The key factors in providing high-quality business
communications
Many IT directors have reservations about using voice over Internet
Protocol (VoIP), believing the network will not be of a high enough
quality to carry both voice and data traffic satisfactorily.
But there is no need to get the jitters about implementing VoIP -
"jitter" (the variable arrival of network packets) is just one of
the quality degradations you might expect from using a data network
for voice.
There are a number of ways companies can prepare their networks to
be of the right technical quality and reliability to ensure
high-quality voice transmission. A whole array of technologies and
tools are available to make sure the network maintains its quality
of service (QoS): a predefined level of performance in a data
communications system. QoS can prevent jitters, round trip delay
and can also prevent packets being lost.
First, a network design based on Lan switches and routers that
supports QoS standards on both the Lan and Wan can provide
consistent high performance.
In addition to the IP telephony gateways, and both physical and
software IP phones, companies should implement jitter buffers to
smooth traffic received from the network, according to Roger Jones,
Avaya business development director, EMEA. This will allow IP voice
to be used over links with patchy or no QoS (for example,
ADSL).
Second, if the network supports "differentiated QoS", then
performance monitoring and analysis tools are useful to warn the
network manager of impending service level agreement violations,
said Mark Orchart, director of European marketing at Spirent
Communications.
"These performance monitoring tools will monitor for delay, jitter
and packet loss," he said. "Where misconfiguration or network
faults lead to degradation in voice quality, test and diagnostics
tools can determine the exact 'service-affecting' error and
recommend a remedial course of action for rapid resolution."
Also, voice packets can be given priority in queuing, with data
packets getting "weighted fair queuing" where data is prioritised
differently to meet specific requirements, without affecting audio
packet quality.
Intelligent routing protocols have been created to prioritise
network traffic. Among these are MPLS (multi-protocol label
switching) and OSPF (open shortest path first). These protocols
intelligently optimise the network traffic, in line with
predetermined parameters.
Most IP products now support multiple data traffic queues in
hardware, and a technique called packet marking, to prioritise
traffic through protocols such as IEEE 802.1p for Ethernet QoS and
Diff-Serv for IP QoS.
There are also a number of packet-shaping and prioritising products
that can enhance network performance. But Jones said, "Care should
be taken when using devices that apply data compression to voice
packets, as they may have already been compressed by the IP phone
or gateway." This would result in a poorer quality audio
signal.
Nick Chrissos, from the EMEA product management team at Cisco
Systems, said, "Features like policy-based routing, which will do
route selection based on the agreed policy, will affect the
optimisation of the traffic within the network.
"All the Cisco routers, depending on the processing power, are able
to implement a number of these features to manage the bandwidth and
traffic limitations over the network.
"The end points within the solution [IP phones and gateways] can
mark packets, to give the network [routers and switches] the
ability to recognise the information and use the correct level of
prioritisation."
BT is an example of an IP network provider that offers a global
MPLS network for voice and data traffic. Ivor Kendall, general
manager IP at BT Retail, said the MPLS network uses a new model of
traffic prioritisation called DSCP (differentiated services code
point) to encode data using a set of bits.
"These bits or settings tell the network which packets are
important to that particular customer to enable the network to
intelligently look after the delivery of packets to the
destination. BT's network maintains this service differentiation
throughout its entire core instead of providing this functionality
only at the access layer," said Kendall.
High-quality VoIP services need highly reliable and scalable VoIP
servers and equipment, but hardware alone will not be enough to
maintain high-quality VoIP services. Good network management
software is also needed to gain visibility of fluctuating traffic,
and robust, rigorous and automated testing is needed to ensure the
service is maintained to the highest levels.
One option is to use products such as Cisco's Network Management
software, which has components to help maintain and monitor the
VoIP network.
Avaya also sells an IP telephony voice-monitoring application as
part of its Integrated Management suite.
Chrissos said products such as the Cisco IP Telephony Environment
Monitor, which is the latest addition to the Cisco Works Suite, are
designed to address the management of the network on both a
reactive and a proactive basis.
