Echoing—The Expectations

Media gateways must tackle echo cancellation issues to ensure voice quality migrates from TDM to IP, says Doug Morrissey

Providing superior voice quality is the most important way carriers can acquire and retain customers. With voice and data converging on to IP, continuing to deliver on the promise of excellent voice quality isn't as easy as it seems.

The goal of VoIP has always been to provide equal or better voice quality than the network it is replacing and deliver services at a lower cost.

Meeting this goal in the local service network has brought new focus on the critical role of echo cancellation in meeting those customer expectations. Equipment manufacturers more than ever need to keep voice quality in mind when selecting modules for handling voice services in gateway equipment designs.

REASONABLE COST
Echo cancellation algorithms have been evolving since the mid 1980s. The challenge has been to provide acceptable quality at a reasonable cost. When VoIP was being developed in the 90s, designers realised that the delay packetisation added to the connection would require echo cancellation. Echo cancellation algorithms had been available in the long-distance equipment market for 20 years and provided a quick solution to the problem. However, what was overlooked was that customer usage and quality expectations of long distance calls differ from local calls. For example, traditional long distance calls were expensive and therefore customers were careful about the surrounding noise environment when placing such calls. Consequently callers avoided or eliminated background noise that might be distracting or make it difficult to hear. Even today, a "bad connection" is not unusual on long distance calls, resulting in callers terminating the call and calling back.

LONG DISTANCE EXPECTATIONS
Traditional long-distance echo cancellation algorithms were optimised around the characteristics of long-distance expectations. For example, when applied to calls with significant background noise, performance is poor. Packet voice is now being placed into all markets, not just long-distance. Customer usage patterns and their expectations of local call
service are very different when using their phones in multiple environments, including those with significant background noise such as shopping malls and in cars.

This means the echo canceller in the VoIP gateway must be designed from the start to operate well with these types of calls. Echo cancellers must be chosen that can effectively handle background noise and minimise clipping or other distortions.

DESIGN-IN QUALITY
Selecting a module that is capable of providing state-of-the-art voice quality is not an easy proposition. The ITU standard for network echo cancellation is G.168-2002. This standard provides the minimal criteria an echo canceller must meet to be considered for traditional network echo cancellation. Simply meeting this standard, however, does not ensure an echo canceller provides the type of voice quality that meets customer's expectations. Extensive subjective testing and close examination is required to ensure the echo cancellation provided will allow serious deployment in carrier's networks.

An independent subjective assessment, ideally by a respected carrier laboratory such as British Telecom (BT Exact) or AT&T, can validate the 'carrier-grade' credentials of the echo cancellation used in a module. 'Carrier-grade' implies that the performance of the echo cancellation algorithm should meet major carriers requirements and that properly designed equipment using such an echo canceller would be approved for use within their network. Equipment designed with products that have demonstrated this type of testing goes a long way towards ensuring the final product will perform acceptably and allow service providers to offer superior voice quality to their customers as they migrate to next-generation networks.

VOICE MODULE
There are many different applications for gateway equipment, and vendors are looking to leverage their investment in integrating a voice module across as many of these applications as possible. The challenge is that given the competitive nature of the market, each solution needs to be cost optimised for its specific application. For example, in the Passive Optical Network (PON) market, there is no need for compression while in the IP-PBX market, customers may desire compression only for connections carried over a WAN, while in the wireless market, compression is needed on every channel. Selecting a module family that is structured to allow optimisation for varying channel densities and features can result in its use as a cost-effective solution across an OEMs offering of voice gateway equipments.

When it comes to selecting a gateway module family there are generally two architectures used – generic DSPs or specialised voice processors. Generic DSP architectures are not optimised for the voice processing functions required for echo cancellation, leading to a less efficient solution or even the inability to provide the same level of quality as a specialised processor. Therefore, echo cancellation experts recommend choosing dedicated voice processors over generic DSPs for echo cancellation.

When echo cancellation is built on dedicated processors, engineers can optimise the algorithm, as well as power consumption/dissipation and device area. Module gateway architecture, with a voice-processing device that provides not only echo cancellation but features such as tone detection, conferencing etc, permits the lowest cost implementation of a G.711 gateway (eg PONS). This same module can easily be expanded to address LBR CODEC requirements (eg IP-PBX). With specialised processors, designers can increase density and MIPS simultaneously while using less power.

Since echo cancellation is required on every channel in the gateway module and is accountable for each customer's call experience, it is probably the single most important factor in determining voice quality in the gateway. When selecting a module family it is important to choose the vendor wisely. The relationship with the vendor does not end once the module is selected. Integration, optimisation, field upgrades and software loads are required as the OEM's product matures and the echo cancellation algorithm evolves. Selecting a partner with a carrier-qualified algorithm, superior expertise in echo cancellation and resources to aid in architecting customised software and is crucial.

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