Voice-over-broadband (VoB) represents the next incremental step in the evolution of the global voice/data network from a circuit- to packet-switching architecture. Emerging VoB access systems, including voice-over-cable and voice-over-digital subscriber line equipment, typically use standard analog phones as the subscriber interface. These access systems-namely, integrated access devices (IADs), smart residential gateways, network interface units (NIUs), cable modems and set-top boxes-require optimized voice interface chip sets to create "derived voice" telephony lines and functions.
The advent of broadband access and VoB has pushed the local loop much closer to the subscriber, collapsing the "last mile" into a "short loop" within the subscriber's premises. Loops in a VoB system are much shorter-typically less than 2,000 feet and often only 100 feet-than a traditional, "plain old telephone service" (POTS) loop, which can stretch 18,000 feet or more. It is easy to see that requirements for these two extremes are quite different and can lead to engineering trade-offs and optimizations. Historically, this circuitry-subscriber line interface circuits coupled with voice codec/filters-has been designed for long-loop analog line cards used in central offices and digital loop carriers.
As VoB system developers have begun to question the need to comply with the stringent performance standards of traditional POTS systems, chip set vendors have responded by optimizing voice ICs for the simpler, short-loop requirements of voice-over-broadband applications.
VoB equipment manufacturers require highly integrated voice chip sets that enable low-cost, low-power, high-density system designs. These manufacturers also want simplified bills of material and reduced component counts. Still, subscribers expect the same level of service and performance from these systems that they have become accustomed to from the public switched telephone network. Price/performance becomes a delicate balance. Voice IC products for VoB applications are influenced by several market requirements. The number of derived voice channels per NIU or IAD can scale from two to 24 lines and even higher. However, two to four channels is the sweet spot for most VoB applications, especially for the small office/home office and residential markets.
The total power being consumed by a VoB system is a critical factor that can determine the product's success or failure in the market. To be competitive, voice IC power specifications should be as low as possible. A key reason why low-power operation is essential is that the VoB equipment must often be battery-backed to provide "lifeline" service-that is, assured telephone access to emergency services such as fire, police and medical-even if there is a main power failure.
The line biasing and ring voltages required in a telephone loop are another source of headaches for designers. Devices that possess an integrated switcher function to self-generate these large negative voltages are becoming increasingly popular for their simplicity and ease of design. In essence, they provide a proven, optimized and "canned" solution.
The shorter length of VoB applications is a consideration that affects both power and noise performance. Short-loop products do not require the same drive and current capability as long-loop devices. A 600-ohm load at 18,000 feet of wire is significantly different than one at 100 feet. This simplifies power-sourcing and dissipation challenges.
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The ability to interface with any commercially available analog telephone is an imperative for voice ICs used in VoB equipment. Subscribers do not want to buy new telephone handsets.
Flexibility is key because many of the VoB standards from such bodies as CableLabs and DSL Forum are still in a very formative stage. In this early market environment, requirements are a mix of historical and new standards, meaning software support in the form of standard APIs and drivers can be extremely valuable in reducing design time.