1. Field of the Invention
This invention relates to a device that controls bands assigned to signal channels in a packet network. In particular, it relates to an ATM multiplexing device that controls bands for virtual channels used for signalling in an ATM network.
2. Description of the Related Art
ATM is widespread as a core technology for making multimedia communication possible. In ATM, data are stored in fixed length packets called cells and transferred. A cell basically consists of a 5-byte header and a 48-byte payload. A VPI/VCI (Virtual Path Identifier/Virtual Channel Identifier) is set in the header as routing information for each cell. The information to be transferred is stored in the payload. For this reason, ATM is able to handle various types of data such as audio data, video data and text data in a unified manner. In addition, in ATM control data are also stored in the cells and transferred.
FIG. 1 is a configuration diagram of an existing ATM system. The ATM multiplexing nodes (AWNs) 501 and 502 each accommodate a PBX, data device, switching device, computer terminal, etc.; data received by them are multiplexed and sent to the ATM network, and, at the same time, cells received from the ATM network are transferred to the devices and terminals determined by the routing information in each cell.
In ATM, first a connection for the purpose of transferring data (a virtual connection) is established by signaling processing. Then the data are transferred through the connection that has been established. In the signaling processing, messages such as SETUP and CONNECT are used. Each of these messages is stored in one or more cells, and are transferred on the ATM network via Virtual Channels (referred to below as VCs) that are used for signals.
The VCs used for signals are normally fixed in advance for each path. In the example shown in FIG. 1, Virtual signal Paths (signal VPs) are fixed on the transmission path that connects ATM multiplexing nodes to one another; and a plurality of signal VCs are fixed on those signal VPs. In this case, a “path” is a transmission path that connects 2 points. For example, in a case in which 2 optical fibers are connected between device A and device B, those 2 optical fibers belong to the same path. However, in a case in which there are transmission paths connected between device A and device B, between device B and device C, and between device C and device A, even though it is possible to go from device A to device B either directly or via device C, those 2 paths are not called the same path.
The bands assigned to each signal VC can be decided upon arbitrarily when the system is constructed. In the example shown in FIG. 1, n kbps have been secured on the transmission path between the ATM multiplexing nodes 501 and 502 as the signal VC for a connection between PBXs. In an existing system, a typical signal VC band would be 64 kbps. It is also possible to perform signaling processing for a plurality of types of media using one signal VC. Standards for signal VCs are being considered by the ATM forum VTOA (STR-VTOA-LLT-01.12).
Let us now simply explain the signaling processing. Here, we show the case in which a call is placed from the telephone 503 to the telephone 506. When the telephone number of the telephone 506 is entered into the telephone 503, the PBX504 posts this number to the ATM multiplexing node 501. The ATM multiplexing node 501 creates a SETUP message in accordance with that number, and then transfers that message to the ATM multiplexing node 502 via the signal VC. The ATM multiplexing node 502 determines the virtual channel to be assigned to the connection between the telephone 503 and the telephone 506 in accordance with the received message, and then returns a response message to the ATM multiplexing node 501 via the signal VC. By this processing, information needed for connecting the telephone 503 to the telephone 506 is set in the ATM multiplexing nodes 501 and 502. Then the PBX 505 calls the telephone 506, to open a communication line between the telephone 503 and the telephone 506. The signal VC that is used to connect the telephone 503 to the telephone 506 is normally maintained, without being cut off, until the communication between the telephone 503 and the telephone 506 is completed.
Normally a signal VC can be shared by a plurality of calls. However, in an existing system, once the band (capacity) for each signal VC is first set, that band cannot be dynamically varied. Consequently, if the number of calls that are made at one time decreases, the amount of band capacity within the signal VC that is not being used increases. That is to say, the efficiency of band use decreases. In the example shown in FIG. 1, the amount of band capacity that has been assigned is enough to process s calls on the signal VC. In this case, if the number of calls that are made at one time is z (z<s), then enough band capacity for s−z calls is unused. This frequently happens, for example, late at night and on holidays.
Meanwhile, when the traffic, or number of calls made at one time, increases, cells are discarded. To avoid this discarding of cells, there is a function in the ATM network that delays the transfer of cells; it is necessary to prevent the transmission rate from exceeding the maximum band capacity of that signal VP by averaging the time between sending of cells that transmit information needed for signaling. However, it is desirable for such delays to be kept as small as possible.
Thus, in an existing type of ATM multiplexing node, it has not been possible for the band capacity of a signal VC to be dynamically varied, causing the efficiency of use of band capacity to decrease, or causing cells to be discarded or delays to occur.