We all are aware with some sorts of communication in our day to day life. For communication of information and messages we use telephone and postal communication systems. Similarly data and information from one computer system can be transmitted to other systems across geographical areas. Thus data transmission is the movement of information using some standard methods. These methods include electrical signals carried along a conductor, optical signals along an optical fibers and electromagnetic areas.
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Suppose a Managing Director of a company has to write several letters to various employees . First he has to use his PC and Word Processing package to prepare his letter. If the PC is connected to all the employee’s PCs through networking, he can send the letters to all the employees within minutes. Thus irrespective of geographical areas, if PCs are connected through communication channel, the data and information, computer files and any other program can be transmitted to other computer systems within seconds. The modern form of communication technologies like e-mail and Internet is possible only because of computer networking.
Computers are powerful tools. When they are connected in a network, they become even more powerful because the functions and tools that each computer provides can be shared with other computers. Networks exist for one major reason: to share information and resources. Networks can be very simple, such as a small group of computers that share information, or they can be very complex, covering large geographical areas. Regardless of the type of network, a certain amount of maintenance is always required. Because each network is different and probably utilizes many various technologies, it is important to understand the fundamentals of networking and how networking components interact.
In the computer world, the term network describes two or more connected computers that can share resources such as data, a printer, an Internet connection, applications, or a combination of these.
Prior to the widespread networking that led to the Internet, most communication networks were limited by their nature to only allow communications between the stations on the network. Some networks had gateways or bridges between them, but these bridges were often limited or built specifically for a single use. One common computer networking method was based on the central mainframe method, simply allowing its terminals to be connected via long leased lines. This method was used in the 1950s by Project RAND to support researchers such as Herbert Simon, in Pittsburgh, Pennsylvania, when collaborating across the continent with researchers in Santa Monica, California, on automated theorem proving and artificial intelligence. At the core of the networking problem lay the issue of connecting separate physical networks to form one logical network. During the 1960s, several groups worked on and implemented packet switching. Donald Davies, Paul Baran and Leonard Kleinrock are credited with the simultaneous invention. The notion that the Internet was developed to survive a nuclear attack has its roots in the early theories developed by RAND. Baran’s research had approached packet switching from studies of decentralisation to avoid combat damage compromising the entire network.
By mid-1968, Taylor had prepared a complete plan for a computer network, and, after ARPA’s approval, a Request for Quotation (RFQ) was sent to 140 potential bidders. Most computer science companies regarded the ARPA-Taylor proposal as outlandish, and only twelve submitted bids to build the network; of the twelve, ARPA regarded only four as top-rank contractors. At year’s end, ARPA considered only two contractors, and awarded the contract to build the network to BBN techologies on 7 April 1969. The initial, seven-man BBN team were much aided by the technical specificity of their response to the ARPA RFQ – and thus quickly produced the first working computers. The BBN-proposed network closely followed Taylor’s ARPA plan: a network composed of small computers called Interface message processor (IMPs), that functioned as gateways (today routers) interconnecting local resources. At each site, the IMPs performed store-and-forward packet switching functions, and were interconnected with modems that were connected to leased line, initially running at 50 kilobit/second. The host computers were connected to the IMPs via custom serial communication interfaces. The system, including the hardware and the packet switching software, was designed and installed in nine months.
The first-generation IMPs were initially built by BBN Technologies using a rough computer version of the Honeywell DDP-516 computer configured with 24 Kilobyte of expandable core memory, and a 16-channel Direct Multiplex Control (DMC) Direct Memory Access unit. The DMC established custom interfaces with each of the host computers and modems. In addition to the front-panel lamps, the DDP-516 computer also features a special set of 24 indicator-lamps showing the status of the IMP communication channels. Each IMP could support up to four local hosts, and could communicate with up to six remote IMPs via leased lines.
The Advanced Research Projects Agency Network (ARPANET), was the world’s first operational Packet Switching network and the core network of a set that came to compose the global Internet. The network was created by a small research team at the Massachusettas Institute of Technology and the Defense Advanced Research Projects Agency (DARPA) of the Defence Department of United States. The packet switching of the ARPANET was based on designs by Lawrence Roberts of the Lincoln Laboratories.
