我一个室友的毕业设计部分内容(数据通信)(doc-文档)

我一个室友的毕业 设计 领导形象设计圆作业设计ao工艺污水处理厂设计附属工程施工组织设计清扫机器人结构设计 部分内容(数据通信)(doc-文档) 我一个室友的毕业设计部分内容(数据通信) Data Communications 1. Data Communications Systems When people talk of installing data communications systems, the first thoughts that come to mind are ,all too often, large processors, front-end processors, concentrators and terminals linked in a complex network. Yet such systems account for only a small percentage of those actually in use or even planned. It is certainly not necessary to think in those terms if one is to obtain many of the benefits of data-communications-based data processing systems. The five basic types of data communication system are (in order of increasing sophistication) (1) off-line data transmission (2) remote batch (3) on-line data collection (4) enquiry-response systems (5) real-time systems. (1) Off-line data transmission is simply the use of a telephone or similar link to transmit data without involving a computer system. The equipment used at both ends of such a link is not part of a computer, or at least does not immediately make the data available for computer processing, that is, the data when sent and/or received are „off-line?. This type of data communication is relatively cheap and simple. (2) Remote batch is the used for the way in which data communication technology is used geographically to separate the input and/or output of data from the computer on which they are processed in batch mode. (3) On-line data collection is the method of using data communications technology to provide input data to a computer(say on a magnetic disk) and processed either at predetermined intervals or as required. (4) Enquiry-response systems provide, as the term suggests, the facility for a user to extract information from a computer. The enquiry facility is passive, that is, dose not modify the information stored. The interrogation may be simple, for example, „RETRIEVE THE RECORD FOR EMPLOYEE NUMBER 1234? or complex. Such systems may use terminals producing hard copy and/or visual displays. (5) Real-time systems are those in which information is made available to and processed by a computer system in a dynamic manner so that either the computer may cause action to be taken to influence events as they occur (for example as in a process control application) or human operators may be influenced by accurate and up-to-date information stored in the computer, for example as in reservation systems. The Elements of a Network At its simplest, a data communications facility consists of some form of input or output unit, a communications link and a host processor. Most networks contain many more than these three basic ingredients, but it is worth remembering that they are all designed to facilitate, expedite or make more efficient the basic functions of inputting, transmitting, processing and outputting data. One of the main hardware devices added to a large computer system for telecommunications is a communications controller or front-end processor, an interface between the computer and the telecommunications channels. In a large computer system, the processor sits in a large cabinet on the computer room floor. The front-end processor is similar to a secretary who serves as an interface to a busy executive. The secretary receives incoming mail, opens it and places it on the executive?s desk. After the executive has worked on the material, the secretary retrieves it, places it in an envelope and mails it. The communications controller receives incoming data communications from a remote terminal and performs certain work on the incoming traffic before sending it on to the computer for processing. After the computer has done the actual data processing, the work is sent back to the controller where it is prepared for transmission back to the remote terminal. The controller or front-end processor is a buffer device between the high-speed computer and the relatively slow telecommunications channel. The controller can analyze incoming data traffic for errors, perform code conversions, speed conversions, etc. Data communications from the computer can be arranged in the proper format for transmission to the distant terminal. The communications controller can consist primarily of hardware components, but most controllers also contain software (computer programs). The tern “front-end processor” is often used to describe controllers which contain software. Within the central processing unit (CPU) is other software designed to interface with the communications controller and the telecommunications environment. All of this software gives the system tremendous capability. However, it also means that such systems can be quite complex to develop install and maintain. Application programs are installed in the CPU for specific data communications applications (remote order entry , airline reservations, etc). The communications controller and the telecommunications software are designed to isolate the application programs from the telecommunications environment. This helps to simplify the work of the application programmers in dealing with computer applications which involve data communications. The communications controller interfaces the telecommunications channel through a hardware device called a modem, which is a conversion device. It converts digital computer pulses into analog information that can be sent over voice telephone lines, the telecommunications channel. Modems have enabled computers to utilize the vast voice telephone network already in existence when