1. Field of the Invention
The present invention relates to a technology for setting bandwidths of optical communication channels in an optical network. More particularly, the present invention relates to a technology for eliminating bandwidth separation occurring in a transmission channel.
2. Description of the Related Art
In large-scale networks such as the ones forming the backbone of the Internet, Metropolitan Area Network (MAN), etc. optical fibers with little loss and broadband have come to be widely used as transmission medium.
As a basic optical transmission technology used in an optical network may be cited Synchronous Optical Network/Synchronous Digital hierarchy (SONET/SDH). The SONET/SDH technology involves hierarchically multiplexing a plurality of low-speed channels (for example, telephone lines, etc.) into a channel and realizing a high-speed and large-capacity optical network. Multiplexing in SONET/SDH is accomplished by using a TDM system in which data is divided into packets according to fixed-length time-slots. The packets are grouped into a frame, and frames are transmitted sequentially. In this way, the bandwidth of each transmission channel on the optical network is divided and utilized.
Generalized Multi-Protocol Label Switching (GMPLS) has recently attracted attention as a technology for controlling transmission channels of the optical network that carry optical signals. GMPLS is Multi-Protocol Label Switching (MPLS), in which high-speed switching is realized by adding a label to an IP packet, generalized to extend the scope of applicability beyond IP networks to encompass TDM networks, optical wavelength multiplexed networks, etc. GMPLS simplifies traffic engineering, which involves control of routes and bandwidths of the optical communication channels (see, for example, “Generalized Multi-Protocol Label Switching (GMPLS) Extensions for Synchronous Optical Network (SONET) and Synchronous Digital Hierarchy (SDH) Control” by E. Mannie, and D. Papadimitriou, [online], December, 2005, retrieved from URL-http://www.ietf.org/internet-drafts/draft-ietf-ccamp-rfc3946bis-01.txt, on Feb. 7, 2006).
How the optical communication channel (hereinafter, “path”) is set up and released in the optical network using GMPLS is specifically explained next. An optical network employing the SONET/SDH system is presented as an example.
When setting up a path, a network administrator enters data such as optical transmission devices to be the start point and the end point of a path, and a required bandwidth into one optical transmission device from a network management system. Once the data is entered, each optical transmission device builds a physical topology table of the network using a protocol such as Open Shortest Path First-Traffic Engineering (OSPF-TE), and, based on the physical topology table, determines optical transmission devices to be passed through by the path and optical fibers to be used. Each optical transmission device then determines the bandwidth for the path after assessing whether the path and the required bandwidth are accepted in the optical transmission devices using a protocol such as Resource Reservation Protocol-Traffic Engineering (RSVP-TE).
The network administrator then enters a cross-connect command (such as ENT-CRS-STS3c) from the network management system, which initiates cross-connection (switching between the optical fibers) in each optical transmission devices. Thus, a path is set up in the optical network.
To release the path, the network administrator enters a release path command. Alternatively, an RSVP-TE protocol is implemented when a release path instruction is given based on the monitored port status.
FIG. 12 is a schematic for explaining the optical network using GMPLS. In the optical network, optical transmission devices 101 to 109 are connected to one another through transmission channels (optical fibers). For setting up a path having a transmission rate of 155.52 Mbps, for example, Optical Carrier-Level 3 (OC-3 in SONET), from the optical transmission device 101 to the optical transmission device 109 of the optical network, the network administrator first specifies the optical transmission device 101 as the start point of the path and the optical transmission device 109 as the end point of the path.
It is assumed that each of the transmission channels (optical fibers) that connect the optical transmission devices 101 and 102, the optical transmission devices 102 and 105, the optical transmission devices 105 and 108, and the optical transmission devices 108 and 109 has a bandwidth of 2.4 Gbps. Because a bandwidth of 155.52 Mbps is ensured in all these transmission channels, a path is set up in the optical network starting from the optical transmission device 101 and ending at the optical transmission device 109 through the optical transmission devices 102, 105, and 108.
However, in the SONET/SDH system, the position of data of each path in a frame is indicated by a pointer set in a header portion of the frame. Consequently, it is not possible to divide up the data pertaining to the same path into a plurality of time slots that are not continuous in the frame. Therefore, repeated setup and release of paths in the optical network described above results in separation of the bandwidths of the transmission channels.
FIG. 13 is a schematic for explaining separation in the transmission channels in the optical network using GMPLS. FIG. 13 depicts a status of part of transmission channels (having five continuous bandwidths) in the optical network after paths have been set up and released between the optical transmission devices 101 and 104. Specifically, FIG. 13 depicts bandwidths 21a to 21e separated from each other in a transmission channel 21, bandwidths 22a to 22e separated from each other in a transmission channel 22, and bandwidths 23a to 23e separated from each other in a transmission channel 23. Such separation occurs, for example, when five paths, including paths 10a and 10b, each having a transmission rate of 155.52 Mpbs are set up in the transmission channel 21, 22 and 23, and after that, paths using the bandwidths 21a, 21c, 21e, 22b, 22c, 22e, 23a, 23d, and 21e (i.e., paths other than the paths 10a and 10b) are released.
In the transmission channel 21, for example, a bandwidth of 466.56 Mbps (the sum of the bandwidths 21a, 21c, and 21e) remains unused. However, a path with a transmission rate of 466.56 Mpbs (such as OC-9 in SONET) cannot be set due to the bandwidths being non-continuous.
Thus, there is lack of flexibility in setting up paths due to separation of bandwidths in the transmission channels.