VoIPWhitepaper.pdf

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White Paper: Voice Over IP Networks

VOICE OVER DATA NETWORKS

Long distance telephone charges continue to be a major expense item for businesses and individuals despite deregulation of the

Public Switched Telephone Network (PSTN). A successful strategy for reducing long distance telephone costs is to utilize the

existing capacity on private and public data networks for voice traffic. This strategy can take two forms:

• Voice over Circuit Switched Data Networks . Historically, much data has been transported over leased telephone lines. A mar-

ket has developed for equipment to transport voice traffic over these same connections, where in most instances the voice traffic

is compressed to reduce bandwidth requirements.

• Voice over Packet Data Networks . Recently, many companies and individuals have switched to packet data networks, includ-

ing Frame Relay and the Internet, to lower costs. The use of the Internet Protocol (IP) over packet data networks for the trans-

mission of voice traffic is discussed in this paper as an emerging tool for reducing long distance telephone costs.

VOICE COMPRESSION FOR CIRCUIT SWITCHED NETWORKS

Many private data networks are constructed with dedicated leased telephone lines. Other expensive switched circuits include

satellite communications links and international long distance telephone lines. When voice is transported long distances across

expensive circuit switched networks, it is often compressed to reduce bandwidth. This process is complicated, however, by the

fact that voice traffic is usually mixed with fax and data modem traffic. To compress mixed traffic, special DSP-enabled compres-

sion systems are required.

Voice Compression and Bandwidth Reduction

Using a DSP-based compression system that has been ported to the Analogic TAP-800 Family of DSP Resource Boards, incom-

ing voice traffic can be compressed using a voice coder (vocoder). If the call is from a fax machine or data modem, the incoming

signal is demodulated and sent digitally to the receiving station. Overall, traffic can be compressed by as much as 12:1, reducing

circuit switched channel costs by the same factor. One end of the system is shown in Figure 1. A similar system at the other end

of the leased line decompresses the voice call and remodulates the fax and data modem traffic. DTMF signaling is also passed

through, making the system transparent to users. Modern voice compression is made possible by new, high-quality parametric

vocoders. Parametric vocoders model the human vocal tract, transmitting only the parameters needed by the model. Vocoder

technology achieves high levels of compression, low delay and near-toll-call-quality reproduction of human speech. These same

advances also make voice transmission over packet data networks possible.

Voice Over IP Networks

Figure 1. Compressed Voice, Data and Fax Traffic on the TAP-802.

SPEECH COMPRESSION FOR PACKET DATA NETWORKS

Voice over Packet Data Networks

Two types of public packet networks are in use today.

• Frame Relay . Frame Relay networks are rapidly replacing the older X.25 networks and adding new customers. Frame Relay

technology not only costs less, it has much lower delays than X.25 because it does less error checking and uses newer, faster

switches. Frame Relay also provides constant delay because it sets up a Permanent Virtual Circuit (PVC) that remains for the

entire call. The PVC is a single path through the Frame Relay network. All the packets in the call will have the same delay and

can’t get out of order. Networks using X.25 switching have too much delay for voice traffic but Frame Relay, using the IP pro-

tocol, can deliver excellent voice quality over very long distances at very low cost.

• The Internet . The world wide Internet is a vast network with a large variety of equipment providing packet switched services.

Most local connectivity is through data routers. Many backbone transmission facilities are being upgraded to Frame Relay and

ATM switches. Although Voice over the Internet has longer delays than Voice over Frame Relay, the quality is acceptable for

most users, and the Internet has the advantage of near universal availability.

Two types of phone-to-phone services are available over packet networks:

• Service Provider applications

• Intranet applications

Each of these applications requires different equipment configurations.

IP VOICE SERVICE PROVIDERS

Voice over IP network service providers fall into two categories:

• International Telephone Companies . These companies provide international telephone service into and out of the US at a dis-

count over what the local PTT or US long distance carrier charges. Today most of these companies make money by aggregating

traffic on leased lines. To further reduce costs, they often provide “call back” service so that calls originate in the US and carry

the lower US tariffs. Many of these companies and many new international telephone companies are looking to further lower

costs by deploying Voice over IP services.