computers needed a telecommunications capability. A telecommunications channel has a modem at both ends of the channel. The modem at the terminal end converts analog information back into digital form. Modems will be discussed in greater detail later in this chapter. The telecommunications channel is the connecting link between the computer and the remote terminals. It is typically a voice telephone channel, and can be a dedicated channel to the terminal or can be dialed-up through the telephone central office. At the distant end of the telecommunications channel is the terminal that communications with the computer. The terminal is the interface between people and the computer. It can be a teleprinter, video display, printer and/or a personal computer. The physical path from the computer to the terminal is similar to a highway .Rules of the road are needed for traffic-data communications-to flow smoothly. Standards and protocols set the speed limit on the highway, assign rights of way, deal with mistakes, etc. Without them there could be no real data flow over the highway. No telecommunications channel is ever totally perfect. Imperfections in the channel and problems with terminals and software can result in errors in data communications. To deal with these problems, many large computer systems have special test equipment. 2. Modems, Concentrators and Multiplexers The term “modem” is a contraction of modulator-demodulator. Modulation is the process of changing some characteristic of an electrical signal—called a carrier—in response to some other signal. A modem takes digital pulses from a computer or terminal and modulates a carrier which is sent to the modem at the other end of the channel. The modulated carrier is a analog signal which can be transmitted over the analog telephone telecommunications channel. The carrier is a type of wave form which resembles alternating current and human speech. There are different ways that a carrier can be altered in response to digital computer information. This can include changing the amplitude, frequency, or phase of the carrier. Changing the frequency is a form of modulation that is easy to understand and widely used in slower-speed modems. In this technique the carrier is modulated at two different frequencies depending on whether a zero or one is being sent to the modem. For example, when a zero is received by the modem, it may send a 1200Hz signal over the telecommunications channel. This may change to a 2200Hz signal when a one is presented to the modem. This back-and-forth changing of frequencies occurs at a rapid rate depending on the transmitted speed of the data. If someone were to listen in on the telecommunications channel, two changing tones would be heard. It is similar to someone whistling over the channel at two different frequencies in response to zeroes and ones from the computer or terminal. Demodulation is the process of converting the varying analog signals back into digital pulses. The demodulator portion of the modem analyzes the variations and decides whether a zero or one is being sent. Electronic circuitry in the modem makes the modulation and demodulation capability possible. At higher data rates more complex modulation techniques are used, phase modulation being common. The actual modulation techniques are usually of more interest to design engineers than users. The end-users of usually of modems are primarily concerned with other characteristics of modems, such as speed. This is the rate at which the modem can send data over the telecommunications channel. Speeds range from approximately 300 bits per second (BPS) to 9,600 BPS and higher. A bit is the basic unit of measure in data communications; it stands for “binary digit.” It is a zero or one which is all the computer understands. The number of BPS is used as a measure of transmission speed. At one might expect, speed costs money. The higher the BPS, the more expensive the modem because it requires more complicated electronic circurity. Slower data rates are often used when a low-speed terminal device is accessed directly by a human operator. For example, a teleprinter where an operator directly types in information to a remote computer might use a 300 or 1,200 BPS. Higher-speed modems might be needed when multiple terminals are using the same telecommunications channel or a high-speed printer is involved in receiving data from the computer. Then 4,800 and 9,600 BPS modems are commonly found. Modems are also classified in terms of the type of telecommunications channels on which they will be used. Low-speed modems--300, 1,200, and 2,400 BPS—are typically used with dial-up telecommunications channels through the telephone central office. Higher-speed modems—4,800, 9,600 BPS and faster—are typically used on dedicated telecommunications channels where a continuous channel links the computer and terminal. The side of the modem which is connected to the computer to the computer or terminal has a 25-pin output connector. The side connected to the telecommunications channel has two or four wires. Modems used in the dial-up telephone network have two wires, the same as a standard telephone set. Modems used on dedicated channels usually have four wires (two pairs). One pair of wires is used for sending data and the other pair for receiving it. Modems come in all sizes and shapes. They can be mounted in a large box that sits on a desk or can be on a small printed circuit board mounted directly inside a terminal. Regardless of their size or shape, they perform the same