(1) Field of the Invention
The present invention generally relates to a frame relay system connecting networks to each other, particularly to a frame relay system which relays frames each of which has a destination address and a frame time-to-live (TTL) as control information. The frame TTL indicates the number of times which a frame can be relayed.
(2) Description of the Related Art
In a frame relay system connecting network to each other, a frame having a format as shown in FIG. 1 is used. That is, the frame has a destination address 1, a frame time-to-live (TTL) 2 and data 3.
A conventional communication is preformed in accordance with a procedure in which a transmission terminal transmits frames, each frame having the frame TTL set at a value, and the frame relay system decrements the value of the frame TTL when relaying each of the frames. The frame relay system has a unit for deleting a frame when the value of the frame TTL reaches zero "0". That is, each of the frames is provided with a term of life so that frames are not infinitely circulating through a network.
A decrement process of the frame TTL in the frame relay system is applied to all the frames which should be relayed. As a result, the delay time of the relay process is increased, so that the relay process is inhibited from being performed at a high speed.
FIG. 2 is a block diagram illustrating a conventional frame relay system. Referring to FIG. 2, the frame relay system has a frame receiving unit 4 and a frame transmitting unit 5. The frame receiving unit 4 receives frames from a network and converts a format of each of the received frames into a format which can be processed in the system. The frame transmitting unit 5 converts a format of each of frame processed in the system into a format which can be output to the network and outputs the processed frame to the network. A set of the frame receiving unit 4 and the frame transmitting unit 5 is provided for each of the networks to which the system is connected. In this example shown in FIG. 2, a set of the frame receiving unit 4 and the frame transmitting unit 5 is provided for each of the networks 1 and 2.
The frame relay system further has a frame relay control unit 10, a path control unit 20 and a destination address extracting unit 6. The frame relay control unit 10 is connected to the frame receiving unit 4 and the frame transmitting unit 5 and carries out a frame relay control process. The frame relay control unit 10 has a frame forwarding block 11 for forwarding frames to the network, a TTL decrement block 12 for decrementing the value of the frame TTL and a header-check-sum calculation block 13.
The path control unit 20 is connected to the frame relay control unit 10 and controls paths. The path control unit 20 has a routing table 21, a table retrieving block 22 and a table forming/updating block 23. The routing table 21 indicates a destination address of each frame and control information corresponding to the destination address. The table retrieving block 22 retrieves that routing table 21. The table forming/updating block 23 forms and updates the routing table 21.
The destination address extracting unit 6 extracts a destination address from each frame received by the frame receiving unit 4. The destination address extracted by the destination address extracting unit 6 is supplied to the path control unit 20.
The conventional frame relay system performs processes in accordance with procedures as shown in FIG. 3. A description will now be given, with reference to FIG. 3, of the frame relay operation.
The frame receiving unit 4 receives a frame from the network and converts the format of the frame into a format which can be processed in the system (a frame receiving process S1).
The destination address extracting unit 6 extracts a destination address from the frame received by the frame receiving unit 4 (S2). The table retrieving block 22 receives the destination address from the destination address extracting unit 6 and retrieves the routing table 21 based on the received destination address (S3). It is checked whether the received destination address corresponds to the present frame relay system (the present station) (S4). The routing table 21 indicates a relationship between destination addresses and output ports as shown in FIG. 4 so that an output port can be decided based on a destination address.
The routing table 21 is formed by deciding a path, namely an output port, corresponding to a destination address in a received path information frame.
When the destination address does not correspond to the present frame relay system, the TTL decrement block 12 decrements a value of the frame TTL to avoid infinitely circulating through the network. If the value of the frame TTL reaches zero "0", the frame is deleted.
The frame forwarding block 11 forwards the frame to the frame transmitting unit 5 corresponding to the output port decided by the table retrieving block 22.
The frame transmitting unit 5 corresponding to the output port converts the frame format used in the system into a frame format which can be output to the network and then outputs the frame to the network.
When it is determined, in step S4, that the destination address corresponds to the present frame relay system, the system determines whether the received frame is a path information frame (S5). If the received frame is the path information frame, the system forms or updates the routing table 21 (S6).
When the frame is relayed with the decrement of the value of the frame TTL, the process time is increased. Thus, to achieve the fast relay process, a method for relaying the frame without the decrement of the value of the frame TTL may be used.
Even if frames are relayed without the decrement of the value of the frame TTL, due to forwarding of frames in accordance with the routing table, the frames can be relayed without errors in a case where the network is formed in a tree-structure. However, if the network structure is complex so that a circular path exits in the network, frames may circulates through the network so as to not reach any stations.
A description will now be given of states where the frame circulation occurs.