Frame Relay Terminology Some common terms are necessary to know when discussing and configuring Frame Relay. Some of the terms on the following list are recalled from Chapter 1, "The Key Components for Modeling an Internetwork": -
Permanent virtual circuit (PVC) ” The logical end-to-end circuit, used for frame transport. A PVC's endpoints are addressed with DLCIs. -
Data-Link Connection Identifier (DLCI) ” A logical number in the range of 16 to 1007 used to identify the PVC between the customer premises equipment (CPE) and the Frame Relay switch. The DLCI is only locally significant, which implies that only the local devices and the Frame Switch that they attach to care what their DLCI numbers are. -
Local Management Interface (LMI) ” Best defined as the signaling standard used between the router and the Frame Relay switch. LMI is used by the switch to learn which DLCIs are defined and what their status is. LMI also supports a 10-second keepalive mechanism that verifies that the PVC is active and data is being exchanged. Three types of LMI are supported on Cisco routers: CISCO, ANSI, and Q933A. If LMI is not configured, the router tries to autonegotiate on all three LMI types: - - CISCO ” LMI type defined by the "big three," Cisco, Digital, and Northern Telecom. This is the default LMI type, if autonegotiation fails. LMI status information is sent on DLCI 0.
- - ANSI ” LMI type defined by ANSI standard T1.617, commonly called Annex D. This is the most common type of LMI found across all Frame Relay networks. LMI status information is sent on DLCI 1023.
- - Q933a ” LMI type defined as ITU-T Q.933, or simply Annex A. LMI status information is sent on DLCI 0.
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Network-to-Network Interface (NNI) ” The standard used for two switches to communicate. NNI is used in both Frame Relay and ATM. In ATM, it is referred to as the Network Node Interface. -
Local access rate ” The clock speed or "port speed" of the connection to the Frame Relay service provider. Usually, this circuit is 56 kbps, 64 kbps, or T1 speeds, but can operate T3 speeds or on High Speed Serial Interface (HSSI). Figure 5-1 illustrates a common Frame Relay network, highlighting the preceding terms. Figure 5-1. Partially Meshed Frame Relay Network  The following terms are called data rate metrics. Frame Relay service providers use data rate metrics to define service-level agreements. The following terms also are used when we configure Frame Relay traffic shaping: -
Committed burst (Bc) ” The number of bits committed to accept and transmit at the CIR. -
Committed Information Rate (CIR) The maximum permitted traffic level per PVC. When this is exceeded, the Discard Eligible (DE) bit is set. The DE bit can be used as an indication to the frame carrier that it can drop that frame if capacity between Frame Relay switches is reached. This value is expressed in bits per second. -
Excess burst (Be) ” The number of bits to transmit after the committed burst value is reached. -
Maximum data rate (MaxR) ” This value is measured in bits per second and is calculated by the following: 
Figure 5-2 illustrates how these metrics relate to each other. Figure 5-2. Frame Relay Data Metrics  |
