|Internet Telephony Over Frame RelayBY JEFF LAWRENCE
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
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
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
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.
VOICE OVER FRAME RELAY
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
VOICE OVER IP
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
INTERNET TELEPHONY OVER FRAME RELAY
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
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
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.