CCNA Study Guide

This Blog highlights CCNA study materials for those pursuing the CCNA EXAM 640-801.It is a must read for those going for the exam and need last minute total recall.

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Thursday, March 23, 2006


Circuit switching establishes a dedicated physical connection for voice or data between a sender and receiver. Before communication can start, it is necessary to establish the connection by setting the switches. This is done by the telephone system, using the dialed number. ISDN is used on digital lines as well as on voice-grade lines. If the local loop is not directly connected to the telephone system, a digital subscriber line (DSL) may be available.

To avoid the delays associated with setting up a connection, telephone service providers also offer permanent circuits. These dedicated or leased lines offer higher bandwidth than is available with a switched circuit. Examples of circuit-switched connections include:

  • Plain Old Telephone System (POTS)
  • ISDN Basic Rate Interface (BRI)
  • ISDN Primary Rate Interface (PRI)

Many WAN users do not make efficient use of the fixed bandwidth that is available with dedicated, switched, or permanent circuits, because the data flow fluctuates. Communications providers have data networks available to more appropriately service these users. In these networks, the data is transmitted in labeled cells, frames, or packets through a packet-switched network. Because the internal links between the switches are shared between many users, the costs of packet switching are lower than those of circuit switching. Delays (latency) and variability of delay (jitter) are greater in packet-switched than in circuit-switched networks. This is because the links are shared and packets must be entirely received at one switch before moving to the next. Despite the latency and jitter inherent in shared networks, modern technology allows satisfactory transport of voice and even video communications on these networks.

Packet-switched networks may establish routes through the switches for particular end-to-end connections. Routes established when the switches are started are PVCs. Routes established on demand are SVCs. If the routing is not pre-established and is worked out by each switch for each packet, the network is called connectionless.

To connect to a packet-switched network, a subscriber needs a local loop to the nearest location where the provider makes the service available. This is called the point-of-presence (POP) of the service. Normally this will be a dedicated leased line. This line will be much shorter than a leased line directly connected to the subscriber locations, and often carries several VCs. Since it is likely that not all the VCs will require maximum demand simultaneously, the capacity of the leased line can be smaller than the sum of the individual VCs. Examples of packet or cell switched connections include:

  • Frame Relay
  • X.25
  • ATM


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