The number of trunk lines needed between linked systems will depend on the maximum number of simultaneous conversations, and whether ‘Pre-Mix Intelligent Link Sharing Mode’ is enabled or not.

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Pre-Mix Intelligent Link Sharing Mode enabled:
With Pre-Mix Intelligent Link Sharing Mode enabled, multiple sources talking to the same remote destination will use only one of the available trunks.

This example will use 1 of the available trunks:
A talking to X
B talking to X (Audio is pre-mixed on System 1 and then trunked to System 2)
C talking to X

This example will use 1 of the available trunks:
A talking to X
A talking to Y (Audio is trunked to System 2 and then split to the destination ports)
A talking to Z

This example will use 3 of the available trunks:
A talking to X
B talking to Y
C talking to Z

Pro: It allows for a more efficient use of available trunks by pre-mixing audio from multiple sources to the same destination onto the same trunk.
Con: The remote matrix can not control the level of each of the pre-mixed sources separately.

Pre-Mix Intelligent Link Sharing Mode disabled:
With Pre-Mix Intelligent Link Sharing Mode disabled, a common source talking to multiple destinations will still use only 1 trunk, but multiple sources talking to the same destination will use a new available trunk for each source.

This example will use 3 of the available trunks:
A talking to X
B talking to X (Audio is trunked to System 2 via 3 separate trunks and then mixed at the destination)
C talking to X

This example will use 1 of the available trunks:
A talking to X
A talking to Y (Audio is trunked to System 2 and then split to the destination ports)
A talking to Z

This example will use 3 of the available trunks:
A talking to X
B talking to Y
C talking to Z

Pro: The remote matrix can control the level of each of the sources separately.
Con: It is not an efficient use of the available trunks. Each new audio route will use an available trunk (even if destination port is the same).