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First Quarter 1998

Internet Telephony Over Frame Relay


At one time, telephony was the network. Circuit switching was the underlying technology for voice, and X.25 packet switching (along with a number of proprietary technologies) was the choice for data communications. As data communications networks evolved from analog to digital technologies, new protocols were defined and deployed. Frame relay technology enabled network performance to move from kilobits per second (Kbps) to megabits per second (Mbps).

Frame relay was followed by Asynchronous Transfer Mode (ATM), which broadened bandwidth to hundreds of megabits per second and higher. In parallel, the Internet developed from the world of academia and the United States Department of Defense into a worldwide communications tool. These networks provided the technologies that became the current foundation for new applications and services.

In the future, the world will be populated by appliances and devices that can be addressed by either a telephone number or an Internet Protocol (IP) address. These devices will be interconnected by a broadband network built on the foundations of frame relay, ATM, and Internet protocols. Each of these protocols has its own advantages and disadvantages.

The explosive growth of these technologies has increasingly highlighted the need to develop solutions that provide the means to transport voice, data, and other traffic seamlessly between what can be very different types of devices and networks. This article will focus in on one aspect of this challenge, specifically the integration of Internet telephony with frame relay.

The frame relay protocol is a data link switching protocol that allows statistical multiplexing of multiple, variable-sized user data payloads (known as frames) over the same physical connection. In the frame relay protocol, each physical connection supports multiple logical connections, each of which can be associated with different user applications.

The connection-oriented nature of the frame relay protocol ensures frames for the same connection will follow the same path through the network. The protocol assumes error-free physical links, so that no error correction or frame retransmission is supported within the network. On error detection, frames are discarded and error recovery is delegated to the applications running in the end devices. The logical connections are identified in each frame by an address header.

Within the network, frames are switched (based on the address header) to the appropriate destination. These features provide a fast switching mechanism for transport of user data and high-bandwidth utilization. In addition, the protocol provides procedures for bandwidth allocation to each logical link and mechanisms for the network to monitor and enforce the traffic contract on each logical link. This means users can negotiate for the bandwidth required by their application to ensure quality of service (QoS) and to manage their costs.

The Internet Protocols (IP) are actually a collection of application protocols, such as Simple Mail Transfer Protocol (SMTP), File Transfer Protocol (FTP), and Hypertext Transfer Protocol (HTTP), and communication protocols, such as User Datagram Protocol (UDP), Transmission Control Protocol (TCP), Internet Control Message Protocol (ICMP), and IP. These application and communications protocols provide connectionless packet-switched services: UDP provides unreliable data transfer; TCP provides support for reliable data transfer. Each IP packet contains addressing information used to route the packet to its final destination. Because each packet has addressing information and the service is connectionless, packets do not have to travel along the same path to reach the same final destination. This means packets may arrive with unpredictable delays.

IP packet sizes are also large. This enables efficient use of network bandwidth, but introduces additional delays in crossing the network. Packets also require very little processing in the intermediate nodes. These features allow the intermediate routers to route the packets around any congestion in the network.

In contrast to e-mail, file transfers, and other types of data transfer, voice has stringent delay and jitter requirements to ensure that a person can understand the person to whom they are speaking. In the traditional telephony network, human speech is converted from analog signals into digital bit streams and transported across the network via circuit switching. At the final destination, these digital bit streams are converted back to analog signals for a person to hear.

There are a number of different techniques used to convert voice to digital bit streams. Pulse Code Modulation (PCM) and Adaptive Differential Pulse Code Modulation (ADPCM) are two of the more well known. These techniques require 64 and 32 Kbps of network bandwidth, respectively. The use of newer speech compression algorithms reduce voice traffic requirements to as low as 4 Kbps, and techniques such as silence suppression and background noise regeneration help deliver high-quality speech.

As the broadband network of the future moves to a combination of circuit switching, packet switching, and cell switching, standards organizations, such as the Frame Relay Forum, ATM Forum, Internet Engineering Task Force (IETF) and European Telecommunications Standards Institute (ETSI), are in the process of defining standards and specifications to support voice over frame relay, voice over ATM, and voice over IP. In addition, there is activity underway to define Signaling System 7 (SS7)-to-IP interworking protocols that will allow seamless voice transport between the telephony and Internet networks. There is no explicit specification for Internet telephony over frame relay, although one of the first complete specifications to emerge out of all of these efforts is the Voice over Frame Relay Implementation Agreement (VoFR IA) from the Frame Relay Forum.

The VoFR IA specifies a flexible and extensible method for transport of voice and other real-time, delay-sensitive traffic over frame relay networks. The IA specifies the compression algorithms, transmission requirements, and other formats and procedures necessary to transport digitized voice payloads in frame relay frames. The IA also extends the frame relay protocol by allowing sub-channelization of the logical connections so that voice payloads from different voice calls can be concentrated over the same logical link. This also allows users to multiplex voice and data payloads over the same logical connection, resulting in high-bandwidth utilization.

