3.3 Developing Service Metrics After gathering requirements for our network, the next step is to analyze these requirements in order to distinguish between various performance levels in the network. We will develop and use performance thresholds and limits to distinguish between low and high performance, and also use performance characteristics to identify predictable and guaranteed performance levels. Performance thresholds and limits and performance characteristics are measured in the system with service metrics. 110 CHAPTER 3 Requirements Analysis: Process Service metrics are either actual measurable quantities in the network or are derived from measured quantities. These service metrics are important, as they are where “the rubber meets the road”-where requirements from all layers in the system are distilled into configurable and measurable quantities. Recall from Chapter 2 that in order for a performance characteristic to be useful, it must be configurable, measurable, and verifiable within the network. This is particularly true when parts of the network are outside the control of the network administrator-for example, when a service provider is used to supply a service such as Frame Relay in the network, or when parts of the network (or the entire network) are outsourced. In cases such as these, service metrics can be used to ensure that you are getting the service you are requesting (and paying for) from the service provider or outsourcing agent. The types of service metrics you use will depend on your design and the types of equipment (network devices) you implement in your network, but at this point in the analysis process, you can influence or require what will be measured in the network and (to some extent) how it will be measured. Service metrics for RMA include: • Reliability, in terms of mean time between failures (MTBF) and mean time between missioncritical failures (MTBCF) • Maintainability, in terms of mean time to repair (MTTR) • Availability, in terms of MTBF, MTBCF, MTTR • Optionally, uptime and downtime (as a percent of total time), error and loss rates at various levels, such as packet error rate, bit error rate (BER), cell loss ratio (CLR), cell misinsertion ratio (CMR), frame and packet loss rates Service metrics for capacity include: • Data rates, in terms of peak data rate (PDR), sustained data rate (SDR), and minimum data rate (MDR) • Data sizes, including burst sizes and durations Service metrics for delay include: • Endtoend or roundtrip delay • Latency • Delay variation Developing Service Metrics 111 As configurable and measurable quantities in the network, service metrics can be described in terms of variables in network devices. There are also mechanisms to configure and measure these variables. As we see in Chapter 7 on network management, current mechanisms to configure and measure service metrics are found within network management platforms that use the simple network management protocol (SNMP) and the common management information protocol (CMIP), both of which access variables described in management information bases, or MIBs.