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General Voice-over-IP (VoIP) Questions

What Bandwidth Do I Require for Each Voice Call?

The bandwidth used varies depending on the compression method chosen. IP Office supports a wide range of compression standards, including the most popular G.723.1 and G.729a. These will occupy approximately 10K and 13K of bandwidth respectively.

What Delay is Acceptable?

Try to keep the overall end-to-end delay to 150 milliseconds or below. An idea of the delay inherent in the network can be measured by carrying out a ping test and dividing the result by two. IP Office has built in echo cancellation to maximize speech quality.

How Do I Minimize Warble and Clipping?

Warble, clipping and some distortion quality problems are symptoms of variable delay and or packet loss. Variability in the delays of traffic is called jitter. Jitter and packet loss may be the result of switches and routers that are either faulty or working outside their design intentions.

IP Office provides jitter buffers to compensate for a moderate amount of jitter found in networks. Voice traffic is quite tolerant of small amounts of packet loss so in most cases this may be ignored. Where packet loss is excessive (greater than 2% say) the cause should be established and fixed. This could be due to a fault or simply an over worked device discarding packets. Significant packet loss can cause perceptible losses in speech, to the extent that no speech may be heard either in one or both directions.

How Do I Minimize Distortion?

Each time speech is converted into a digital signal and back again, tiny difference from the original creeps in. The more times this happens on a single call, the bigger those differences can become. These differences can become perceptible as distortion. Ideally, the path speech takes should only require one ‘analog to digital to analog’ conversion and this will be the case in many instances. Exceptions to this occur when making calls to mobile telephones or voice mail systems where the analog to digital to analog conversion may occur twice (once on IP Office and once on the mobile network, etc).

Different encoding methods will have different effects. IP Office supports a range of encoding methods to allow you to choose the one with the right quality versus bandwidth for your network. In general multiple conversions should be minimized wherever possible.

How Do I Minimize Delay Induced Echo?

Delay in a network originates from a number of different sources and phenomena. A primary source of delay is the process of converting speech to VoIP traffic. The IP Office supports a number of standards based encoding methods to allow the optimum trade off between quality and bandwidth to be made. IP Office incorporates integral echo cancellation to minimize the effect of echo introduced in the VoIP conversion process.

Another source of delay comes from data and voice traffic queuing at the ports of switches, routers, gateways and or bridges that make up the network. It is possible that the traffic queuing at a port is minimal and no action needs to be taken. This would be the case if the available bandwidth far exceeded the demand. To overcome queuing bottlenecks in the network, IP Office prioritizes voice traffic using a standard known as DiffServ. This marks each IP packet carrying voice with a flag so that routers, etc. can force packets containing voice to the front of the transmission queue. An alternative method of prioritization that can be used by switches and routers, with an equally satisfactory result, is to look at what protocol is being used and prioritize this. All voice traffic is carried using two easily identifiable protocols, RTP and RTCP. Both methods are equally good, choose whichever method is the most cost effective and easiest to implement and manage.

A similar source of delay can be attributed to specific network nodes that convert from one network medium to another. For example T1 trunk lines may be carried across a high speed DSL like connection and converting from the high speed link back to T1 in the access gateway takes time to perform. Any VoIP traffic being carried through this link is therefore subject to the delay introduced by this conversion step. The delay may be minimized by ensuring that an appropriate QoS mechanism is enabled in the gateway to prioritize the VoIP traffic. IP Office incorporates integral echo cancellation to help minimize the effect of this kind of delay introduced by the network.

Delay can also be introduced as a consequent of collisions occurring on particular segments of the LAN. Collisions result when two devices on a shared switch port or segment try to transmit simultaneously. This causes all devices to stop transmitting for a period of time. This is a normal characteristic of many older Ethernet networks and, if occasional, may pass unnoticed. The more devices sharing a switch port, and the busier they are, the greater the opportunity for collisions. This is simply resolved by reducing the number of devices on each port, or by dedicating a port to each VoIP device. If you are just using VoIP to link two IP Offices together, it’s well worth dedicating a port to each IP Office and router at either end of the link as the cost implications are likely to be minimal. In this regard it is important to dimension a network to cope with existing traffic demands as well as any future increases in traffic carrying capability.