Avoid the jitters by ensuring networks are fully prepared for VoIP implementation

The key factors in providing high-quality business communications

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.


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