In this protocol, all LPs synchronize after every Q simulation time instants, where Q is termed the simulation quantum. This periodic global synchronization affects only the semantics of receive statements in the simulation model: upon executing a receive statement, an LP accepts the earliest matching message only if it has a receive timestamp less than the simulation time instant at which the current quantum terminates. Otherwise, the LP remains blocked, and its simulation time is updated to the time instant at which the current quantum terminates. The receive statement is re-executed in the following quantum.
The quantum protocol is safe only
if the simulation quantum, Q, is less than the minimum
communication latency, L, of the target machine. The proof
of this statement is as follows:
Messages sent from one LP to another
in one quantum are guaranteed to have been deposited in the
recipient's message queue before
the next quantum starts, because the periodic global synchronization
flushes messages in transit. In the
pth quantum, which covers the simulation interval 
,
an LP can select only messages which have a receive timestamp less than
. Consequently, an upper bound on the send timestamp of a selected
message is 
. If Q<L, 
. Consequently, in the
pth quantum, only messages that have been sent in the previous or
earlier quantum can be selected for acceptance. Because of the global
synchronization, all such messages must already be present in the
LP's queue. Consequently, the message selection cannot be invalidated
eventually.