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Network architectures
The term architecture is commonly used today to describe networks. A network architecture, such as illustrated in Fig.1.7, describes the components in the network, how they operate , and what form they take . A network encompasses hardware, software, data link controls (DLC), Standards, topology, and protocols.
1. hardware
The physical equipment in the network is referred to as the hardware. Typical of such equipment are computers, modems, telephone, terminals , machines, controllers, and the like.
2. Software
Software consists of programs, procedures, and sometimes, associated documentation of the operation of the communication system. Communications software (CSW), application programs (AP), network control programs (NCP), operating system (OS), and database management system (DBMS) are examples of software.
3. Protocols
A protocol defines how network components establish communications, exchange data, and terminate communications. A protocol is essentially a formal set of conventions, or rules, between communicating processes about of messages to be exchanged. Handshaking and line discipline are also protocols. To make implementation and usage more convenient in sophisticated networks, higher-level protocols may use lower-level in a layered fashion.
4. Data link controls
A data link consists of electronic equipment such as electronic devices, terminals, and machines and the interfacing circuits that are operating in a particular way to permit information to be exchanged between installations. The specific method of operation is defined by transmission codes, transmission modes, and direction control.
There are two major categories link control protocol in use today:
(1) Asynchronous, in which bit streams are transferred at fixed rates with the transmitter and receiver operating in synchronization by the clock.
(2）The flow of date to and from the many points and links within the network must be controlled and orderly. The sending and receiving sites must sequencing of the messages being transmitted among all users. The connection path between sits is usually shared by more than one user, as multipoint configuration.
A multi-point line, sometimes called a multi-drop line, is a communications line having several subsidiary controllers that share time on the line under the control of a central site.
Date line controls provide for all these needs. They manage the flow of data messages across the communications path, or links. In essence, they are special kinds of protocols consisting of a combination of software and hardware located at each site in the network. Through a communication link, the DLC protocol pro-vides for the reliable interchange of information between data terminal equipment. Typical DLC functions in the network are the following:
Synchronizing the sender and receiver
1) Controlling the sending and receiving of data
2) Detecting and recovering transmission errors between two points
3) Maintaining awareness of link conditions
4) Topologies
Topology describes the surface layout the elements comprising the communications network-----that is, it is the shape of the system. Communication system may be implemented in a variety of ways. typical communication system are telephone lines, radio wave links, networks, serial connections, parallel connections and power line system modulations. The choice of topology depends on factors such as cost, response time, throughput, capacity, load sharing, and capacity needs.
5. Local area networks
A local area network (LAN) is a communications network operating in a local area. It extends from several hundred to several thousand feet within a building or other facility. A LAN is owned and operated by an individual or organization and is, therefore, not subject to regulation by either the federal communications commission (FCC) or the state Public Utility Commission (PUS). A LAN is a means of connecting various types of equipment for the purpose of sharing resources and communicating in a distributed processing environment, as illustrated in fig .1.8. that is , all device that must communicate with one another in a CIM environment can be tied together though some form of common interface such as a LAN .
An effective LAN has specific characteristics. Typical of these are the following:
① Is in a moderate-sized geographic area (office, laboratory, building, factory, industrial complex, or campus, for example)
② Has medium –to high –sped data channel(s)
③ Provides reliable communications
④ Is owned and used by a single organization
⑤ Connects independent devices rather than a device and its peripherals
A LAN usually has four major components that serve to transport data between end users: user workstation, protocol control logic. medium interface, and physical path.
6. User workstation
The user workstation is used to accomplish an application. The station can be anything from a word processor to a mainframe computer.
(1) Protocol control logic
A protocol control logic takes user is information and converts it to a protocol that can move the LAN network to reach the desired location , the protocol logic also controls the LAN and provides for the end user is access to the network .
（2) Medium Interface
The medium interface function generates the electrical signals to be transmitted on the LAN the interface is between the path the protocol logic can take several forms, typical of which are CATV (cable television) taps , infrared diodes for infrared path , microwave antennas, or complex laser-emitting semiconductors for optic fibers.
(3) Physical Path
A LAN is path may consist of coaxial TV cables, coaxial bade-band cables, twisted pair of wires, optic fibers, and microwaves. Cable TV coaxial cable is used in many networks because it has a high capacity, a very good signal-to-noise ratio, low signal radiation, and low error rates.
(4) Manufacturing Automation Protocol
There are many types and sizes of networks. Some LAN systems are owned by an organization and are confined to the premise of that organization. Others are owned by national or international public networks that may be accessed by the user. In many case, LAN systems may also connect to public networks and be able to access them.
Network are also connected in several configurations (topologies) , as seeing in Fig .1.9.Each topology can constitute an entire network or can be just a portion of a large network .
A star network is a centralized network in which each device is connected virtually to a central controlling point.
A ring network, sometimes called a loop, is a network in which each device is connected to the next in sequence in a closed, circular fashion. This topology has two possible paths, one in each direction.
A bus network is a topology that consists of a single shared line to which all devices are connected.
In a hierarchy network, a central root node has over the entire network. The secondary nodes attached to the root can be front-end processors (EEP) that in turn connect to tertiary nodes of lesser significance. The hierarchy can consist of a large computer that has overall of several smaller control devices connected to their own terminals.
A multiply connected topology requires that there be at least two paths between any pair of nods to assure availability and reliability m the event of failure one path or node.
The unconstrained network occurs most commonly. The term unconstrained merely means that the configuration is not restricted to one specific kind of connection.
Many LAN systems are available today. All use either base-bands, broad-bands, or both, in a base-band, the line is pulsed, as in the on/off conditions. Because these pulses are still the square wave direct current, they require no additional special handling or modulation, which makes the process relatively inexpensive, it is impossible to mix voice and data unless the analog voice signals have been previously digitized.
The broadband technology requires a conversion process and is therefore more expensive. The advantages gained are that voice, video, and data can all be sent on the same channels. Facilities can be shared through assignment of frequency channels to allow all devices to operate simultaneously. Popular modulation and multiplexing techniques can be used because a wide range of frequencies is available.
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