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Establish architecture to address VoIP QoS

Posted: 01 Apr 2008 ?? ?Print Version ?Bookmark and Share

Keywords:IP network design? quality network architecture? video voice communications?

Increased attention to voice quality in IP networks is placing more focus on the evolving standards that are needed by quality initiatives.

The use of quality monitoring and advanced real-time algorithms requires a standardized architecture in which quality metrics can be exchanged between endpoints and communicated to a quality monitoring agent for more comprehensive analysis.

There are many architectural and technological options currently available to implement this functionality. For core carrier networks, a standardized quality architecture is needed before any large-scale, next-generation QA system can be deployed. This article describes several of the most pertinent quality architecture issues as they relate to IP networks, and existing circuit switched and cellular networks.

In today's complex IP network topology, it is common for a user's connection to traverse 10 or more different "networks" to establish a complete channel. These various networks may involve IP or ATM technologies, PSTN and various cellular RF air interfaces.

A comprehensive quality architecture is needed to address various handoff issues among the networks. Existing mechanisms are typically limited to allocating bandwidth and minimizing delay, are simply inadequate. Establishing a complete and quality architecture requires distributed intelligence throughout the network and across service provider boundaries. Placing intelligence in network endpoints is the initial step in realizing this goal.

The most commonly conceived endpoint instrument in any telephone call is the handset. It is easy to visualize two IP phones as the endpoints in a conversation. These IP phones could also transmit statistics on call quality.

Intelligence within IP phones and wireless handsets provide the most reliable and objective indicator of the user experience. This is because it is the closest possible monitoring point to the user, where both packet and content impairment data are accessed. In the case of IP phones, statistics can be easily accessed through the IP network. A variety of protocols are available for this purpose. In the case of cellphones, quality metrics can be sent as data and require minimal bandwidth over today's cellular networks.

The design of the PSTN precludes quality reporting from a plain old telephone service (POTS) telephone because no data channel is associated with the instrument. There are no mechanisms for gathering user quality experience metrics.

Placing a call from any phone to another requires multiple gateways within today's communications landscape. Multiple types of gateways are involved in these complex connections. Sometimes these gateways perform the complex task of converting from traditional circuit switched technology to IP technology. In this case, both the PSTN and IP connections are effectively terminated, and the gateway is in an ideal position to report on voice quality from both directions.

As mobile networks have evolved, several voice coding techniques have been developed. This evolutionary process is still going on today with many new codecs under development. When subscribers to a particular mobile network want to talk with a subscriber who may be on a different wireless network, transcoding is likely to be required. This process tears apart and reassembles the voice packet. This is another point in the network where quality monitoring can take place in an unobtrusive manner.

This user quality monitoring functionality requires unobtrusive technologies that will not unduly affect the user experience itself or the processing load on the endpoint device. This scalable processing functionality is most often provided by a DSP. Because of their programmability and flexibility, DSPs are well suited to the changing landscape of today's communications infrastructure. As new codecs are developed and as features evolve to meet the demands of new applications, the flexibility of DSPs will be put to good use.

Quality metrics
Probes can also be deployed in a network to collect and analyze various quality metrics. Today, these probes can be embedded as software into endpoints, allowing for more comprehensive and real-time fault isolation, and an economic and scaleable way of monitoring performance on the edges of the network. In addition, specialized agent probes can be downloaded to endpoints when certain faults are detected.

In addition, new quality management overlay systems for the IP network are evolving. These quality management overlay systems are capable of expert analysis of call quality data and are designed to integrate into existing simple network management protocol (SNMP)-based management architectures. These overlay quality systems allow the service provider to proactively identify degradations in the quality of the user experience and take remedial actions.

The figure illustrates how industry standards can be used to overlay a quality management system on an IP network. Shown in the figure are embedded software probes such as industry-standard VQMon agents in IP phones, monitoring call quality in real time. These agents create RTCP XR messages that are exchanged every 5-15s to provide both quality feedback. They also allow more accurate estimates of quality, since each endpoint can incorporate information on the quality of the service provided by the other endpoint such as echo level. At the end of a call, a SIP QoS report is sent to a call quality management system. Voice Quality Manager can then provide comprehensive summaries and analysis via SNMP to network managers.

Shown are embedded software probes, such as industry-standard VQMon agents in IP phones, monitoring call quality in real time.

Ensure quality now
Today, there exist many of the capabilities of a quality architecture that could be overlaid onto IP networks. Industry standards have been developed and approved, enabling endpoints to monitor and accurately report quality data on a regular basis. Being based on open standards enhances the scalability of an overlaid quality architecture, a critical feature for rapidly expanding IP networks.

Moreover, such quality architecture must be able to rapidly respond to ongoing conditions. Interrogating endpoints in real time to determine the root cause of a fault or degraded service could initiate corrective actions to mitigate the degraded conditions immediately. Midlevel points in the network would collect and manage the quality data generated at the endpoints. Of course, the deployment of such architecture must be cost-effective and its impact on the performance of endpoints should be negligible.

Even though all of the pieces for quality architecture are already available, establishing an architecture to address voice quality will not happen overnight. It is likely that regulatory and privacy issues as well as inter-carrier issues (i.e. the unwillingness to share quality metrics) will slow the process.

The nature of IP and the desire for a higher quality user experience has shown itself to be a driving force in moving standards forward to address quality issues.

- John Warner
High Density Product Manager

Frank Fruth
Multimedia Application Software Development Director
Texas Instruments Inc.

Alan Clark
Founder and CEO
Telchemy Inc.

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