VoIP and QoS Sensor Types

The sensors in this section can monitor Quality of Service using PRTG's own QoS sensor as well as Cisco IP-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 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, 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 ( < 20 - 50 ms) and PDV (< 100 ms) and "zero" measurements for packet loss, duplicated and out-of-order packets.

Two sensors are available:

  • QoS (Quality of Service): Monitors VoIP relevant network parameters by testing network connection quality between two probes
  • Cisco IP SLA: 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 "quality of service" 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 remote 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.

As mentioned before, measurement takes place 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 (the local probe on the PC running the PRTG Core can also be used as one end). If any firewalls, packet filters or NAT systems are en-route you must configure them as necessary so that the UDP packets can reach the target probe. In PRTG new QoS sensors must be created with a "probe device" as the parent device. The UDP packets will be sent from this probe to the target probe. During the creation of the sensor you are going to choose the target probe that the UDP packets shall be sent to for measurement. To get started right click a probe device, choose "Add Sensor" and then choose "QoS (Quality of Service)" from the "VoIP and QoS" group. On the next web page you can configure the sensor. Please choose a probe from the "Target probe" drop down. The list shows the probes with the IP address that is currently used by each probe to connect to the core. Nevertheless you must enter the IP address manually because the target probe's IP from the perspective of the sending probe may be different (e.g. due to NAT). You also have to choose a UDP port number (please use one port number per QoS sensor). With the settings for number and for size of the packets you can configure the test data stream, 1000 packets of 172 bytes is good for a 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 have a look at sound quality for VoIP traffic.

If you haven't done so already you must create a device for the Cisco device that you want to monitor. Remember to enter SNMP credentials because PRTG uses SNMP to get the data from the switch. The second step is to create a new sensor on that device, choose "IP SLA" from the "VoIP and Infrastructure" section and follow the instructions on the screen. A few moments later you will see the current measurements in PRTG's user interface.

This feature is only available in the more expensive boxes from Cisco. 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: ICPIF (Calculated Planning Impairment Factor), MOS (Mean Opinion Score), Avg. 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: MOS (Mean Opinion Score) und ICPIF (Calculated Planning Impairment Factor).

For MOS, Cisco conducted a panel test where a wide range of listeners have 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 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)


For more information about these measurements see "IP SLAs - Analyzing Service Levels Using the VoIP Jitter Operation" on the Cisco website: http://www.cisco.com/en/US/docs/ios/12_4/ip_sla/configuration/guide/hsvoipj.html


Keywords: voip,qos,ip-sla,quality of service,Voice over IP,jitter,packet delay variation,packet loss