On a logical connection, a predefined sub-channel is reserved for the end-to-end transportation of signaling information. These extensions, in conjunction with frame relay's bandwidth allocation and traffic management capabilities, will allow users and network providers to leverage and extend the functionality of their existing frame relay infrastructure.

There is a similar effort underway to specify the formats and procedures to support voice over IP (also known as Internet telephony or VoIP). The intent behind these specifications is similar to that of the VoFR IA -- to allow users and network providers to leverage and extend the functionality of their existing Internet infrastructure. In addition to the delay and jitter issues introduced by the IP protocol, the unique openness of the Internet creates a special challenge to ensure that effective security solutions are available to transport sensitive voice, data, or other traffic. As these issues are addressed and service quality, security, and reliability improve, individuals, businesses, and organizations will be able to justify the migration of some or all of their voice traffic to the Internet.

However, as the industry is still working towards generally accepted standards for Internet telephony, currently available solutions are mostly vendor-specific and vary widely in service quality. Widespread deployment of high-quality Internet telephony and multimedia services will depend on the end-to-end deployment within the Internet of bandwidth allocation, QoS, and class of service features available in the Resource Reservation Protocol (RSVP).

Security and other issues are being addressed to some extent by Virtual Private Network (VPN) activities. As appliance and device mobility grows, interworking between the Internet and telephony networks will become essential. Today, a number of proprietary Internet-to-telephony gateway solutions are available from vendors that provide some of this functionality. Availability of standards will continue to fuel the growth of this market segment.

Service providers and carriers have been offering frame relay services used primarily by businesses and organizations for some time. Changing services or infrastructure can be costly and time consuming. As Internet telephony develops, many businesses and organizations may find it advantageous and cost effective to deploy Internet telephony over their existing frame relay services and infrastructure. A viable model for reaching this goal is fairly straightforward.

Most frame relay networks currently support the transport of IP packets encapsulated in frame relay frames as specified in the Frame Relay Forum's Multiprotocol over Frame Relay Implementation Agreement and the IETF's RFC 1490 specification. IP packets are carried over frame relay with just the overhead of the existing frame relay address header and an additional 2-byte header to distinguish IP packets from other data. The multiprotocol mechanism can easily be used to transport VoIP traffic transparently using the existing frame relay network. Some service providers are using this model for deployment of a new suite of Internet telephony services.

The VoFR IA provides an additional boost to this synergy by improving the delay characteristics of the frame relay network. Without the VoFR IA, any voice data would have to compete for access to the frame relay network with other data traffic carried on the same physical connection. When long frames arrive before a voice frame, the local delay introduced because of the wait for frame transmission completion would be unacceptable. The VoFR IA provides mechanisms to circumvent this problem at the end devices.

On a physical link used to transport voice and non-voice payloads, standards-based frame fragmentation can be used to turn long frames into shorter frames. Any voice frames on a logical link on the same physical link can then be interleaved with the short frame fragments by assigning higher priority to the voice frames. This reduces delay and jitter and ensures that the QoS is sufficient for voice connections.

The Internet protocols were designed to connect user applications and end devices independently of any underlying network architectures and protocols. Deployment of the Internet protocols in devices will become ubiquitous in homes, businesses, and organizations. As this occurs, it will become essential that the Internet protocols are able to run over broadband networks built upon frame relay, ATM, and in its native mode, of course -- IP. Each of these technologies has their own particular cost, performance, QoS, security, reliability, scalability, and flexibility benefits and issues.

Frame relay is a proven, understood, widely deployed and still evolving networking technology. It is an important technology for network managers as they seek to build geographically distributed networks for their businesses and organizations. Most frame relay networks offer Permanent Virtual Circuit (PVC) support. Some equipment vendors are beginning to offer Switched Virtual Circuit (SVC) support. Efforts are currently underway to define extensions to the frame relay protocol to further improve the quality of real-time, delay-sensitive traffic and enable additional interworking capabilities with ATM networking technology.

Internet telephony is still in its infancy, but it is likely that it will become widely available. Its success will depend on the deployment of a number of enabling technologies. Specifically, the success of Internet telephony depends on providing the level of service individuals, businesses, and organizations have come to expect from the existing telephony network.

As the demand for the transport of voice, data, fax, video, and other information grows, new solutions will be developed to integrate these technologies and provide services that are lower cost, higher quality, more scalable, and easier to manage for the user. One such solution, Internet telephony over frame relay, is a natural evolution of the demand for more varied and better coordinated communications. It will provide a solution that will work well for many businesses and organizations as they build their networks.

Jeff Lawrence is chairman, CEO, president, and director of Trillium Digital Systems, Inc., a leading provider of communications software solutions for computer and communications equipment manufacturers. Trillium develops, licenses, and supports standards-based communications software solutions for SS7, ATM, ISDN, Frame Relay, V5, IP, and X.25/X.75 technologies. Trillium is also developing solutions for high-availability, interworking, wireless, mobility, Software on Silicon, Java, and other emerging technologies. Additional company and product information is available on the Web at www.trillium.com.

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