Packet switching is the dominant basis for data communications worldwide and it was a new concept at the time of the conception of the ARPANET. Data communications had been based on the idea of Circuit Switching, as in the traditional telephone circuit, wherein a telephone call reserves a dedicated circuit for the duration of the communication session and communication is possible only between the two parties interconnected.
With packet switching, a data system could use one communications link to communicate with more than one machine by collecting data into Datagram and transmit these as Packet onto the attached network link, whenever the link is not in use. Thus, not only could the link be shared, much as a single PostBox can be used to post letters to different destinations, but each packet could be routed independently of other packets.
Systems Network Architecture (SNA) is IBM’s proprietary Computer Network architecture created in 1974. It is a complete Protocol Stack for interconnecting Computer and their resources. SNA describes the protocol and is, in itself, not actually a program. The implementation of SNA takes the form of various communications packages, most notably Virtual telecommunications access method (VTAM) which is the mainframe package for SNA communications. SNA is still used extensively in banks and other financial transaction networks, as well as in many government agencies. While IBM is still providing support for SNA, one of the primary pieces of hardware, the IBM 3745/3746 communications controller has been withdrawn from marketing by the IBM Corporation. However, there are an estimated 20,000 of these controllers installed and IBM continues to provide hardware maintenance service and micro code features to support users. A strong market of smaller companies continues to provide the 3745/3746, features, parts and service. VTAM is also supported by IBM, as is the IBM Network Control Program (NCP) required by the 3745/3746 controllers.
IBM in the mid-1970s saw itself mainly as a hardware vendor and hence all its innovations in that period aimed to increase hardware sales. SNA’s objective was to reduce the costs of operating large numbers of terminals and thus induce customers to develop or expand Interactive terminal based-systems as opposed to Batch Processing systems. An expansion of interactive terminal based-systems would increase sales of terminals and more importantly of mainframe computers and peripherals – partly because of the simple increase in the volume of work done by the systems and partly because interactive processing requires more computing power per transaction than batch processing.
Hence SNA aimed to reduce the main non-computer costs and other difficulties in operating large networks using earlier communications protocols. The difficulties included:
A communications line could not be shared by terminals whose users wished to use different types of application, for example one which ran under the control of CICS and another which ran under Time Sharing Option.
Often a communications line could not be shared by terminals of different types, as they used different “vernacular” of the existing communications protocols. Up to the early 1970s, computer components were so expensive and bulky that it was not feasible to include all-purpose communications interface cards in terminals. Every type of terminal had a Hardwired Control communications card which supported only the operation of one type of terminal without compatibility with other types of terminals on the same line.
The protocols which the primitive communications cards could handle were not efficient. Each communications line used more time transmitting data than modern lines do.
Telecommunications lines at the time were of much lower quality. For example, it was almost impossible to run a dial-up line at more than 300 bits per second because of the overwhelming error rate, as comparing with 56,000 bits per second today on dial-up lines; and in the early 1970s few leased lines were run at more than 2400 bits per second (these low speeds are a consequence of Shannon-Hartly Theorm in a relatively low-technology environment). Telecommunications companies had little incentive to improve line quality or reduce costs, because at the time they were mostly monopolies and sometimes state-owned.
As a result running a large number of terminals required a lot more communications lines than the number required today, especially if different types of terminals needed to be supported, or the users wanted to use different types of applications (.e.g. under CICS or TSO) from the same location. In purely financial terms SNA’s objectives were to increase customers’ spending on terminal-based systems and at the same time to increase IBM’s share of that spending, mainly at the expense of the telecommunications companies.
SNA also aimed to overcome a limitation of the architecture which IBM’s System/370 mainframes inherited from System/360. Each CPU could connect to at most 16 “channels” (devices which acted as controllers for peripherals such as tape and disk drives, printers, card-readers) and each channel could handle up to 16 peripherals – i.e. there was maximum of 256 peripherals per CPU. At the time when SNA was designed, each communications line counted as a peripheral. Thus the number of terminals with which powerful mainframe could otherwise communicate is severely limited.
SNA removed link control from the application program and placed it in the NCP. This had the following advantages and disadvantages:
Localization of problems in the telecommunications network was easier because a relatively small amount of software actually dealt with communication links. There was a single error reporting system.
Adding communication capability to an application program was much easier because the formidable area of link control software that typically requires interrupt processors and software timers was relegated to system software and NCP.
