Call Setup Delay Measurement

The SBC is generally deployed as one element in a series of elements that comprise an overall telecommunications solution. Call setup delays in such an environment is a key performance indicator. As such, it is important to track the component of call setup delay attributable to the SBC for use with routine monitoring as well as for troubleshooting system problems. The SBC is equipped to enable measurement of the delay it introduces on a call-by-call basis for SIP-to-SIP calls. This internal instrumentation makes these measurements in a manner that has minimal impact on the SBC.

The SBC records measurements such as:

• Request Latency—Delay through SBC itself in the “forward” direction.
• Response Latency—Delay through the SBC itself in the “reverse” direction.
• Downstream Latency—Delay between forwarding the request and receiving the associated response due to all downstream elements.
• Upstream Latency—Delay between request and response as seen by the element sending the initial INVITE to the SBC.
• Overload Status—In order to properly interpret the four call setup delay measurements, it is important to know the overload status of the SBC when the measurements were taken.

These five measurements are included in the Call Data Records (CDRs) recorded for each SIP-to-SIP call. Call setup delay information is recorded for simple calls, two-stage calls, calls involving call transfer, and calls involving call recursion (i.e., the response involves 3xx). The delay measurements include SBC delays to the extent that they are part of the call setup process. The granularity of the SBC call setup delay measurements is one millisecond.