ccna 1 practice skills

ccna 1 practice skills.Data Communications Equipment (DCE) A Frame Relay switch is a DCE device. DCEs are also treated as data circuit termination devices. Typically DCEs are located in a service provider's networkAccess Link A leased line between a DTE and a DCEAccess Rate (AR) The rate of the access link that is controlled by the clock. This choice affects the price of the connectionCommitted Information Rate (CIR) The rate at which bitstreams are sent over the VC, based on a commercial contract between the customer and the providerData Link Connection Identifier (DLCI) A Frame Relay header used to identify the Frame Relay address of the VCNon-Broadcast Multiple Access (NBMA) A network that does not support broadcasting, but can connect more than two devicesLocal Management Interface (LMI) This protocol is used to manage connections between DTEs and DCEs. SVC signaling messages, PVC status messages, and survival times are LMI messages275 Chapter 13 Understanding Frame Relay Concepts Frame Relay is defined in documents developed by the International Telecommunications Union (ITU) and the American National Standards Institute (ANSI). In the 1990s, Frame Relay was first defined as a new type of network. Initially, in the 1990s, the Frame Relay Forum defined many of the original specifications, and gradually, the ITU and ANSI adopted many of these standards. Even after hearing some of the ideas and key terms related to Frame Relay, the next section looks more deeply at the core functions of Frame Relay: VirtualCircuit (VC), the LMI protocol, framing, and Frame Relay addressing.Virtual circuits exhibit significant advantages over point-to-point leased lines only. In turn, these major advantages are closely related to virtual circuits. A typical Frame Relay network with three sites is shown in Figure 13-3.Figure 13-3 Typical Frame Relay Network with 3 Sites A virtual circuit defines a logical path between two Frame Relay DTEs. The term "VirtualCircuit" itself explains the concept very well. Like a point-to-point circuit, it provides the ability to transmit data between two endpoints over a WAN. However, there is no physical circuit directly connected between the two endpoints, so it is virtual. For example, R1 is connected to two VCs, one of which has R2 at the other end and R3 at the other end, and R1 can send communication data directly to either of these two routers through the corresponding VCs. The VCs share the access link and the Frame Relay network. For example, both VCs use the same access link to terminate Rl. R1 can send a Frame Relay frame to R2, followed by another frame to R3, both of which are sent over the same physical access link. Not only does a client router share its access link across many VCs, but many users share the same Frame Relay network. Initially, users on leased-line networks were reluctant to migrate to a Frame Relay network because they would be competing with other customers in the Frame Relay cloud to use the network capacity provided by the service provider. To address these concerns, Frame Relay networks were designed using the concept of Committed Information Rate (CIR). Each VC has a CIR, which is the minimum bandwidth that the service provider guarantees to each particular VC. This allows users to migrate from a leased line to Frame Relay and receive at least the same CIR as the previous leased line. A major advantage of Frame Relay over leased lines is that Frame Relay provides connectivity to each site with only one access link between each router and the Frame Relay provider. Interestingly, even for a 3-site network, the cost of using Frame Relay can be cheaper than using a point-to-point link. This is because the access link to the nearby Frame Relay provider's point of entry (PoP) is usually quite short. For larger enterprise WANs, the overhead of Frame Relay and other multi-layer access WAN technologies is higher. Assuming an organization has 100 sites that need to be connected at any two points, how many leased lines are needed? 4950! On top of that, if point-to-point leased lines are used, each of the organization's routers would need to have 99 serial interfaces. With Frame Relay, the organization could have 100 access links to the local Frame Relay switch, one to each router, giving a total of 4950 VCs running between them. This would require very few actual physical links and only one serial interface per router. The service provider is better off building its own Frame Relay network overhead than leased lines. Also as expected by users, Frame Relay