Multiplex communications – Wide area network – Packet switching
Patent
1989-09-25
1991-10-15
Olms, Douglas W.
Multiplex communications
Wide area network
Packet switching
34082551, 34082552, 370 941, H04J 302, H04J 324
Patent
active
050581082
DESCRIPTION:
BRIEF SUMMARY
times out and if the data waiting flag is set, the device server transmits a message through the virtual circuit. Until the server circuit timer times out, the device server is inhibited from transmitting a new message over the virtual circuit. Furthermore, the data waiting flag inhibits the device server from transmitting any messages until the data waiting flag is set, indicating it has received a message from the node in which the response requested flag is set or that is has new slot data from the service users to send.
The response requested flag allows the node to immediately acknowledge a message received from the device server, whether or not it has data to send immediately, and to reserve for itself the ability to transmit a second message which has data if no data is sent in the initial message, which second message can be sent even if it receives no intervening message from the device server. The response requested flag in conjunction with the data waiting flag allows the node to force the device server to acknowledge a message that includes data when the server circuit timer next times out, whether or not the device server has data to send. The server circuit timer establishes a minimum delay period after the device server transmits a message before it transmits a subsequent message, thereby allowing other device servers to transmit messages through virtual circuits which they have established over the communications link. The device server and the node thus have the benefit of both a timer based system, based on the server circuit timer, and an event driven system, based on the response requested and data waiting flags.
BRIEF DESCRIPTION OF THE DRAWINGS
This invention is pointed out with particularity in the appended claims. The above and further advantages of this invention may be better understood by referring to the following detailed description taken in conjunction with the accompanying drawings in which:
FIG. 1 is a general block diagram of a local area network constructed in accordance with this invention;
FIG. 2A is a diagram illustrating the contents of a service advertising message transmitted by the service providers in the network depicted in FIG. 1, and FIG. 2B is a diagram of a data base established by the service users in the network of FIG. 1 in response to the service advertising message depicted in FIG. 2A;
FIG. 3A is a diagram depicting a virtual circuit and service sessions, that is useful in understanding the operations of the network depicted in FIG. 1;
FIGS. 3B and 3C are diagrams depicting databases used by the service providers and service users in the network depicted in FIG. 1;
FIG. 4 is a state diagram useful in understanding the operation of the virtual circuit depicted in FIG. 3A;
FIG. 5 is a state diagram useful in understanding the operation of the service sessions depicted in FIG. 1;
FIGS. 6A through 6D depict various formats of virtual circuit messages transmitted through the virtual circuit depicted in FIG. 3A;
FIGS. 7A through 7D depict various formats of session slot messages transmitted between corresponding sessions in the device servers and nodes as depicted in FIG. 3A; and
FIG. 8 depicts the timings of messages transmitted through the virtual circuits as depicted in FIG. 3A.
DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT
FIG. 1 illustrates a local area network 10 in which a plurality of service users, generally indicated by reference numeral 12, communicate with a plurality of service providers, generally indicated by reference numeral 14, over a common communications link 16. A communications link 16 may take the form of any one of a number of communications lines and interface circuitry which transfer data between the service users and service providers in bit serial or parallel form. For example, the communications link may take the form of a coaxial cable and interface circuitry which transmits messages using the well-known Ethernet local area network protocol. In that protocol, data is organized into messages having a predetermined format
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This is a continuation of copending application Ser. No. 07/338,485 filed on Apr. 13, 1989, now abandoned, which is a continuation of Ser. No. 07/178,430 filed on Apr. 6, 1988, now abandoned, which is a division of Ser. No. 07/088,063 filed on Aug. 21, 1987, now U.S. Pat. No. 4,823,122, which is a division of Ser. No. 07/027,033 filed on Mar. 19, 1987, now abandoned, which is a continuation of Ser. No. 06/616,553 filed on June 1, 1984, now abandoned.
Duffy Darrell
Lauck Anthony
Mann Bruce
Strecker William
Chin Wellington
Digital Equipment Corporation
Olms Douglas W.
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