Implementing Quality of Service (QOS) is Critical in Your VoIP Installation

Implementing QOS has huge benefits for your VoIP application. Don’t underestimate the importance of setting this up properly. Proper configuration can save customers from a difficult experience as well as keep your support costs down.

What Does QOS Do?

QOS sets the priority for data packets on your LAN. The LAN has packets from a diverse set of applications all traveling through a limited amount of bandwidth. Voice occupies a very small portion of the bandwidth. Since the voice packets are delivered in a time sensitive manner, it is important that they do not get interrupted or delayed. If they do, the audio quality on the call can deteriorate to a noticeable degree.

Voice Packets vs. Regular Data Packets

Voice Packets are distinguished from other data packets by a designation in the voice packet Header. This allows the data switch to know how to prioritize the individual packets to avoid delaying voice packets. Networks always try to deliver data on a best efforts basis. If there is bandwidth available, the data switch will try to pass all of the packets through as soon as it gets them using all of the available bandwidth. If this happens, the voice packets can be momentarily blocked by all of the other data. Even though this may only take a few seconds, it is enough of a delay to cause the phone call to experience audio interruptions as packets are delivered too late to be able to be used. By prioritizing the voice packets, you insure that the voice will never be interrupted. Since the voice is a very small percentage of total bandwidth, there is no noticeable effect on all of the other data packets.

An example would be that 10 people on the LAN are trying to download a 20 meg file at the same time. In a normal 100 base T network that could completely block all data traffic for a brief time. By prioritizing the voice packets to always take priority over the data packets, the voice is delivered without delay because the downloaded file makes room for the voice packets with little or no perceptible delay to the downloads.

How Do I Set up QOS?

QOS is set up in the data switch. The IPitomy server will have settings that it uses to identify the data packets. These settings are set to CS3 by default. The data switch will need to be configured to give the highest possible priority to these data packets. Switches use a variety of QOS labels so you will have to determine the scheme (specification) of the switch to be used. Since IPitomy uses the DSCP Class label, just match that label in the switch to the switch’s highest priority (this may be a digit or “Highest” as in the Netgear FS728TP). It’s important to know that no other devices on the network are utilizing that Class ID. If there are, change them or the IPitomy PBX under PBX Setup/SIP/Advanced. The Class ID used for voice traffic must not be used by other, non-voice data devices.

Note: QOS can only be set on the LAN [in the data switch(es)], it is not relevant on the WAN (Internet) since this media is routed by “hops” for which you have no control. The exception to this is private WAN’s like MPLS where the network provider may be able to configure QOS point-to-point.

VOIP (RTP) works best when QOS is set on the LAN. As a rule, always implement QOS.

For more information on setting up QOS, see the article in IPitomy’s WIKI.