cisco ccna devnet certification

cisco ccna devnet certification..DCE: NT1 (for BRI) or CSU/DSU (for PRI).Figure 24.3 shows an example of an analog circuit-switched connection. In this connection, PPP or HDLC encapsulation is typically used, and SLIP is rarely used because it has no authentication capabilities and only supports the use of IP as a transport protocol.Packet-switched connections Leased lines and circuit-switched connections use physical circuits to establish a connection between two sites. With a leased line, the same circuit path is always used, while with a circuit-switched connection, the circuit path is re-established each time a phone call is initiated, so the circuit path used may be different each time. Packet-switched connections use logical circuits to establish a connection between two sites; these logical circuits are called virtual circuits (VCs) o One advantage of logical circuits over physical links is that they are not dependent on a specific physical link, but can be established using any available physical link. Another advantage of a logical circuit is that multiple logical circuits can be established using the same physical circuit, so that only one physical link to the carrier is required to connect to multiple sites. With leased lines, this is simply not possible: a separate physical circuit is required to connect to each site, resulting in a much higher cost than a solution using logical circuits. Technologies that use packet switching and logical circuits include ATM, Frame Relay, SMDS and X.25. In terms of cost, packet switching solutions fall between circuit-switched solutions and leased lines. Frame Relay and ATM are losing market share. With the advent of cable modems, DSL, fiber to the home and other technologies, the use of Frame Relay and ATM has declined dramatically. Due to the emergence of reliable digital circuits, there is currently little use of X.25Olong-distance telephone networkModem RoutersTelephone Telephone Intercom Router Modem Switchswitch modulatorTelephone Circuit Switching Connection Diagram 24.3 Analog Circuit Switching ConnectionOf the four aforementioned technologies that use packet switching and logical circuits, X.25 is the oldest, an ITU.T standard ° X.25 is a network layer protocol that can be used for both synchronous and asynchronous physical circuits, providing great flexibility in terms of connection methods. X.25 was actually developed for unreliable media and provides error detection and correction as well as traffic control at both the data link layer (via the balanced link access process [LAPB]) and the network layer (via X.25). In this sense, it performs functions similar to those that TCP provides to IP at the transport layer. Because of the high overhead, X.25 is only suitable for unreliable asynchronous connections; for synchronous digital connections, it is much more efficient to use other protocols (Frame Relay or ATM).Frame Relay is a digital packet switching service that can only be used at the data link layer for synchronous digital connections. It does not perform error correction and traffic control like X.25 because it uses (error less) digital connections. However, Frame Relay checks for errors and discards invalid frames, but leaves the job of retransmitting the discarded information to a higher layer protocol (such as TCP).