical signals having different respective wavelengths, each said optical signal carrying data, and wavelength-division-multiplexing the optical signals; and a plurality of optical receivers connected to the optical transmitter by way of a passive optical network for receiving the wavelength-division-multiplexed optical signals, each receiver having wavelength selection means operable in dependence upon control information sent from the transmitter to the receiver concerned (for example by way of the passive optical network) to select one of the optical signals of the said plurality, and also having detection means for processing the selected optical signal to derive therefrom the data carried thereby.
In such a network the downstream capacity of the passive optical network can be shared flexibly by the different optical receivers.
For example, in the optical transmitter the data to be transmitted to the optical receivers may be allocated to the optical signals of the said plurality dynamically in dependence upon the respective amounts of data which it is desired to transmit to the different optical receivers in a particular time frame. In the optical transmitter, data destined for the optical receivers may, for example, be buffered in queues corresponding respectively to the different optical receivers and the amounts of data for the different optical receivers can then be determined from the queue fill levels.
The control information is preferably sent from the optical transmitter to the optical receiver concerned by way of the passive optical network but may alternatively be sent by way of further communications paths linking the transmitter to the receivers. In this case, the control information could be embodied in radio signals, or in electrical signals carried by dedicated landlines.
To reduce the amount of control information required to be transmitted, the control information preferably specifies only changes in optical signal selection to be made by the optical receivers.
The control information may be carried as overhead information by the optical signals. This keeps the cost of the optical receivers down because the control information can be received through the selected optical signal and detected using the same detector that detects the data.
In one embodiment, the control information relevant to a given optical receiver is carried as overhead information by all of the optical signals. This keeps the design of the optical transmitter simple because it does not need to keep track of the optical signal that the given optical receiver has currently selected. However, the broadcast of the control information on all optical signals is wasteful of the downstream capacity and accordingly in another embodiment the control information relevant to a given optical receiver is carried as overhead information only by the optical signal currently selected by the said given optical receiver.
In this case the optical transmitter preferably has selection storing means for storing the respective current optical-signal selections made by the optical receivers, and overhead information adding means operable, when control information is to be transmitted to one of the said optical receivers, to determine from the stored current optical-signal selections the optical signal of the said plurality that is currently selected by that optical receiver, and to cause the control information to be carried as overhead information only by the determined optical signal.
In another technique for increasing the throughput of data, two or more of the said optical signals may be used to carry simultaneously, as overhead information, different co

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