et sent from the originating end- through the RTD metric in point at the customer location Cisco IOS Software; it is pro- to the terminating endpoint at vided per call and is also avail- the service provider and back able through IP-SLA probes. Jitter 50-100 Jitter is a measurement of the Jitter is measured in the per-call change in the delay of one pack- statistics; the maximum jitter et to another during a call. detected during the call is Packet loss 1 percent Packet loss is the number of This metric can be monitored or lower packets lost during any given by SNMP in Cisco IOS call, including UDP and TCP Software; it is provided per call packets. and can also be tracked with IP- Uptime 99.999 Uptime is the percentage of When uptime is measured, percent time that a path is available for planned outages should be the customer to complete a call accounted for, and it should be to the PSTN. measured as the number of unplanned minutes of outages and monitored with trouble continues 130 SIP Trunking Table 7-1 Voice-Quality Monitoring Attributes (continued) Metric Definition Method to Monitor Answer seizure Varies The ASR can be recorded as the ASR can be measured by a sum- rate (ASR) or number of calls made divided mary of call activity at the end call success by the number of calls that of the month. The specific rate (CSR) complete a voice path. This value of ASR is not as impor- number varies greatly because tant as whether there are large of calling numbers that are unas- swings in the ASR from one signed or busy. CSR is the per- month to another that might centage of calls successfully indicate a problem with end-to- completed through a service end network connectivity. provider. The CSR rate should be more than 99 percent. The ASR rate is typically approxi- mately 60 percent. Scalability and High Availability One of the attractive cost benefits of SIP trunking is the technical ability to centralize PSTN access for the enterprise into a single large pipe. Doing so, however, creates several design considerations,