| 摘要 |
A local area data network (26) interconnects devices (12, 14, 20, 30) which make up a video surveillance system (10). The devices attached to the local area network are divided into distinct classes (12, 14, 30) which are polled with different degrees of frequency. Devices are dynamically reassigned from class to class depending on each device's current requirements for network access. Group polling is also employed, with resolution of contention by means of a probing algorithm. Negative responses to polling messages are formed as a series of line transitions that require less time than transmission of a single data byte. Polling messages have a distinct length in terms of bytes that is shorter than any other message transmitted on a network. Polled devices (12, 14) are permitted to transfer data directly to peer devices in response to polling messages.
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