Frame Relay, part 1 - Page 2

By Cisco Press | Posted Jan 9, 2002
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Frame Relay Terms and Concepts
Frame Relay is a frame-switched technology, meaning that each network end user, or end node, will share backbone network resources, such as bandwidth. Connectivity between these end nodes is accomplished with the use of Frame Relay virtual circuits (VCs). Figure 15-1 illustrates the components of a Frame Relay WAN.

Figure 15-1: Frame Relay WAN
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(Click image for larger view in a new window)

Table 15-1 defines the common and relevant Frame Relay terms.
Table 15-1: Frame Relay Terms and Definitions

Acronym Definition
Bc Committed burst. Negotiated tariff metric in Frame Relay internetworks. The maximum amount of data (measured in bits) that a Frame Relay internetwork is committed to accept and transmit at the committed information rate (CIR). Bc can be represented by the formula Bc = CIR Tc.
Be Excess burst. Negotiated tariff metric in Frame Relay internetworks. The number of bits that a Frame Relay internetwork will attempt to transfer after Bc is accommodated. Be data is generally delivered with a lower probability than BC data because Be data is marked as discard eligible (DE) by the network.
BECN Backward explicit congestion notification. A Frame Relay network in frames traveling in the opposite direction of frames that are encountering a congested path sets this bit. Data terminal equipment (DTE) receiving frames with the BECN bit set can request that higher-level protocols take flow control action as appropriate, such as the throttling back of data transmission.
CIR Committed information rate. Rate at which a Frame Relay network agrees to transfer information under normal conditions, averaged over a minimum increment of time. CIR, measured in bits per second (bps), is one of the key negotiated tariff metrics.
DCE Data communications equipment. The DCE provides a physical connection to the network, forwards traffic, and provides a clocking signal used to synchronize data transmission between DCE and DTE.
DE Discard eligible. If the Frame Relay network is congested, DE-marked frames can be dropped to ensure delivery of higher-priority trafficin this case, CIR-marked frames.
DLCI Data-link connection identifier. Values used to identify a specific PVC or SVC. In Frame Relay internetworks, DLCIs are locally significant. In a Frame Relay LMI extended environment, DLCIs are globally significant because they indicate end devices.
DTE Data terminal equipment. Device at the end of a User-Network Interface (UNI) that serves as either a data source or destination.
FECN Forward explicit congestion notification. Bit set by a Frame Relay network to inform the DTE receiving the frame that congestion was experienced in the path from origination to destination. The DTE that is receiving frames with the FECN bit set can request that higher-level protocols take flow control action as appropriate, such as throttling back data transmission.
LMI Local Management Interface. Set of enhancements to the basic Frame Relay specification. LMI includes support for keepalive mechanisms, verifying the flow of data; multicast mechanisms, providing the network server with local and multicast DLCI information; global addressing, giving DLCIs global rather than local significance; and status mechanisms, providing ongoing status reports on the switch-known DLCIs.
NNI Network-to-Network Interface. Standard interface between two Frame Relay switches that are both located in either a private or public network.
PVC Permanent virtual circuit. Frame Relay virtual circuit that is permanently established (does not require call-setup algorithms).
SVC Switched virtual circuit. Frame Relay virtual circuit that is dynamically established via call-setup algorithms. Usually found in sporadic data transfer environments.
Tc Tc is a periodic interval. This interval is triggered anew when data is incoming to the network. When there is no data traffic when time Tc has elapsed, a new interval does not begin until new data traffic is sent to the network.
UNI User-Network Interface. Frame Relay interface between a Frame Relay switch in a private network (such as a customer premise) and a public network (such as a service provider). Sometimes referred to as a Subscriber Network Interface (SNI).

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