Appendix C. Voice Call Admission Control Reference

Suppose that you and a passenger are inside a vehicle traveling on a northbound highway. The highway consists of four regular lanes and one high-occupancy-vehicle (HOV) lane. To gain entry into the HOV lane, your vehicle must contain two or more people. In this instance, the HOV lane represents a priority queue, whereas the remaining four lanes represent lower-priority queues. As traffic begins to build in the four regular lanes, you decide to merge into the HOV lane. Over the next few miles/kilometers, the HOV lane maintains a constant rate of speed while the four regular lanes begin to stall. You are now enjoying the benefits of a priority queue. As you continue traveling northbound, you notice more vehicles, meeting the requirements of two or more passengers, begin to merge into the HOV lane. Over the next few miles/kilometers, your speed decreases until the HOV is now stalled.

In this instance each vehicle in the HOV lane has met the criteria to be placed in the high-priority queue. The expectation is that the HOV lane will move quickly. Contrary to these expectations, as more vehicles enter the HOV lane the slower the lane becomes. How do you prevent this issue from arising?

This scenario illustrates the need to have some mechanism in place to limit the amount of traffic that can gain access into the priority queue, ensuring that a consistent flow of traffic can be maintained across a network link. This concept is called call admission control (CAC), which is the subject of this chapter.