Multiplex communications – Wide area network – Packet switching
Patent
1993-06-01
1994-07-12
Olms, Douglas W.
Multiplex communications
Wide area network
Packet switching
370 853, 370 941, H04J 314, H04L 1228
Patent
active
053295196
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method and apparatus for monitoring packet-based LANs (local area networks) that utilize the media access control method known as CSMA/CD (Carrier Sense Multiple Access with Collision Detection).
BACKGROUND ART
As is well known, in a CSMA/CD LAN a number of stations are interconnected through a single shared cable (electrical or optical) logically configured as a bus (though possibly physically configured as a bus, tree or star). Data is transmitted between the stations in packets and generally signals are normally only present on the LAN when user data or control data is being transferred (that is, the LAN is idle--no signals present--in the gaps between packets). A station wishing to transmit data first checks that the LAN has been idle long enough for all transmissions to have propagated across the LAN; providing this check shows the LAN to be idle, the station then proceeds to transmit its data. It is, of course, still possible for two stations to start transmission at approximately the same instant and in this case there will be a collision between the two packets being transmitted. Each transmitting station listens out for any such collision and if one does occur, the station erases transmission after having first transmitted a jam sequence that ensures the collision is detected by all the stations concerned. The stations responsible for transmitting the colliding packets will then attempt to retransmit their data after a short, random, delay.
If the LAN is functioning correctly, collisions should only occur within a predetermined time interval following the start of transmission by any particular station. The duration of this time interval is dependent on the longest propagation delay between any pair of stations on the network (or where the network is made up of several segments interconnected by repeaters, between any pair of stations on the relevant network segment), the time taken by a station to launch a packet, and the time taken for a station to detect a collision. In practice, where a LAN is designed in accordance with a particular technical standard, this predetermined time interval will normally be determined with reference to the maximum values of the defining parameters as set down in the technical standard.
In some LAN fault conditions, packet collisions may occur after the initial time interval where such collisions are expected. It is possible to observe such collisions using network analyzers; however, such items of equipment are expensive and complex, requiring much skill to set them up to observe late collisions.
It is an object of the present invention to provide a method and apparatus that facilitate the monitoring of late collisions on CSMA/CD LANs.
DISCLOSURE OF THE INVENTION
According to one aspect of the present invention, there is provided apparatus for monitoring a CSMA/CD LAN, the said apparatus comprising: the LAN whilst operational; the presence of a packet on the LAN and to generate a first output signal in response thereto; packet collision on the LAN and to generate a second output signal in response thereto; and output signals and operative to provide a late-collision output indication when the second output signal indicates the occurrence of a packet collision subsequent to the elapse of a predetermined time interval from the start of the last preceding packet as indicated by the first output signal.
The late-collision output indication, which is preferably a visual output, enables a user of the apparatus to readily discern the presence of late collisions and therefore of a fault on the LAN.
Advantageously, the late-collision detector means comprises timing means operative to generate an inhibit signal in response to said first output signal indicating the start of a packet on the LAN, said inhibit signal having a duration, from the time said first output signal indicates the start of the last preceding packet, corresponding to said predetermined time interval; and gating means connected to receive said secon
REFERENCES:
patent: 4779088 (1988-10-01), Restrepo
patent: 4916694 (1990-04-01), Roth
patent: 5012467 (1991-04-01), Crane
patent: 5185735 (1993-02-01), Ernst
patent: 5267237 (1993-11-01), Townley
Hewlett -Packard Company
Kizou Hassan
Olms Douglas W.
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