Communications: electrical – Continuously variable indicating – With meter reading
Patent
1986-12-11
1988-10-18
Griffin, Robert L.
Communications: electrical
Continuously variable indicating
With meter reading
3408252, 34082514, 370 86, 370100, 375107, H04L 700, H04L 1116, H04J 306
Patent
active
047790873
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a loop transmission system with frame synchronization control. The system according to the present invention is useful, for example, for frame synchronization control in which a frame synchronization cancellation signal is used for the control for taking-in or bypassing a node in a loop transmission line in a local area network (LAN) in association with computers distributed along the LAN.
DESCRIPTION OF THE RELATED ART
In general, in order to protect against errors in frame synchronization signals in a loop transmission system with frame synchronization control used, for example, for a LAN, the frame synchronization signals are monitored at regular intervals at which the frame synchronization signals are inherently expected to occur.
In this case, usually the synchronization state is maintained even if, for example, absence of the frame synchronization signal is detected once or so. Only after the absence of the frame synchronization signal has been detected a predetermined number of times is synchronization decided as lost and a new synchronization operation performed. Similarly, synchronization is not confirmed upon detection of receipt of a single frame synchronization signal, for example. Only after detection of the receipt of a predetermined number of the frame synchronization signals with predetermined timings is the frame synchronization decided as restored and the normal operation of reception and transmission of information enabled.
Assume that a supervisory device and a plurality of nodes are connected by a transmission loop to constitute a loop transmission system. The information transmitted through the transmission loop consists of a sequence of frames, each of the frames consisting of a frame header having frame synchronization signal (EP) and data.
Sometimes, a node in the transmission loop is bypassed because of a shutdown of the power source of the node in question. In this case, dummy data is produced in the supervisory device and the dummy data transmitted through the transmission loop.
When the node in question detects the dummy frame synchronization signal FP(D) in the frame header of the dummy data, the input portion and the output portion of the node in question are short-circuited so that the node in question in the loop transmission system is bypassed. In this case, the bypassing instruction is included in the frame header in addition to the synchronization signal
Since the bypassing process is carried out at a timing within the duration of the dummy data, the length of transmission is reduced by the short-circuiting of the input portion and the output portion of the node in question. Hence, in appearance, a certain length of region is eliminated from the dummy data.
Due to this, there is a problem that, at the nodes downstream of the node in question, after the detection of a frame synchronization signal (FP), the next frame synchronization signal cannot be detected because of a timing error due to the reduction of length of the dummy data, so that the transmission and the reception of information in a number of frames from the frame subsequent to the frame synchronization signal in question are prevented until the restoration of synchronization by a synchronization operation. Furthermore, there is a problem that the nodes upstream of the node in question continue to transmit information regardless of the failure of synchronization until the failure of synchronization is acknowledged, while the nodes downstream of the node in question cannot receive such information transmitted from such upstream nodes.
Similarly, failure of synchronization can occur when a node is taken into the transmission loop.
When the taking-in of a node which has been bypassed to the transmission loop is informed from this node to the supervisory device, for example, by telephone, the supervisory device delivers dummy data. When the dummy frame synchronization signal FP(D) is detected by the node awaiting the taking-in, the short-circuit betw
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IEEE Global Telecommunications Conference; vol. 1; 11/28/83 to 12/1/83; "Synchronization Design Criteria for a Token Ring", H. Meyr et al.; pp. 13.4.1-13.4.9.
Awazu Tomohiki
Fukuda Haruki
Matsuda Masahiro
Tazaki Kenshi
Fujitsu Limited
Griffin Robert L.
Smith Ralph E.
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