basic functions. Modems have to be compatible with each other. At lower speeds, modems from different vendors are often compatible. At higher speeds, compatibility is often more of a problem since different vendors often use different modulation techniques. The death of the modem has been predicted for years, yet they are still a fast-selling item. The move toward all-digital telecommunications is taking place at a slower rate than was anticipated by many people and so modems continue to be needed for the existing analog channels. The price of modems has continued to decrease over the years. Along with decreasing prices has come a move on the part users toward higher-speed modems. Once 300 BPS was considered adequate for dial-up use, and now users are moving up to 1,200 and 2,400 BPS modems. The higher speeds mean that less time is spent on dial-up calls. Particularly on long distance calls, the faster modems can pay for themselves in terms of reduced charges. Modems are very reliable. The failures with them tend to occur in the first few hours of operation, and from then on the failure rate is low. The modem has made data communications possible, and they will be around for quite some time to come. In many cases (indeed in most cases in Europe) the attachment of a modem to any given communications link is not entirely at the discretion of the user but rather at the command of the relevant telecommunications authority. This control is enforced by maintaining a register of approved modems which can be attached to any given type of communications link. Most modems are attached directly to the communications link in use and remain permanently in one physical position. Some modes are, however, constructed instead as acoustical couplers, which makes them much more portable. The acoustic coupler modulates the digital input in such a device, which is physically a small ?black box? with a rest or cradle on to which a standard telephone handset can be placed, is often combined with a simple keyboard/hard-copy printer unit to provide a terminal of average briefcase size and portability. Such acoustically coupled terminals are being increasingly used because of their convenience, being able to operate in offices, meeting rooms, etc.— anywhere where a normal telephone is available. Concentrators Conceptually the simplest way of linking a number of remote locations to a single host processor is to provide a separate communications link to each remote location. The cost of the communications links in this type of network can quickly become prohibitive and various approaches have been adopted to overcome the prohibitive and various approaches have been adopted to overcome the problem—two of the approaches being concentrators and multiplexers. The concentrator is effectively a computer in its own right which—as its name implies—concentrates the data traveling to or from a number of remote locations into „bulk loads?. Thus a concentrator may „collect? data from a number of relatively slow-speed terminals using appropriate low speed and probably asynchronous links and interleave them for transmission over a higher performance link to the host processor. Being intelligent, concentrators may be programmed to perform a variety of tasks, which frequently include data validation, and for this purpose storage devices containing a variety of files may be attached. Similarly for data flowing output from the host processor, that is, towards the terminal, output formatting may be performed, for example, to provide appropriate screen layouts for visual display units. Finally the possibility of using concentrators to provide at least some of the essential back-up and/or fail-safe options in a network should be mentioned. Thus a concentrator may perform sufficient processing to enable „its? part of the network to continue essential processing if one or some of the communications link(s) to it be come inoperable. Furthermore, it may store essential data if a link or device on either the inboard (host processor) or outboard (terminal) side of it goes down. Multiplexers Compared with a concentrator, a multiplexer is basically an unintelligent unit which performs only the basic role of reducing total communications link costs but has no other function in the network. In this sense the multiplexer plays a solely economic role with regard to data communications links whereas, as we have seen, a concentrator may have a functional (back-up, data validation, ect.) as well as an economic role. This distinction will probably become less clear as an increasing number of multiplexers are based on programmable minicomputers and the development of hybrid devices seems likely. There are two main techniques used in multiplexing: frequency division and time division. The trend is towards the latter at the expense of the former. 