Below is a Definition of Queueing Theory, a glossary, and vocabulary. Knowing the concepts below will help you better understand these articles on Queues.
- Queue: A line (or buffer or inventory) feeding a number of servers
- Server: An operation fed by a queue.
- Arrival rate (λ): Mean number of arrivals per unit time (usually per hour or day).
- Service rate (μ): Mean number of customers that can be served at 100% utilization by each individual server per unit time (usually per hour or day). At the individual workstation level, the service rate will equal capacity.
- Channels (M): The number of parallel operations connected to an individual queue. For example, if each queue has 2 operations then it will have two channels.
- Utilization (u): A measure of how busy the system is. It is generally defined as the ratio of throughput to capacity. Note that u = λ/(Μμ) if λ < Μμ, i.e. the utilization is less than 100%. (Also, note that while the Greek letter μ or mu looks a bit like u, they are in fact two different variables.)
- Phase: A queue and its connected servers, or routes to a server.
- Balking: When a person, who would otherwise have entered a line, decides not to enter it.
- Reneging: When a person, who has entered a line, later decides to leave it without being served.
- Interarrival Time: The time between when one customer arrives at a queue and when the next customer arrives.
- Service Time: The time it takes for one particular server to complete a customers service. The average service time will be the same as the cycle time.
And now for some equations you’ll need to know.
- CV: The coefficient of variation. This is a measure of a random variables variability. For a random variable x, CVx is defined as Standard Deviation (x) x mean (x) CV = .
- CVIAT: The coefficient of variation of the interarrival time. The greater the CVIAT, the lumpier the arrival rate.
- CVST: The coefficient of variation of the service time. The smaller the CVST, the more consistent a server is.
- Lq: The average number of people in a line awaiting service.
- Wq: The average length of time a customer waits before being served.
After you have become familiar with the terms above, go ahead and read the articles on Queueing Theory.