PRTG 9 Manual: Monitoring Quality of Service

PRTG can monitor the Quality of Service in a network with its own QoS sensor as well as Cisco IP Service Level Agreement (SLA). Slight variations of network parameters like jitter, packet loss, or packet delay variation (PDV) usually have only little effect on TCP based services (e.g. HTTP, SMTP, etc.). But for UDP based services like Voice over IP (VoIP) and video streaming a steady stream of data packets is crucial. The sound quality of a VoIP call drops dramatically when UDP packets are not received in a timely fashion, or if packets are lost or out-of-order. As a rule of thumb for good quality of service (in a VoIP perspective) you would want low measurements for jitter (less than 20 to 50 ms) and PDV (less than 100 ms) and preferably zero measurements for packet loss, duplicated packets, or packets in wrong order.

For Quality of Service measurements, two sensors are available:

  • Cisco IP SLA Sensor
    Monitors VoIP relevant network parameters through IP SLA results from Cisco devices (via SNMP).

QoS (Quality of Service) Sensor

The QoS Sensor is used to monitor the quality of a network connection by measuring the following parameters:

  • Jitter in ms according to RFC 3550
  • Packet delay variation (PDV) in ms according to RFC 3393
  • Lost packets in %
  • Out-of-order packets in %
  • Duplicated packets in %

The measurements are taken by sending UDP packets between two probes. This means that you can test any network connection in your network by simply placing a remote probe on (or near) each 'end' of the connection and measuring the connection quality between them. This is the perfect tool to find network issues that can affect VoIP sound quality or video streaming 'hiccups'.

Monitoring Quality of Service with PRTG

Monitoring Quality of Service with PRTG

As mentioned before, measurement are taken between two probes. So the first step is to place two PCs running a remote probe on (or near) both ends of the connection that you want to monitor. As an alternative, the local probe on the PC running the PRTG core can also be used as one end. If any firewalls, packet filters or Network Address Translation (NAT) systems are en route, you must configure them as necessary so that the UDP packets can reach the target probe.

In PRTG, create a new QoS sensor on a Probe Device. Please find details about settings in the QoS (Quality of Service) Sensor section. With the settings for number and for size of the packets you can configure the test data stream. 1,000 packets of 172 bytes each is a good start, but if your applications use larger packets you may want to enter other values here. Try to configure the test streams with parameters similar to that of the UDP services you are using across this connection.

Cisco IP SLA Sensor

Wikipedia describes IP SLA as a feature included in the Cisco IOS Software that can allow administrators the ability to Analyze IP Service Levels for IP applications and services. IP SLA uses active traffic-monitoring technology to monitor continuous traffic on the network. This is a reliable method in measuring over head network performance. IP-SLA is mostly used to monitor the sound quality of VoIP traffic.

If you haven't done so already, please add a device in PRTG for the Cisco device that you want to monitor. Then create a new Cisco IP SLA sensor on this device. Please find details about settings in the Cisco IP SLA Sensor section.

This feature is only available in the more expensive Cisco devices. If you don't have IP SLA capable routers/switches you can still get similar information with PRTG's QoS sensor (see above) which does not require any special hardware—just two PCs running Windows. If you do own hardware which supports IP SLA then PRTG brings you probably the least-cost monitoring solution for IP SLA. Most vendors charge extra for IP SLA support (a thousand bucks and more). Following Paessler's long term policy we simply include this as one of our sensor types. With PRTG you can even use the Freeware Edition to monitor IP SLA!

PRTG monitors the following parameters: Calculated Planning Impairment Factor (ICPIF), Mean Opinion Score (MOS), Average Jitter, Packets Lost, Packets Out Of Sequence, Packets Late, Average Round Trip Time (RTT), DNS RTT, TCP RTT, Transaction RTT. Especially two of these parameters are interesting for VoIP: Mean Opinion Score (MOS) and Calculated Planning Impairment Factor (ICPIF).

Voice over IP

For Mean Opinion Score (MOS) measurements, Cisco conducted a panel test where a wide range of listeners judged the quality of voice samples sent using particular codecs, on a scale of 1 (poor quality) to 5 (excellent quality). The Cisco device calculated the corresponding value for the current network connection based on the network parameter measurements like jitter and packet loss. The values and their meanings are:



Expected Quality Impairment






Perceptible, but not annoying



Slightly annoying






Very annoying

The second interesting parameter ICPIF is the sum of measured impairment factors minus a user-defined access Advantage Factor that is intended to represent the user's expectations, based on how the call was placed (for example, a mobile call versus a land-line call) (quoted from Cisco's website).

Upper Limit for ICPIF

VoIP Call Communication Quality


Very good






Limiting case


Exceptional limiting case


Customers likely to react strongly (complaints, change of network operator)


Paessler Knowledge Base: Where can I find more information about Cisco IP SLAs, VoIP, and QoS?


Sensor Technologies—Topics


Keywords: VoIP,VoIP Technology

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