Specifying Availability Requirements

You should encourage your customers to specify availability requirements with precision. Consider the difference between an uptime of 99.70 percent and an uptime of 99.95 percent. An uptime of 99.70 percent means the network is down 30 minutes per week, which is not acceptable to many customers. An uptime of 99.95 percent means the network is down 5 minutes per week, which may be acceptable, depending on the type of business. Availability requirements should be specified with at least two digits following the decimal point.

It is also important to specify a timeframe with percent uptime requirements. Go back to the example of 99.70 percent uptime, which equated to 30 minutes of downtime per week. A downtime of 30 minutes in the middle of a working day is probably not acceptable. But a downtime of 30 minutes every Saturday evening for regularly scheduled maintenance might be fine.

Not only should your customers specify a timeframe with percent uptime requirements, they should also specify a time unit. Availability requirements should be specified as uptime per year, month, week, day, or hour. Consider an uptime of 99.70 percent again. This uptime means 30 minutes of downtime during a week. The downtime could be all at once, which could be a problem if it's not during a regularly scheduled maintenance window, or it could be spread out over the week. An uptime of 99.70 percent could mean that approximately every hour the network is down for 10.70 seconds. Will users notice a downtime of 10.70 seconds? Certainly some users will, but for some applications, a downtime of 10.70 seconds every hour is tolerable. Availability goals must be based on output from the first network design step of analyzing business goals, where you gained an understanding of the customer's applications.

Try doing the math yourself for a goal of 99.80 percent uptime. How much downtime is permitted in hours per week? How much downtime is permitted in minutes per day and seconds per hour? Which values are acceptable in which circumstances?

Five Nines Availability

Although the examples cited so far use numbers in the 99.70 to 99.95 percent range, many companies require higher availability, especially during critical time periods. Some customers may insist on a network uptime of 99.999 percent, which is sometimes referred to as five nines availability. For some customers, this requirement may be linked to a particular business process or timeframe. For example, the requirement may refer to the monthly closing of financial records or to the holiday season for a company that sells holiday gifts via catalog and web orders. On the other hand, some design customers may need, or think they need, five nines availability all the time.

Five nines availability is extremely hard to achieve. You should explain to a network design customer that to achieve such a level, redundant equipment and links will be necessary, as will extra staffing possibly, and extremely reliable hardware and software. Some managers will back down from such a requirement once they hear the cost, but, for others, the goal may be appropriate. If a company would experience a severe loss of revenue or reputation if the network were not operational for even very short periods of time, five nines availability is a reasonable goal.

Many hardware manufacturers specify 99.999 percent uptime for their devices and operating systems and have real customer examples where this level of uptime was achieved. This may lead a naive network design customer to assume that a complex internetwork can also have 99.999 percent uptime without too much extra effort or cost. However, achieving such a high level on a complex internetwork is much more difficult than achieving it for particular components of the internetwork. Potential failures include carrier outages, faulty software in routers and switches, an unexpected and sudden increase in bandwidth or server usage, configuration problems, human errors, power failures, security breaches, and software glitches in network applications.


Some networking experts say that 80 to 90 percent of failures are due to human errors, either errors made my local administrators or errors made by service provider employees (or the infamous backhoe operator). Avoiding and recovering from human errors requires skill and good processes. You need smart people thinking about availability all the time and processes that are precise without stifling thought. Good network management and troubleshooting play a role. Network management tools should provide immediate alerts upon failures and enough information for a network administrator to make a quick fix.

Consider a network that is used 24 hours a day for 365 days per year. This equates to 8760 hours. If the network can be down only 00.001 percent of the time, it can be down for only 0.0876th of an hour or about 5 minutes per year. If the customer says the network must be available 99.999 percent of the time, you better make it clear that this doesn't include regularly scheduled maintenance time or you better make sure that the network will have the capability to support in-service upgrades. Inservice upgrades refer to mechanisms for upgrading network equipment and services without disrupting operations. Most internetworking vendors sell high-end internetworking devices that include hot-swappable components for in-service upgrading.

For situations where hot-swapping is not practical, it may be necessary to have extra equipment so there's never a need to disable services for maintenance. In some networks, each critical component has triple redundancy, with one being active, one in hot standby ready to be used immediately, and one in standby or maintenance. With triple redundancy, you can bring a standby router down to upgrade or reconfigure it. After it is upgraded, you can then designate it as the hot standby, and take the previous hot standby down and upgrade it. You can then switch from the active to the hot standby and upgrade the active.

Depending on the network design, you may load share among the redundant components during normal operations. The key design decision is whether your users can accept degraded performance when some of the components are unusable. If all this sounds too complicated or expensive, another possibility is not to do it all yourself, but put resources at collocation centers that can amortize the highly redundant equipment over many customers.

The Cost of Downtime

In general, a customer's goal for availability is to keep mission-critical applications running smoothly, with little or no downtime. A method to help you, the network designer, and your customer understand availability requirements is to specify a cost of downtime. For each critical application, document how much money the company loses per hour of downtime. (For some applications, such as order processing, specifying money lost per minute might have more impact.) If network operations will be outsourced to a third-party network management firm, explaining the cost of downtime can help the firm understand the criticality of applications to a business's mission. Specifying the cost of downtime can also help clarify whether in-service upgrades or triple redundancy must be supported.

Mean Time Between Failure and Mean Time to Repair

In addition to expressing availability as the percent of uptime, you can define availability as a mean time between failure (MTBF) and mean time to repair (MTTR). You can use MTBF and MTTR to calculate availability goals when the customer wants to specify explicit periods of uptime and downtime, rather than a simple percent uptime value.

MTBF is a term that comes from the computer industry and is best suited to specifying how long a computer or computer component will last before it fails. When specifying availability requirements in the networking field, MTBF is sometimes designated with the more cumbersome phrase mean time between service outage (MTBSO), to account for the fact that a network is a service, not a component. Similarly, MTTR can be replaced with the phrase mean time to service repair (MTTSR). This book uses the simpler and better-known terms MTBF and MTTR.

A typical MTBF goal for a network that is highly relied upon is 4000 hours. In other words, the network should not fail more often than once every 4000 hours or 166.67 days. A typical MTTR goal is 1 hour. In other words, the network failure should be fixed within 1 hour. In this case, the mean availability goal is as follows:

4000 / 4001 = 99.98 percent

A goal of 99.98 percent is typical for many companies.

When specifying availability using MTBF and MTTR, the equation to use is as follows:

Using this availability equation allows a customer to clearly state the acceptable frequency and length of network outages.

Remember that what is calculated is the mean. The variation in failure and repair times can be high and must be considered as well. It is not enough to just consider mean rates, especially if you depend on external service agents (vendors or contractors) who are not under your tight control. Also, be aware that customers might need to specify different MTBF and MTTR goals for different parts of a network. For example, the goals for the core of the enterprise network are probably much more stringent than the goals for a switch port that affects only one user.

Although not all customers can specify detailed application requirements, it is a good idea to identify availability goals for specific applications, in addition to the network as a whole. Application availability goals can vary widely depending on the cost of downtime. For each application that has a high cost of downtime, you should document the acceptable MTBF and MTTR.

For MTBF values for specific networking components, you can generally use data supplied by the vendor of the component. Most router, switch, and hub manufacturers can provide MTBF and MTTR figures for their products. You should also investigate other sources of information, such as trade publications, to avoid any credibility problems with figures published by manufacturers. Search for variability figures as well as mean figures. Also, try to get written commitments for MTBF, MTTR, and variability values from the providers of equipment and services.

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Project Management Made Easy

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