3. Telecommunication Channels The telecommunications channel is the link among modems and ultimately the link between the computer and terminals. The channel is normally designed for human speech ( analog communications). When the need for the remote accessing of computers via telecommunications first arose, a gigantic telephone network was already in existence. The modem was developed to adapt this existing network to computer communications. One of the most widely used channels is a pair of wires to the local telephone company central office switch. A telephone number is assigned to the modem just as a telephone number is assigned to a telephone set. Both the terminal and computer could be connected to the same central office switch. For the terminal user to access the computer, he simply dials up the telephone number for the computer. A dial-up connection is easily established on an as-needed basis. A dial-up channel is typically a half duplex channel. This means that data can be sent in only one direction at a time. If both ends try to send data at the same time, there is a collision on the channel. Some modems use a frequency division multiplexing technique to obtain a full duplex capability over a dial-up channel. With full duplex operation, the modems can send and receive at the same time. This is another method to obtain maximum utility of what is readily available. When modems are connected over a dial-up channel, they go through a routine called handshaking. This is where the modems communicate with each other. The procedure has to occur on every dial-up call. If problems arise on a dial-up call, it can be disconnected and redialed. This is what people do when they get a bad telephone connection. It is also a quick and easy procedure to use when problems occur on dial-up data communications. Normally, when a full-time telecommunications channel is needed from the terminal to the computer to the computer, it is obtained In the form of a dedicated channel. Such a channel consists of a telephone circuit which does not pass through the central office switch, even though the wires do go through the central office. Rather, the wires are cross-connected on the distribution frame. Thus, the terminal and computer are always connected to each other. Dedicated channels are often used when higher data speeds are needed, for higher volumes of traffic, or when the application requires a dedicated channel. For example, a terminal at a travel agency might be on a dedicated channel to provide instant access to information. A dedicated channel saves the time of dialing each call, waiting for a connection to be established, for handshaking to occur, ect. This can be important in some computer applications. The dedicated channel can be a two-or fout-wire path. Two wires provide a half duplex operation similar to a dial-up connection; four wires enable the modems to have full duplex operation. In the latter, the modems can send and receive at the same time. This can increase the volume of data which is sent over the channel, and the handshaking routine can be avoided. The dedicated channels for voice communications are sometimes used by organizations which have a heavy volume of telephone traffic between two facilities. They can handle higher data speeds because switching equipment is not in the path. Also, the wires can be arranged for the best possible performance because they are dedicated to one specific connection. However, in case of problems on a dedicated channel, the user can not simply dial-up another path. The problems have to be located and corrected. Dedicated channels can fail for a number of reasons including accidental damage to the wires or an accidental disconnect on the distribution frame. All-digital dedicated telecommunications channels are now available. They do not require the use of modems, but small interfacing devices are on each end of the channel. They provide the 25-pin connections to the computer and terminal. The signal is sent digitally from end to end. These channels are designed for data communications and permit higher speeds with fewer errors. While they pass through the central office, they have their own special equipment which is intended to handle digital computer pulses. These channels will not pass an analog telephone conversation and are intended only for digital communications. 4. Terminals for Data Communications Terminals are only the tip of the data communications iceberg, but are familiar to most people. Terminals are similar to telephone sets in that they provide people with access to a very sophisticated capability. One of the most widely-used terminals for data communications is a teleprinter, It consists of a keyboard for sending data and a printer for receiving data from the computer. Teleprinters were used on the telegraph network as a replacement for the telegraph key. The original ones were rather large and contained electro-mechanical components. Gradually, teleprinters have become smaller and mainly electronic. Teleprinters are often called “dump” terminals because they do no local processing of the data. They simply send and receive. However, some of them have been made smarter by the inclusion of a very small computer. That permits the terminal to perform some local editing on data before it is sent to the computer. A more sophisticated terminal is a cathode ray tube and a keyboard. These are called CRTs or video terminals. They permit the user to display information directly on a screen in response to commands entered on the keyboard, and are widely used when data needs to be observed but a printed copy is not needed. An airline reservation operator can use a video terminal to enter a request for flight information and to assign seat. Such terminals are normally used on dedicated telecommunications channels, often at the speeds of 4,800 or 9,600 BPS. The higher speeds are needed to quickly fill the screen with information. A video display can have its own modem and dedicated channel. However, when multiple displays are in the same facility, a local communications controller is often installed. Multiple video terminals are connected to the controller, which is connected to the modem. The local controller serves as a traffic manager for the