Increasingly, libraries affiliated with academic institutions and major corporations are becoming involved in discussions on the use of local area networks to meet campus-wide data communications needs. Some 20 universities and several hundred corporations have installed, or are in the process of installing, LAN systems. The initial investments range from $500,000 to $7 million. Among academic users are Brown, Carnegie-Mellon, Lehigh, Rochester Institute, Pepperdine, and Stanford.
A local area network (LAN) is a facility which provides data and voice and! or video communication within a single building or a close cluster of buildings, although in some rare cases a LAN can be utilized over distances as great as 20 miles. A LAN uses "local" communications facilities rather than those provided by a common carrier or public communications service. Thus, LANs do not utilize public telephone systems, public access long distance networks, or other public communication channels. This does not mean that they cannot connect to public communications services-some do so using "gateways" but rather that these boundaries define the LAN.
Typically the computing resources of an academic institution include a number of different types of mainframes, minicomputers and microcomputers from a variety of vendors. They are rarely single vendor environments. The ability of the more sophisticated LANs to support linkages among machines in a multivendor environment is a major advantage. Multivendor LAN interfaces typically offer "bit carrier' capabilities as a minimum, and at a maximum, provide true file interpretation and linkage between dissimilar devices.
LAN linkage in a multivendor situation may also offer other advantages, greatly reducing modem and data line expenses, increased sharing of expensive resources, reduced need for communication controllers and optimized utilization of ports on a host processor.
LANs have many different applications. In an office situation, a LAN can readily replace much of today's paper flow with electronic documents. In this situation the LAN can transmit budget material generated by personal computer-based spreadsheets among the personal computers themselves and on to a host computer. Computer resources can be shared campus-wide, with each terminal or micro on a campus being able to access every computer. High-speed printers, plotters, CAD (computer aided design) units, and other special devices can be shared among many users. Data bases can be made available throughout a campus.
A review of the significant design concepts of LANs may be helpful in understanding their use. The important aspects are:
- Circuitry-the physical media need
- Access Methods-the approach used to handle access to the LAN by the devices connected to it
- Topology-the physical design of the LAN
- Bandwidth-the measure of the throughput capacity of the LAN
Circuitry. Although a variety of LAN circuitry is possible, the most common circuit types are coaxial cable and twisted pair telephone-type wires. In the future, fiber optic cable will be more common. LANs usually accommodate the most common established forms of data transmission. A LAN typically transmits data in small chunks, or packets, and adds transmission routing, addressing, and sequencing data to each packet. "Packet switching" transmission is used because it provides better utilization of the network circuits than techniques which transmit an entire message in a continuous string (message switching).
Access methods. To transmit data effectively, a LAN must prevent data collisions which could occur when two or more devices want to access and send information on a single channel at the same time, Strategies used to avoid collisions include Carrier Sense Multiple Access / Collision Detection (CSMA/CD) and token passing. Under the first method, a device trying to send a message first checks whether the line is "busy." If not, it proceeds to send its packet, or packets. If the packets are received correctly, communication continues normally. When two devices attempt to send simultaneously, a collision is detected and each device waits its own randomly determined time period before retransmitting. Token- passing access methods permit a device to transmit only when it possess a logic "token." The token is passed between devices in the network in a predetermined sequence.
Both methods are intended to allow interconnected devices to communicate efficiently without requiring a master network controller of the type usually employed in computer terminal networks. Where a LAN network controller is present, it typically directs the communication flow on the overall network level rather than at the device level. The connected devices are responsible for their own data stream control and communication session management. In this regard, LAN communication requires a minimum level of local intelligence at the connected device or the cable interface unit.
Topology. The topology of a LAN usually follows one of three configurations:
- Star
- Data Bus, or
- Ring.
In a star configuration, all devices are connected to a central control point using a single line for each connection. Many star LANs are PBX-based, rather than being independent and using their own coaxial cabling. They are, therefore, operationally dependent upon the reliability and loading of the central PBX controller. Although recent advances in transmission technology have increased the throughput of PBX-based lines, they often have a relatively slow transmission capacity--56 kilobits/ second-for each twisted pair wire.
In a data bus structure, all connected devices typically tap into a single line or bus. The data bus cable is usually Open-ended. Information packets flow outward in both directions from each point of connection. Data bus LANs are usually based on coaxial cable and may utilize dual parallel cable--one cable for traffic flowing in each direction.