With the advent of APPN, routing functionality was the responsibility of the computer as opposed to the router (as with TCP/IP networks). Each computer maintained a list of Nodes that defined the forwarding mechanisms. A centralized node type known as a Network Node maintained Global tables of all other node types. APPN stopped the need to maintain APPC routing tables that explicitly defined endpoint to endpoint connectivity. APPN sessions would route to endpoints through other allowed node types until it found the destination. This was similar to the way that TCP/IP routers function today.
Connection to non-SNA networks was difficult. An application which needed access to some communication scheme, which was not supported in the current version of SNA, faced obstacles. Before IBM included X.25 support (NPSI) in SNA, connecting to an X.25 network would have been awkward. Conversion between X.25 and SNA protocols could have been provided either by NCP software modifications or by an external protocol converter.
A sheaf of alternate pathways between every pair of nodes in a network had to be predesigned and stored centrally. Choice among these pathways by SNA was rigid and did not take advantage of current link loads for optimum speed.
SNA network installation and maintenance are complicated and SNA network products are (or were) expensive. Attempts to reduce SNA network complexity by adding IBM Advanced Peer-to-Peer Networking functionality were not really successful, if only because the migration from traditional SNA to SNA/APPN was very complex, without providing much additional value, at least initially.
The design of SNA was in the era when the concept of layered communication was not fully adopted by the computer industry. Applications, Database and communication functions were come together into the same protocol or product, to make it difficult to maintain or manage. That was very common for the products created in that time. Even after TCP/IP was fully developed, X Window system was designed with the same model where communication protocols were embedded into graphic display application.
SNA’s connection based architecture invoked huge state machine logic to “keep track” of everything. APPN added a new dimension to state logic with its concept of differing node types. While it was solid when everything was running correctly, there was still a need for manual intervention. Simple things like watching the Control Point sessions had to be done manually. APPN wasn’t without issues; in the early days many shops abandoned it due to issues found in APPN support. Over time, however, many of the issues were worked out but not before the advent of the Web Browser which was the beginning of the end for SNA.
1.4 X.25 and public access
Following on from DARPA’s research, packet switching networks were developed by the International Telecommunication Union (ITU) in the form of X.25 networks. In 1974, X.25 formed the basis for the SERCnet network between British academic and research sites, which would later become JANET. The initial ITU Standard on X.25 was approved in March 1976.
The British Post Office, Western Union International and Tymnet collaborated to create the first international packet switched network, referred to as the International Packet Switched Service (IPSS), in 1978. This network grew from Europe and the US to cover Canada, Hong Kong and Australia by 1981. By the 1990s it provided a worldwide networking infrastructure.
Unlike ARPAnet, X.25 was also commonly available for business use. X.25 would be used for the first dial-in public access networks, such as Compuserve and Tymnet. In 1979, CompuServe became the first service to offer electronic mail capabilities and technical support to personal computer users. The company broke new ground again in 1980 as the first to offer real-time chat with its CB Simulator. There were also the America Online (AOL) and Prodigy dial in networks and many bulletin board system (BBS) networks such as The WELL and FidoNet. FidoNet in particular was popular amongst hobbyist computer users, many of them hackers and radio amateurs.
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In 1979, two students at Duke University, Tom Truscott and Jim Ellis, came up with the idea of using simple Bourne shell scripts to transfer news and messages on a serial line with nearby University of North Carolina at Chapel Hill. Following public release of the software, the mesh of UUCP hosts forwarding on the Usenet news rapidly expanded. UUCPnet, as it would later be named, also created gateways and links between FidoNet and dial-up BBS hosts. UUCP networks spread quickly due to the lower costs involved, and ability to use existing leased lines, X.25 links or even ARPANET connections. By 1983 the number of UUCP hosts had grown to 550, nearly doubling to 940 in 1984
1.6 Uses of Computer Networks
Computer networks have many uses in present life. However, the usage goes on increasing from day to day, More and more people use networks for their corresponding applications and thus increasing the area of usage. However, we categorize the usage of computer network as follows
The global here is to make all programs equipment and especially data available to anyone on the network without regard to the physical location of the resource and the user.
Always all the files could be replicated on one or more machine. So if one of them is unavailable the other copies could be used for the reference.