terminals attached to it. It permits the video terminals to have their turn at sending and receiving from the remote computer. The controller is able to allocate time so that the users all think they have instant access to the computer. Cluster video controllers often have small computers built into them that enable them to perform more sophisticated functions. When video terminals are used, there may be a need for printed copy. This can be provided by a printer. It is a terminal which receives data from the computer and prints it on paper. A cluster controller might have one or more common printers to serve all of the video terminals. Printers often place heavy traffic demands on the system. The controller has to ensure that the printer does not overload the channel and thus prevent the video terminals from getting access. A printer can also have its own dedicated channel. This might be the case when vast amounts of data are to be the printer on a constant basis. Computers can also be used as terminals. A medium-sized computer in a field location might do local data processing and then send the data to a large central computer for further processing. The connection between the two computers can be via a dial-up or a dedicated channel, depending on the volume of data to be sent and received. Personal computers have also become terminals for data communications with large remote computers. The personal computer typically uses a dial-up channel to call the remote computer. A modem can be built into the personal computer or it can be in a separate unit. Special programs are used to handle data communications with the remote computer. One terminal that is often overlooked is a telephone set with a pad on it. In some applications, users can dial-up a computer and send data by pressing the buttons on the telephone pad. This application does not require modems since the pad generates tone signals which are analog in nature and pass directly over the channel. 5. Standards and Protocols Standards and Protocols make possible the transmission of computer communications over the telecommunications channel. These are the rules of the road. There are various aspects to these rules, but only a few will be considered here. Data communications have to be coded for transmission. Sometimes the same code used in the computer is used for communications. In other cases, the internal computer code is converted to another code for data communications. EBCDIC(Extended Binary Code Decimal Interchange) are among the codes used in data communications. As discussed before, codes are the zeroes and ones which represent numbers and letters. In addition, codes are used to create control characters which are not normally considered by the user. For example, one control character may tell a printer that when it reaches the end of a line to go to the next line and start printing again at the far left side of the paper. Control characters are inserted without the user having to be concerned about them. Codes are described in terms of the number of bits that it takes to make up a character. Computer codes typically consist of seven or eight bits. Within a computer, a character is typically handled in parallel fashion. This means that all the bits which make up a character are transferred at the same time. Transmission over a telecommunications channel is in serial fashion, one bit at a time. The front-end processor at the central computer site converts data from parallel to serial form for transmission over the channel. Encoded data can be sent asynchronously or synchronously over the channel. In asynchronous transmissions, every character stands on its own. A start-and-stop bit surrounds each character. This enables the terminal. And computer to remain in synch by analyzing one character at a time. Asynchronous transmissions are usually used in low-speed applications, typically over a dial-up telecommunications channel. It is well-suited for applications where a human operator would type in letters and numbers at varying rates of speed. Each character can be considered on its own. Asynchronous transmissions have a significant overhead since the start-and-stop information goes with every character. At higher speeds this overhead can become a burden and synchronous communications are typically used. In synchronous communications, data is sent in blocks. Each block is preceded by information which indicates the start of a block. The end of each block contains information which indicates that it is the end of a block of data. Synchronous communications enable data to be sent with less overhead, but involve a more complex process and are normally used at higher speeds with sophisticated applications and over dedicated channels. A number of speeds have become industry standards. They are 300,1200,2400,4800 and 9600 BPS. Higher speeds of 14.4 kilobits per second (KBPS), 19.2 KBPS, 56 KBPS and higher are becoming more common. All-digital telecommunications channels will make higher data transmission rates more common. The 25-pin connector which connects the modem to the modem to the computer or terminal is normally an RS232 interface. This is a standard interface developed by the Electronic Industries Association (EIA). It has helped to simplify the connection of terminals and computers to modems. The RS232 standard concerns itself primarily with the electrical connections on each pin, their purpose, etc. The handshaking routine between modems is related to the requirements of the RS232 standard to establish a connection