System information

CHAPTER
Troubleshooting Frame Relay Connections 18-397
18
Troubleshooting Frame Relay
Connections
Frame Relay was originally conceived as a protocol for use over ISDN interfaces. Initial proposals
to this effect were submitted to the International Telecommunication Union Telecommunication
Standardization Sector (ITU-T), formerly the Consultative Committee for International Telegraph
and Telephone (CCITT), in 1984. Work on Frame Relay was also undertaken in the American
National Standards Institute (ANSI)-accredited T1S1 standards committee in the United States.
There was a major development in Frame Relay’s history in 1990 when Cisco Systems, StrataCom,
Northern Telecom, and Digital Equipment Corporation formed a consortium to focus Frame Relay
technology development and accelerate the introduction of interoperable Frame Relay products.
This consortium developed a specification conforming to the basic Frame Relay protocol being
discussed in T1S1 and ITU-T, but extended it with features that provide additional capabilities for
complex internetworking environments. These Frame Relay extensions are referred to collectively
as the Local Management Interface (LMI).
Frame Relay Technology Basics
Frame Relay provides a packet-switching data communications capability that is used across the
interface between user devices (for example, routers, bridges, host machines) and network
equipment (for example, switching nodes). User devices are often referred to as data terminal
equipment (DTE), whereas network equipment that interfaces to DTE is often referred to as data
circuit-terminating equipment (DCE). The network providing the Frame Relay interface can be
either a carrier-provided public network or a network of privately owned equipment serving a single
enterprise.
As an interface to a network, Frame Relay is the same type of protocol as X.25 (see Chapter 19,
“Troubleshooting X.25 Connections”). However, Frame Relay differs significantly from X.25 in its
functionality and format. In particular, Frame Relay is a more streamlined protocol, facilitating
higher performance and greater efficiency.
As an interface between user and network equipment, Frame Relay provides a means for statistically
multiplexing many logical data conversations (referred to as virtual circuits) over a single physical
transmission link. This contrasts with systems that use only time-division-multiplexing (TDM)
techniques for supporting multiple data streams. Frame Relay’s statistical multiplexing provides
more flexible and efficient use of available bandwidth. It can be used without TDM techniques or on
top of channels provided by TDM systems.
Another important characteristic of Frame Relay is that it exploits the recent advances in wide-area
network (WAN) transmission technology. Earlier WAN protocols such as X.25 were developed
when analog transmission systems and copper media were predominant. These links are much less
reliable than the fiber media/digital transmission links available today. Over links such as these,
link-layer protocols can forgo time-consuming error correction algorithms, leaving these to be