Roger Jones's customer had a problem. The firm had Cisco
implement one part of itsvoice over internet protocol(VoIP)
system and Avaya do the other. Sound quality was suffering, and
callers were hearing jittery audio.
"There had been mismatch between Cisco and Avaya QoS," says the
business development director within Avaya's technology and
consulting group. There were supposed to be three streams of
traffic in the VoIP system - one for voice, and two sets of
signalling traffic (one for the gateway and another for handset
signalling).
"The customer had only set up two," he says. "That was a classic
example where a network readiness test had been done but they
needed to do one extra test.
As more companies' traditional PBX systems reach end of life and
they decide to move to VoIP, situations like these will continue to
crop up, but VoIP needn't be such a trial. Implementing the
technology can be relatively painless as long as you follow some
basic guidelines.
Dan Davies, consultant manager at specialist unified
communications integrator Unified Group, says that any VoIP
implementation should start with a formal requirements gathering
phase, so designers know what the business wants to do with the
system.
"Users really don't know what their call patterns look like,
which makes it very difficult. The answer to 'how much bandwidth do
you need?' is 'how many calls [will be made], and how compressed do
you want them to be?'"
Victor von Schlegell, president of Michigan-based VoIP services
firm Appia Communications, says that customers often don't know
what systems can do and set out to save money without considering
loftier goals.
"Before they buy, they should understand what convergence really
is and how it can help them," he says. For example, receptionists
may be used to telling callers to redial another number to find
someone working in another location. VoIP makes inter-office
routing possible - but does the customer know that?
When the customers are ready, the designer has to be sure the
network is ready. A network readiness assessment is crucial, says
Davies.
He will put network probes on a system for around a week, to
emulate VoIP calls between themselves and other nodes. This gives
the design team an understanding of issues such as packet latency,
and enables it to produce a summary of recommendations.
Security measures should be a part of those recommendations.
Putting voice and data across the same network makes the
infrastructure an even more critical attack vector. Those who don't
secure their VoIP implementation could potentially look forward to
unauthorised access to PBX systems, call tapping, and denial of
service attacks.
There are some ways to avoid these dangers. Encrypting the voice
content is important to avoid eavesdropping, says Jones. "We're
using standards-based SIP phones and we want to encrypt the voice
to that phone. We do it on a per-call basis," he says.
Avaya uses certificates sent to the phones, which means that it
is also important to encrypt the signalling data. The Session
Initiation Protocol normally used to control calls is a verbose,
human-readable language, which is often sent in cleartext. Using
TLS encryption protects both that, and the certificates used to
encrypt the voice.
Other measures include using endpoint security technologies such
as 802.1x, to verify devices connecting to the network. IP phones
should also be logged out by default, and default passwords in
their embedded web server software should be configured to be more
secure.
The Unified Group recommends using a virtual Lan to segregate
VoIP traffic for security purposes. However, Davies points out that
it isn't quite that simple. Many companies must consider softphone
users, who will be calling from PCs on the regular data Lan rather
than from IP handsets that are easy to connect with the VLan.
VLans can also be useful for quality of service (QoS), which is
a crucial attribute in networks serving low-latency applications
like VoIP. QoS is a particular concern across Wans where bandwidth
is often constrained. VoIP solutions can be implemented across the
open internet if you have a high-speed broadband connection (the
higher the speed, the better), but it can be ropey. Von Schlegell
doesn't recommend that any office with more than three phones
attempts an open internet VoIP setup.
Ideally, customers will have a dedicated Wan service provider,
which a VoIP implementer would talk with to find out how Wan
systems have been configured. Many such companies are moving to the
to DiffServ model for QoS, where bits are set in a certain area of
the packet header to signify different levels of service.
DiffServ is becoming particularly important as VoIP solutions
spread across multiple sites, Davies says. "802.1p is a layer two
QoS mechanism, but as soon as a packet needs to traverse between
subnets, through a router or layer 3 switch, 802.1p is useless," he
warns. "Then you have to use a layer 3 QoS mechanism and DiffServ
is becoming a standard there."
Following guidelines such as these could keep your VoIP signals
crisp. Ignoring them could create a 'London Bus' syndrome in your
network, where VoIP packets arrive too late, all at once, or not at
all. And unfortunately, unlike public transport, you can't shrug
your shoulders and assume that there'll be another packet along in
a minute.
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