Introduction to local-area networks.
An LAN can be defined as a reliable, high-speed communication system that permits interconnection of information-processing equipment in a peer-to-peer or master-slave mode, or a combination of the two, within a limited geographical area. Typically this is an industrial manufacturing or processing plant.
Functionally, an LAN is a trunk line with drop lines and nodes. The latter can be programmable logic controllers, numerical controls, or computers.
In the process of automating, plant management has employed terminals, computers, and programmable devices (like programmable logic controllers and personal computers) from different vendors. These devices must communicate with each other to transfer the information needed to manage and coordinate the process. LANs provide the media for this information transfer.
Four basic elements characterize a LAN: The media, the topology, the signaling method, and the access method.
The media provide the physical channel used to interconnect nodes on a network (Figure 1). Media are classified as bounded (wires, cables, and optical fibers) or unbounded (the "air waves" over which radio, microwave, infrared, and other signals are broadcast).
A network topology is created by geometric arrangements of the trunks and nodes that make up a network.
The signaling method is the procedure for transmitting over a network.
The access method describes the technique by which the nodes actually gain the use of the common trunk or channel to transmit messages.
Several different media have been used for LANs. While it appears that fiber-optic cable is the best choice for a transmission medium, a closer look indicates that current technology limits its use. Connectability to the trunk line with minimum signal loss limits its use in an industrial environment.
As technology progresses, fiber optics will become one of the most viable transmission media. Twisted-pair wire does not offer low signal attenuation for the higher data rates. Based on readily available technology, coaxial cable provides the optimum performance. Topologies
Three different topologies or trunk and node configurations have been used for LANs. These are the star, ring, and bus (Figure 2).
The star configuration uses a centralized control scheme. Cable lengths from the peripheral nodes to the central nodes can be very long in the factory environment. Failure of the central node can shut down the entire LAN.
The ring configuration is a good distributed control scheme, but generally requires a lot of cable and has limited expansion capability. A node fault can disable the network unless a bypass scheme is used.
The bus configuration incorporates relatively sophisticated flow control and contention schemes. It accommodates distributed control and supports peer-to-peer communications.
The cable length from peripheral nodes to the trunk cable are short; this configuration offers ease of expansion. An important consideration is that a single node failure does not shut down the entire system. Signaling and access methods
Two signaling methods predominate in LANs: baseband and broadband. In baseband, signals are not modulated onto a carrier, and remain in their original unmodulated form. A single channel is transmitted over a single cable.
With broadband LANs, a single physical channel on a single cable is divided into a number of smaller independent frequency channels. Each frequency channel can be used to transfer different forms of information, such as voice, data, and video.
Two of the more popular access methods are token passing and CSMA/CD (carrier sense multiple access with collision detection).
In token passing (Figure 3), special addressable bit patterns or packets called tokens--each is several bits in length--travel from node to node. Possession of the token gives the node exclusive access to transmit its message, avoiding conflict with other nodes that wish to transmit.
This method minimizes collisions on the network. It supports real-time control in the plant environment, since node access can be predicted, and the time element can be incorporated into system design.
In a ring, the token circulates around the ring from node to node in a physical ring pattern. In a bus, a logical ring is formed, but the token does not have to move from one node to its adjacent node. It need only circulate in the same pattern. Not having to flow in a physical ring permits greater flexibility in network design.
The CSMA/CD technique anticipates collisions and uses them as a part of the design to allocate the channel. Each node has the ability to detect traffic on the channel. The nodes do not transmit when they detect that there is traffic.
While it is transmitting, each node detects collisions by monitoring the energy level of the channel. Collision of messages changes the energy level. When a node detects a collision, the node stops transmitting for a random period of time.
This method doesn't support a real-time plant-control system, since nodes must back off a random amount of time before trying to regain access. Token-passing yields a relatively predictable update time for each node, while update times can only be estimated with CSMA/CD. Standards vital
Communications standards are important because they are necessary for multivendor support. They provide a common reference point for product designers and system integrators. Standards also provide for a high degree of system compatibility, because all manufacturers are designing to the same specifications. This helps ensure a high degree of harmony in control/communications function and operation.
In addition, standardization enhances LAN maintainability, since people need familiarize themselves with just one standard. Ultimately, standardization reduces users' costs, thanks to economies of scale, increased vendor volume, and increased research.
Establishment of a standard LAN lets all devices in the plant communicate via a single trunk line. That reduces the number of proprietary networks required, and also the time and cost for installation and debugging.
Various standards organizations throughout the world share a common objective to fill the need for standards by developing and promulgating them at the technical level. These include the International Standards Organization, European Computers Manufacturing Association, and the Consultative Committee for International Telephone and Telegraph. Many international standards have been developed by these organizations, and many are used in the US.
In this country, we have ANSI, the American National Standards Institute; EIA, the Electronic Industries Association; and the NBS, the National Bureau of Standards. NBS has become predominant in communications standards in recent years.
