The variety of applications for LANs is wide. To provide some insight into the types of requirements that LANs are intended to meet, the following sections discuss some of the most important general application areas for these networks.
Personal Computer LANs
A common LAN configuration is one that supports personal computers. With the relatively low cost of such systems, individual managers within organizations often independently procure personal computers for departmental applications, such as spreadsheet and project management tools, and for Internet access.
But a collection of department-level processors won't meet all of an organization's needs; central processing facilities are still required. Some programs, such as econometric forecasting models, are too big to run on a small computer. Corporate-wide data files, such as accounting and payroll, require a centralized facility but should be accessible to a number of users. In addition, there are other kinds of files that, although specialized, must be shared by a number of users. Further, there are sound reasons for connecting individual intelligent workstations not only to a central facility but to each other as well. Members of a project or organization team need to share work and information. By far the most efficient way to do so is digitally.
Certain expensive resources, such as a disk or a laser printer, can be shared by all users of the departmental LAN. In addition, the network can tie into larger corporate network facilities. For example, the corporation may have a building-wide LAN and a wide area private network. A communications server can provide controlled access to these resources.
LANs for the support of personal computers and workstations have become nearly universal in organizations of all sizes. Even those sites that still depend heavily on the mainframe have transferred much of the processing load to networks of personal computers. Perhaps the prime example of the way in which personal computers are being used is to implement client/server applications.
For personal computer networks, a key requirement is low cost. In particular, the cost of attachment to the network must be significantly less than the cost of the attached device. Thus, for the ordinary personal computer, an attachment cost in the hundreds of dollars is desirable. For more expensive, high-performance workstations, higher attachment costs can be tolerated. In any case, this suggests that the data rate of the network may be limited; in general, the higher the data rate, the higher the cost.
Back-End Networks and Storage Area Networks
Back-end networks are used to interconnect large systems such as mainframes, supercomputers, and mass storage devices. The key requirement here is for bulk data transfer among a limited number of devices in a small area. High reliability is generally also a requirement. These are some typical characteristics:
High data rate. To satisfy the high-volume demand, data rates of 100 Mbps or more are required.
High-speed interface. Data transfer operations between a large host system and a mass storage device are typically performed through high-speed parallel I/O interfaces, rather than slower communications interfaces. Thus, the physical link between station and network must be high speed.
Distributed access. Some sort of distributed medium access control (MAC) technique is needed to enable a number of devices to share the medium with efficient and reliable access.
Limited distance. Typically, a back-end network will be employed in a computer room or a small number of contiguous rooms.
Limited number of devices. The number of expensive mainframes and mass storage devices found in the computer room generally numbers in the tens of devices.
Back-end networks are commonly found at sites of large companies or research installations with large data-processing budgets. Because of the scale involved, a small difference in productivity can mean millions of dollars.
Consider a site that uses a dedicated mainframe computer. This implies a fairly large application or set of applications. As the load at the site grows, the existing mainframe may be replaced by a more powerful one, perhaps a multiprocessor system. At some sites, a single-system replacement won't be able to keep up; equipment performance growth rates will be exceeded by demand growth rates. The facility will eventually require multiple independent computers. Again, there are compelling reasons for interconnecting these systems. The cost of system interrupt is high, so it should be possibleeasily and quicklyto shift applications to backup systems. It must be possible to test new procedures and applications without degrading the production system. Large bulk-storage files must be accessible from more than one computer. Load leveling should be possible to maximize utilization and performance.
Obviously, some key requirements for back-end networks are the opposite of those for personal computer LANs. High data rates are required to keep up with the work, which typically involves the transfer of large blocks of data. The equipment for achieving high speeds is expensive. Fortunately, given the much higher cost of the attached devices, such costs are reasonable.
A concept related to that of the back-end network is the storage area network (SAN). A SAN is a separate network to handle storage needs. The SAN unties storage tasks from specific servers and creates a shared storage facility across a high-speed network. The collection of networked storage devices can include hard disks, tape libraries, and CD arrays. Most SANs use Fibre Channel, which is described in another article in this series. Figure 1 contrasts the SAN with the traditional server-based means of supporting shared storage. In a typical large LAN installation, with a number of servers and perhaps mainframes, each has its own dedicated storage devices. If a client needs access to a particular storage device, it must go through the server that controls that device. In a SAN, no server sits between the storage devices and the network; instead, the storage devices and servers are linked directly to the network. The SAN arrangement improves client-to-storage access efficiency, as well as direct storage-to-storage communications for backup and replication functions.
The use of storage area networks (SANs).
High-Speed Office Networks
Traditionally, the office environment has included a variety of devices with low- to medium-speed data transfer requirements. However, new applications in the office environment have been developed for which the limited speeds (up to 10 Mbps) of the traditional LAN are inadequate. Desktop image processors have increased network data flow by an unprecedented amount. Examples of these applications include fax machines, document image processors, and graphics programs on personal computers and workstations. Consider that a typical page with 200 picture elements, or pels (black or white points), per inch resolution (which is adequate but not high resolution) generates 3,740,000 bits (8.5 inches x 11 inches x 40,000 pels per square inch). Even with compression techniques, this generates a tremendous load. In addition, disk technology and price/performance have evolved so that desktop storage capacities in the gigabyte range are typical. These new demands require LANs with high speed that can support the larger numbers and greater geographic extent of office systems as compared to back-end systems.
The increasing use of distributed processing applications and personal computers has led to a need for a flexible strategy for local networking. Support of premises-wide data communications requires a networking service that's capable of spanning the distances involved and that interconnects equipment in a single (perhaps large) building or a cluster of buildings. Although it's possible to develop a single LAN to interconnect all the data-processing equipment on the premises, this is probably not a practical alternative in most cases. There are several drawbacks to a single-LAN strategy:
Reliability. With a single LAN, a service interruption, even of short duration, could result in a major disruption for users.
Capacity. A single LAN could be saturated as the number of devices attached to the network grows over time.
Cost. A single-LAN technology is not optimized for the diverse requirements of interconnection and communication. The presence of large numbers of low-cost microcomputers dictates that network support for these devices be provided at low cost. LANs that support very low cost attachment will not be suitable for meeting the overall requirement.
A more attractive alternative is to employ lower-cost, lower-capacity LANs within buildings or departments and to interconnect these networks with a higher-capacity LAN. This latter network is referred to as a backbone LAN.
The factory environment is increasingly being dominated by automated equipment: programmable controllers, automated materials-handling devices, time and attendance stations, machine vision devices, and various forms of robots. To manage the production or manufacturing process, it's essential to tie this equipment together. And, indeed, the very nature of the equipment facilitates this. Microprocessor devices have the potential to collect information from the shop floor and accept commands. With the proper use of the information and commands, it's possible to improve the manufacturing process and to provide detailed machine control.
The more a factory is automated, the greater the need for communications. Only by interconnecting all the devices and by providing mechanisms for their cooperation can the automated factory be made to work. The means for interconnection is the factory LAN. Key characteristics of a factory LAN include the following:
Ability to handle a variety of data traffic
Large geographic extent
Ability to specify and control transmission delays
Factory LANs are a niche market requiring, in general, more flexible and reliable LANs than are found in the typical office environment.