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10What is the definition of resource-based CAC?

Resource-based CAC calculates the resources required to facilitate and protect the call on each leg the call traverses toward the destination and then attempts to reserve these resources for use by the voice conversation.

11For a gatekeeper to provide CAC, what must be configured for each link that requires protection?

Each link must be associated with a zone for a gatekeeper to provide CAC.

12What is IntServ and how does it work?

The integrated services (IntServ) model includes provisions for best-effort traffic, real-time traffic, and controlled-link sharing. In the IntServ model, an application requests a specific level of service from the network before sending a data flow. If the requested level of service can be achieved from the network, a reservation is placed in each intermediary router to guarantee the requested level of service for the life of the flow.

13Name the messages used by RSVP to provide resource reservation and CAC.

Path and resv message

14What level of service must be in place to provide CAC?

Guaranteed-delay

Q&A

Call Admission Control Concepts

1Why is CAC needed in an environment where LLQ has been properly implemented?

LLQ provides classification, marking, and prioritization of voice packets, but does not differentiate between voice streams. The priority queue can become overburdened with a larger number of voice conversations than expected, leading to degradation in all voice conversations. CAC provides the protection needed to guarantee the quality of service for established voice conversations by limiting the

number of simultaneous voice conversations allowed in the priority queue. In short, LLQ protects voice from data, whereas CAC protects voice from voice.

816 Appendix A: Answers to the “Do I Know This Already?” Quizzes and Q&A Sections

2Name four possible measures that a CAC mechanism can take in the event that the resources are not available to proceed with the call.

•Reroute call to alternate LAN/WAN link.

•Reroute call to alternate PSTN link.

•Return call to originating PBX for rerouting.

•Return a reorder tone, or fast-busy, to the caller.

3How does a common channel signaling circuit, such as PRI, react to CAC?

CCS circuits have the capability to send out-of-band signaling to tear down the circuit and reroute the call in the event that a CAC mechanism indicates that the resources are not available to proceed with the call.

4How does a channel-associated signaling circuit, such as E&M or T1 CAS, react to CAC?

CAS circuits carry signaling in band. This forces the call to be hairpinned back on another channel in the event that a CAC mechanism indicates that the resources are not available to proceed with the call.

5What is the difference between payload bandwidth and the bandwidth per call required for a voice conversation?

Payload bandwidth is the amount of bandwidth required to digitize the analog voice stream, whereas the bandwidth per call for a voice conversation takes the Layer 2 overhead, IP, UDP, and RTP headers, and the payload into consideration.

Local-Based CAC

6What is the definition of local-based CAC?

Local CAC mechanisms base the availability of network resources on local nodal information such as the state of the outgoing LAN or WAN link.

7Name three of the five available local-based CAC mechanisms.

•Physical DS0 limitation

•Max-Connections dial-peer configuration

•VoFR, Voice-Bandwidth configuration

•Trunking condition

•Local Voice Busyout

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8What Cisco IOS command is used to enable physical DS0 limitation?

Physical DS0 limitation is not an IOS command; it is a design methodology that limits the number of physical DS0 connections into the gateway.

9What Cisco IOS command is used to enable CAC on a VoFR network? frame-relay voice-bandwidth bandwidth-in-bps

10What type of circuit, CAS or CSS, will Local Voice Busyout (LVB) be most beneficial for?

A CAS circuit is better suited to use LVBO because a CAS circuit can reroute the call in the event that the resources are not available to proceed with the call.

Measurement-Based CAC

11What is the definition of measurement-based CAC?

Measurement-based CAC techniques look into the packet network to gauge the current state of the network

12What is the difference between LVBO and Advanced LVBO?

Whereas LVBO provides for busyout based on local conditions of the originating gateway, AVBO adds the capability to trigger an SAA probe to one or more configured IP destinations.

13What is the difference between an SAA packet and a ping packet?

SAA packets can be IP/TCP, or IP/UDP or (most common) IP/UDP/RTP packets with sizes, frequency, and ToS all set to accurately synthesize the protocol being observed; whereas a ping packet is an ICMP best-effort packet that does not resemble a voice packet in size or protocol.

14What measurements are gathered by SSA probes for PSTN fallback?

Delay and loss measurements

15What Cisco IOS command is used to allow the destination node to participate in measurement based CAC

rtr responder