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Etkin modem haberleşmesi sağlayan nesneye dayalı bir yazılım tasarımı

Design of an object oriented software for efficient modem communications

  1. Tez No: 14353
  2. Yazar: FEZA BUZLUCA
  3. Danışmanlar: PROF.DR. EMRE HARMANCI
  4. Tez Türü: Yüksek Lisans
  5. Konular: Bilgisayar Mühendisliği Bilimleri-Bilgisayar ve Kontrol, Computer Engineering and Computer Science and Control
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1991
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 83

Özet

ÖZET Bilgi çağını yaşayan günümüz dünyasında veri iletişiminin önemi her geçen gün artmaktadır. Verileri bir yerden başka bir yere aktarmank için etkin ve ucuz bir iletim ortamı arndıgında da akla ilk olarak dünyanın her tarafını saran telefon şebekesi gelmektedir. Yaklaşık 100 yıldan bu yana kullanılması nedeniyle ülkelerin en ücra köşelerine kadar uzanan telefon hatlarından veri iletişiminde yararlanılmasının maliyeti düşüreceği düşü nülmüştür. Bununla birlikte telefon hatları sayısal verileri aktarmak için uygun olmadığından bilgisayarlar ve sayısal bilgileri kullanan diğer elemanlar telefon hattına bir modem cihazı ile bağlanmaktadır. Ancak fiziksel iletim ortamında oluşan arıza ve gürültü nedeniyle bazı bilgilerin hatalı ya da eksik iletilmesi sorunu ortaya çıkmaktadır. Yapılan tez çalışmasında, modemler aracılığı ile telefon şebekesi üzerinde birbirine bağlanan iki bilgisayarın hatasız ve etkin bir şekilde haberleşmesini sağlayan bir yazılım gerçekleştirilmiştir. Bu yazılım bir yönetim/denetim birimi, üç adet de katman modülünden o- luşmaktadır. Tez çalışmasının büyük kısmını veribagı katmanının yazılımı oluşturmktadır. iki düğüm arasında hatalardan arındırılmış bir iletim oluşturmakla görevli bu katman CCITT'nin LAPB protokolünden yararlanılarak tasarlanmıştır. iletişim sırasında oluşan hataları düzeltmek için izlenen yol şöyle açıklanabilir: Veriler önce paketlere ayrılır ve her pakete bir numara verilerek karşı tarafa gönderilir. Hatalı iletilen yada alıcısına hiç ulaşamayan paketler tekrar gönderilir. Ayrıca oluşan hataların sıklığına göre paket boyları değiştirilerek etkin bir iletişim sağlanmaktadır. Söz konusu yazılım“full-duplex”(iki taraf da aynı anda veri gönde- rebilmektedir) iletişime destek verebilecek şekilde ta sarlanmıştır. Tez çalışmasında oluşturulan yazılım nesneye dayalı (object oriented) özelliğine sahip Turbo C++ dili kullanılarak yazılmıştır. Bu dilin seçilme nedeni donanı mın etkin bir şekilde kullanılmasına destek vermesi ve verimli programlar yazılmasını sağlamasıdır.

Özet (Çeviri)

