1. Field of the Invention
The present invention relates to an optical wavelength division multiplexing network which multiplexes and transmits optical signals having a plurality of different wavelengths.
2. Description of the Related Art
FIG. 25 shows an example of the constitution of a conventional optical wavelength division multiplexing network. The network shown in FIG. 25 has a ring structure comprising two or three layers. The network of FIG. 25 will be explained in separate sections comprising a network (1) 11, a network (2) 12, a network (3) 13, a network (4) 14, and a network (5) 15. The network (1) 11 has a ring structure, and is provided at the highest level. The network (1) 11 comprises at least one center node 21 and two or more (three in FIG. 1) remote nodes 22, 23, and 24. The network (2) 12 is a ring network comprising a node (#4) 24, which is one of the remote nodes of the network (1) 11, and is provided below the network (1) 11. The network (3) 13 comprises a tree-shaped structure centered on a node (#41) 25, which is one of the nodes of the network (2) 12, and is provided below the network (2) 12. The network (4) 14 comprises a ring-shaped structure centered on a node (#3) 23, which is one of the remote nodes of the network (1) 11, and is provided below the network (1) 11. The network (5) 15 comprises a tree-shaped structure centered on a node 27, which is one of a plurality of nodes 23, 26, 27, and 28 of the network (4) 14, and can conceivably be provided below the network (4) 14. Optical network units (ONU) (they are also called optical service units.) 51 to 56 comprise the subscribers of each home, business office, and the like, and are provided in the network (3) 13 or the network (5) 15.
In FIG. 25, the nodes of network (1) 11, the network (2) 12, and the network (4) 14, are connected by optical fiber transmission paths 60, 60, . . . which comprise a plurality of optical fibers. The ONUs are connected to the nodes of the networks (3) 13 and (5) 15 by optical fiber transmission paths 70, 70, . . . which comprise a single optical fiber. Equipment for signal termination electrically processes transmission signals, which have been converted from optical signals to electrical signals, and are provided at the nodes 21 to 28.
In this explanation, traffic from subscribers in the network (3) 13 or the network (5) 15 is assumed to be 1.5 Mb/s. The traffic from the subscribers is multiplexed at the subscriber office (node 25 or node 27), and transmitted to the node (#4) 24 in the network (2) 12 or the remote node (#3) 23 in the network (1) 11 at a higher transmission speed of, for example, 52 Mb/s. At the node (#41) 25 and the node 27, traffic sent from other nodes in the network (2) 12 or the network (4) 14 is combined with the multiplexed traffic from the subscribers, and transmitted to the next node in the network (2) 12 or the network (4) 14 at an even high transmission speed. Transmission speed conversion and the like is also carried out at the nodes in the network (1) 11. That is, electrical processing is carried out at each node.
In conventional optical networks such as that shown in FIG. 25, when starting a new high-speed access service for users at a speed of, for example, approximately 150 Mb/s or 1 Gb/s, transmission apparatuses which carry out electrical processing for multiplexing the traffic must be provided at each node, since there are several users belonging to the network (3) 13 or the network (5) 15. Consequently, the initial expenditure is considerable. Moreover, depending on the region, there may be fewer users per node, leading to a drawback of expenditure efficiency.