Call Admission Control Overview

Call admission control (CAC) mechanisms extend the capabilities of other quality of service (QoS) methods to ensure that voice traffic across the network link does not suffer latency, jitter, or packet loss that can be introduced by the addition of other voice traffic. CAC achieves this task by determining whether the required network resources are available to provide suitable QoS for a new call, before the new call is placed. Simply put, CAC protects voice conversations from other voice conversations. Figure 8-1 demonstrates the need for CAC. In this example, if the WAN link between the two private branch exchanges (PBXs) has sufficient bandwidth to carry only two Voice over IP (VoIP) calls, admitting the third call will impair the voice quality of all three calls.

Figure 8-1 VoIP Network Without CAC

IP Network Supports 2 Calls Max!

Figure 8-1 VoIP Network Without CAC

IP Network Supports 2 Calls Max!

The Third Call Degrades Voice Quality for All Calls

Similar to the earlier HOV example, the reason for this impairment is that the employed queuing mechanisms do not provide CAC. If packets exceeding the configured or budgeted rate are received in the priority queue, in this case more than two calls, these packets are just tail dropped from the priority queue. There is no capability in the queuing mechanisms to distinguish which IP packet belongs to which voice call. As mentioned in Chapter 4, "Congestion Management," both Low Latency Queuing (LLQ) and IP RTP Priority police traffic inside the low-latency queue when the interface is congested, so any packet exceeding the configured rate within a certain period of time is dropped. In this event, all three calls will experience packet loss and jitter, which can be perceived as clipped speech or dropped syllables in each of the voice conversations.

The addition of CAC preserves the quality of the voice conversations in progress by rejecting a new call when insufficient network resources are available to allow the new call to proceed.

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