Nortel Networks CG030601 manual Introduction to Frame Relay, Permanent virtual circuits

Configuration Guide

Contivity Secure IP Services Gateway

Frame Relay on Contivity Secure IP Services Gateway

Introduction to Frame Relay

Frame relay is a high-speed, packet-switching WAN protocol that connects geographically dispersed LANs. A public network provider usually offers frame relay. However, private organizations can acquire and manage their own frame relay networks as well.

Frame relay is a connection-oriented protocol. This means it relies on end-to-end paths between devices connected across the network. It implements these connections using permanent virtual circuits (PVCs).

Frame relay assumes that networks use transmission lines with low error rates, such as digital transmission media. Therefore, frame relay provides only basic error detection with no error recovery. This minimizes the processing required for each packet, allowing frame relay networks to operate at high speeds with few network delays.

Because frame relay performs only basic error checking, end stations running upper-layer protocols such as the Internet Protocol (IP) are responsible for resending packets that did not transmit correctly the first time.

Permanent virtual circuits

A permanent virtual circuit (PVC) is a dedicated logical path that connects two devices over a network. When configured, a PVC is always available to the connected devices; a PVC does not require setup before data can travel across the network, nor does it need to be disconnected after data has passed. Because many PVCs can coexist for one physical line, devices can share the bandwidth of the transmission line.

Frame Relay packets

The structure of a frame relay packet is shown in the following figure.

Figure 1 Frame Relay packet

The packet’s header field includes the following components:

Data link connection identifier (DLCI)

The DLCI is the virtual circuit identification number. The frame relay network uses the DLCI to direct basic data flow. You configure the DLCI for PVCs.

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