CCNA Exam Cram[tm] 2 (Exams 640-821, 640-811, 640-801)
| Frame Relay is a high-speed, packet-switching WAN protocol. Packet-switching protocols enable devices to share the available network bandwidth. As its name implies, Frame Relay operates at Layer 2 of the Open Systems Interconnection (OSI) model and runs on nearly any type of serial interface. Frame Relay encapsulates packets from the upper layers of the OSI model and switches them through the provider's network. Frame Relay services have been streamlined to gain more throughput. Services such as flow control, robust congestion management, and error checking are left to upper-layer protocols such as Transmission Control Protocol (TCP); however, Frame Relay does include some error checking and congestion management. Frame Relay uses cyclic redundancy checking ( CRC ) to perform error checking quickly. CRC produces a frame check sequence (FCS), which is appended to each frame that is transmitted. When a node receives the frame, it calculates a new FCS (based on the data portion of the frame) and compares it with the one contained in the frame. If the values are different, the frame is dropped. Frame Relay manages congestion through the use of a discard eligibility bit. This bit is set to a value of 1 if the frame has lower importance than other frames; the DTE device is responsible for setting the bit and sets the bit to 1 for frames that have lower importance than other frames. Switches within the WAN provider's network may discard frames to manage congestion. However, the switches only discard frames with the discard eligibility bit set to 1; frames with bits set to 0 are still transmitted. This feature protects against critical data being dropped during periods of network congestion. Virtual Circuits
Communication in a Frame Relay network is connection oriented, and a defined communication path must exist between each pair of DTE devices. Virtual circuits provide the bidirectional communication within Frame Relay networks. In essence, a virtual circuit is a logical connection established between two DTE devices. Many virtual circuits can be multiplexed into one physical circuit, and a single virtual circuit can cross multiple DCE devices within the Frame Relay network. Virtual circuits can be grouped into two categories: switched virtual circuits (SVCs) and permanent virtual circuits (PVCs) . SVCs are temporary connections and can be used when only sporadic data communication is necessary between DTE devices. SVCs require the connection to be set up and terminated for each session. Conversely, PVCs are permanent connections. They support frequent and consistent data communications across a Frame Relay network. When the PVC is established, DTE devices can begin transmitting data when they are ready. PVCs are used more widely in Frame Relay networks than SVCs. DLCI
A data link connection identifier ( DLCI ) serves as the addressing scheme within a Frame Relay network. The service provider assigns a DLCI for each PVC, and the DLCI is locally significant within the network. In other words, the DLCI must be unique within the network like an Internet Protocol (IP) address. Two DTE devices that have a PVC established between them may or may not use the same DLCI value.
LMI
Local Management Interface ( LMI ) is a set of enhancements to the Frame Relay protocol specifications. Developed in 1990 by four companies (nicknamed the " Gang of Four "), LMI extensions offer several features for better management of complex Frame Relay networks. These extensions include global addressing, virtual circuit status messaging, and multicasting.
The LMI global addressing extension enables a DLCI to have global instead of local significance. With LMI, DLCI values are unique within a Frame Relay network, and standard address resolution protocols, such as Address Resolution Protocol (ARP) and reverse ARP (or inverse ARP), as well as discovery protocols can be used to identify nodes within the network. Virtual circuit status messaging improves the communication and synchronization between DTE and DCE devices. The status messages, which are similar to hello packets, report on the status of PVCs. LMI multicasting enables multicast groups to be assigned. Multicasting reduces overhead by allowing route updates and address resolution messages to be sent to specific groups of DTE devices.
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