SUMMARY DESIGN OF AN OBJECT ORIENTED SOFTWARE FOR EFFICIENT MODEM COMMUNICATIONS A number of importnant developments in the field of computer technology have increased the use of comput ers. So, many organizations have a substantial number of computers in operation, often located far apart. To achieve a more powerful system it has been decided to connect these separate computers. When two computers owned by the same company or organization and located close to each other need to com municate, it is often easiest just to run a cable between them. However, when the distances are large, or there are many computers, or the cables would have to pass trough a public road or other public right of way, the costs of running private cables are usually prohibitive. The cheapest solve of this problem is the using of the existing public switched telephone network (PSTN). The switched public telephone network were designed many years ago, with completely different goal in mind: transmitting the human voice in a more or less recognizable form. The PSTN have bandwidth of from 400 to 3400 Hz or simply 3000 Hz. This means that a tele phone line will not pass very low frequency signals which may arise, for example, if the data stream to be trans mitted is made up of a continuous string of binary Is or 0s. When data are transmitted using the transmission lines from an existing PSTN, it is first necessary to convert the electrical signals output by the source com puter into a form that is acceptable to the PSTN. The circuit that performs this operation is known as a modu lator and the circuit, performing the reverse function a demodulator. Since each side of a data link must normal ly both send and receive data, the combined device is known as a modem. viIn any form of digital system, the loss or corruption of a single bit of information can be criti cal. It is thus essential when designing a communication facility for a distributed system to ensure that adequate precautions are taken to detect and, if necessary, cor rect for any possible loss or corruption of information during transmission. The main aim of this study is to design a software which includes a set of rules and control procedures to ensure the reliable (free of errors and duplicates) exchange of messages between two computers connected over telephone lines. Several techniques are used to provide error con trol in communication systems. Some of them are de scribed in section 2. In the earlier days of computer communication, it has meant that only closed communities of computers (that is, from the same manufacturer) could communicate with each other in a meaningful way. So universal intercon- nectability was not possible. In attempt to alleviate this problem» the ISO (International Standards Organiza tion) formulated a reference model to provide a common basis for the coordination of standards developments. The ultimate aim was to allow an application process in any computer that supported the applicable standards to freely communicate with an application process in any other computer supporting the same standard, irrespective of its origin of manufacture. This model was termed the ISO reference model for Open Systems Interconnection (OSI). A communication subsystem is a complex piece of software. This software can not be designed with a sin gle program, because the resulting communication subsys tem will be difficult to test and modify. To overcome this problem, the ISO have adopted a layered approach for the reference model. The complete communication subsystem is broken down into seven layers. Each layer performs a well-defined function in the con text of the overall communication strategy. It operates according to a defined protocol by exchanging messages, both user data and additional control information, with corresponding layer in a remote system. Each layer has a well-defined interface between itself and layer immedi ately above and below it. VI iAn important part of this study consists of the 2nd layer, the data link layer. The task of this layer is dividing data into frames and providing reliable, ef ficient communication between two physically connected equipment. The most common used error control strategy is automatic repeat request. In this scheme a small control message is returned to acknowledge correct receipt (or otherwise) of each transmitted frame. There are two de rivatives of this scheme. Idle repeat request (send-and-wait or stop-and- wait) is the simplest automatic repeat request scheme. It requires a minimum of buffer storage for its implemen tation, but it is also the least efficient in its use of transmission capacity. In this thesis is continuous repeat request scheme used. With this scheme the link utilization is much im proved at the expense of increased buffer storage re quirements. The operation of this protocol: - The sender sends I-Frames (Information) without waiting for an ACK-frame (Acknowledge) to be returned. - The sender retains a copy of each I-frame transmitted in a retransmission list. - The receiver returns an ACK- frame for each correctly received I-frame. - Each I-frame contains a unique identifier which is re turned in the corresponding ACK-frame. - On receipt of an ACK-frame the corresponding I-frame is removed from the retransmission list by the sender. When an error does occur, two alternative proce dures may be followed; Selective retransmission: The sender detects the out-of- sequence ACK-frame and retransmits just the unacknow ledged frame (s). Go-back-N: The receiver detects the receipt of an out-of- sequence I-frame and requests the sender to retransmit all outstanding unacknowledged I-frames from the last correctly received, and therefore acknowledged, frame. In this study go-back-n strategy is used, because I- frames have to come in correct sequence. HDLC (High-level Data Link Control) is a protocol that has been defined by ISO for use in the data link layer. CCITT (Comite Consultatif Internationale de Tele- graphique et Telephonique) then adopted and modified HDLC for its LAP (Link Access Protocol) as part of the x.25 viiinetwork interface standard, but later modified it again to LAPB, to make it more compatible with later version of HDLC. In this study LAPB is adopted for modem communica tions. To specify this communication software the extend ed finite state machine model is used. With this tech nique, each protocol machine (sender or receiver) is al ways in a specific state at every instance of time. Transitions between states take place as a result of an event occurring; for example a frame becoming ready to send, a frame received or a timer expiring. By an ex tended finite state machine a large number of states can be grouped together for purposes of analysis. All other states can be thought as transient, just steps on the way to one of the main states. The specification and description of the telecom munication system is achieved by using of SDL (Specifica tion and Description Language). The basis for SDL is the concept of communicating finite state machines called process. An SDL system is set of blocks. Blocks are connected to each other and to the environment by channels. Within each block there are one or more pro cesses. These processes communicate with one another by signals and are assumed to execute concurrently if the hardware is suitable. A process will wait in a state until it receives a valid signal from its environment. Then it will perform a transition to another state. Dur ing the transition it may perform actions; these may ei ther be manipulation of information local to the process or sending signals to other processes or the external environment of the system. The software structure of the communication system consists of a management unit, three layers and primitive queues between these units. The details of this struc ture is described in 4th chapter of the study. The management unit is the main process of the communication software. Services provided by this unit are; user interface, control of primitive queues, memory management, activation of layer processes, timer functions, and calculating error statistics to increment or decrement frame size. The logical exchange of information and control between the adjacent units of the communication system is represent by primitives in an abstract way. These primi tives are of the two general types. The request ixprimitive type is used when a higher layer is requesting a service from the next lower layer. The indication pri mitive type is used by a layer providing a service to notify the next higher layer of any specific activity which is service layer. This primitive may be the result of an activity of the lower layer related to the primi tive type at the peer entity. The 3rd layer has the duty to divide files into frames. The size of these frames depends on error rate of the transmission. When the error rate increases, the management unit sends“decrement”primitive to the 3rd layer; and by a low error rate frame sizes are increment ed. The task of the 2nd layer (data link layer) achieving reliable and efficient communication between two physically connected units. In this study the frame structure of the HDLC is used to providing services of the 2nd layer. The protocol of this layer is designed with continuous repeat scheme and“go back n algorithm”. The 1st layer (physical layer) realizes modem con trol functions. Serial communication processes, which are sending and receiving data via serial communication adapter, are included in this layer. Object-oriented programming method is used for designing and implementing the communication software system. Major goals are to increase software extensibil ity and reusability and to control the complexity and cost of software maintenance. When object_oriented pro gramming is used, the design phase of software develop ment is linked more closely to the implementation phase. In this study each layer consists of a base class, which includes global data and of its derived classes. Each derived class represents one of the states of the finite state machine. When a process of an layer is ac tivated by the management unit, the transition procedure of the instantaneous state is called. The importnant features of object-oriented programming, encapsulation, inheritance, and polymorphism to design an efficient com munication software. The object-oriented approach is described in the 5th and 6th chapters of this study.

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