Optical communications – Multiplex – Optical switching
Reexamination Certificate
2000-09-22
2004-05-11
Pascal, Leslie (Department: 2633)
Optical communications
Multiplex
Optical switching
C398S057000, C398S063000
Reexamination Certificate
active
06735393
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical networks, signal routing network elements for such networks and methods for implementing optical networks. Especially the invention concerns an all-optical network, passive wavelength routers for such networks, and methods for implementing all-optical networks. Uses of the invention are also disclosed.
2. Discussion of the Background
An optical network consists basically of nodes with optical transmitters and receivers, optical fibre as the transmission line connecting these nodes, and signal routing/switching network elements (NE) at the nodes. An all-optical network is a network where the signal remains in the optical domain all the way from the transmitting to the receiving end, i.e. where no conversion from the optical to the electrical domain (or vice versa) takes place at the intermediate nodes. In today's backbone network the transmission between nodes is done optically, yet the network itself is actually not all-optical in that all the processing and routing functionality at the node is done in the electrical domain. The signal is converted to the electrical domain at each node and the header information that is incorporated in the digital signal serves to direct it forward to the next node in order to reach its final destination. In this respect all the routing, i.e. the path-finding, through the network is done at levels other than the optical. The introduction of optical add-drop multiplexers (OADM) makes it possible to connect some nodes directly, by-passing that is some of the intermediate nodes and creating a direct optical path for a certain signal to reach the node it is destined for. However, this is far from the vision of the all-optical network where all the routing functionality will be done in the optical domain. The implementation of all-optical networks will have significant consequences that have been extensively analysed within the field of optical communications. The introduction of optical network functionality at the node is the major area of interest in the field of optical communications and the realisation of good performance all-optical or nearly all-optical networks is the main motivation behind nearly all of the work carried out in the field.
Wavelength Division Multiplexing (WDM) is used to multiplex many optical channels in one fibre. At the receiving end the signal is divided back to its original constituents and each wavelength is received on a separate receiver that is accordingly tuned to the right frequency/wavelength. The number of channels in a WDM transmission system has increased dramatically in the past three years. Fixed wavelength transmitters are used and WDM systems with two hundred WDM channels in one fibre, have been announced by systems producers. WDM is today used for capacity increase yet the real driving force behind its tremendous development is the prospect of optical networking. The wavelength of the channel is used to identify the signal in the optical domain so that it can be directed as necessary without the need for demultiplexing and reading of the content of the signal itself.
One of the main building blocks of the (all-) optical WDM network is the optical cross-connect (OXC). This is a controllable NE that is used at the node to direct optical signals to the right output. Any optical signal that arrives at a certain fibre input to the node will be switched over to the fibre output that will ensure that this signal follows the right path towards its end destination. A typical OXC ought then to be able to switch each one of those (100) wavelengths from each one of the input fibres (minimum of two) to the right fibre output (between a minimum of two output fibres). OXCs are just becoming commercially available, primarily electro-optic versions of these with far from ideal performance and quite high cost. The sheer size, cost and complexity are such that it has not been proved possible to realize an OXC of admissible specifications/performance for a real implementation. The complexity and cost of an OXC increases dramatically as a function of size, the size being defined as a function of the total number of wavelengths as well as the number of fibre inputs and outputs. In addition, unrealistically stringent performance requirements are placed upon the components the OXC (e.g. optical switches). It is quite likely that better OXC may be realized in the future, yet it has become clear that technical limitations will render it practically impossible to realize a large high capacity all-optical network that comprises cascades of large OXCs. This constitutes a considerable limitation for the realisation of optical networks which has been a major delaying factor to their introduction.
Recently, a hard-wired OXC that functions as a passive wavelength router was described. See Chen et al., “Fiber Bragg Grating-Based Large nonblocking Multiwavelength Cross-Connects,” Journal of Lightwave Technology, Vol. 16, NO. 10 (October 1998), the entire content of which is hereby incorporated by reference. In this device, each wavelength from each input is directed to a predetermined output, and at the same time, signals carried by the same optical wavelength originating from different inputs are directed to distinct outputs , a functionality that is central in a cross-connect as indeed its name implies.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method for the realisation of a network where no switching elements, such as the OXC or others, are included at the node.
Some known networks can be considered switchless network. For example, a simple ring network with non configurable OADMs is a switchless network. In the case of a star network, a star wavelength demultiplexer has been used to provide dedicated wavelength connections from node to node. These two network architectures can be used in the access or the regional area network. However, in the backbone network area and for larger scale networks, mesh architectures are practically required. Star networks suffer unacceptable protection problems to be implemented in the backbone area where full protection is required and make a very inefficient use of the available fibre infrastructure. OADM rings offer low functionality in relation to a mesh, resulting in long transmission lengths and a rather wasteful use of the optical bandwidth. In order to realize a mesh optical network, an OXC is generally required exactly because many choices of direction are present for each signal at each node. One of the main contributions of the present innovation is to devise a method for the realisation of a switchless mesh network with the possibility to attain the performance that is required from today's networks. This features simultaneous connections between all node-pairs, high bandwidth between the nodes, 1+1 (1:N) protection, dynamic bandwidth allocation, and efficient use of the bandwidth available in the fibre. Obviously, lower performance is also attainable.
In accordance with a first aspect of the invention there is provided an optical network comprising a plurality of nodes, optical fibre transmission lines connecting the nodes, at least one transmitter and/or at least one receiver in each node, and at least one wavelength router. The router is operative to route at least one wavelength band that contains more than one consecutive optical channel. The optical network can be a mesh, a ring, a star or a bus network, or combinations of these.
In accordance with a second aspect of the invention there is provided an all-optical network comprising a plurality of nodes, optical fibre transmission lines connecting the nodes, at least one transmitter and/or at least one receiver in each node, and at least one passive wavelength router, wherein the network is a transparent/passive medium with a set of fixed wavelength dependent rules, with a plurality of paths connecting the nodes, the network being a mesh network, a path of an optical signal propagating throug
Pascal Leslie
Telenor AS
Tran Dzung
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