Voice network monitoring: what metrics to use and for what?

19 October 2022

The monitoring of voice networks provides information that prevents attacks, prevents fraud, anticipates service drops and detects anomalous patterns.

Although there are elements that support advanced metrics (even with other protocols such as MEGACO, MGP, ISUP, etc) in the SIP world they are usually measured in SBC and softswitches. These are the most common metrics:

  • Active calls (incoming, outgoing, traversal)
  • CAPS call attempts (incoming, outgoing, traversal)
  • Calls established per second
  • Registered users and REGISTER transactions per second
  • Response time and transaction time
  • SIP messages per second

IETF RFC 6076 – first specific to measuring SIP

In this article we will focus on the metrics defined by the standards. Although the ITU-T E.411 and E.781 already define conventional telephony metrics, based on circuits, that can be used for IP telephony, it is the IETF RFC 6076 standard, defined in January 2021, the first specific to measure SIP. It is characterized by measuring with SIP factors unrelated to the protocol: network problems, equipment performance, etc. And most importantly, it defines the measurements but does not provide reference values. The most common measures are:

  • RRD (Registration Request Delay): Time between the REGISTER request and the reception of the 200 OK. Valid to measure the efficiency of the SIP Registrar.
  • IRA (Ineffective Registration Attempts): Records with response 400, 401, 407, 5xx, 6xx or timer expirations. Valid to measure the efficiency of the SIP Registrar.
  • SRD (Session Request Delay): to measure the efficiency of establishing sessions.
    • Successful: Time between the INVITE request and the reception of the 180 (or 200 OK).
    • Failed: Until receipt of 4xx, 5xx or 6xx (with some exceptions)
  • SDD (Session Disconnect Delay): Time from the BYE request to receiving a 200OK or a Timeout.
  • SDT (Session Duration Time): Average duration of the sessions.
    • Successful: Time between the 200 OK after the INVITE until the reception of the BYE at the end of the call.
    • Failed: Until call failure (timeout,…)
  • SER (Session Established Ratio): Percentage of INVITEs with a 200 OK response compared to the total number of call attempts (excluding 3xx). Similar to ITU E.411 ASR.
  • SEER (Session Establishment Effectiveness Ratio) Percentage of INVITES that receive a conclusive response (200, 480, 486, 600, 603) with respect to the total (without 3xx).
  • ISA (Ineffective Session Attempts) Percentage of sessions that end in 408, 500, 503 or 504. with respect to the total.
  • SCR (Session Completion Ratio) Percentage of complete sessions with respect to the total requested. Similar to Call Completed Ratio (CCR).

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