Multiplex communications – Wide area network – Packet switching
Patent
1987-05-29
1989-05-02
Pellinen, A. D.
Multiplex communications
Wide area network
Packet switching
370 94, H04J 312
Patent
active
048274739
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
1. Technical Field
The present invention relates to a packet switching system and, more particularly, to a packet switching system for achieving high-speed and large-capacity packet switching on communications lines conforming to the recommendation X.25 of the International Telegraph and Telephone Consultative Committee (C.C.I.T.T.).
2. Background Art
A packet switching system, the importance of which as a data communications system is generally recognized, is particularly effective for short-burst traffic. As packet switching has a concept of logical multiplexing and uses a line only when data is actually present, it contributes to efficient use of the line. Since packet switching further has error recovery and flow control functions, it permits reliable transmission and reception of data as well as communication between terminals having different speeds.
Despite these many advantages, a packet switching system commercially available at present has the disadvantages that it can handle a line speed of at most 64 Kbps, and that the processing capacity of a whole system is only a few thousand packets per second (pps), or tens of pps per line. Consequently, no commercially available packet switching systems can handle high-speed short-burst traffic of 1 to 10 megabits per second (Mbps) used by local area networks (LAN's), resulting in inability to expand burst characteristics in a given local area.
Obviation of these disadvantages requires a packet switching system capable of handling line speed in the order of megabits per second. Methods for accelerating packet switching have been proposed in U.S. Pats. Nos. 4,491,945 and 4,494,230. These proposed methods include simplifying the protocol used in packet switching, eliminating the flow control and error recovery functions from within the packet switching system and having these functions performed between terminals. These methods however, involve the following problems, and therefore cannot be used for a packet switching system for commerical data communications.
Thus, since plural combinations of packet terminals simultaneously communicate via plural logical channels, the error recovery functions have to simultaneously cover the plural combinations of packet terminals, and those functions, including CRC (cyclic redundancy check) and request-for-retransmission functions performed between the terminals, have to be independent for each logical channel. As a result, the whole system becomes complicated, resulting in higher cost and lower speed of the terminals.
On the other hand, the protocol prescribed by the C.C.I.T.T. recommendation X.25 leaves such error recovery functions to the X.25 Level 2 (Data Link Procedure). This ensures the reliability of link-by-link data transfers between terminals and the switching system, so that the terminals need not be conscious of logical channels in the performance of the CRC or request-for-retransmission functions and their loads are less than a terminal-by-terminal protocol would impose. This consideration points to a problem inherent in the assignment of error recovery functions to the terminals from the viewpoint of alleviating the protocol loads on the terminals.
A packet terminal on the transmitting side, after transmitting a data frame (packet), has to keep that transmitted data frame until a signal of "acknowledgment" is received from the packet switching system or the other terminal in preparation for a possible request for retransmission due to an error. Meanwhile, the number of packet buffers on the receiving side limits the number of outstanding frames of packets (denominated in k) that can be transmitted before a signal of "acknowledgment" arrives. Now, among the values of k, line delay d (sec), data frame length l (bits), data transfer velocity v (bps) and line utilization frequency .xi., there is a fundamental relationship of .xi.=(l/2d).multidot.(k/v). The line delay, which is about 10 ms (milliseconds) on a terrestrial line, is 250 ms or so on a satellite line. Then, when a flow control is effec
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Rothauser et al., "Meshed-Star Networks for Local Communication Systems", pp. 25-40, Aug. 29, 1980.
Restorick et al., "A Multi-Microprocessor Design for Use in a Packet Switched Network", pp. 811-816, Sep. 10, 1982.
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Anezaki Mitsugu
Ishizuka Toshio
Tsuzuki Kazuo
Yoshida Yoshinori
Evans Geoffrey S.
NEC Corporation
Pellinen A. D.
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