Distributed Systems

1. Distributed Systems Lecture no. 1 Evening

2. Distributed System: An Introduction • What do you thing what can be a Distributed System? • What is the alternative of Distributed System? • What can be the drawbacks of a Centralized System? • Ok so if we want to define Distributed Systems it will be: A Distributed System is one in which components located at networked computers communicate and coordinate their actions only by passing messages. • Can you name any three examples of Distributed Systems?

3. Introduction • The world is revolutionized by the advancements that have taken place in various fields. • Examples of such areas are computers specially internet, communication particularly telecommunication and information technology. • Just imagine the technology today, it is beyond imaginations. • You have high speed technology and devices that are capable to operate on high data rates. What makes devices to have these qualities embedded?

4. Data and Signals • You must know the difference between Analog and Digital Signals. • Periodic and Non Periodic Signals. • Representation of Analog and Digital Signals. • How to calculate Peak Amplitude? • Example is: The power we use at home has a frequency of 60 Hz. How can you determine the period of this wave? The answer to the above example is 0.0166 x 10 Power 3 milliseconds.

5. Transmission Impairment • Signal Travels through transmission media which are not perfect. • This imperfection cause signal impairments, that means that the signal at the beginning of the medium is not same as the signal received at the end of the medium. • It will be more worst if the medium is wireless. • What information you have sent is not received and the factors can be attenuation, distortion and noise.

6. Diagrammatic Representation of Impairment

7. Attenuation • It means Loss of Energy. • Whenever a signal travel through medium it losses some of its energy in overcoming the resistance of the medium. • That is the reason a wire carrying electric signal gets warm. • To compensate this loss amplifiers are used to amplify the signal.

8. Decibel • To demonstrate whether a signal has lost or gained strength, Engineers use the unit of decibel. • The dB means the relative strengths of two signals. Or it can be one signal at two different points. • Note that dB is negative if the signal is attenuated and positive if the signal is amplified. NB: Power is square of voltage.

9. Examples • Suppose a signal travels through a transmission medium and its power is reduced to one half, how will you calculate the power loss? • A signal travels through an amplifier, and its power is increased 10 times. How can you calculate the amplification? • In first example the answer will be negative because it is attenuation and the second example answer will be Positive because of the amplification.

10. Why Decibels • One of the reasons why engineers use dB to measure the changes in strength of the signal is that dB numbers can be added or subtracted, when we measure several points. • Have a look in the example below and you will find how easy it is to find the attenuation or amplification of the overall system.

11. Distortion • Distortion means if the signal changes its form or shape. If you have a composite signal made of different frequencies than it will cause distortion. • Every signal has its own propagation speed through a medium and therefore it will cause its own delay while receiving at the final destination.

12. Noise • There are so many types of Noise. It can be thermal, induced noise, cross talk, and impulse noise. • Thermal noise is the random motion of electrons in a wire which creates an extra signals not created by a transmitter. • Induced noise comes from sources such as motors and appliances. These devices act as a sending antenna. • Cross talk is the effect of one wire over another. One wire act as a sending antenna and other as receiving. • Impulse noise is a spike (A Signal with High Energy in a Very short Time) it comes from power lines, lightning etc.

13. Types of Information • Categorically speaking all information can be either Analog/ Digital • If I say the systems are Digital and the real time signals are Analog, Am I right or wrong. Can you justify that? • Here we need to have a debate what are the advantages of Analog and Digital Signals? • You all have your notebooks, please write at least 3 advantages of Digital Information and 3 disadvantages of Analoginformation. I am not asking to compare just individual advantages.

14. Advantages of Digital Information • A Digital Signal which comprises bits can be easily processed by computers. Once the computers are involved in processing, much of the work by humans are avoided. • For instance to manipulate a beautiful drawing you need to have drawing masters. If it is a computer even a kid can make it as he/ she likes. The condition is if the kid knows how to operate or use the system. • The Digitized information can easily be copied and the copies also have the originality of it.

15. Advantages of Digital Information (Continued) • The noise that interrupts the information and it is always in analog form if we are considering the wireless transmission and reception can’t modify the digitized information. • The network like internet and intranet are available to transmit the digital information to a long distance. You have High speed digital systems these days they are capable to operate on high data rates. • Example is ISDN, Integrated Services Digital Networks Technique.

16. Radio Basics • Once we talk about wireless systems most of the time we deal with RF Signals, Can anyone tell what are RF Signals? • The Radio Spectrum is a term that scientists, engineers and policymakers use to classify a vast and otherwise undifferentiated swath of EM Energy that exists in the Universe. • This form of Energy makes possible the development and use of technology such as broadcast radio and TV, Remote controlled Toy Air Planes, Geographic Positioning System (GPS) and Mobile Phones etc.

17. Radio Basics (Continued) • In 20th Century, the entrepreneurs developed first reliable radio system and the spectrum quickly became recognized as a radically new mean by which human being can communicate. • Initially there was comparatively little or no need to differentiate between various sections of radio spectrum because there was relative modest demand. • Modest demand means there was less products available at that time. • But with the passage of time problem was created and the problem was?

