Cisco QoS FAQ: Call Admission Control and QoS Signaling

Q1. Why is CAC needed in an environment where LLQ has been properly implemented?

Figure: Legacy VoIP Network with CAC

call-admission-control-qos-signaling

Answer: 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.

Q2. Name four possible measures that a CAC mechanism can take in the event that the resources are not available to proceed with the call.

Answer:
• 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.

Q3. How does a common channel signaling circuit, such as PRI, react to CAC?

Answer: 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.

Q4. How does a channel-associated signaling circuit, such as E&M or T1 CAS, react to CAC?

Answer: 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.

Q5. What is the difference between payload bandwidth and the bandwidth per call required for a voice conversation?

Answer: 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

Q6. What is the definition of local-based CAC?

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

Q7. Name three of the five available local-based CAC mechanisms.

Answer:
• Physical DS0 limitation
• Max-Connections dial-peer configuration
• VoFR, Voice-Bandwidth configuration
• Trunking condition
• Local Voice Busyout

Q8. What Cisco IOS command is used to enable physical DS0 limitation?

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

Q9. What Cisco IOS command is used to enable CAC on a VoFR network?

Answer: frame-relay voice-bandwidth bandwidth-in-bps

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

Answer: 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

Q11. What is the definition of measurement-based CAC?

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

Q12. What is the difference between LVBO and Advanced LVBO?

Answer: 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.

Q13. What is the difference between an SAA packet and a ping packet?

Answer: 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.

Q14. What measurements are gathered by SSA probes for PSTN fallback?

Answer: Delay and loss measurements

Q15. What Cisco IOS command is used to allow the destination node to participate in measurement based CAC

Answer: rtr responder

Q16. What is the definition of resource-based CAC?

Answer: 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.

Q17. Which resources can resource availability indication (RAI) currently monitor?

Answer: DS0 and DSP resources.

Q18. Which Cisco CallManager call-processing model utilizes location-based CAC?

Answer: A centralized call-processing model.

Q19. For a gatekeeper to provide CAC, what must be configured for each link that requires protection?

Answer: Each link must be associated with a Zone for gatekeeper to provide CAC.

Q20. What is IntServ and how does it work?

Answer: 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.

Q21. Name the messages used by RSVP to provide resource reservation and CAC.

Answer: Path and resv message

Q22. What level of service must be in place to provide CAC?

Answer: Guaranteed-delay

23 Which RSVP profile should be configured to classify traffic for the priority queue within LLQ?

Answer: The ip rsvp pq-profile uses the voice-like profile to classify voice traffic.

Cisco QoS FAQ: Call Admission Control and QoS Signaling

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