Enabling High Availability for Voice Services in Cable Networks

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White Paper

Enabling

High Availability

for Voice Services

in Cable Networks

When customers place a telephone call, they expect it to go through on the first attempt—and they expect it to continue without disruption. This user expectation requires a very high level of availability, which has traditionally been measured as network availability. Certainly, network availability is a primary concern of cable operators. But as cable operators deploy IP-based voice telephony services, they want to achieve availability levels comparable to the public switched telephone network (PSTN). To deliver a PSTN-equivalent user experience, it is essential to measure both network availability and availability of the voice service.

All advanced cable services place stringent requirements on the network for availability, latency, jitter, and other parameters that contribute to a user’s experience (Figure 1). This paper focuses on the availability aspect of these requirements.

Figure 1

Among the Advanced Cable Services, PSTN-Equivalent Voice Has the Most Stringent Requirements for the Factors that Affect a User’s Experience PSTN-Grade; Has Stringent; Effort; Loss; Best-Effort; PSTN-Equivalent; PSTN-Equivalent; On-Demand; High-Definition

The concept of service availability is also important for services such as commercial high-speed data, broadcast video, and video on demand (Figure 2). With its leading, comprehensive set of products and technologies for IP-based data networks, voice over IP (VoIP), and video, Cisco Systems®offers cable operators the solutions that enable high availability today for advanced cable services. However, because PSTN-equivalent voice

PSTN Equivalent Voice Network Requirements Jitter Latency Packet loss Best effort Availability <0.25% <40ms Best effort <150ms 99.94%

Example: PSTN GradeVoicehas Stringent

Network Requirements Jitter Latency Packet Loss Best Effort Availability <0.25% <40ms Best Effort Best Effort <150ms 99.94%

Best Effort Data Best Effort

Other Applications / Services Have similar Network Requirements PSTN Equivalent VoIP On Demand Video High Definition TV Digital Video Commercial Services Online Games PSTN Equivalent Voice Network Requirements Jitter Latency Packet loss Best effort Availability <0.25% <40ms Best effort <150ms 99.94%

Example: PSTN GradeVoicehas Stringent

Network Requirements Jitter Latency Packet Loss Packet Loss Best Effort Availability <0.25% <40ms Best Effort Best Effort <150ms 99.94%

Best Effort Data Best Effort

Other Applications / Services Have similar Network Requirements PSTN Equivalent VoIP On Demand Video High Definition TV Digital Video Commercial Services Online Games Other Applications / Services Have similar Network Requirements Other Applications / Services Have similar Network Requirements PSTN Equivalent VoIP On Demand Video High Definition TV Digital Video Commercial Services Online Games

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Cisco Systems, Inc.

Figure 2

The Different Advanced Cable Services have Different Requirements for Service Availability

How Many Nines? Measuring True Network and Service Availability

“Five nines” is the classic measurement of PSTN availability, meaning the network is up 99.999 percent of the time. However, in the document VoIP Availability and Reliability Model for the PacketCable Architecture (Document: PKT-TR-VoIPAR-V01-001128), PacketCable notes that the popular myth of five nines (99.999 percent) for end-to-end PSTN reliability is incorrect. To calculate a truly equivalent value for PSTN availability, the document breaks down the subsections of the PSTN and draws a direct analogy to an equivalent, IP-based network. Based on this analysis, PacketCable determined that the end-to-end availability of a VoIP network should be greater than 99.94 percent.

In addition to uptime, other service-availability metrics are important because they directly affect the user experience. For voice, these metrics are the number of existing calls that are dropped and the number of ineffective attempts for new calls. As defined in the PacketCable document, the end-to-end network should have service-availability metrics of no more than 125 calls dropped per million and 500 ineffective attempts per million. These values are exactly the same as the PSTN requirements for network and service availability as set forth in the Bellcore

GR series specifications.

When evaluating network and service availability, cable operators need to consider the end-to-end network and the Cisco®_{portion, not just individual network elements. Figure 3 presents a reference architecture for an end-to-end} cable network; the Cisco portion of the network is shown in the highlighted box.

