Voice Over Frame Relay

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 Voice Over Frame Relay  





Frame relay is becoming the transmission technology of choice for wide area network (WAN) users worldwide who are looking for a WAN technology that is low cost, fast, flexible, and standards based. Vendors and carriers have both reported growth rates of 350% from 1994 to 1995.


Optimized for data traffic, frame relay’s explosive growth has been driven predominantly by local area network (LAN) internetworking, SNA migration, and remote access. As customers continue to express interest in frame relay, vendors and customers are pushing for other applications and traffic to run over a frame relay network.

In 1995 the first generation of equipment was introduced that supported voice over frame relay. These offerings met with moderate acceptance due largely to a lack of standards and lack of support. A year later, new vendors have begun supporting voice over frame relay and a new wave of customers has emerged who want to realize the financial benefit of incorporating voice/fax along with data into their frame relay networks.

The second generation of equipment does a better job of addressing the quality issues associated with voice compression. Additional buffer space to help with jitter, coupled with more advanced compression algorithms and better prioritization schemes, have alleviated some of the issues. However, there is still a lack of standards. As of this writing, this lack of standards makes all voice-over-frame relay (VOF) offerings proprietary solutions and, consequently, an issue for many end users.

In the 1990s, when the corporate world is downsizing and rightsizing, the prospect of lowering monthly telecommunications infrastructure costs is economically very attractive to many businesses. In many cases the cost issues are paramount, but there are other issues to be addressed as well—among them, ongoing infrastructure requirements, business continuity, and network management. A brief discussion of when it may be appropriate to incorporate voice into the data network will address many of these issues.

Business Perspective on Frame Relay

Frame relay today provides primary connectivity for LAN-to-LAN and IBM SNA/SDLC (System Network Architecture/synchronous data link control) connectivity, accounting for almost 95% of current network utilization. Unfortunately, many users of frame relay technology have yet to recognize the full economic advantage of its architecture.

Business Continuity Concerns.  Combining voice over frame and potentially eliminating a major portion of the network is financially attractive. In fact, with the latest compression techniques it can cost as little as half a cent per minute. However, users must consider that if the frame relay link is lost, then both voice and data communication is now lost and a remote site is potentially without any communications.

Business continuity and application criticality must be considered. Many dial backup solutions may be applicable—integrated services digital network (ISDN), in many cases, or plain old telephone service, for example. In addition, for truly critical sites, users should consider multiple access paths for the cable into the building.

Network Management.  Network management must also be considered when users look to combine multiple networks into one. The combination of voice/fax/data over the same physical link elevates the need for a robust network management system and for working with a carrier who provides valuable network information. Some carriers are expanding this part of their business and are offering access to their network management data.

The best candidates for voice over frame relay today are customers with existing frame relay data networks whose data requirements are a few DS0s, allowing them to take a relatively simple approach of looking at the equipment costs and the additional DS0 cost for adding voice.

For many of the carriers, combining voice traffic over the frame relay network presents issues of pricing as well as network management.


For public network frame relay service, many carriers have chosen a cautious approach to promoting and supporting VOF technology. This is due primarily to concerns about guaranteeing quality of service (QOS) specifications associated with voice traffic.

Of those carriers that offer support for VOF (see Exhibit 1), they are typically discussing tariffing data link connection identifiers (DLCIs) for voice to ensure acceptable delay. This would enable voice to travel alone on a dedicated virtual circuit, minimizing or eliminating many of the problems. (Note: The majority of carriers, as of this writing, do not provide voice over frame relay as a service offering. Many of the carriers have announced plans for voice over frame relay service, but do not have definitive service offerings.)

Many leading switch vendors offer the ability to support different levels of QOS for voice using different DLCIs. As these devices become more prevalent and intelligent, the carriers will be able to provide higher-quality service, and users will gain more network options.

One challenge for end-user companies that have many sites is the interoperability between the carriers. The network-to-network interface (NNI) agreement covers the very basics and does not address voice issues and some network management issues.

Today, few carrier-based frame relay offerings support all the traditional voice applications, including interconnecting private branch exchanges (PBXs), use of off-premises extensions (OPX), and private line auto ringdown (PLAR). When available, such offerings can make frame relay networks an attractive option for everything from tying together telecommuters into a virtual office to linking the branch office and regional office locations. In this implementation, the home or small branch office can be connected with a single pipe that provides voice communications integrated with the office PBX and voice mail, as well as integrated data communications with the office network and host computers.

This configuration often creates an easier frame relay design and implementation because the end stations are typically homes back to a central office location. This means advanced functionality requirements of frame relay, such as switched virtual circuit (SVC) support for voice, are not required, as all voice calls need to be first routed to the office PBX.





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