August 1998

By Tony Rybczynski

With the rapid growth of frame relay services, and with the introduction of frame relay multimedia capabilities, the very nature of frame relay is changing. Frame relay is no longer seen as a data-only service. Technologies are now emerging that will allow frame relay to transport high-quality voice.

In the beginning — actually, in the mid-80s — frame relay was a best-effort data service, the success of which hastened the acceptance of carrier ATM infrastructures, which were required to support frame relay’s growth. With the emergence of ATM, standards were developed to allow service interworking between routers using frame relay services and routers using ATM services. Such interworking meant that routers at branches using frame relay services (at, say, 56 Kbps) could fully interwork with routers at head office locations, which might be connected at OC3 speeds using ATM.

In the early 90s, the only practical way to support enterprise voice traffic was by using circuit-based techniques (that is, 64-Kbps circuit switching and TDM multiplexing). Branch office networking usually involved combinations of frame relay, ISDN, and leased lines, with frame relay as the technology of choice for bursty data traffic between LANs and transaction-service terminals and their hosts. Packet switching (SNA, X.25, IP, and frame relay) was a data-only solution. Most corporations had disparate voice and data networks with no sharing of bandwidth between these two traffic types.

Such sharing is technically possible with ATM, which, by design, supports real-time and non-real-time voice, data, and video networking. However, T1, the minimum standard speed for ATM, is too expensive at most branch locations. The use of frame-based solutions is the option that provides dynamic bandwidth and, given recent developments in packetized voice technologies and standards, can deliver good quality voice.

VOICE CONVERGENCE AT THE BRANCH
Branch office requirements are being driven by traffic growth and the proliferation of IP applications, so it is reasonable to consider running voice over IP. In addition, voice over IP leverages the installed intranet router base.

A key concern about voice over IP, much discussed, is voice quality. However, even if voice quality were not a concern, voice over IP would have another problem: it is not very efficient in its use of bandwidth.

To keep packetization delays at a reasonable level, voice encapsulated over IP has a typical payload of tens of bytes. Therefore, with a 16-byte RTP (Real Time Protocol) header on top of a standard 24-byte IP header, the overheads can be as high as 80–100 percent. In many cases, branch routers are connected over frame relay networks, which brings the overhead of running voice over IP over frame relay into the 120-percent-plus range over access facilities, which are already an economic and bandwidth bottleneck. Therefore, a good strategy is to encapsulate voice directly onto frame relay, thus bringing overheads into the 40–50-percent range. The Frame Relay Forum has standardized this approach in what is called FRF.11.

ENABLING TECHNOLOGIES
But how is voice quality delivered over what was originally designed as a data networking solution? The answer lies on two fronts: new frame relay standards-based services and a new generation of FRADs (Frame Relay Assembler/ Disassemblers or Access Devices) which take advantage of these new frame relay networking capabilities.

And what are these new frame relay standards-based service developments? Frame relay service providers are introducing:

• Multimedia classes of services. These services improve end-to-end performance for delay-sensitive traffic. Multiple priority queuing supported on a per virtual circuit (VC) basis at the user’s interface into the network, across the network, and at the egress user interface ensures that the application performance requirements are met. This significantly enhances the ability of frame relay networks to provide consistent performance for real-time applications such as voice.
• Switched virtual circuit (SVC) operation. This capability eases configuration and management with further reduction in latency for voice and improved scalability for LAN traffic. SVCs improve latency by providing cut-through switching across the network. For voice applications, the network is acting as a virtual tandem PBX providing one-hop switching between each pair of PBXs. A key requirement is to interface to a broad range of PBX signaling systems (for example, ISDN, Q.SIG)
• Pre-standard capabilities. Work is now being done in anticipation of standards that will allow voice over frame relay to interwork with voice over ATM, further enhancing the alignment of frame relay service definitions and ATM service definitions. This extends the concept of frame relay to ATM service interworking for data-to-voice networking.

All of these service developments make voice over frame relay very attractive. It’s only a matter of time before most service providers offer these standards-based capabilities.

GETTING CONNECTED
At the customer premises, two complementary developments have taken place to take advantage of these service developments while supporting high-growth IP traffic: First, we have the introduction of fourth-generation FRADs (or multiservice FRADs); Second, the introduction of enterprise network switches.

Multiservice FRADs
Multiservice FRADs are multimedia branch-office access devices that preserve the native characteristics of diverse traffic types. At the same time, these devices integrate their bit streams in the most efficient way possible.

The first generation of FRADs were data-only devices. Next came what some call internetworking FRADs, which integrated IP routing functionality. Then came voice FRADs, which integrated voice support. All three of these generally have several common networking attributes: They use frame relay permanent virtual circuits or PVCs, operate at a single class of service for voice and data, and require a FRAD at both ends. For voice operation, these FRADs provide fragmentation of data frames on an end-to-end basis to deal with latency requirements.

Multiservice FRADs support routing, legacy data, and voice. In addition, multiservice FRADs differ from traditional FRADs in several respects. Specifically, multiservice FRADS allow for support of:

• Class of Service (CoS). In fact, frame relay multimedia CoS is available from a growing number of service providers.
• SVCs. As with CoS arrangements, SVCs constitute a service that is supported by a growing number of service providers.
• Local fragmentation. This capability, between the multiservice FRAD and the central office frame relay switch, avoids the overhead of current end-to-end fragmentation schemes.

The Frame Relay Forum is evolving the currently defined frame relay interface standard to support local fragmentation. With local fragmentation, a graphics file transfer would not be allowed to disrupt an ongoing voice call. Local fragmentation is already supported in some multiservice FRADs and CO frame relay switches. The multiservice FRAD continues to support the rich set of adaptation capabilities that FRADs support, including backup configurations using ISDN switched connectivity.

Enterprise Network Switches
Enterprise network switches provide highly scalable solutions at large sites, both for branch access and backbone connectivity for multiple large sites. In addition, an enterprise network switch provides interworking between voice over frame relay and voice over ATM.

An enterprise network switch is a class of product that has evolved over the last few years. It provides network consolidation functions in the form of extensive service adaptation capabilities (that is, for all traffic leaving the building or campus) and supports integrated frame relay and ATM SVCs, voice switching, and multiprotocol switching for IP. Large, central site enterprise network switch solutions avoid the cost and complexity of having multiple FRADs deliver the high level of required connectivity.

THE NEW FRAME RELAY
The introduction of frame relay multimedia capabilities and the acceptance of carrier ATM infrastructures are changing the very nature of frame relay offerings. Over time, frame relay will be seen increasingly as a lower speed multimedia access ramp onto carrier ATM networks rich in Quality of Service (QoS) options. User connectivity to these network services can be provided via multiservice FRADs and enterprise network switches, both critically important in allowing enterprises to develop integrated multimedia networking to extend their market reach, enhance customer service, and gain a competitive advantage — in short, to power their businesses.

Tony Rybczynski is director of Strategic Marketing and Technologies for Nortel's Enterprise Data Networks business unit, which delivers high-performance data networks globally. This business unit creates new alternatives to increasingly complex data network infrastructures, and offers them through direct and indirect sales channels. For more information, visit the company's Web site at www.nortelnetworks.com.   E-mail questions or comments to the author at [email protected].