ent types of throughput. - Network throughput: It is the average rate of successful messages deliv- ered in a communication channel. The channel is usually a network link or node. - System throughput: It is the aggregate of data rates delivered to all ter- minals in a network. System throughput is also referred to as aggregate throughput. Normalized throughput: Throughput expressed as a percentage is called normalized throughput or channel utilization. Several attributes linked to throughput like maximum achievable throughput and peak measured throughput help the operator in network planning and quality measurement. Error rate: This attribute is a measure of the data blocks with error. The data blocks can be bits in which case the term used is bit error rate, it can be words corresponding to work error rate or error in data packets measured as packet error rate. Error is mostly expressed as a ratio of error data blocks against the total number of data blocks. Total and average value of error rate is also used as an attribute for measuring errors. Network failure: Different flavors of network failure is monitored for per- formance measurement purpose. The most common ones include dropped calls to identify the total number of calls dropped, link down to identify disruption of service in connection links, packets lost or packets dropped corresponding to the number of packets that required resending, and node down to identify the time duration when a network element was down caus- ing network failure in offering a specific service. Some of the attributes monitored by service quality management module are (see Figure 20.9): MTTR: This attribute is a measure of the availability of the system. For sys- tems that have a backup facility where a secondary machine takes up control without delay when the primary goes down, the MTTR is zero. Here the system has zero MTTR while the individual devices (primary and secondary) have nonzero MTTR. Mean time to recover: It is the average time a system or network device/ element take