A CDE Definition
(Frame Relay Access Device or Frame Relay Assembler Disassembler) A communications device that formats outgoing data into the format required by a frame relay network. It strips the data back out at the other end. It is the frame relay counterpart to the X.25 PAD. See frame relay.
A high-speed packet switching protocol used in wide area networks (WANs). Providing a granular service of up to DS3 speed (45 Mbps), it has become popular for LAN to LAN connections across remote distances, and services are offered by most major carriers.
Frame relay (FR) is much faster than X.25, the first packet-switched WAN standard, because frame relay was designed for reliable circuits and performs less error detection (X.25 was never widely used in the U.S.). Frame relay does not process the packets; it relays them from the switch's input port to the output port, hence the name.
The FRAD (Frame Relay Access Device)
Attachment to a frame relay network is made via a FRAD on the customer's premises, which may be a separate device or software in the router. The FRAD connects to a switch port on the service provider's network via the User-to-Network Interface (UNI). All traffic for one customer generally travels over the same line, which is typically a multiple of 64 Kbps. Frame relay switches interconnect via point-to-point lines or an ATM backbone.
Permanent and Switched Circuits
Frame relay provides Permanent Virtual Circuits (PVCs) and Switched Virtual Circuits (SVCs). They are logical connections provisioned ahead of time (PVCs) or on demand (SVCs).
Connections are identified by a Data Link Connection Identifier (DLCI) number that is significant only to the local FR switch, which changes the number as it passes the packet on to its destination. The receiving switch uses a different DLCI for its end of the same connection. Every DLCI requires a Committed Information Rate (CIR), which is a pledge on the part of the network to provide a certain amount of transmission capacity for the connection. CIRs are adjusted with experience.
Voice Over FR
Voice can be packetized to travel over a frame relay network, often providing significant cost savings with some sacrifice in voice quality, depending on network configuration. In 1998, the Frame Relay Forum finalized the Voice Over FR specification. FRF.11 defines the formats, and FRF.12 subdivides large frames in order to interleave real-time voice with data on slow connections.
A Superb Resource
"Frame Relay for High-Speed Networks" by Walter Goralski is must reading not only to learn about frame relay, but about wide area networking in general. Goralski factors in history, trends and related networking technologies. Published by John Wiley & Sons, Inc., ISBN 0-471-31274-6.
A Frame Relay Network
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