between a computer and terminal. Protocols are rules of how the data communications traffic will flow over the telecommunications channel. They are agreements on how data will be sent back and forth. One widely-used protocol for synchronous data communications is BSC (Binary Synchronous Communications). It was developed by IBM and has become a de facto industry standard. With BSC, data is sent in one direction at a time and one block transfer will take place. Even though the transfer is half duplex, a four-wire dedicated channel is typically used. This is done because after each block is received the sending location must be advised of this fact. Having a full duplex channel speeds up this acknowledgement and the transfer of data. Newer protocols called HDLC (High-level Data Link Control) have come into existence. These protocols permit multiple blocks of data to be sent before an acknowledgement is required. Also, full duplex communications can take place over the channel providing a four-wire communications channel is available. The newer protocols are also better suited for transmissions over satellite channels, however, the older protocols will remain in existence for many years. Organizations are often in no rush to convert because of the expense involved in replacing equipment and software. 6. Error Detection Techniques There are a number of reasons for problems in data communications. For example, noise on the telecommunications channel can affect communications. When two people talk to each other, they can often ignore noise on the line, but this same noise can destroy the bits which make up data communications. The faster the speed of data, the more damage that can be done by even an instant of noise on the channel. However, not only noise can affect data communications. Cross-talk—where a conversation on one telephone channel is picked up on another channel—can also have a negative impact on data communications. Also attenuation—the loss of signal power—can cause errors. When problems exist on a dial-up data call, the call can be disconnected and redialed. This is done at the start of a transmission when it is obvious that there are problems. However, problems often occur on a dial-up call after an initially good connection has been established. A brief instant of noise might affect only a few bits of data, but it is important for the user to know when this has happened. There are a number of ways to deal with data communications errors. One technique, called party checking, is a widely-used error detection technique, particularly in asynchronous communications. Party relates to the evenness or oddness of the number of ones in the data code for any particular character. An illustration will help to clarify this technique. The letter A in the ASCLL code consists of the following seven bits: 1000001. If an even parity code scheme is used, the number of ones must come out even. In this case the number of ones is already even, so a final zero is added to give 10000010. The letter C in ASCLL is 1100001. The number of ones is odd. To make the parity even, a final one must be added to give a transmitted character of 11000011. If a single error occurs in one of the transmitted data bits (a switch of 1 to 0 or vice versa) the parity bit check will catch the error. It cannot state which bit is in error, only that there is an error. However, if two bits are switched, the parity bit will be fooled, so it is not a totally perfect technique. However, the switching of multiple bits might also result in a character that does not exist in the ASCLL code and would thus be detected as an error. Parity is a simple yet rather effective error detection technique. However, it adds an additional overhead bit for each seven bits of data that is transmitted. This increase the time for transmission and reduces the total volume of data that can be sent in a given period of time. When data is transmitted in blocks, which is typically the case with synchronous communications, other error detection and correction techniques are used. One of these calls for the receiving location to send an acknowledgement after each block is received. The received location after checking for errors, acknowledges that the block of data is all right and requests the next block of data. If errors are detected, the receiving location asks for a retransmission of the last block of data . This block-checking technique helps to reduce the amount of overhead. It can be quite effective on telecommunications channels where line conditions are good and the likelihood of errors is low. However, on telecommunications channels where line conditions are poor and the error rate is high, there may be constant retransmissions of blocks of data. The HDIC protocols allow multiple blocks to be sent before any acknowledgement is needed from the receiving location. The latter can advise required. This approach is often particularly effective when using satellite communications. Through trial and error, users determine whether a particular telecommunications channel works effectively or not. When error rates become too high, other channels or protocols are adopted. The move to all-digital telecommunications channels will help to reduce errors because the channels are designed for digital pulses. In an analog channel, an analog signal is amplified and reamplified throughout the transmission path. In a digital channel, new digital pulses can be generated rather than just amplifying the old ones. The final result is improved transmission performance