Frame Relay

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William Mohawk

P.O. Box 1612

Santa Monica, CA 90406

(310) 585 - 2634

Frame Relay is a Consultative Committee for International Telegraph and Telephone (CCITT) and American National Standards Institute (ANSI) standard that defines a process for sending data over a public data network (PDN). It is a high performance, efficient data technology used in networks throughout the world. Frame Relay is a way of sending information over a WAN by dividing data into packets. Each packet travels through a series of switches in a Frame Relay network to reach its destination. It operates at the physical and data link layers of the OSI reference model, but it relies on upper-layer protocols such as TCP for error correction. Frame Relay was originally conceived as a protocol for use over ISDN interfaces. Today, Frame Relay is an industry-standard, switched data link-layer protocol that handles multiple virtual circuits using High-Level Data Link Control (HDLC) encapsulation between connected devices. Frame Relay uses virtual circuits to make connections through a connection-oriented service.

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. A Frame Relay network can consist of computers, servers, etc. on the user end; Frame Relay network devices such as switches, routers, CSU/DSUs, or multiplexers.  As you have learned, 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).

Frame Relay can be used as an interface to either a publicly available carrier-provided service or to a network of privately owned equipment. You deploy a public Frame Relay service by putting Frame Relay switching equipment in the central office of a telecommunications carrier. In this case, users get economic benefits from traffic-sensitive charging rates, and don't have to spend the time and effort to administer and maintain the network equipment and service.

No standards for interconnecting equipment inside a Frame Relay network currently exist. Therefore, the support of Frame Relay interfaces does not necessarily dictate that the Frame Relay protocol is used between the network devices. Thus, traditional circuit switching, packet switching, or a hybrid approach combining these technologies can be used.

Frame Relay provides a means for multiplexing many logical data conversations. The service provider's switching equipment constructs a table mapping DLCI values to outbound ports. When a frame is received, the switching device analyzes the connection identifier and delivers the frame to the associated outbound port. The complete path to the destination is established before the first frame is sent.

 

KEY  CONCEPTS
CSU	Channel Service Unit; the digital interface device that connects end-user equipment to the local digital telephone loop.  The concept of CSU/DSU works sort of like a modulator - demodulator in that the process converts digital - to - analog and analog - to digital signals for transmission.
DSU	Digital Service Unit; a device used in digital transmission that adapts the physical interface on a DTE device to a transmission facility such as T1 or E1. The DSU is also responsible for such functions as signal timing. The concept of CSU/DSU works sort of like a modulator - demodulator in that the process converts digital - to - analog and analog - to digital signals for transmission.
FRAME	Logical grouping of information sent as a data link layer unit over a transmission medium. Often refers to the header and trailer, used for synchronization and error control, that surround the user data contained in the unit.  Sometimes used interchangeably with the term packet.  The Frame is read by each corresponding layer of the OSI model and striped of its appropriate information as it heads up the layer stack.
ROUTER	Network layer device, often a dedicated computer specifically designed for the job, that uses one or more metrics to determine the optimal path along which network traffic should be forwarded. Routers forward packets from one network to another based on network layer information. Occasionally called a gateway, though this is somewhat dated.  Routers are also used to segment or sub-divide a local are network (LAN).
SWITCH	Network device that filters, forwards, and floods frames based on the destination address of each frame. The switch operates at the data link layer of the OSI model.  Please refer to the page on switching on this same website.