VOICE NETWORKS

From the March 2004  issue of Communications News

Improve Your VoIP Deployment

Not all LANs are ready for voice over IP. In this report about the converging of voice and data, two experts outline the steps and tools necessary to successfully deploy voice over IP onto an enterprise’s existing network.

by Paula Daley

Implementing voice over Internet protocol (VoIP) can have a substantial, positive impact on and enterprise. Cost savings from IT staff efficiencies, reduction of international toll charges, workforce mobility and productivity-enhancing applications, such as multimedia call centers and unified messaging, are all possible–but up-front planning and management are the critical success factors.

What management features are important when converging voice onto an IP network? Mainly, a pre-deployment assessment of the IP network for VoIP readiness, and an integrated management view of data, voice and quality of service (QoS).

First, assess the existing IP network for its readiness to handle the new demands of VoIP. The introduction of VoIP will add to network traffic volume and raise the stress level of the IT department. Being more delay sensitive than data, voice degradation is immediately apparent to users and nothing generates more complaints. People are accustomed to good voice service and if VoIP introduction becomes associated with negative calling experiences, the convergence project can be doomed.

Be sure to run the assessment end-to-end, through everything in the voice traffic path, to obtain valid results. Testing solely over the LAN can give a false sense of security. With the LAN’s higher bandwidth, you can expect the assessment to deliver positive data, while the more limited bandwidth and greater variety of traffic paths in the wide area network are likely to expose deficiencies. The following items are important to assess:

• network utilization, delay, jitter, and packet loss, with jitter (the variation in the difference between packet arrival and departure times) a critical factor in voice quality;

• variations in traffic volume on a daily basis;

• capabilities of your network devices, such as support for QoS and maintenance of continuous QoS across the network; and

• voice quality in the assessment phase, where voice-quality measurement identifies network characteristics that may be adequate for data traffic, but are inadequate for voice traffic.

Predeployment assessment pinpoints problems in the current production network, helping to justify network upgrades and focus capital investment dollars. As some companies have learned the hard way, predeployment assessment is less painful than recovering IT integrity after the fact. The assessment phase simplifies the move to production network management, since many of the tools used for assessing the network are the same ones that are essential for ongoing management.

QOS PROVIDES PRIORITY DELIVERY
A converged IP network is both dynamic and shared, with data and voice contending for the same resources and requiring some over-arching means of allocation based on constantly changing network conditions. QoS is that mechanism, giving specified applications priority in network delivery over others. QoS should be implemented in the converged network and managed within the same system as data and voice. An integrated management view of data, voice and QoS is the tripod that lends stability to the entire network.

Most IP networks started their life as data networks. By the time voice is added to the network, a data-management tool is often established. Not wanting to disrupt the management already in place for data traffic, customers sometimes opt for a separate voice-management tool. The introduction of QoS adds to the confusion and is sometimes considered itself to be an enabling mechanism for the convergence of data and voice traffic. Partitioning the management of data, voice and QoS is a common pitfall, reducing the value of management overall. The value is in the integration of the data to show how the factors contribute to the conditions in the network.

Voice-quality measurement is important in the predeployment assessment and as an on-going management tool. A standard metric for measuring voice quality is the mean opinion score (MOS) rating. MOS was originally calculated subjectively, by eliciting the opinions of a large number of listeners on a range of quality factors. Ways of mapping MOS ratings to objective testing techniques have been developed and are commercially available.

When the MOS rating is satisfactory over the LAN but degraded below an acceptable level over the WAN, the problem can be resolved by trading off tonal quality and conversation timing against voice clarity. The trade-off is accomplished by using different encapsulations available in the phone and managed by the phone switch.

On a LAN, a high-bandwidth encapsulation (codec G711) can give analog-quality voice. If a call traverses a WAN, however, that same codec would result in an unintelligible conversation, or even a dropped call, due to both bandwidth limitations and inconsistent packet delivery times (jitter). Codecs such as G723 and G729 are preferable for use over a WAN. By performing data compression and accepting varying jitter, the receiver can smooth out the traffic–much like traffic shaping for the human ear.

A well-thought-out QoS implementation is the "make-or-break" factor in successful network convergence. A converged network should allocate resources among different traffic types–classifying, shaping, policing and scheduling traffic–to satisfy such metrics as bandwidth, jitter, delay and packet loss. These trade-offs are based on the classes of service established by the customer, are associated with QoS metrics and are governed by service-level agreements for each class. Using QoS as the arbiter, the various traffic types can be treated according to volume and quality needs, in the context of a single network.

Just as important as deploying QoS is managing QoS within an integrated management structure. The QoS policies govern both voice and data servicing. If the voice service degrades, is it because a router is down or are the voice policies suboptimal? An integrated management approach can facilitate the answer to that question.

In summary, avoid:

• managing voice and data independently, which will only result in conflicts over network resources that decrease overall service quality;

• adding bandwidth only as a means of ensuring voice quality–a shortcut that will eventually be exposed by WAN limitations and increased usage; and

• making excuses for implementing and managing QoS.

Instead, successful strategies include:

• assessing the network before VoIP deployment to determine its capacity and capabilities, and to pinpoint upgrades needed for successful deployment;

• implementing voice-quality measurement at the assessment phase and continuing to use it after IP-telephony services are rolled out;

• implementing QoS end-to-end; and

• establishing a single, integrated management system for voice, data and QoS.

For more information from Concord Communications:
www.rsleads.com/403cn-253