The Cisco portion of the network excludes the cable modem and hybrid fiber-coaxial (HFC) plant at one end and the local exchange and access networks at the point where the cable network connects to the PSTN. To meet the PacketCable guidelines, Cisco recommends that its portion of the network have uptime of greater than 99.9665 percent, with fewer than 65 calls dropped per million and fewer than 275 ineffective attempts per million. Based on these values, a cable operator can be confident of achieving PSTN-equivalent availability when deploying IP-based voice telephony services today.

High-Speed Data Voice Video

Residential High-Speed Data (Best Effort) Commercial High-Speed Data Services Availability Requirement Second/Teen Line Voice PSTN Equivalent Voice Video on Demand Broadcast Video

High-Speed Data Voice Video

Residential High-Speed Data (Best Effort) Commercial High-Speed Data Services Availability Requirement Second/Teen Line Voice PSTN Equivalent Voice Video on Demand Broadcast Video

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Figure 3

Availability Metrics in the Cisco Portion are Key Contributors to End-to-End Network Availability]

A Leading Perspective for Assuring High Availability

Unlike vendors that offer products only for selected services or areas in a cable operator’s network, Cisco Systems offers voice solutions that cover that network from an HFC plant to a PSTN gateway. With this systemwide perspective, Cisco can deliver an integrated approach to service availability—identifying optimal features and implementing them on the most appropriate platforms.

Cisco continues to develop high-availability features that will help reduce outage time for a voice call if the network fails. These features include fundamental capabilities such as support for software and hardware redundancy; routing-optimization technologies for fast convergence; and fiber transport technologies with features for rapid failure recovery such as Dynamic Packet Transport (DPT).

Cisco offers additional, innovative features for high availability, including Cisco Nonstop Forwarding (NSF), Cisco Stateful Switchover (SSO), and in-service software upgrades. Planned for delivery in phases throughout 2003–2004, these features will be implemented as appropriate in Cisco routers, switches, PSTN gateways, and cable modem termination systems (CMTSs).

As shown in Figure 4, all of these Cisco features will further reduce a cable operator’s defects-per-million (DPM) value for calls dropped and ineffective attempts.

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Cisco Systems, Inc.

Figure 4

Cisco Plans to Release Additional High Availability Features by Year End 2004, Incrementally Reducing the Values for the Two Key Voice Service Metrics.

Fast Convergence for Layer 3 Routing

Within the IP network, the Layer 3 routing architecture—and its ability to recover (converge) quickly from a failure—is an important aspect of high availability. If a component or link failure causes a Layer 3 topology change, the routing architecture should enable fast convergence to prevent dropped calls, meaning a convergence time of less than three seconds (the calls-dropped threshold). Fast convergence also helps reduce mean time to repair (MTTR)—a measurement of how quickly a router recovers from a failure—which has a direct impact on service availability metrics.

Cisco’s leading technologies for IP routing enable Layer 3 convergence in less than three seconds (and in certain cases even faster). This reduced router convergence time is possible because of Cisco’s enhancements to shortest-path-first (SPF) calculation algorithms, exponential timers, and subsecond hellos.

The Impact of Operational Practices on High Availability

Day-to-day operational activities and procedures can have a significant, ongoing impact on network and service availability. Cisco has helped some of the largest service providers in the world to improve their operational practices and associated impact on service availability. For example, in a Cisco consulting engagement for a major U.S. service provider, a joint team focused on several aspects of network operations, including:

• Regression testing of changes in network elements to minimize unplanned outages • Processes and procedures for planned outages to deploy hardware and software upgrades • Network management practices that can proactively address availability issues

• Architecture and planning to consider network and service availability

• A systemwide troubleshooting methodology and escalation process for fast response to outages

As a result of this engagement, Cisco helped the service provider reduce DPM for data services from more than 2000 to fewer than 100—a 95 percent improvement.

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With its focus on service and network availability, Cisco Systems leads the industry in delivering cable network solutions that support the stringent requirements of voice telephony. This focus is implemented through high-availability technologies and advanced Layer 3 routing in Cisco networking products and Cisco IOS®_{Software, as well as guidance for cable operators on network design and operational procedures.} An end-to-end Cisco network can help cable operators meet PacketCable specifications for network and service availability today—and deliver the positive user experience that can make voice telephony services a market success.

For More Information

To learn more about Cisco products and services for cable operators, visit: http:/www.cisco.com/go/cable.