Now we will take the time to review the various WAN technologies you may counter on your Cisco CCNA certification exam.

A leased line connection is a dedicated circuit between two sites. The speed of a leased line can run from 2400 bps all the way up to 45 Mbps. The two most common types of leased lines are a T1 and a DS-3. The speed of a T1 is 1.54 Mbps. The speed of the DS3 is 45 Mbps. The nice thing about a leased line is this communication link is private and you're the only one who uses this communication line. However it is also one of the most expensive technologies to use in the WAN arena. Another drawback is if you are connecting five remote sites to your main hub site, you will have to purchase a separate and distinct leased line from each remote site to your hub site and have a separate WAN interface at your hub site for each of the remote sites. This can get very expensive. But at the same time since you are not sharing this leased line with anybody else, you don't have to worry about them intruding upon your bandwidth. Leased line circuits typically run the point-to-point protocol, high-level data Link control protocol, or serial line Internet protocol. These are covered more in depth in our full blown CCNA CBT DVD video series.

A circuit switched connection differs from a dedicated leased line connection in that the connections are established via dial-up and thus are not always active. Common examples would be an analog dial up or digital ISDN connection. Another difference between a leased line connection and circuit switched connection is that with a leased line connection you generally pay a flat rate per month regardless of how much or little you use the line, in contrast with the circuit switched connection you were charged based upon the amount of data you pass over the circuit.

Two common types of circuits which connections are as follows: asynchronous serial connections which is your standard telephone service or more commonly referred to as plain old telephone service or POTS. This service provides a maximum of 53 Kbps of bandwidth. Synchronous serial connections include BRI and PRI dialup connections. Both of these technologies offer a higher bandwidth throughput than POTS. Circuit-switched connections usually use the point to point protocol, high-level data Link control protocol, or serial line Internet protocol.

Packet-switched services allows you to connect to your service providers shared network much like as if you connect a bunch of computers to a hub. When you have a bunch of computers connected to a hub, if one computer is sending a lot of data it can consume the bandwidth available and impact the other computers on the network. The same is true for packet switched services. Examples of packet-switched technologies are X.25, frame-relay, and ATM. In using each of these technologies you are sharing the bandwidth provided by your service provider with their other customers. Since this is a shared environment, the service providers able to provide you more bandwidth for less money.

With the previously mentioned leased lines and circuit-switched connections, a physical circuit is used to make the connection between the two sites. With a leased line, the same circuit path is always used. With circuit-switched services, the circuit path is created every time a connection is made, making it highly possible that the same circuit path will not be used for every connection. In contrast packet switched connections use logical circuits. These logical circuits are referred to as virtual circuits. An advantage of logical circuits is that you can build multiple logical circuits over the same physical circuit, thus with a single physical connection to your provider you can connect to multiple remote sites. Remember in the leased line example above, if we wanted to connect or five remote sites to our corporate office, we had to have a separate physical serial interface on the router in our corporate office for each of the remote sites. However with packet-switched services we are able to have one physical connection to our service provider which enters us into the cloud. Across of this physical connection, there can be multiple virtual circuits.

The first packet-switched technology we will discuss is X.25. This network layer protocol can run across both synchronous and asynchronous circuits. It provides an error detection and correction at the data link layer by using LAPB. Because of its high overhead, it is best used in asynchronous reliable connections that need error correction.

The next packet switched technology we will discuss is frame relay. Frame relay is a digital packet-switched service that runs across asynchronous connections at the data link layer it has minimal error checking in that can detect errors and drop some bad frames but it is up to another protocol such as TCP to resend the dropped frames.

The final packet-switched technology will discuss is ATM. This also uses digital circuits, but unlike frame-relay and X.25, it uses a fixed length cell that is 53 bytes in length. ATM offers many features such as quality of service, guaranteed bandwidth, and limited number of errors.

We hope this article covering your basic CCNA certification exam WAN topics was helpful to you. You will find many more CCNA articles on our website along with our CCNA video tutorials and CCNA lab kits.