ANALOGIC

• Internet Service Providers (ISP’s) . The Internet’s growth continues with usage doubling every nine months. However, compe-

tition for new customers is fierce and low cost is a selling point. To improve margins, ISPs are seeking new services to provide

to users. Many ISPs are looking to add voice as a new service and a new source of revenue. Note that in the US, ISP’s provide

low, often fixed-cost access to the Internet because they are considered “enhanced service providers” by the US Federal

Communications Commission (FCC). As such, they are exempt from the access changes that the Regional Bell Operating

Companies (RBOC’s) charge long distance telephone companies. This situation may change in the future but a considerable

cost advantage over long distance switched circuit technology is expected to remain.

Service Providers deliver IP Voice telephone services to individuals or corporate customers over Frame Relay networks or the

Internet. This IP Voice application involves placing a phone call over the local PSTN to the Service Provider (SP) at its Local

Point of Presence (POP) . (See Figure 2.) At the Local POP, the voice data is compressed, packetized and sent over the Internet or

a Frame Relay network to a Remote POP. The voice data is compressed, not only to save bandwidth but also to allow the voice

data to fit in between other data packets in the data packet network. At the Remote POP, the call is placed to the customer over

the remote PSTN. This phone-to -phone connection bypasses the long distance telephone carrier, providing substantial cost sav-

ings to the service provider, who passes these savings on to the customer. The savings are particularly dramatic for international

calls.

Figure 2. Phone to Phone Service Over Packet Data Networks.

Inside the POP

Telephone calls made to the service provider arrive by T1 (North America) or E1 (Europe and much of the rest of the world) digi-

tal telephone connection. A T1 line handles up to 24 simultaneous telephone calls using TDM multiplexing. An E1 handles 30

phone calls. The T1/E1 connection is terminated in the IP Voice Gateway on a T1/E1 telephony board (see Figure 3 below). The

IP Voice Gateway is an industrial-grade Pentium PC with plug-in telephony boards connected by either the MVIP or SCSA local

TDM bus.

Voice Over IP Networks

Figure 3. IP Voice Gateway for Service Providers.

The voice data is passed from the T1/E1 board through the local TDM bus (MVIP or SCbus) to the TAP-802, where it is com-

pressed. The compressed voice data is then sent to the Pentium processor via the ISA Bus where it is packetized and sent to a

router through an Ethernet connection. The router places the voice data on an IP packet data network, either the Internet or a

Frame Relay network.

DSP Software

The compression software on the TAP-802 is more than just a vocoder. It also includes an echo canceller, Voice Activity Detector

(VAD) and DTMF tone detector/generator. The DSP Speech Compression System that has been ported to the TAP-800 family of

DSP resource boards is shown in Figure 4 (next page).

ANALOGIC

Figure 4. DSP Compression System Block Diagram .

IP VOICE OVER INTRANETS

A business with remote offices can use its own intranet facilities for IP Voice services to save on long distance telephone changes.

The savings will be especially dramatic for traffic to international facilities. Any company with access to a packet data network

including Internet access is a candidate for installing an Intranet Voice Gateway.

The Intranet Voice Gateway

An Intranet Voice Gateway system employing the TAP-802 DSP Resource Board is shown in Figure 5. In this system, the

Intranet Voice Gateway is a PC attached to a local PBX. The connection from the PBX to the gateway is completed by a com-

mercial voice board with either an SCSA or MVIP bus interface. The user calls an internal telephone (PBX) extension connected

to the gateway and is greeted by a voice prompt that is provided by the voice board. The user is asked to enter an access code and

the phone number for the call, using a telephone key pad. The gateway then compresses the voice data in the TAP-802, using a

DSP Compression System, packetizes the compressed voice data on the host, and completes the call through a packet data net-

work to another gateway at another company location. The process is the same as in a Service Provider system except that the

Gateway is accessed from a PBX internal telephone instead of a local telephone call to a Service Provider.

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