Enable VoIP Quality of Service from the Design Platform

The need for a 'Quality Overlay'
In the past, the goal of IP Quality of Service (QoS) initiatives has been to guarantee a certain level of service for a particular service. These QoS schemes attempted to guarantee a minimum throughput speed or a maximum delay time, and they provide a framework for preferential treatment to certain customers or types of traffic. They did not guarantee a fault-free environment nor protect against network packet loss or jitter.

Thus, QoS initiatives typically do not guarantee a quality user experience for real-time applications such as voice and video. Also, they do not enable remedial actions in the event of service degradation. Another critical piece that is missing from these approaches is a view of the signaling layer that is used throughout the network to establish and maintain user connections. In today's complex IP network topology, it is not uncommon for a user's connection to traverse 10 or more different "networks" to establish a complete channel. These various networks may involve IP or Asynchronous Transfer Mode (ATM) technologies, the Public Switched Telephone Network (PSTN), and a variety of cellular radio frequency (RF) air interfaces.

A comprehensive quality architecture is needed to more effectively address these and various handoffs issues among the networks. Existing mechanisms, which typically are 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.

Endpoints
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. Besides the call itself, these IP phones could also transmit statistics on call quality. However, the definition of an endpoint should be extended to include any point in the total communications infrastructure involving any media or any technology, including cable, PSTN, wireless and others, where a transition in protocol or coding occurs. Under this definition all gateways and transcoding nodes qualify as endpoints. Figure 1 provides an illustration of endpoints for the purposes of this discussion.

Intelligence within IP phones and wireless handsets provide the most reliable and objective indicator of the user experience because this is the closest possible monitoring point to the user where both packet and content impairment data is accessed. In the case of IP phones, statistics can easily be accessed through the IP network. A variety of protocols are available for this purpose. (See sidebar located at the end of the article: "Protocols for compiling user experience statistics on IP networks,") In the case of mobile cell phones, 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 there is no data channel associated with the instrument. For the most part this is not needed because the 4kHz POTS voice channel is dedicated bandwidth with a fixed delay. This is not to say that problems don't arise in this environment; however, there are no mechanisms for gathering user quality experience metrics. For the most part, a POTS device is an "on/off" technology; it either works or it doesn't.

Placing a call from any phone to any other phone requires multiple gateways within today's communications landscape. For example, calls from an IP phone in a modern digital home can be connected to remote and often primitive villages in far away lands. Multiple types of gateways are involved in all of these complex connections. Sometimes these gateways perform the complex task of converting from traditional circuit switched technology, as found in the PSTN, 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, broadband or even the PSTN 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.

The user quality monitoring functionality described above 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 Digital Signal Processor (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 features and functionality evolves to meet the demands of new applications, the flexibility of DSPs will be put to good use.

Reporting quality metrics
In addition to the quality data gathered at the endpoints, probes can be deployed in a network to collect and analyze a variety of quality metrics. Historically these metrics have been used for troubleshooting. Active or passive probes strategically placed in the network could acquire the data needed for troubleshooting. Today, these probes can be embedded as software into endpoints, allowing for more comprehensive and real-time fault isolation and providing for 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.

Whether it is from an endpoint or from a network probe, equipment vendors and network operators can choose from any of several protocols for reporting quality metrics. New standards for this purpose are currently being developed and existing standards are evolving. (See sidebar below) In addition, new quality management overlay systems for the IP network are evolving which can collect information from intelligent endpoints on the quality of real-time voice or video services. These sorts of quality management overlay systems are capable of expert analysis of call quality data and are designed to integrate into existing SNMP-based management architectures. These overlay quality systems allow the service provider to proactively identify degradations in the quality of the user experience and to take remedial actions.