Question

Differentiate among open, closed and semi-open queues.

Answer 1

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If you look very closely, the arrow are going out... in network C.

←⋅←⋅

So CC is "open", while BB has information going in and out. It is mixed:

←⋅←←⋅←

AA is closed since no information is coming in or out:

⋅←⋅⋅←⋅

⋅→⋅C←⋅B→⋅A⋅→⋅C←⋅B→⋅A

Please look at the exercise on box algebraby knot-theorist Lou Kaufmann, or this article on the little 2-cubes operad by Jim Stasheff

Users↔Proxy ↔Video ServerA↔AdminBCUsers↔Proxy ↔Video ServerA↔AdminBC

Here are the definitions from your notes:

Open model in which jobs departing from one queue arrive at another queue (or possibly the same queue)Closed Closed queueing network: No external arrivals or departures; Total number of jobs in the system is constantMixed Open for some workloads and closed for others ⇒ Two classes of jobs.

Network C is open to external information. A is closed there nothing going in and out. B is open to user jobs but closed to admin jobs.