Type of connection

A network consist of two or more devices connected through links.A link is a communication pathway that transfers data from one device to another. Fro visualizations purposes, it is simplest to imagine any link as a line drawn between to points.For communication to occur, two devices must be connected in some way to the same link at the time.There are two possible types of connections: point-to-point and multipoint.A point-to-point connection provides a dedicated link between two devices, and the entire capacity of the link is reserved for transmission between these two devices.A multipoint connection is one in which more than two specific devices share a single link. In a multipoint environment,the capacity of the channel is shared, either spatially or temporally.

Network Criteria

First of all i want to make a introduction that what the network is. A network is a combination of hardware and software that sends data from a location to an other. The hardware consists of the physical equipment that carries signals from one point in the network to another. The software consists of instructions that make the services that we expect from a network possible.
The network must be able to meet a number of criteria.Three most important are performance, reliability and sequrity

                   Performance

Performance can measured in many ways, including transit  time and response time.Transit time is the amount of time required for a message to travel from one device to another.Response time is elapsed time between an enquiry and a response.The performance of a network depends on a number of factor, including the number of users,the type of transmission medium, the capabilities of the connected hardware, and the efficiently of the software

                  Reliability

In addition to accuarcy of delivery, network reliability is measured by frequency of failure, the time it takes to recover from a failure, and network’s robustness

                  Sequrity

Network security issues include protecting data from unauthorized access, damage and change, and implementing policies and procedures for recovery from breaches and data losses.We are going to discus later for advanced sequrity.

The Fourth Generation(1971-Now)

Large scale integrated (LSI) and very large scale integrated(VLSI)circuit were developed that contained hundreds of million of transistors on a tiny chip. In 1971 Ted Hoff of Inteil  developed the microprocessor, which packaged an entire CPU, complete with memory, logic, and control circuits on a single chip. The microprocessor and VLSI circuit technology caused a radical changes in computers, in their size, appearance, cost, availability and capability, and they started the process of miniautorization: the development of smaller and smaller computers. Also during this time, computer’s main memory capacity increased, and cost decreased, which directly affected the types and usefulness of software that could be used. Software applications like word processing, electronic, painting and drawing programs, desktop publishing, database management programs, painting and drawing programs and so forth became commercially available, giving more people reasons to use computer.

The Third Generation(1964-1970)

In the third period, the integrated circuit(IC) that is a complete electronic circuit that packages transistor(signal bridges) and other electronic components on a small silicon chips, replaced traditional transistorized circuity. Integrated circuits are cost-effective because individual components don’t need to be directly to the computer’s system board. The use of magnetic disk for secondary data storage become widespread, and computers began to support such capabilities as multiprogramming and timesharing. Minicomputers,priced around $18000, were being widely used by the early 1970s and were taking some of the business away from the estabilished mainframe market. Processing that formerly required the processing power of a mainframe could now be done on a minicocomputer.

The Second Generation(1959-1963)

By early 1960,transistors and some other solid-state devices that were much smaller than vacuum tubes were being  used for much of the computers circuity.A transistor is an electronic switch that alternately allows or does not allow electronic signals to pass.Magnetic cores, which looked like very small metal washers of the main memory.Removable magnetic disk packs, stack of disk connected by a common spindle ,were introduced as storage devices. Second generation machines tended to be smaller,  more reliable, and significantly faster than first generation computers.

The First Generation(1944-1958)

In the earliest general-purpose computers, most input and output media were punched cards and magnetic tape. Main memory was almost exclusively made up of hundreds of vacuum tubes through one computer used a magnetic drum for main memory.These computers were somewhat unreliable because the vacuum tubes failed frequently. They were also slower than any microcomputer used today,produced a tremendous amount of heat, and were very large. They could run only one program at time.ENIAC and UNIVAC(The Universal Automatic Computer), which was used in U.S. Bureau of Census from 1951 to 1963. Today you could purchase a microcomputer chips with with the same processing power for less than $100

ENIAC

The first large-scale electronic computer,the grandparent of today’s handheld
machines, was the Electronic Numerical Integrator and Computer or ENIAC,whic become operational in 1946. ENIAC contained approximately 18000 light-bulb-size electronic vacuum tubes that controlled the flow of electric current.It weighed 30 tons and occupies about 1800 square feet of floor space ,a huge machine by today-s standard. It was able to moltiply four numbers in the then-remarkable time of 9 milliseconds(that is 9/1000 of a second). From that start, computers have developed through four so called generations, or stages, each one characterized by smaller size, more power, and less expense than is predecessor.