Administrators of VoIP systems have to be equipped with tools that
give them information about the status of the traffic at any point
on the network. This data has to be intelligently compiled to be
valuable for use in optimising the network and expanding it if
needed.
Such software can give the network manager tools to look at the
specific IP voice traffic on the network, examining the round-trip
delay, jitter and packet loss on an individual extension, a group
of extensions or the whole network.
BT has developed a service called Application Assured
Infrastructure, where it deploys probes, robots and monitoring
tools to simulate user application traffic. It can then report in
real-time and analyse the network traffic from business and VoIP
applications.
In terms of protecting and securing the voice network, VoIP is an
application that exists on top of a data network that has already
been deployed. So most of the company's existing security devices
and applications should provide protection for an IP PBX
server.
But in addition, firewalls should become "applications-aware", so,
for example, remote workers wanting to connect to the corporate
network over the virtual private network, using the IPSec protocol,
will need to be able to access the full range of PBX
services.
Jones outlined three further issues that IT security personnel need
to address to secure IP telephony systems:
- If real-time IP voice traffic is passing through a security
device, that device will have to be able to deal with latency and
jitter. "The performance of the security devices needs to be
analysed prior to deploying IPT," he said.
- Placing call processors and gateways on the IP infrastructure,
makes them more vulnerable to hackers, denial of service and
eavesdropping attacks. "A traditional PBX is typically not
connected in this way," said Jones. "An example is if you look at
the ways the call processors can be isolated from the corporate IP
network and, of course, the internet. The products need to be
hardened to withstand denial-of-service attacks."
- The third issue is eavesdropping. "It is an easy process to
sniff packets from an unsecured network," said Jones. "It is also
easy to decode VoIP calls with a number of freely available tools.
This means that any intercepted call can be listened to, converted
to a .wav file and e-mailed to anyone."
Jones added that IT directors need to ensure that their IP PBX
supports media encryption, so that the packetised voice is
encrypted and only authorised users can decrypt the contents of the
call. "Do not use an IP telephony system without media encryption,"
he warned.
Network managers should make sure they carry out some simple
procedures so their network's voice/data quality remains high: for
example, managing the number of IP calls that are placed across an
IP Wan link.
Also, using call admission control can limit the bandwidth used by
voice, and can, if necessary, allow calls to be routed across the
public switched telephone network system to ensure good quality.
This can be done on a call-by-call and extension-by-extension
basis.
When using an IP software phone on a PC, the IT department should
make sure that employees have a good quality USB headset or
handset, which will improve call quality for remote users.
They could also implement change control on the IP network to
prevent users from starting to use a new high-bandwidth
application, for example peer-to-peer file sharing, which could
harm voice traffic.
Also, the use of virtual Lans can separate voice traffic from other
traffic, particularly broadcast-intensive applications, and this
will keep the quality high.
VoIP implementation: the network manager's shopping
list
In terms of the hardware and software required to assemble a
reliable VoIP network, managers should make sure they have certain
key items on their shopping list.
The list should include centralised call processing servers,
such as the popular Cisco Call Manager; a set of IP phones, either
hardware IP phones that can plug into the network, or
software-based IP "softphones".
PSTN Gateways will provide connections between the IP network
and the public switched telephone network and time division
multiplexing (TDM) systems, and additional hardware resources such
as conference bridges might be required. If the IP network is
distributed over multiple sites, additional hardware might be
needed to achieve a high level of redundancy and bandwidth
management.
The architecture itself is likely to be different to a
traditional TDM-based PBX. Therefore, the underlying network
infrastructure needs to support industry standard quality of
service, and protocols such as IEEE 802.1p and Diff-Serv, and
standards-based power over Ethernet (POE, IEEE 802.3af).
Network managers should also put a range of software tools on
their shopping list to assure the reliability of their VoIP network
and services. Application-specific firewalls can secure the network
and allow remote workers full access to services.
Network management tools can give the network manager visibility
of the VoIP network, and help them to understand and troubleshoot
the service as it is being used. In addition, software analysis and
diagnostic tools can help the network manager to maintain a
high-quality network.