The envisaged future telecommunications network includes electronic edge nodes and an optical core that comprises a plurality of optical cross connectors. Currently, an optical cross connector has no practical means for storing optical signals. This severe limitation has implications regarding the structure and operation of an optical-core network.
Wavelength-division-multiplexed (WDM) fiber links, each carrying several wavelength channels, connect the edge nodes to the optical cross connectors. The optical cross connectors may also be interconnected by WDM fiber links. In multi-hop routing, individual wavelength channels routed through the network may traverse several optical cross connectors. Such multi-hop routing within the optical core precludes the use of time sharing scheme, such as TDM (time-division multiplexing) switching or burst switching, due to the difficulty of coordinating the switching times among the bufferless optical core nodes.
In a switching node having a plurality of input ports and output ports, where signals arriving at the input ports are generally uncoordinated in the time domain, being generated by independent sources, and have arbitrary destinations, contention resolution is required because input signals arriving at two or more input ports may require connection to a single output port. Contention resolution can be based on a primitive method where one of the signals competing for an output port is accepted while the remaining signals are blocked, i.e., denied a path through the switching node. The blocked signals would be discarded if the respective input ports have no storage facility. Otherwise, the blocked signals can be buffered until given permission to proceed at subsequent intervals of time. Alternatively, contention is avoided by scheduling the transfer of data from input to output.
An edge node that establishes a path through a core node is said to subtend to the core node. An edge node may subtend to some or all of the core nodes of a network. When an edge node subtends to two or more core nodes, it selects a core node to establish a path to a given destination edge node according to some merit criterion. The edge nodes sending data traffic through a core node are said to be subtending edge nodes of the core node.
The path availability for signals arriving at a given input port and destined to several output ports of a switching node do not necessarily occur according to the same order in which the signals arrive to the switching node. Thus, a simple sequential storage facility is not useful to deploy, and, currently, there are no practical means for buffering incoming optical signals and selectively retrieving any signal for switching to a respective output port when a path across the switching node becomes available.
The use of interconnected optical cross connectors in the core of the network leads to an inefficient rigid network. Traversing two or more optical nodes is problematic for the following reasons:                (1) As described above, the absence of buffers makes time sharing of wavelength channels virtually impossible.        (2) The required paths for edge-node pairs must be established, through the core, one at a time. Resources must then be reserved during the path setup phase, which may take a considerable amount of time, several-hundred milliseconds for example. During the path setup phase, the reserved channels are kept idle.        (3) Paths can be blocked due to mismatch of links of sufficient vacancies. For example, there may be several candidate routes, each having two or more hops, from a source edge node to a sink edge node through a plurality of interconnected optical cross connectors. If each candidate route has one link that is fully reserved, a free end-to-end path can not be found even when all other links in the set of candidate routes are lightly loaded. This is conventionally called mismatch blocking. To reduce the mismatch probability, the occupancy of the optical core must be kept at a relatively low level, which can be realized by known internal-expansion techniques but may be considered costly.        (4) Wavelength conversion is often needed in multi-hop optical paths.        
To circumvent these difficulties, an obvious solution is to disallow the use of consecutive optical nodes in any path. With this approach, complemented with time coordination between each core node and each of its subtending edge nodes, the buffers at each edge node can be exploited for contention resolution. A time-locking technique is detailed in Applicant's copending U.S. patent application Ser. No. 10/054,509, filed on Nov. 13, 2001 and titled “Time-Coordination in a Burst-Switching Network”. With time locking, all edge nodes subtending to a core node can time their transmission to arrive at the optical core node at any desired instant of time. Time locking is feasible when a path between two electronic edge nodes traverses only a single optical core node. This enables adaptive channel switching at the optical core nodes without the need to allow large idle periods between successive path changes. Time locking also enables the use of time-sharing schemes; TDM or burst switching. It also allows the network core to operate at a high mean occupancy, 0.95 for example. Without time locking, time sharing of network transport resources is not possible, and channel switching would require that the core be operated at a lower mean occupancy, of 0.7 for example.
A complete network is defined herein to be a network that can allocate paths of a total capacity of C (bits per second) from any set of channels collectively having a capacity of C (bits per second) and emanating from any group of source nodes to any set of egress channels, collectively having a capacity that equals or exceeds C, and belonging to any group of sink nodes. For example, a source node can transfer its entire traffic to a sink node of at least equal capacity. Therefore, any spatial variation of traffic loads can be accommodated and the need for traffic engineering is virtually eliminated.
The capacity of a node is defined herein as the total data rate that can be switched from the input ports to the output ports of the node. The degree of a node refers to the number of ports in the node. In an asymmetrical node, where the number of output ports differs from the number of input ports, an input-side degree of the node refers to the number of input ports and an output-side degree refers to the number of output ports.
An efficient wide-coverage high capacity complete network that does not allow traversing two consecutive optical nodes would take the form of a composite-star network, or a plurality of composite-star networks as described in Applicant's U.S. patent application Ser. No. 09/624,079 filed on Jul. 24, 2000, and titled “Multi-dimensional Lattice Network”.
To be complete, the composite star network must be uniform, having identical optical-core nodes and identical edge nodes. To enable wide coverage, each edge node must have a high degree, having several links to the optical core, and each core node must be able to accommodate a large number of subtending edge nodes.
To enable a high capacity, each edge node must have a large number of input ports and a large number of output ports, with each input or output port supporting a channel of a high bit rate, 10 Gb/s for example. The edge node should also be provided with a high-speed controller to enable fast scheduling that can follow traffic-intensity variation. A versatile high capacity edge node is described in Applicant's copending U.S. patent application Ser. No. 10/025,982, filed Dec. 26, 2001, and titled “Universal Edge Node”.
A complete network, as defined above, responds gracefully to wild spatial traffic variation. However, this may be realized at the expense of some added optical-link mileage, as explained in a paper titled “Comparison of two optical-core networks”, authored by Blouin et al., Journal of Optical Networking, Vol. 1, No. 1, January 2002.
If the spatial traffic distribution can be determined with a reasonable level of confidence, then it would be possible to relax the strict requirement of network completeness and employ, instead, an incomplete network that is still tolerant to considerable spatial traffic variation.
It is desirable, therefore, to find a structure of an incomplete network that employs edge nodes of different capacities and degrees, and core nodes of different capacities and degrees, while still realizing a high capacity wide-coverage network that also provides high performance under diverse traffic conditions. An optimum incomplete network may include edge nodes of widely varying capacities and degrees and core nodes of different capacities and degrees to reduce fiber-link mileage. There is a need to realize such a network without significantly affecting network performance and agility.
Thus, a non-uniform incomplete composite-star network structure that can realize network economy and control simplicity is required and methods of route selection in such a structure are needed.