From Cisco Press' Network Consultants Handbook we present Chapter 15, serialized in its entirety -- all you need to know about Frame Relay on a Wide Area Network.
Network Consultants Handbook - Frame Relay
by Matthew Castelli
Frame Relay is a Layer 2 (data link) wide-area networking (WAN) protocol that operates at both Layer 1 (physical) and Layer 2 (data link) of the OSI networking model. Although Frame Relay internetworking services were initially designed to operate over Integrated Services Digital Network (ISDN), the more common deployment today involves dedicated access to WAN resources.
ISDN and Frame Relay both use the signaling mechanisms specified in ITU-T Q.933 (Frame Relay Local Management Interface [LMI] Type Annex-A) and American National Standards Institute (ANSI) T1.617 (Frame Relay LMI Type Annex-D).
Frame Relay is considered to be a more efficient version of X.25 because it does not require the windowing and retransmission features found with X.25. This is primarily due to the fact that Frame Relay services typically are carried by more reliable access and backbone facilities.
Frame Relay networks are typically deployed as a cost-effective replacement for point-to-point private line, or leased line, services. Whereas point-to-point customers incur a monthly fee for local access and long-haul connections, Frame Relay customers incur the same monthly fee for local access, but only a fraction of the long-haul connection fee associated with point-to-point private line services. The long-haul charges are typically usage-based across the virtual circuit (VC).
The long-haul fee associated with point-to-point private (leased) line services is sometimes known as the inter-office connection fee. Service providers generally file a tariff with the FCC regarding these fees, comprising a base cost plus a per-mile charge.
X.25 was designed for use over less reliable transmission medium than what is available in the marketplace today. Due to this unreliable nature, X.25 took on the error detection and correction (windowing and retransmission) mechanisms within the protocol stack. This resulted in higher overhead on the network, yielding less available bandwidth for data throughput.
Frame Relay is a packet-switched technology, enabling end nodes to dynamically share network resources.
Frame Relay was standardized by two standards bodiesinternationally by the International Telecommunication Union Telecommunication Standardization Sector (ITU T) and domestically by ANSI.
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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Table 15-1 defines the common and relevant Frame Relay terms.
Table 15-1: Frame Relay Terms and Definitions
|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).|
Figure 15-2 illustrates some of the Frame Relay terminology used. The remainder of the terms will be illustrated where appropriate throughout this chapter.
Figure 15-2: Frame Relay Terminology, Part I
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Part 2 of this chapter will cover Frame Relay components.