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Frame relay is defined as a communications protocol as well as a
packet data service. We will describe frame relay as a packet data
service because of its ability to create a Wide Area Network (WAN) that can
integrate an enterprise infrastructure.
The frame relay service is a high speed packetized data service that
consists of physical and logical components. The physical components include
Frame Relay Assemblers Disassemblers (FRADs), access circuits, and frame
relay ports. The logical components consist of permanent virtual circuits (PVCs).
Frame Relay Assemblers Disassemblers (FRADs) are devices, such as routers,
that assemble data into frame relay packets and that transmit the packets
through the local access circuits to which they are connected. The local
access circuits, whose bandwidths can range from 56 Kbps to 1544 Kbps, are
digital circuits that connect the FRADs to the frame relay network
provider's site. A frame relay port, whose bandwidth equals that of the
local access circuit, physically connects the local access circuit to the
frame relay network.
Frame Relay's logical component differentiates it from the traditional
high speed data services that are physical in nature. The logical component
of frame relay consists of Permanent Virtual Circuits (PVCs). PVCs are the
logical connections that allow the various sites, having physical network
access, to communicate among one and other. Each logical connection is
defined in the frame relay provider's network and in the FRAD equipment.
Logical connections in the frame relay providers network are software
defined connections that can create many types of network topologies (i.e.,
hub- and-spoke, full mesh, partial mesh). Because the connections are
defined using software, companies may provision logical connections in a few
hours as opposed to the few weeks that are necessary with traditional high
speed data services like private lines. In addition to provisioning the
logical connections within the frame relay provider's network, the FRADs
must be programmed to send the data to the correct permanent virtual
circuit. Every PVC provisioned in the frame relay network must also be
programmed into the FRAD. FRADs are programmed using data link connection
identifiers (DLCIs) that associate an identification number to each PVC.
Each PVC has a Committed Information Rate (CIR) that is the bandwidth
associated to the logical connection. Frame relay networks have the ability
to transmit data at a rate higher than that of the CIR. This allows a
"bandwidth-on-demand" feature that improves the performance of bandwidth
intensive applications.
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