Dynamic Call Admission Control Overview

Dynamic call admission control (CAC) provides enhanced control over WAN bandwidth. When dynamic CAC is configured on an interface responsible for providing call bandwidth, the TGM550 informs the Media Gateway Controller (MGC) of the bandwidth limit available for voice packets on the interface and requests the MGC to block new calls when the bandwidth is exhausted.

Dynamic CAC is especially useful in situations where a primary link is unavailable and a backup link with less bandwidth is active in its place. Without dynamic CAC, the MGC cannot detect the switchover to the backup link or the resulting changes in network topology and available bandwidth. As a result, the MGC continues to admit calls at the bandwidth of the primary link, causing network congestion and possible jitter, delay, and loss of calls.

Supported Interfaces

Dynamic CAC must be configured on each Services Router interface responsible for providing call bandwidth. You can configure dynamic CAC on the following interface types:

• Fast Ethernet
• Gigabit Ethernet
• ISDN BRI
• Serial interfaces with PPP or Frame Relay encapsulation

Bearer Bandwidth Limit

The dynamic CAC bearer bandwidth limit (BBL) configured on an interface specifies the maximum bandwidth available for voice traffic on the interface. The TGM550 reports the BBL to the MGC. When the call bandwidth exceeds the BBL, the MGC blocks new calls and alerts the user with a busy tone.

You can configure the dynamic CAC activation priority value on interfaces to specify the order in which the interfaces are used for providing call bandwidth. The interface with the highest activation priority value is used as the primary link for providing call bandwidth. If the primary link becomes unavailable, the TGM550 switches over to the next active interface with the highest activation priority value, and so on.

Rules for Determining Reported BBL

The TGM550 uses the following rules to report the BBL of the interfaces on which activation priority is configured to the MGC. The reported BBL (RBBL) allows the MGC to automatically control the call bandwidth when interfaces responsible for providing call bandwidth become available or unavailable.

1. Report the BBL of the active interface with the highest activation priority. For example, if one interface has the activation priority of 200 and a BBL of 1500 Kbps and another interface has the activation priority of 100 and a BBL of 1000 Kbps, the RBBL is 1500 Kbps.
2. If more than one active interface has the same activation priority, the BBL is reported as the number of interfaces times their lowest BBL . For example, if two interfaces with the same activation priority have BBLs of 2000 Kbps and 1500 Kbps, the RBBL is 3000 Kbps (2 x 1500 Kbps).
3. If the interface with the highest activation priority is unavailable, the TGM550 reports the BBL of the active interface with the next highest activation priority.
4. If all the interfaces on which dynamic CAC is configured are inactive, the RBBL is 0. The MGC does not allow calls to go through when the RBBL is 0.

Dynamic CAC works in conjunction with the Avaya Communication Manager (CM) Call Admission Control: Bandwidth Limitation (CAC-BL) feature. If you configure dynamic CAC on WAN interfaces, you must also configure CAC-BL on Avaya CM. For more information about configuring CAC-BL, see the Administrator Guide for Avaya Communication Manager.