These days, if you're starting a home or small office, chances are you
might be considering one of the VoIP providers rather than getting
another analogue phone line put in. However VoIP call quality and
reliability depends on a number of factors. Here are some tips to
improving your VoIP call quality.
There's a glossary at the end of this article.
Choose the Right Handset
Whether you choose to use an IP handset or an analogue telephone
adapter, make sure you choose one with a proven track record. There are
so many manufacturers, and many new ones entering the market each
month, that it can be a difficult task to weed out the chaff. Many
newer manufacturers ship their phones with pre-release firmware that
can result in all sorts of issues, ranging from configuration
difficulties through to dropped calls and choppy audio. A good
community site for checking user reviews of handsets and ATAs is
www.voip-info.org. This community site presents a great deal of
information about VoIP in general.
Choose the Right Codec
A codec (shortened version of enCOder/DECoder) is the software
algorithm that is used to convert your analogue audio speech into an
(often compressed) digital form. There is a range of codecs available
for VoIP. Most handsets and ATAs support at least two.
Generally speaking, the trade-off in choosing a codec is between
bandwidth and CPU utilisation. A codec that requires a smaller
bandwidth usually taxes the CPU more. Choosing what codec to use
depends mostly on whether you are connecting to a PABX on your local
LAN (usually the case in larger offices), or, as is more often the case
with small or home offices, you are connecting to a "Virtual PABX" or
broadband phone provider on the Internet. If you're connecting to your
local LAN there are no bandwidth constraints so you should choose a
full-rate codec such as G711. If you connect over the Internet however,
you most certainly require some level of compression. In this case, a
codec such as G729 or iLBC can give good results.
Because the Internet is a public, uncontrolled network, data travelling
across it often takes a circuitous, unordered route. Packets of data
often arrive out of order, and with varying levels of delay (in VoIP
jargon, this is referred to as jitter). Some codecs deal with jitter
better than others; for example, G729 is particularly resilient to
jitter. In some cases, packets are lost altogether. Virtually all VoIP
codecs react badly to packet loss.
When using a high-compression codec, which necessarily taxes the CPU
more, it's important to ensure you have a robust phone with the latest
firmware installed (see note above regarding handset selection). Poorly
written firmware can often result in choppy audio when using a high
compression level because the CPU simply can't keep up with the
requirements of the algorithm.
Prioritise Your Internet Traffic
Selecting a good handset and the most appropriate codec is no use if
your VoIP calls are having to compete with other Internet traffic on
your broadband connection. You could be having a perfectly crystal
clear phone call with a client one moment which is suddenly interrupted
with all sorts of drop-outs and distortion because your computer is
busily downloading several MB of email.
Most modern SOHO routers have a facility to prioritise various kinds of
traffic competing for use of the connection. The simplest form enables
you to give a higher priority to traffic destined for a particular IP
address and port number or range. You should set this to be the IP
address and audio UDP port range for your VoIP provider (they will be
able to provide you with this information). Another simple form of
prioritisation is by internal IP address - however this requires that
your phone handset have a static IP address (as opposed to being
automatically assigned one by your router).
Some modern routers are smart enough to recognise VoIP traffic, and
automatically prioritise it over other kinds of traffic. Indeed, some
manufacturers are now coming out with inexpensive devices that connect
between your router and your broadband modem. These devices require no
configuration and will automatically prioritise voice traffic.
Yet other routers provide what is called QoS (Quality of Service).
These are special tags that can be attached to IP packets that are read
by some ISP's routers and used to prioritise traffic within the ISP's
network (once it has left your broadband modem). Most ISPs that also
offer broadband phone services support some sort of QoS. It's best to
consult with the ISPs technical department about exactly how to set
this up. While valuable, QoS is not going to help unless you have
already sorted out your traffic priority internally using one of the
above methods.
Glossary
Michael Pollitt is an experienced SOHO business operator, with several successful SOHO businesses, including ConferenceOne, a telephone conferencing service, and LB Broadcast Systems, a developer of closed captions software.
