Tài liệu Frame Relay

Frame Relay is a high-performanceWAN protocol that operates at the physical and data link layers
of the OSI reference model. Frame Relay originally was designed for use across Integrated Services
Digital Network (ISDN) interfaces. Today, it is used over a variety of other network interfaces as
well. This chapter focuses on Frame Relay’s specifications and applications in the context of WAN
services.
Frame Relay is an example of a packet-switched technology. Packet-switched networks enable end
stations to dynamically share the network medium and the available bandwidth. Variable-length
packets are used for more efficient and flexible transfers. These packets then are switched between
the various network segments until the destination is reached. Statistical multiplexing techniques
control network access in a packet-switched network. The advantage of this technique is that it
accommodates more flexibility and more efficient use of bandwidth. Most of today’s popular LANs,
such as Ethernet and Token Ring, are packet-switched networks.
Frame Relay often is described as a streamlined version of X.25, offering fewer of the robust
capabilities, such as windowing and retransmission of lost data, that are offered in X.25. This is
because Frame Relay typically operates over WAN facilities that offer more reliable connection
services and a higher degree of reliability than the facilities available during the late 1970s and early
1980s that served as the common platforms for X.25 WANs. As mentioned earlier, Frame Relay is
strictly a Layer 2 protocol suite, whereas X.25 provides services at Layer 3 (the network layer) as
well. This enables Frame Relay to offer higher performance and greater transmission efficiency than
X.25 and makes Frame Relay suitable for current WAN applications, such as LAN interconnection.
Initial proposals for the standardization of Frame Relay were presented to the Consultative
Committee on International Telephone and Telegraph (CCITT) in 1984. Due to lack of
interoperability and lack of complete standardization, however, Frame Relay did not experience
significant deployment during the late 1980s.
A major development in Frame Relay’s history occurred in 1990 when Cisco Systems, Digital
Equipment, Northern Telecom, and StrataCom formed a consortium to focus on Frame Relay
technology development. This consortium developed a specification that conformed to the basic
Frame Relay protocol that was being discussed in CCITT but extended the protocol with features
that provide additional capabilities for complex internetworking environments. These Frame Relay
extensions are referred to collectively as the Local Management Interface (LMI).
Since the consortium’s specification was developed and published, many vendors have announced
their support of this extended Frame Relay definition. ANSI and CCITT have subsequently
standardized their own variations of the original LMI specification, and these standardized
specifications now are more commonly used than the original version.