Advanced analog voice solutions in a VoIP world

VoIP holds out the promise that the wired desktop will converge into one major appliance, allowing companies to do away with traditional analog telephones and FAX machines and avoid installing both Ethernet and twisted-pair copper lines throughout the building.

However, many enterprises already have existing Ethernet and legacy copper lines. In this situation, the cost savings that IT managers are looking for in an IP- based phone exchange are not found in new capabilities such as auto reconfiguration, call handling, voice mail filtering and caller ID, but rather in the transport arena. Shunting voice traffic over enterprise LANs produces immediate savings in the form of less expensive long-distance bills. This reduction in cost can be significant, on the order of 30-50%, and may be achieved via media converters that support the existing analog telephones and convert voice to Ethernet traffic.

For all the benefits of supporting standard Plain Old Telephone System (POTS) interfaces in an IP-PBX, there are some system design considerations that need to be taken into account. The support of legacy connectivity requires additional line card interfaces to connect to analog subscriber loops. Considerations from the system-level include additional cost and space requirements of analog interfaces, as well as the scalability of those interfaces. The silicon components used for the POTS line cards can be the most significant factor in minimizing these design impacts.

The silicon components required to terminate POTS interfaces and perform the necessary media conversion functions include the Subscriber Line Interface Circuit (SLIC), pulse code modulation CODEC, DSP and micro-controller. Of these, the SLIC and CODEC provide the most essential functions in the PBX analog subscriber loop interface, including 2-wire to 4-wire signal conversion, battery feeding, ring generation, A/D conversion, filtering and PCM encoding. The DSP is typically used for voice compression, line echo cancellation and time division multiplexing to packet conversion, while the micro-controller performs supervisory and control functions for the line card components and higher-layer transport and routing protocols, such as H.323 or SIP.

The SLIC essentially manages the high-voltage interface between the analog POTS line and the rest of the line card. POTS lines require nominal 48-volt DC power during periods where the receiver is on-hook and 60 volts DC or more when ringing signals are applied to the receiver. In a PBX environment, where subscriber loop lengths are typically under one kilometer, the SLIC must be able to apply the appropriate battery voltages and ring signals to the subscriber line, as well as supervise the status of the subscriber terminal, regulate current flow, and signal the initiation/termination of a call to the network. Another critical function of the SLIC is the hybrid, which converts the voice and signaling traffic from a high voltage differential, 2-wire signal to a 3.3- volt single-ended 4-wire signal that can be passed to and from the CODEC. Important performance indicators of the SLIC, in enterprise environments, are the 2-wire return loss, especially around 1kHz, where most of the human voice spectrum falls, and the longitudinal balance, which gives an indication of noise immunity from imbalanced line voltages.

The basic functions of the CODEC are to filter the signals from the SLIC, convert them between analog and digital formats, and encode or decode them for interfaces to the other system elements. Traditionally, the CODEC performed some compression of the voice signal but the increased use of advanced compression algorithms in VoIP applications has given rise to the need for a linear encoding format within the CODEC. That would provide a more accurate sampling of the voice signal prior to its compression within the DSP. Given that most equipment manufacturers will likely market their IP-PBX system in multiple countries, it is typical to use a programmable CODEC that can internally match the system interface to the appropriate telephone network. Another desirable feature for the CODEC is the ability to perform integrated test and diagnostic functions (ITDF), which determine line condition and performance. Integrating ITDF on the CODEC reduces space and cost requirements on a PBX line card by eliminating the need for external devices.

While loading the SLIC and CODEC with advanced functionality adds to the scalability of a POTS interface card, tight integration between these two components is the key to substantial cost and space savings. An integrated CODEC/SLIC chipset can minimize board space by incorporating passive elements into silicon and increase signal quality by optimizing the internal filtering to the characteristics of the two devices. This integration can be taken a step further by changing the traditional partitioning of the functions between the devices to maximize feature and process strengths.

Traditionally, the pure analog functions relating to subscriber line termination and monitoring were incorporated in the SLIC. The CODEC incorporated analog-to-digital conversion and the digital portion of functions such as filtering, PCM encoding/decoding and compression. Because of the inherent cost advantages of digital CMOS processes over high voltage analog processes, significant cost benefits can be achieved with CODEC/SLIC chipsets that incorporate the supervision and detection functions into the CODEC and simplify the SLIC to a pure analog front end. In addition to providing a more cost-effective solution, this partitioning further centralizes the intelligence of the analog interface in the CODEC to ease software complexity.

CODEC/SLIC chipsets that provide an advanced feature set, tight integration and an aggressive price point offer a new, effective solution to resolve the issues and considerations involved in backward compatible IP-PBX systems. Through new developments in partitioning, the cost and board space requirements of analog line cards can be reduced substantially from previous solutions.

See related chart

Furthermore, the tight integration of analog and mixed-signal building blocks provides an off-the-shelf solution that can be scaled upward or downward to satisfy system port densities. By effectively minimizing cost and space requirements while promoting scalability in the overall system design, integrated CODEC/SLIC chipsets facilitate POTS connectivity for IP-PBX systems and provide a clear roadmap for legacy analog voice support in the emerging enterprise VoIP market.