Two levels of priority are supported on the network: "high priority" is used for traffic for which the system has reserved bandwidth and to which it has given guaranteed QoS ("Quality of Service"), while "low priority" is used for "best-effort" traffic.
Unlike other networks, high priority traffic can use up to 97% of the network bandwidth and "best effort" traffic can safely occupy all of the remaining capacity, including bandwidth that has been reserved but is not used by the flow for which it was reserved. Low priority messages that have been accepted onto a segment will not be discarded although they may of course be lost if cable faults occur.
Failure of a link is detected immediately, and connections can be quickly re-routed to avoid the failed link. An alarm is also raised, so that the break can be repaired as a matter of urgency.
The following service classes are supported:
This class of service is intended for safety-critical data such as alarms, also for important mmedia streams such as "on air" signals for broadcasters. It is only supported where two completely separate routes are available, for instance between two nodes on a ring (where one route goes around the ring in each direction) or between sites that have connections via two separate wide area networks.
Two copies of each data package are transmitted, one via each route. Both routes are high-priority, and the maximum transmission rate is registered with the Connection Admission Control mechanism before any packages are sent.
This class of service is intended for audio and video, both point-to-point and broadcasts.
One copy of each data package is sent, on a high-priority route, and the maximum transmission rate is registered with the Connection Admission Control mechanism, before any packages are sent.
When a link fails, the all nodes will be notified within a few milliseconds; if the break stops transmissions reaching their destination, renegotiation with the Connection Admission Control mechanism is required. The interruption at a receiver when this occurs should be a small fraction of a second.
The third class service is intended for all data messages other than safety-critical alarms.
One copy of each data package is sent, on a low-priority route. Within a segment, low priority data uses the capacity not required for high priority data and can be overtaken by high priority data. It can use all the resources not required for other traffic and will not be discarded by the system.
Low-priority data will not impede high-priority traffic, so no Connection Admission Control is required for third class traffic. A "fairness" mechanism prevents any one node from flooding a segment with enough data to prevent other nodes from transmitting.
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