Now that VoIP is commonplace in business, the focus has moved from implementation to management, particularly the management of VoIP Quality.
The concept is simple. VoIP traffic is a series of ethernet packets. The packets must arrive in the same order as they left and not be damaged or lost in transit to ensure that both parties can talk comfortably. One of the primary measurements of VoIP Quality is that of VoIP Jitter, something that can seriously affect VoIP traffic quality.
Unlike the PSTN, VoIP uses the Internet as a transmission medium. Therefore, the quality of your conversation depends entirely on the quality of the Internet connection between the two parties. Simply put, Jitter is where packets arrive in the wrong order or are dropped entirely, leading to an unsatisfactory conversation.
Cisco defines Jitter as a specific form of latency on the network, specifically “a variation in the delay of received packets”.
Causes of Jitter
Precisely what causes Jitter can be complicated. It can arise from a single cause or be the culmination of several. Typical causes include:
- Network Congestion. Probably the single most common cause. If you lack the bandwidth to support concurrent voice and data traffic, your voice packets will be delayed or dropped. That compromises call quality.
- WiFi. In many installations, WiFi is an addition to an existing network. A typical WiFi access point can handle up to 50 devices simultaneously, each WiFi connection sharing the access point’s bandwidth to the network.
Even if there are only a small number of active WiFi devices connected to an access point, most users are probably suffering degraded network connections, especially if it is a 100Mb connection.
- Configuration. VoIP needs to be configured at all levels from the Core to Access layers, and usually at a switch port level to support the VoIP VPNs. The network also requires careful design to support QoS, or VoIP traffic is given priority in traffic management algorithms. Configuration errors can stop VoIP entirely or result in poor performance.
- Hardware. Most networks grow gradually over time. If a piece of equipment does not support VoIP or is damaged somehow, VoIP will suffer. An old switch or router or a bad ethernet cable connection can stop it in its tracks.
Jitter needs measurement because in the management dictum, “If You Can’t Measure It, You Can’t Manage It”.
The first thing is that like measles if you have it, it’s obvious. Users will be complaining that their calls are a shambles, bits missing, words jumbled and n the wrong order, and long delays in responses.
You must check for internal Jitter on your internal network and external Jitter on your Internet connection. It is not much you can do about Jitter on an Internet connection running on an external provider network.
Take the following steps:
- Map the problem against the network layout. If complaints indicate that Jitter is only between specific extensions or over particular network segments, that gives somewhere to start investigations.
- Ping. Use Ping to test the send/receive delay between two endpoints. A long ping time indicates a potential problem. Cisco says 300ms round-trip Ping, 150ms mouth to ear ping time, and 30ms Jitter delay is the maximum acceptable for VoIP.
- Check switch and router configurations.
- Check cable connections.
- Use network-monitoring software to measure ping times and calculate Jitter.
If the problem is entire with the external provider, drop the call and try again to see if you get a faster connection.
Check and Update hardware
Check that all connectivity equipment is working correctly and has up to date software supporting the VoIP protocols you are using. Check hardware and software firewalls to make sure they don’t apply bandwidth limitations.
Check QoS and other Configurations
If you are using QoS, make sure it is appropriately configured consistently through the network. Beware of codecs. Some VoIP implementations use them to convert voice to and from data. If they are resource-hungry, they may introduce extra latency.
Provide more bandwidth.
It sounds simple, but it can have the best effect.
Jitter can turn an effective VoIP environment into an unusable one. Fortunately, it is easily detected and cured.