Allen-Bradley bases its product development on the IEEE 802.2 and 802.4 spec, the General Motors MAP spec, and the A-B Gold Book specification (IEEE stands for Institute of Electrical and Electronic Engineers; MAPS, for Manufacturing Automation Protocol Specification). These are the dominant standards influencing today's major LAN offerings.
The ISO Open System Interconnect (OSIe reference model (Figure 4) provides an architecture for exchanging information among systems that are open to one another through mutual use of standards. All manufacturers have these standards available to them and can therefore develop compatible products.
The model is a powerful tool for describing, designing, implementing, standardizing, and using LANs, too, but is only a reference, not a standard per se. It doesn't provide specifics on how a communication network is actually implemented. The model
The ISO-OSI model (Figure 5) is a seven-layered architecture. Each layer has a name indicating the functions performed at that level to provide a defined set of services to the layer above it. A layer in turn requests and uses the services of the layer below it. Each also has a peer-to-peer protocol relationship with the corresponding layer in a connected system. If you take all the functionality for all seven layers and interconnect them by appropriate protocols, they will provide all the functionality you need to implement a computer network.
Here's how the layers fit into the model:
The first three--physical, data link, and network--are responsible for establishing the reliable circuit. Layer 4, the transport layer, coordinates and controls data flow. The remaining three layers--session, presentation, and application--interpret the data.
Let's take two users, System X and System Y, each using architecture based on this model. They are joined by an interconnecting medium such as cable or radio waves. Each layer talks to its peer layer through its own protocol.
At present, the first two layers have become standardized by IEEE 802.2 and 802.4. Layer 4, Transport, is standard per NBS and ISO. Interim standards for the upper five layers are being developed at national and international levels to make the model operational.
A few years ago, IEEE commissioned the 802 Committee to develop a family of standards for LANs. The standards developed for the first two layers define four types of media-access technologies and their associated physical media.
The IEEE 802 family includes 802.1, which defines the relationships among the other 802 standards and the ISO-OSI reference model; 802.2, defining the protocol for the link protocol layer; 802.3, the standard that deals with bus topology using CSMA/CD over broadband or baseband network; and 802.4, defining the bus topology using the token-passing access method, for baseband and broadband systems.
There's also 802.5, defining the ring topology using token-passing over baseband, and 802.6 defining the metropolitan LAN--a spec concerned with the use of a LAN over a large area such as a city. GM's MAP spec
Two years ago, General Motors Corp introduced its MAP specification, MAPS, the company's approach to the application of LAN's in manufacturing. MAPS provides guidelines that allow communication among diverse intelligent devices in a standardized, cost-effective manner.
The spec supports the ISO-OSI reference model, and conforms to the IEEE 802.4 spec. MAPS includes the NBS Class Four transport protocol, and also defines some of the functionality for the application layer (layer 7) on the ISO-OSI model.
We have developed a specification complementary to the MAPS document to meet the needs of users across a broad range of industries. These include automotive and machine tool, food processing, forest products, chemical and petrochemical, and mining and metals. This is known as the Allen-Bradley Gold Book.
Our company has selected the medium of coaxial cable; well-defined, readily available, offering low error rate and large bandwidth.
Bus topology was chosen because it is the least expensive and most reliable under the standard. It offers high data rates over long distances without need for signal regeneration, plus high flexibility for expansion. This topology allows connection of unlimited nodes on each channel along a 20,000-ft cable, expandable with an amplifier. Failure of a node on a bus topology does not result in network failure.
We selected broadband signaling because it's based on currently available cable tV technology. Broadband supports multiple channels on a single cable; you can have closed-circuit and broadcast TV, video surveillance systems, voice information, and data information all on one cable.
Broadband also supports plantwide communications at 5 and 10 MB/sec, and offers a high degree of noise immunity, an important feature on the plant floor.
Token-passing provides a predictable response time to support real-time control. The nodes can access the network in a collision-free environment. This provides reliability and consistent throughput under high loading. Contents of A-B's broadband net
The A-B broadband system consists of a Data Gateway connecting the A-B Data Highway to the broadband network; the head-end remodulator for controlling on-line network operations; a Network Interface Controller (NIC) for interfacing intelligent, high-speed devices to the LAN; and a terminal communication server to interconnect up to eight nonintelligent RS-232 devices to the broadband system.
The system also includes a network virtual terminal server with RS-232/422 ports to support terminals such as personal computers (availability in the third quarter of 1985), and a network manager for configuration, monitoring, and on-line performance management of the system.
The network manager not only monitors on-line performance, but also does off-line modeling and configuration. A computer--a DEC, IBM, Hewlett-Packard, or other--can interface to the broadband LAN through a network interface controller.
The system includes off-the-shelf 802.4 broadband products, open to all vendors, with internetworking capability to extend capacity up to the office environment, throughout the shop floor, and down to proprietary subnetworks.
As standards develop for upper levels of the ISO-OSI reference model, we will incorporate them into the broadband communications products to ensure that they conform to the latest standards, and that the system remains open to other vendors.
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|Author:||Jones, Robert L.|
|Publication:||Tooling & Production|
|Date:||Jul 1, 1984|
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