Clustering is a fault-tolerant server technology that is similar to redundant servers, except each server takes part in processing services that are requested

Grid computing is another load-balanced parallel means of massive computation, similar to clusters, but implemented with loosely coupled systems that may join and leave the grid randomly

Circuit switching is the second type of WAN technology up for discussion. Telecommunication providers have used circuit switching since 1891, when a Kansas City undertaker patented the first one. The most common form of circuit switching is the plain old telephone service (POTS).

POTS — This humble voice-grade analog telephone service is used for voice calls and for connecting to the Internet and other locations via modem. Modem speeds can vary from 9,600bps to 56Kbps. Although the POTS service is relatively inexpensive, very reliable, and widely available, it offers only low data speeds.

Packet-switched networks share bandwidth with other devices. They are considered more resilient and work well for on-demand connections with bursty traffic. Packet-switched protocols packetize data in much the same manner as the Ethernet or a Token Ring would. The data is placed into a frame structure. Let’s look at some different types of packet-switching protocols and services:

X.25 — Although it might sound like a government secret spy plane, X.25 is actually one of the original packet-switching technologies. It was developed in 1976 and operates at the physical, data link, and network layers of the OSI model. Once used extensively, X.25 is no longer widely used. X.25 has speeds of up to 56Kbps and is considered reliable.

To make sure the right infrastructure is deployed, the following questions should be asked:

What applications will be used on the network?
—Demanding applications require high performance. Ten Gigabit Ethernet, Gigabit Ethernet and Asynchronous Transfer Mode (ATM) are three possible choices.

What amount of bandwidth is needed?
—Modern LANs demand increasing amounts of bandwidth. Virtual machines and increased Internet traffic raise the demand for bandwidth. Whereas 10Mbps connectivity with hubs was once sufficient, 100Mbps switched connectivity is now seen as the minimum.

The term firewall has been used since the 1990s and describes a device that guards the entrance to a private network. Firewalls were developed to keep out unauthorized traffic. Firewalls have undergone generations of improvements so that today several different types of firewall exist.

These include the packet filter, application proxy, circuit proxy, and stateful inspection. Packet filter firewalls operate at Layer 3 of the OSI model. Packet filters look at the packet header to determine whether to block or pass traffic. Packet filters can be thought of as the first generation of firewalls. They inspect the TCP/IP headers and make a decision based on a set of predefined rules. Packet filters simply drop packets that do not conform to the predefined rule set. These rules can include the following:
. Source IP address
. Destination IP address
. TCP/UDP source port
. TCP/UDP destination port
. TCP flags (SYN, FIN, ACK, and so on)

The Application Layer
Layer 7 is known as the application layer. Recognized as the top layer of the OSI model, this layer serves as the window for application services. This is the layer that users are most knowledgeable of. The application layer serves as the interface for applications, such as email and web browsers. Without the application layer, email and the Web would not exist and our computers would be unable to interpret and sort the data transmitted by other computers. Layer 7 is not the application itself, but rather the channel through which applications communicate. Think of this in terms of preparing to send a present to a friend—the application layer would be equivalent to buying the gift.

The Presentation Layer
Layer 6 is known as the presentation layer. Consider the gift analogy from the preceding section. At Layer 6, this is when you are now ready to take the gift to the post office. It will require packaging. Although some might be content in placing the gift in a paper package, the post office will require a specific type of box if you want to send the gift by priority mail. This means that the presentation layer is concerned about presentation. Data must be formatted so the application layer can understand and interpret the data. The presentation layer is skilled in translation; its duties include encrypting data, changing or converting the character set, and performing protocol conversion. Data compression is also performed at the presentation layer.