Computer Networks IV

NETWORK DEVICES

The types of machines that can be connected to a network include PCs, intelligent workstations, dumb terminals, host computers, clients, and file and other types of servers. File servers control network activity such as printing and data sharing, as well as controlling security. Important factors to consider in selecting a file server include its speed, processor performance, memory, hard drive capacity, and most importantly, its compatibility with network software.

NETWORK COMPUTERS.

A corporate trend since the mid-1990s has been toward so-called network computers (NCs), a variation on the long-established notion of dumb terminals supported by a powerful central system. Spurred by advances in Internet technology, IT managers found that they could save on the high cost of buying and maintaining full-featured PCs for every desktop when only a handful of corporate applications were used, and these could conceivably be retrieved from (or run off) a central computer, the server. Advances in software and data portability, such as HTML documents on the Web and Sun Microsystems' platform-independent Java language, encouraged the idea that NC users could simply download whatever programs and files they needed from a central repository, rather than storing such information locally on each computer.

SERVERS.

Servers are computers that run software to facilitate various kinds of network activities; the software packages that enable such activities are sometimes also called servers. A single physical computer may host a number of server-related processes. The three main types of server functions are file servers, network servers, and printer servers. File servers can be run in either a dedicated or a nondedicated mode. Nondedicated file servers can be used as work stations as well, although workstation functions can take up much of the processor's capacity, resulting in delays for network users. Also, if a workstation program causes the file server to lock up, the entire network may be affected and suffer a possible corruption of data. One compromise for a small office is to use a nondedicated file server as a workstation for a light user. A disk subsystem can increase the performance of a file server in large network applications. Network servers are used to facilitate network activities, such as processing e-mail, while printer servers manage traffic on networked printers.

STORAGE AREA NETWORKS.

Highlighting the need for network storage space, particularly for critical system backups, has been the development of a relatively new set of network technologies known as storage area networks (SANs). SANs, which are high-speed networks of storage devices that can work in conjunction with any number of servers and other network devices, can be deployed as a solution to the inefficiencies of maintaining a host of separate disk subsystems. Although most companies of any size perform routine system backups, the process of backing up as well as restoring data can be slow and cumbersome—a competitive liability for companies that depend heavily on their systems being available 24 hours a day. SANs are used to reduce this liability and improve efficiency.

OTHER NETWORK DEVICES.

Connecting devices such as bridges, routers, and gateways are used to subdivide networks both physically and logically, to extend the range of cabling, and to connect dissimilar networks. Connecting devices can be used extend the range of cabling or to subdivide networks into segments, which is useful for isolating faults. Repeaters simply extend the physical distance that network data can travel by receiving and retransmitting information packets. They do not provide isolation between the components they join. Connecting devices are classified according to the functional layer at which they operate.

Bridges operate at OSI layer two (also known as the data link layer; see Figure 2). They are used to isolate segments from a network backbone, to connect two networks with identical lower layers, and to convert one lower level technology into another. They can be configured to transmit only appropriate messages (filtering).

Routers operate at layer three (network layer). They can also be used to isolate network segments from a backbone, but unlike bridges, they can connect segments with different lower-layer protocols. Software exists which can perform this function, though not usually as fast. "Brouters" are a hybrid between bridges and routes that operate at layers two or three.

Gateways operate at layer four (transport layer) or higher. They are required for minicomputer or mainframe access from PCs and are much more complex and costly than other connecting devices. They are capable of converting data for use between dissimilar networks.