vb.net barcode generator One to Five 9s Availability is often expressed numerically as a percentage in Software

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One to Five 9s Availability is often expressed numerically as a percentage
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of uninterrupted productive time containing from one to five 9s. For instance, 99 percent availability, or two nines, equates to a certain amount of availability and downtime, as does 99.9 percent (three nines), and so on. Table 2-5 offers calculations for each of the five 9s based on 24-hour, yearround operation.
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How Does a Switch Achieve Five 9s Five 9s are not the result of divine guidance given to Bellcore in the 1970s, but rather a process of engineering resulting in a high level of reliability. Reliability is enhanced when each component is replicated in a system. This is called redundancy. If one unit
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Table 2-4 Costs of down time
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Avg. cost per hour of downtime $6,500,000 $2,600,000 $1,150,000 $113,000 $90,000
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Nelson, Gene. Architecting and Deploying High Availability Solutions: Business Drivers and Key Considerations. Compaq white paper. October 1998. Available online at ftp://ftp.compaq. com/pub/supportinformation/papers/ecg0641198.pdf.
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Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
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The Public Switched Telephone Network (PSTN)
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Table 2-5 Availability and downtime: How the five 9s are calculated
Availability 90% (one 9) 99% (two 9s) 99.9% (three 9s) 99.99% (four 9s) 99.999% (five 9s) Downtime 36.5 days per year 3.65 days per year 8.76 hours per year 52.55 minutes per year 5.25 minutes per year
2
fails, its replicated unit takes over. Redundancy is usually expressed in terms of a ratio of 1:1 where one replicated unit exists for every primary unit or N11 redundancy where there are N(N 1) replicated units per primary unit. Another mechanism to enhance reliability is to ensure no single point of failure (SPOF) exists on the system. That is, every mechanism has a backup in the event of the failure of one unit. Hot standby refers to having a replicated unit take over the functions of its primary unit for either planned or unplanned outages. Continuously available (reliable) systems rely exclusively on active replications to achieve transparency in masking both planned and unplanned outages.13
Network Equipment Building Standards (NEBS) In addition to the five 9s, the
other buzzword for reliability in the Class 4 market is Network Equipment Building Standards (NEBS). NEBS address the physical reliability of a switch. NEBS parameters are contained in Telcordia specification SR 3580, which contains the requirements for performance, quality, safety, and environmental metrics applicable to network equipment installed in a carrier s central office. Most North American carriers require equipment in their central offices or switching locations to be NEBS compliant. Tests include electrical safety, immunity from electromagnetic emissions, lightning and power faulting, and bonding and grounding evaluations. Between five 9s and NEBs, the Class 4 and 5 vendors have developed, through decades of experience, a reliable product that delivers superb uptime, and rapid recovery capabilities. As a result, service providers are often reluctant to experiment with new technologies.
The Five 9 s Pilgrimage: Toward the Next Generation of High Availability Systems. A white paper from Clustra at www.clustra.com.
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
The Public Switched Telephone Network (PSTN)
The Public Switched Telephone Network (PSTN)
Scalability For financial reasons, a Class 4 or 5 switch must offer flexibil-
ity in scalability. Ideally, a service provider starts out with a chassis and a minimum capability in terms of DS0s (one DS0 is one phone line) and then adds more capacity as demand increases. This is considered a scalable solution and is preferable to a solution that requires either another chassis or a whole new system (known as a forklift upgrade) when demand grows beyond the initial installation. Nortel s DMS-250 Class 4 switch, for example, scales up to approximately 100,000 DS0s. The other metric for scalability is the call-processing power of the switch. The DMS-250, for example, can process 800,000 BHCAs.14 Another financial reason for scalability is to achieve a low cost per DS0 in purchases and operations. By buying a large switch with many DS0s, a service provider can negotiate a lower price per DS0 and improve the odds of being profitable in that market. For these financial reasons, scalability is important to service providers in both the legacy and converging markets. In terms of BHCAs, a Class 4, such as DMS-250, offers call-processing power at 800,000 BHCAs. The first softswitches offered no more than 250,000 BHCAs.
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