School of Business Eastern Illinois University Network Management 1 (Week 16, Tuesday 12/5/2006) © Abdou Illia, Fall 2006
Learning Objectives • Generating Useful Statistics • Availability • Reliability • Centralized Network Management
Generating Useful Statistics • Statistics: Data about network operation or network devices operation • Example: Availability of a modem, Reliability of a Hub, transmission speed, etc. • Statistics are very helpful for network management • Could help identifying problems in Network operation • Could be used to demonstrate the need to invest in technology Q: What kind of tools, already introduced in class, can be used to generate useful statistics?
Availability • Availability: probability that a particular component or system will be available during a fixed time period • Availability is function of: • Mean time between failures (Given by manufacturer or generated based on past performance) • Mean time to repair (Found in studies or in our archives) • Mean time between failures (MTBF) is the average time a device or system will operate before it fails. • Mean time to repair (MTTR) is the average time necessary to repair a failure
Availability • Standard equation: • A(t) = a/(a+b) + b/(a+b) x e-(a+b)t • in which: a = 1/MTTR • b = 1/MTBF • e = natural log function • t = the time interval • Approximation equation: • Availability% = (Total available time – Downtime)/Total available time
A(t) = a/(a+b) + b/(a+b) x e-(a+b)t Availability Suppose we want to calculate the availability of a modem that has a MTBF of 3000 hours and a MTTR of 1 hour. The availability of this modem for an 8-hour period is: a = 1/1 b = 1/3000 = 0.00033 A(8 hours) =1/(1 + 0.00033) + 0.00033/(1 + 0.00033) x e-(1 + 0.00033)8 = 0.9997 + 0.00033 x 0.000335 = 0.9997 Q: What will be the availability of the modem if the Approximation equation is used?
Availability • A component has been operating continuously for three months. During that time, it has failed twice, resulting in downtime of 4.5 hours. Calculate the availability of the component during that three-month period using the Approximation method.
Availability • To calculate the availability of a system of components: • Calculate the availability of each component • Find the product of all availabilities • Example: If a network has tree devices with availabilities of 0.992, 0.894, and 0.999, the availability of the network is: 0.992 x 0.894 x 0.999 = 0.886
Reliability • Reliability: probability that a component or system will be operational for the duration of a transaction time t. • Reliability is function of: • Mean time between failures • Transaction time • Mean time between failures (MTBF) is the average time a device or system will operate before it fails. • Transaction time is the time interval of operation to complete a given transaction.
Reliability • Reliability is defined by the equation: • R(t) = e-bt • in which: b = 1/MTBF • t = the time interval of the operation
Reliability What is the reliability of a modem if the MTBF is 3000 hours and a transaction takes 20 minutes, or 1/3 of an hour (0.333 hours): R(t) = e-bt b = 1/MTBF = 1/3000 t = 0.333 R(0.333 hours) = e -(1/3000)(0.333) = e -0.000111 = 0.99989 Q: If a component has a MTBF of 500 hours and a transaction takes 4 seconds, calculate the reliability of the component
Summary Questions See slides # 6, 7, 11