Small computers have a much better price / performance ratio than larger ones .Mainframes are roughly a factor of ten faster than personal computers, but they cost Thousand times more. This imbalance has caused many system designers to build systems Consisting of personal computers, with data kept on more than one machine
A computer network can provide a powerful communication medium among widely separated employees. Using a network, it is easy for two or more people who live far apart to write a report together. When one person makes a change, the other can easily look into that and convey his acceptance.
Access to remote information:
Many People, pay their bills, manage their accounts, Book tickets, electronically. Home shopping has also become popular, with the ability to inspect the on-line catalogs of thousands of companies. There are also cases where people are able to get information electronically.
Email: Electronic Mail or E-Mail is an application through which a person can communicate With another person present anywhere. E – Mail is used today by millions of people and they Can send audio or video in addition to text.
WWW (World Wide Web) : A main application that falls into the application category is access to information systems like the current World wide Web, which contains information about arts, books, business, cooking, government, health so on.
1.7. Data Transmission Modes:
Data communication circuits can be configured in a huge number of arrangements depending on the specifics of the circuit, such as how many stations are on the circuit, type of transmission facility, distance between the stations, how many users at each station and so on. Data communication circuits can however be classified as either two point or multipoint . A two-point configuration involves only two stations, whereas a multipoint configuration involves more than two stations. Regardless of configuration, each station can have one or more computers, computer terminals or workstations. A two point circuit involves the transfer of digital information from a mainframe computer and a personal computer, two mainframe computers, two personal computers or two data communication networks. A multipoint network is generally used to interconnect a single mainframe computer to many personal computers or to interconnect many personal computers. Coming to transmission modes, there are four modes of transmission for data communication circuits namely-
3. Full Duplex
In a simplex mode, the transmission of data is always unidirectional. Information will be sent always only in one direction Simplex lines are also called receive-only, transmit-only, or one-way-only lines. A best example of simplex mode is Radio and Television broadcasts.
Fig. 1.1 Simplex Communication
In the half-duplex mode, data transmission is possible in both the directions but not at the same time. When one device is sending, the other can only receive, and vice-versa. These communication lines are also called two-way-alternate or either-way lines.
Fig. 1.2 Half Duplex Communication
In the full-duplex mode, the transmissions are possible in both directions simultaneous, but they must be between the same two stations. Full-duplex lines are also called two-way simultaneous duplex or both-way lines. A good example for full-duplex transmission is a telephone
Fig. 1.3 Full Duplex Communication
Types of Data Transmission Modes
There are two types of data transmission modes. These are:
1. Parallel Transmission
In parallel transmission, bits of data flow concurrently through separate communication lines. Parallel transmission is shown in figure below. The automobile traffic on a multi-lane highway is an example of parallel transmission. Inside the computer binary data flows from one unit to another using parallel mode. If the computer uses 32-bk internal structure, all the 32-bits of data are transferred simultaneously on 32-lane connections. Similarly, parallel transmission is commonly used to transfer data from computer to printer. The printer is connected to the parallel port of computer and parallel cable that has many wires is used to connect the printer to computer. It is very fast data transmission mode.
2. Serial Transmission
In serial data transmission, bits of data flow in sequential order through single communication line. Serial dat& transmission is shown in figure below. The flow of traffic on one-lane residential street is an example of serial data transmission mode. Serial transmission is typically slower than parallel transmission, because data is sent sequentially in a bit-by-bit fashion. Serial mouse uses serial transmission mode in computer.
Synchronous & Asynchronous Transmissions
In synchronous transmission, large volumes of information can be transmitted at a time. In this type of transmission, data is transmitted block-by-block or word-byword simultaneously. Each block may contain several bytes of data. In synchronous transmission, a special communication device known as synchronized clock’ is required to schedule the transmission of information. This special communication device or equipment is expensive.
In asynchronous transmission, data is transmitted one byte at a ‘time’. This type of transmission is most commonly used by microcomputers. The data is transmitted character-by-character as the user types it on a keyboard.
An asynchronous line that is idle (not being used) is identified with a value 1, also known as ‘Mark’ state. This value is used by the communication devices to find whether the line is idle or disconnected. When a character (or byte) is about to be transmitted, a start bit is sent. A start bit has a value of 0, also called a space state. Thus, when the line switches from a value of 1 to a value of 0, the receiver is alerted that a character is coming.
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