Optical TDMA network

Details Optical TDMA network Optical TDMA network

1998-03-06

2001-05-22

Chan, Jason (Department: 2633)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200, C359S199200, C359S199200, C359S199200

Reexamination Certificate

active

06236485

ABSTRACT:

The present invention relates to an optical network and in particular to a network suitable for carrying TDMA (time division multiple access) signals. A network embodying the present invention might be used, for example, in an access network within a broadband optical telecommunications system, or in a local area network (LAN) for interconnecting computer systems.
As discussed in our co-pending European application no.9604020.9 (BT case A25109), the increasing power of computer systems in terms of processor speeds and storage capacity has made it possible for conventional personal computers to handle multimedia applications which involve real-time video and computer graphics. The high bandwidth data associated with such applications place heavy demands on the network and the performance of conventional LANs has failed to keep pace.
Optical networks using synchronous TDMA potentially offer a far higher bandwidth than conventional LANs and so might be used as a high speed LAN. However, in practice, current proposals for optical TDMA networks suffer a number of limitations.
A key design problem in the construction of optical TDMA networks is the need to distribute an optical clock signal to the different nodes on the network. This clock signal is required to enable a receiver in a node to read data from an appropriate slot, and to enable a transmitter to write to the appropriate slot. Conventionally, the optical clock signal has been transmitted together with the data. The clock signal is distinguished from the accompanying data by, for example, a different polarization state or by a different wavelength. Each node then requires a stage for separating the clock from the data. One example of this conventional approach to optical TDMA is disclosed in the paper by J G Zhang in Pure Appl. Opt. III (1994) 609-614. This discloses a network which uses a star topology and in which a clock signal at the frame rate is transmitted at a different wavelength to the data. In this system, the need to demultiplex the incoming frames to recover the clock adds undesirably to the complexity of the node and places constraints on the network topology.
A further example of a prior art network is disclosed in the paper by Nowatzyk & Prucnal published in the proceedings of 22 Annual International Symposium on Computer Architecture, June 1995. This proposes the use of a clock or “framing pulse” which is distinguished from the data pulses by a different polarization state. The receiver section of each node then requires a device to separate the data from the framing pulse. However, using such polarization multiplexing techniques, it is found that polarization characteristics of all the optical elements of the network need to be tightly controlled if the network performance is avoid degradation by interaction between the clock and the data.
European patent application number 87308169.9 (EP-A-0268355) discloses an optical communications network in which an optical synchronization signai is transmitted on one optical, modulated at the customer premises, and transmitted onto a second optical line which is located separately from the first optical line. However, while such an approach might have been possible for use at relatively lower bit rates, recent work has been directed to achieving much higher bit rates, in excess of 1 Gbit per second. At these higher bit rates schemes such as that disclosed in EP-A-0268355 fail as a result of timing variations between the two optical lines.
According to a first aspect of the present invention there is provided an optical network comprising:
(a) an optical bus including at least two co-located optical fibre waveguides;
(b) a plurality of transmitter nodes, each transmitter node including a transmitter stage connected to the optical bus and arranged to receive an optical clock signal from one of the fibre waveguides and arranged to write an optical data signal, in a time slot synchronized to the optical clock signal, onto another one of the fibre waveguides; and
(c) a least one receiver node, the receiver node including a receiver stage for receiving an optical data signal which was output onto the optical bus by the transmitter stage of a transmitter node.
The present invention provides an optical network having a construction and topology markedly different from those of conventional networks. Through using a bus including two optically separate but physically co-located waveguides, the invention provides a local clock at each node which provides an accurate timing reference for reading or writing data, irrespective of the position on the bus of the node. The different waveguides may be different fibres located within a single cable, or may be different cores of a single fibre. The use of separate optical paths avoids the problems of crosstalk and susceptibility to noise associated with conventional networks in which clock and data are multiplexed on a single path using, for example different polarization states. It is found that by co-locating the waveguides timing errors can be reduced to low levels, allowing operation at very high bit rates.
Commonly, a transmitter node and receiver node may both be present at a single location. For example, this would be the case where the transmitter and receiver nodes are part of a network interface for a computer which is connected to an optical LAN and which needs both to transmit and to receive data via the LAN. The invention is not however limited in this respect, and also encompasses implementations in which transmitter nodes and receiver nodes are at different locations.
Preferably the transmitter stage includes a modulator and is arranged to generate the optical data signal by modulating the clock signal received from the one of the waveguides.
The preferred implementation of the present invention further simplifies the construction of the network nodes, by avoiding the necessity for a local optical source for data generation. Instead, the transmitter in a node takes an optical clock signal from one fibre in the bus, modulates it, and places the signal back on another fibre in the bus in the appropriate time slot.
Preferably the transmitter stage is arranged to receive an optical clock signal from a first one of the optical fibre waveguides which in use carries a write clock, and the receiver stage is arranged to read an optical clock signal from a second one of the optical fibre waveguides which in use carries a read clock.
The use of separate clocks carried on different fibres enables savings in the power budget over the extent of the network. An optical data signal produced by modulating the write clock may be carried on the same waveguide as the read clock, in which case the write and read clocks are arranged to have different wavelengths. Alternatively, the data may be carried separately from the two clock signals on a third co-located optical fibre waveguide.
The receiver nodes may be coupled to the bus via a star network. As described in further detail below, this offers the advantage of greater fault tolerance and robustness.
According to a second aspect of the present invention, there is provided a method of operating an optical network comprising an optical bus comprising at least two co-located optical fibre waveguides, a plurality of transmitter nodes connected to the optical bus, and a receiver node, the method comprising:
(a) at a transmitter node, receiving an optical clock signal from one of the fibre waveguides, and writing an optical data signal, in a time slot synchronized to the optical clock signal, onto another one of the fibre waveguides; and
(b) at a receiver node, receiving an optical data signal which was output onto the optical bus in step (a).
The present invention also encompasses a local area network (LAN) suitable for interconnecting a plurality of computer systems and using a network in accordance with the first aspect.


REFERENCES:
patent: 4568931 (1986-02-01), Biolley et al.
patent: 5073980 (1991-12-01), Prucnal et al.
patent: 209 329 (1987-01-01), None
patent: 268 355 (1988-05-01), Non

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