STORAGE ACCESS METHODS
How data files are stored in secondary storage varies with the types of media and devices you are using. Data files may be stored on or in sequential-access storage, direct-access storage, or random-access storage.
Punched cards, paper tape, and magnetic tape are examples of sequential-access storage media. When operating in a sequential environment, a particular record can be read only by first reading all the records that come before it in the file. When you store a file on tape, the 125th record cannot be read until the 124 records in front of it are read. The records are read in sequence. You cannot read just any record at random. This is also true when reading punched cards or paper tape.
Direct-access storage allows you to access the 125th record without first having to read the 124 records in front of it. Magnetic disks and drums are examples of direct-access storage media. Data can be obtained quickly from anywhere on the media. However, the amount of time it takes to access a record is dependent to some extent on the mechanical process involved. It is usually necessary to scan some (but not all) of the preceding data.
Random-access storage media refers to magnetic core, semiconductor, thin film, and bubble storage. Here, a given item of data can be selected from anywhere in storage without having to scan any preceding items. And, the access time is independent of the storage location.
Q.27 Punched cards, paper tape, and magnetic tape use what storage access method?
A network can be defined as any system composed of one or more computers and terminals; however, most are composed of multiple terminals and computers. In this section you will learn how this allows dissimilar computers to work together as a team.
LOCAL AREA NETWORKS (LANs)
In local area networks (LANs), various machines are linked together within a building or adjacent buildings. Figure 4-12 shows an example of a LAN. A LAN allows dissimilar machines to exchange information within one universal system. With the ability to communicate, the dissimilar machines act as a team. The information that exists in one system can be reused without being reentered via keyboard or disk into another separate system. A universal system for the integration and exchange of information is connected to all input devices. The entire system is usually housed within the same building or the same geographic area. A local area network is made up of a communications facility (for example, a coaxial cable, such as that used for cable television) and interface units creating a link for the computers and terminals to the communications facility. Two designs can be used: broadband or baseband.
Figure 4-12. - Local area network system.
A baseband communications channel uses the basic frequency band of radio waves and a coaxial cable. This coaxial cable has one channel, which is like a party line. Only two machines can use this cable at one time, even though many have the channel available, but there is no central switching unit to route traffic over the network.
A more expensive channel, called a broadband communications channel, can handle more advanced applications. This includes transmission of voice as well as data and text. Because of the use of a controller to route traffic for a large number of simultaneous users, the users are able to share one of the many individual channels of the system.
WIDE AREA NETWORKS
Wide area networks provide for global connections and are sometimes referred to as global networks. Organizations are able to send information from city to city, across the nation, and to other countries throughout the world, through the expansion of local area networks into larger network configurations. Combinations of telephone lines, microwave radio links, and satellites are used by these larger telecommunications networks to send information. In 1965, the first successful communications satellite for business applications was launched. It was not the only try, it was preceded by many more primitive satellites. With the launching of larger and more complex satellites, the size and complexity of earth stations have been shrinking. Since satellite services' costs have been steadily decreasing, it is becoming more cost effective to employ them for business-type uses.
Since both signals and data can be transmitted and received through cables (communications lines), we refer to them as input/output channels. And when we transmit data directly to a computer over long distances, it becomes necessary to add two other devices, one at each end of the communications line. These devices are called modems (fig. 4-13). The word modem is an acronym for modulator/ demodulator (combines first syllable of each word). A modem converts the digital signal produced by your terminal or the computer to an audio signal suitable for transmission over the communications line. The modem at the other end of the line converts the audio signal back to a digital signal before it is supplied to the computers or your terminal. If this conversion were not carried out, the digital signal would degenerate during transmission and become garbled.
Figure 4-13. - Modem.
The physical link or medium that is used to carry (or transmit) data from one location to another is a communications channel. It allows remotely located input/output devices to communicate directly with the computer's central processing unit (cpu). Telephone lines (often referred to as land lines) are a frequently used type of communications channel.
In a simple data communications system, terminals and other remote I/O devices are linked directly to one or more cpu's to allow users to enter data and programs and receive output information. Interface elements (those devices that serve to interconnect), such as modems are used to bridge and control the different data communications environments. Modems are used to permit the system to switch back and forth from computer digital data to analog signals that can be transmitted over communications lines.
A modem never knows exactly when to expect data; therefore, it must be given some type of signal warning that data is about to be transmitted. This gives the modem time to get itself aligned and in synchronization with the incoming signal. Special characters, known as message characters, provide this warning and are placed in front of and behind the data to mark the beginning and ending of the message. Two methods are used: asynchronous and synchronous.
With asynchronous transmission, each character of data must be surrounded by message characters. As a result, more total bits must be transmitted (transferred) than would be necessary if the synchronous method were used.
With synchronous transmission, only a single set of start and stop message characters is needed per block of data, thus allowing more characters to be transmitted per second. As you can see, synchronous transmission is more efficient and faster. However, it has the disadvantage of requiring a more complex and expensive modem than does asynchronous transmission.
You should be aware that whenever data is transferred between devices, it also involves an exchange of prearranged signals. This is known as handshaking. These signals, in combination with a prearranged pattern of message characters, define the rules for exchanging data over a communications line. The exact rules depend upon each individual computer manufacturer, the telephone company, and the related devices (the modems) that make up the computer system. Protocol is the term used for the specific set of rules that govern handshaking and message characters.
In the system illustrated in figure 4-14, data to be sent to the main computer's cpu is entered through a remote online user terminal (far left). As the data is keyed, it is keyed in digital form and sent to a nearby modem to be converted into an analog signal suitable for transmission. This converted data is then transmitted over the telephone (or land) lines to another modem, that is located near the main computer system's cpu. The data, now in digital form, can be sent directly to the cpu for processing. The same route is followed when information is sent from the cpu back to the remote terminal.
Figure 4-14. - Modems used in network system.
Data communications and networks expand our use of computer technology by providing a means for computers and other machines to talk to each other.
Q.30 Any system composed of one or more computers and terminals can be defined as what?