res a knowledge of how



res a knowledge of how changes can spawn other changes within a device and / or a system. A consequence of these difficulties is that the performance of COV systems can be unpredictable.

Convert Polarsoft BACnet data to the protocol intergrate

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The offline Subscriber problem


A subscriber may be offline when a notification is sent. If the subscription required confirmed notification then this could present a temporary but significant loss of bandwidth while the COS server waits the timeout period before sending the next notification. If no confirmation was required then there is no way of knowing that the subscriber was offline and even if it wanted to the COS server device cannot signal this in anyway.

The COV system does not require vendors to manage notifications sent to offline subscribers. Thus the COV server does not have to queue them and resend them. Thus, it is possible for a COV client to lose its data synchronization with the Server. The only way around this is to use occasional polling. However, if the data client was a logging system then the damage is already done and the log records will not be made.



Specific Limitations of BACnet COV

Subscriptions may be lost on reset

The BACnet spec does not require vendors to maintain the subscriptions if the device resets. Thus a reset may result in the loss of all subscriptions. Now the system is dependent on the frequency of the re-subscription by the data client.  That frequency is a vendor choice too and some systems don't send periodic re-subscriptions.

Single/Multiple Subscriptions

A single data client can potentially subscribe more than once to the same data object. This is possible because in identifying itself, in the subscription, the client provides two elements of information - identifier and handle. The identifier  tells the server where to send responses. The handle is a number allocated by the client for some internal purpose. Changing the handle is enough to make a subscription unique and hence it is possible to have multiple subscriptions to