Getting the most out of packet voice - Packet voice software permits cheap voice communication over existing data networks - Technology Information

Communications News, Sept, 1997 by Matt Noah

Where once there was only the public-switched telephone network (PSTN) for handling voice communications, the Internet and corporate data networking are the catalysts for the hottest voice application since the answering machine -- packet voice.

Packet voice (PV) makes sense in today's technological landscape. It is much more cost-efficient to use existing and emerging data networks based on frame relay, ATM, and IP protocols than to always use the public switched telephone network (PSTN) for voice communications.

This is not to say the PSTN is obsolete. PV will not replace the PSTN in our lifetime, but it will complement the PSTN, supplement the PSTN, and make voice communications more accessible, much less expensive, and more useful in a variety of applications.

Compressed voice got its start on data networks in point-to-point TDM networks. Compression was necessary to use bandwidth efficiently. In a few short years -- with the advent of frame relay -- fully-meshed networks for small, medium, and large corporate (enterprise) customers have emerged.

The trend toward carrier use of PV is now emerging. Carriers see PV as both a competitive advantage and a threat to existing PSTN revenue.

A number of factors are leading to the surge in packet voice. Frame relay, ATM, and IP infrastructure are rapidly expanding. Voice compression algorithms have matured to the point where "toll quality" voice is achieved at 8,000 bits per second.

The DSP chips used to process the complex compression algorithms are ever more powerful and relatively inexpensive. Global telecommunications reform is opening up new and large markets for competitive voice services. Quality of service specifications are being agreed on by the standards bodies to facilitate time-critical applications such as voice and video.

The six-node network diagram on this page illustrates how a typical international corporation might use a PV network based on frame relay. An ATM network example would be identical. Each frame relay access device (FRAD) carries both voice and data traffic. For illustration purposes, only voice connections to the FRAD input are shown.

The diagram illustrates a number of points: * Corporate communications can avoid expensive international long-distance charges. Any phone on a PBX in New York, for example, can be connected via frame relay to any phone on a PBX in Geneva without ever using the PSTN. * It is possible to call from New York to local numbers in Tel Aviv by first traversing the frame relay network from New York to Tel Aviv and then "hopping off" on to the PSTN. * The same network which is carrying corporate data is now carrying corporate voice communications; a very cost-effective proposition. * The FRADs are flexible enough to handle T1 in New York and E1 in Geneva. Translating between the two signaling protocols becomes the work of the FRADs.

The key to successful deployment and use of PV is knowing how to engineer and design a network of devices which can provide the quality of service level expected by users.

It starts with a voice compression algorithm which is "toll quality" and low bit rate. Toll quality implies quality at least as good as 32 Kbps Adaptive Differential Pulse Code Modulation. For frame relay, this means allocating an appropriate amount of committed information rate (CIR) and segmenting larger data frames.

It means having a sufficient capability with private and/or switched virtual circuits. It means choosing equipment which accounts for network jitters and provides line echo cancellation, voice level settings, and PBX-like dialing plans.

Beyond these fundamentals, the differentiating voice features of a packet network access device include the ability to: * detect fax and data modem signals and act appropriately. * service a variety of physical voice connections. * respond to congestion on the network via rate fallback schemes and/or dropped packet interpolation/extrapolation schemes. * operate with telephone requirements and standards in a great number of nations. * generally behave in a manner that does not disrupt the end users' telephone usage.

Customers want access to digital and analog line facilities. For analog lines, this entails connections to phones, connections to a central office, and E&M connections to analog PBXs.

For digital lines, the use of T1 and E1 facilities are desired. The use of basic primary rate interface and other common channel signaling systems such as DPNSS and QSIG are becoming more important.

The latest frontier for voice communications has been the Internet. Quality of service issues dominate the outlook for long-haul transport on the Internet. However, private IP networks can be engineered to work much better.

Of more interest is the interworking between the Internet and PSTN and private IP networks and the PSTN. In this regard, Internet-based telephones can be seen as an access to the PSTN.

The author is an engineering director at ACT Networks in Camarillo, Calif. He may be contacted at matt@acti.com, by phone at 805-384-2411 or by fax at 805-383-1035.