Information Technology Reference
In-Depth Information
18
CHAPTER
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.
A major development in Frame Relay's history occurred in 1990 when Cisco Systems, StrataCom,
Northern Telecom, and Digital Equipment Corporation formed a consortium to focus Frame Relay
technology development and to 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 it 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, and 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 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 performed at higher