A ring network connects devices in a fashion similar to a data bus LAN, but connects the ends together to form a continuous loop. Because of this circular shape, information packets can be routed in one direction around the system and all data will be received by all connected devices. Many such LANs use token passing and place a token at the start of a packet to indicate whether it is already full or may be loaded with more data.
Bandwidth. The terms "baseband" and "broadband" describe the transmission capacity of the LAN circuit and thus the total net throughput provided to the user. Under a baseband approach, the entire capacity of the LAN or the data channel is utilized by one user at a given moment. While the speed of transmission is quite fast, typically 2 to 15 megabits/second, the entire capacity of the system is dedicated to one user at a given moment.
With a broadband approach, the capacity of the system is divided into segments, or channels. Each channel may be assigned to a different use. In this manner, hundreds of users may concurrently use the LAN capacity for widely different applications. Because the transmission capacity is divided into multiple segments, each carrying a major communication load, the total capacity of the broadband LAN is far greater than that of a baseband LAN.
A local area network may use several different design approaches. For example, a broadband LAN may be installed as a spine or backbone to provide high capacity linkage through a facility. Baseband LANs may then be attached to this backbone to handle connections, especially those with low volume users.
A major factor in deciding on the design of a LAN is the applications which are to be supported. The throughput capacity requirements in bits per second for various types of LAN applications are:
| Telex | 55 |
| Data from a voice grade phone line | 2,400-4,800 |
| Telephone speech | 64,000 |
| High-fidelity stereo music | 1,000,000 |
| Compressed digital video | ,000,000 |
| Broadcast color video | 92,000,000 |
The total distance to be covered can be important, particularly if a baseband LAN is planned. In some cases, a baseband LAN requires repeaters every 500 yards to boost signal strength. With broadband LANs, distance is less of a limiting factor, but they do require signal amplifiers to overcome long-distance signal loss.
In the case of a relatively complex LAN installation, planning and design typically account for one-third of the budget, hardware one-third, and installation one-third. In coming years, LAN hardware components, including those borrowed from the CATV industry, should generally drop in per-unit price. The planning/design and installation labor cost will probably continue to rise, however.
LAN vendors typically fit into one of three categories:
- Office equipment suppliers who developed local area network products to tie together their product family, or so they could coexist in a multi-vendor environment.
- Advanced telephone system and/or PBX manufacturers who have become involved in data switches and LANs as extensions of their business.
- Independent communications suppliers and CATV equipment manufacturers with LAN products as their primary product offering.
Suppliers generally approach the LAN market in quite different ways which reflect their origins.
Office data processing and word processing systems suppliers generally developed LAN products which "protected" their installed base of products. At the same time, they continued to promote or extend existing communications approaches. The LAN products from such firms tend to be well developed and reliable, but not designed to offer "breakthrough" capabilities or sophisticated network services such as automatic code conversion. They generally comply with existing communications standards and are rapidly moving toward supporting multivendor LAN installations.
Suppliers from a PBX background usually provide LAN products derived from data switch methodologies. These LANs tend to promote ease of use and low cost workstations, sometimes at the expense of functionality. In many cases, their approach is to attempt to displace more costly personal computers with less functional devices that provide digital voice linkage and host computer data access. The emphasis is toward large numbers of low-cost "dumb" workstations interconnect through intelligent PBXs.
The third group of LAN suppliers, firms mostly in the LAN and CATV component business, are often smaller organizations. These companies have tended to accelerate the development of LAN products up to the limits of their resources. The LAN products from these firms are not always the lowest cost alternative, but are quite flexible and may be the only real choice for unusual requirements.
In terms of likely future developments, at least three major areas of significance stand out:
- Physical circuitry
- Network intelligence, and
- Standards.
LAN physical circuitry will include increasing use of fiber optics. The trend toward increasingly complex personal computer color graphics and multiple windowing will increase bit loads and therefore encourage the use of fiber optics to handle the traffic.
LANs will probably provide more "value added" services in regard to network intelligence with the LAN of the future moving beyond the role of being a high volume bit carrier to providing more automatic data conversion services and interpretation during transmission. LAN network management services will offer increased security provisions and possibly some form of encryption. Additionally, a dynamic storage management capability is needed. Future LANs must be able to automatically move data files to their highest volume point of usage. The increasing size of local data disk storagemagnetic, optical, or other --should make such dynamic LAN data management capabilities realistic and cost-effective.
LANs will most likely adapt to new standards--either official or de facto--as these emerge. They will accommodate related new product trends such as personal computer graphics and other options.
