Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Patent
1996-09-27
1999-10-05
Lefkowitz, Edward
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
359129, 359137, 359167, H04B 1020
Patent
active
059633506
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND TO THE INVENTION
1. Field of the Invention
The present invention relates to an optical telecommunications network and to switching structures suitable for use in such a network.
2. Related Art
Optical transmission systems potentially offer a basis for telecommunications networks of very high capacity. It is envisaged that this capacity will be used to provide connections for large numbers of systems to high bandwidth services such as high bit rate data communications, video on demand VoD or video telephony. However, in practice when it comes to realising such a high capacity network, the design of appropriate switching technologies is found to be a major constraint on the performance of the network. With current technologies, the switching structures are electronic (e.g. ATM switches) and are therefore limited to capacities considerably lower than those which can be achieved in the optical domain. Control and management of the switching function also provides further difficulties and there is a need to minimise software costs for call processing, network intelligence, network management and service management.
In order to minimise the use of active switches and the associated constraints discussed above, it has been proposed to use passive optical networks PONs. However hitherto, such PONs have been used for multiplexing and consolidation of access traffic to a local switch node with customer to customer connections still being provided by switching centrally at the nodes.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention there is provided an optical telecommunications system comprising a plurality of transparent passive optical networks (TONs) each providing connections to a plurality of terminals, and the head-ends of the TONs being connected to a common central switching node, characterised in that each terminal includes means for selecting a wavelength channel, for forming a connection with another subscriber station within the respective TON, or within another of the plurality TONs and in that the central switching node includes an optical spatial/wavelength switch arranged to provide connections between subscribers connected to different TONs.
The present invention uses amplified PONs (Transparent PONs or `TONs`) to provide a network with sufficient capacity for distributed switching to be carried out. Wavelength channels and, preferably, time slots are allotted dynamically to individual customer-customer connections. The use of time slots is preferred, but may be dispensed with where a network is operating at relatively low capacity. This distributed switching function combined with spatial cross-switching at a node connecting a plurality of TONs provides a system capable of covering an entire regional or national area.
Preferably the switch is a time- and wavelength-shared rearrangable space switch, more preferably comprising a plurality of stages connected to an optical bus. A rearrangable switch is one which does not inherently provide fully non-blocking cross-connection, but which functions as a non-blocking switch by re-assigning e.g. space or wavelength channels to on-going connections when necessary.
Preferably each of the plurality of stages includes an array of 2.times.2 optical changeover switches. Preferably said array of changeover switches is provided by a spatial light modulator SLM array arranged between a wavelength demultiplexer and a wavelength multiplexer and coupler/splitters. In embodiments of this aspect of the invention switching is performed in a distributed manner, with customer terminals dynamically allocated their transmitter and receiver wavelength channels and time-slots, under centralised processor control. A pair of sending and receiving customer terminals are allocated a common wavelength and time-slot for the duration of their connection. Such distributed channel allocation is used in combination with a central switch node, to connect the appropriate spatial fibres at the correct time and wavelength. The switch node employs
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British Telecommunications public limited company
Lefkowitz Edward
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