Frame Relay

Frame Relay is a standardized wide area network technology that specifies the physical and logical link layers of digital telecommunications channels using a  packet switching methodology. Originally designed for transport across Integrated Services Digital Network (ISDN) infrastructure, it may be used today in the context of many other network interfaces (http://en.wikipedia.org/wiki/Frame_Relay). 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. 

A major development in Frame Relay's history occurred in 1990 when Cisco Systems, StrataCom, Northern Telecom, and DEC 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 (now called ITU-T) 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).  ANSI and ITU-T have standardized their own variations of the original LMI specification, and these standardized specifications now are more commonly used than the original version (http://netcert.tripod.com/ccna/wan/frelay.html).

Frame Relay Information

• Frame Relay is both a Data Link layer encapsulation type implemented on the router and a Physical service provided by a telecommunications company.
• Frame Relay is a packet switching and encapsulation technology that functions at the Data Link and Physical layers of the OSI model and runs on nearly any type of serial interface.
• Access to Frame Relay networks is made through private leased lines at speeds ranging from 56 Kbps to 45 Mbps.
• Frame Relay is a connection oriented packet-switching mechanism that establishes VCs between endpoints.
• The ITU-T and ANSI define Frame Relay as a connection between the DTE (Data Terminating Equipment) and the DCE (Data Communications Equipment).
• DCE is switching equipment, supplied by a telecommunications provider, that serves as a connection to the public data network (PDN).
• DTE is also known as customer premise equipment (CPE), because it is the equipment that belongs to, and is maintained by the PDN customer.
• If you connect your Cisco router to a Frame Relay switch (provided by phone company), the Cisco router is the CPE (DTE) and the Frame Relay switch is the DCE.

Frame Relay is a protocol standard for LAN internetworking which provides a fast and efficient method of transmitting information from a user device to LAN bridges and routers.

The Frame Relay protocol uses a frame structured similar to that of LAPD, except that the frame header is replaced by a 2-byte Frame Relay header field. The Frame Relay header contains the user-specified DLCI field, which is the destination address of the frame. It also contains congestion and status signals which the network sends to the user.

The Frame Relay frame is transmitted to its destination by way of virtual circuits (logical paths from an originating point in the network) to a destination point. Virtual circuits may be permanent (PVCs) or switched (SVCs). PVCs are set up administratively by the network manager for a dedicated point-to-point connection; SVCs are set up on a call-by-call basis.

The designers of Frame Relay aimed to provide a telecommunication service for cost-efficient data transmission for intermittent traffic between local area networks (LANs) and between end-points in a wide area network (WAN). Frame Relay puts data in variable-size units called "frames" and leaves any necessary error-correction (such as re-transmission of data) up to the end-points. This speeds up overall data transmission. For most services, the network provides a permanent virtual circuit (PVC), which means that the customer sees a continuous, dedicated connection without having to pay for a full-time leased-line, while the service-provider figures out the route each frame travels to its destination and can charge based on usage(http://en.wikipedia.org/wiki/Frame_Relay).

Advantages of Frame Relay

Frame Relay offers an attractive alternative to both dedicated lines and X.25 networks for connecting LANs to bridges and routers. The success of the Frame Relay protocol is based on the following two underlying factors:

• Because virtual circuits consume bandwidth only when they transport data, many virtual circuits can exist simultaneously across a given transmission line. In addition, each device can use more of the bandwidth as necessary, and thus operate at higher speeds.
• The improved reliability of communication lines and increased error-handling sophistication at end stations allows the Frame Relay protocol to discard erroneous frames and thus eliminate time-consuming error-handling processing.

These two factors make Frame Relay a desirable choice for data transmission; however, they also necessitate testing to determine that the system works properly and that data is not lost.