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Fundamentals of VoIP Monitoring & Troubleshooting Part 2

Level 8

In Fundamentals of VoIP Monitoring & Troubleshooting Part 1 we spoke about the difficulties with reactively troubleshooting VoIP related problems and how Call Detail Records (CDRs) can be used to fill the gap in time. When an end user experiences a problem it can be mins, hours and even days before you’re notified and the troubleshooting process begins. Visibility into what took place during that call is paramount and the metrics gathered from CDR’s can help.

Let’s start with the following call quality metrics:

Network Jitter: Real-time voice communications over the network are sensitive to delay in packet arrival time or packets arriving out of sequence. Excess jitter results in calls breaking up. Jitter can be reduced to a certain extent by using jitter buffers. Jitter buffers are small buffers that cache packets and provide them to the receiver in sequence and evenly spaced for proper playback. Buffer lengths can be modified; however, if jitter buffer is increased too much then the call will experience an unacceptable delay. Consequently, a reduction in buffer turns results in less delay but more packet loss. Jitter is measured in milliseconds (ms).

Latency: Latency, or lag, is the time delay caused in the transmission of a voice packet. Excess latency results in delay, packet drops, and finally to echo. Latency is measured in milliseconds (ms)

Packet Loss: Packet loss occurs when one or more packets of data fail to reach their destination. A single packet loss is referred to as “packet gap”, and series of packet loss is known as” burst”. Packet loss can occur for a variety of reasons including link failure, high congestion levels, misrouted packets, buffer overflows and a number of other factors. Packet loss causes interrupted playback and degradation in voice quality. Packet loss can be controlled using packet loss concealment techniques within the playback codec.

MOS: Mean Opinion Score is a numerical value to indicate the perceived quality of the call from the user’s perspective of the received call after compression, transmission, and decompression. MOS is a calculation based on the performance of the IP network and is defined in the ITU-T PESQ P.862 standard and is expressed as a single number in the range of 1 to 5, where 1 is lowest perceived quality and 5 is the highest. The above metrics are important to monitor and control in order to keep call quality at an acceptable level. It’s also important to note that the above metrics can vary depending where they’re captured. As a best practice it’s a good idea monitor these metrics end to end within your VoIP network. In our next post we’ll talk about how you can capture these statistics from the perspective that matters most – between two VoIP phones after a failed call. 

For more information on monitoring and troubleshooting VoIP please read our white paper here.

Level 15

Bookmarked.  Good reference material.

Level 21

I was always taught to provide sufficient bandwidth first.  If there's no lack of bandwidth, then QoS doesn't kick in, no matter your settings.

OK, that's not the world I live in, and management may not look at available WAN services before opening a site or moving VoIP to an existing site in a remote location, where bonded T1's may be the only option.

Our organization would likely benefit greatly from Orion's VoIP monitoring solution.  Just have to get the folks on board for it.

Level 18

This gets to be fun with you work with digital radios and experience multipathing or terrain induced dropouts. 

Level 21

That DOES sound complex.  I was just happy to skip SSB to Columbia or Argentina or occasionally to Germany from my car back in the '70's.


Level 18

at least you were getting 12 watts on opposed to the usual 5 watts AM unless you were running an amp....then I don't want to know about it.

I can get DX to South America, Europe, etc on a regular basis with my ham radio with cruising to and from work...usually easier on 20m or 17m in the afternoon.

Level 21

Nothing like trying to talk past someone wearing boots.  Don't know if/when the FCC ever enforced the rules on those guys.  They sure mess up life for the rest of us.