18. Need to Divide Radio Spectrum • One reason for dividing is that the number of frequencies available for transmission are limited and we have to manage them properly. • Just think about mobile networks. Even in Pakistan you have different providers. Are they operating on the same frequency (we are not talking about the frequency band we are discussing the spectrum allocation) • Things will get clearer in the coming lecture so don’t worry for the time being 

19. Electromagnetic Energy • To begin to understand how radio energy is used for human communication it is helpful to start with a simple rule of physics: when material vibrate they transfer energy to surroundings. • There can be different form of waves, For the time being we just say that energy is transferred in the form of waves. • If we take the example of sound wave, it is transmitted through the air by means of ‘compression’ wave like for example when we speak our mouth expels air from our lungs.

20. Electromagnetic energy (continued) • In the previous analogy air act as a medium that enables that Compressional waves to propagate from one place to another. • Not necessarily sound waves only travel through air. Just think about a real time scenario while you are speaking and other person is listening is there only air that is capable to pass on the sound wave to the receiver or there are other objects as well?

21. Frequency and wavelength • I expect that you all know about wavelength and frequency. • The length of the radio wave is a property that allows us to classify the radio spectrum according to the frequency. • During transmission or reception, the number of energy vibrations reaching an antenna each second is measured as the frequency of the signal. • One vibration per second is known by the term HZ, Hertz.

22. Units of Frequencies

23. Examples of Distributed Systems • The Internet : A Distributed Approach • An Intranet: which is a portion of the Internet managed by an organization • Mobile Computing • The main motivation of the Distributed system was to share the resources and we all know that resources are very limited and to use and manage them effectively and efficiently is always a challenge. • Now how to manage the resources is again a challenge, either you can manage with the help of a server or they may be working as objects accessed by other clients.

24. Challenge of the Distributed System • Yes that is a reality there are different challenges while designing a Distributed System, the major challenges when we consider the construction of the Distributed Systems are heterogeneity of its components openness. • Can you explain what is meant by the above point? • Openness deals with the scalability and security. If I say that the major consideration for the components to be added are scalability and security, Am I right or wrong?

25. Introduction • Networks of Computers are almost every where these days. • What can be a master network? Hint: A network that has connected many networks. • Examples of Networks are Mobile Networks, Industrial Networks, Campus Networks, Home Networks, In car networks etc. • All of the above mentioned networks are separate from each other? Yes or No

26. Introduction to Computing • All networks are different but they can be synchronized with each other. What makes it possible is the Internet. • Mobiles are different network from computer networks but if you install Skype on mobile phone and on your personal computer you can have a successful communication, How? By the Use of Internet. • We define a distributed system as one in which hardware or software components located at networked computers communicate and coordinate their actions only by passing messages.

27. Consideration • Computers that are connected by a network may be spatially separated by any distance. They may be on separate communication systems in the same building or the same room. • Our definition (used by the book) of distributed systems has the following significantly consequences; • Concurrency • No Global Clock • Independent Failures

28. Concurrency • In a network of computers, concurrent program execution is normal. I can do my work on my computer while you can do your work on yours. Sharing resources such as web pages or files when necessary. • The capacity of the system to handle shared resources can be increased by increasing more resources (for examples adding computers to a network or adding servers to a computer network) • Your concept will get clearer with time how this extra capacity can be introduced to a network.

29. No Global Clock • When program needs to cooperate they coordinate their actions by exchanging messages. • Close coordination often depends on a shared idea of the time at which the program action occurs. • But it turns out that there are limits to the accuracy with which the computers in a network can synchronize their clocks. • Limitations say when the windows leave any updates on their server individual user get updates at different times when they login their individual systems.

30. Independent Failures • All computer systems can fail and it is the responsibility of the system designers to plan for the consequences in case of failures. • That is why the most important element to consider while designing a network is to consider the back up plan in case of failure. • If it is a distributed system, fault in the Network can result in the isolation of the components that are connected to it. For example if there is a print server in a network and the print server is failed how can you get prints, Disaster isn’t it?

31. Independent failures (Continued) • It doesn’t mean that the computers stop running, they send their commands. • In fact the programs on them doesn’t detect whether the network has failed or has become unusually slow. • Similarly the failure of a computer or the unexpected termination of the program somewhere in the system is not immediately made known to the other components to which it communicates.

32. A typical Portion of the Internet

33. Aim of the Lecture • Further understanding to the lecture will help you in understanding and getting a clear view of the nature of the Distributed Systems. • The challenges that must need to be addressed to ensure that they are successfully designed or not. • You will get the information related to distributed systems, the components from which they are constructed and their purposes.

34. Examples of Distributed Systems • Our examples will be based on familiar and widely based networks. • The internet, intranets, and the emerging technology of networks based on mobile devices. • They are designed to exemplify the wide range of services and applications that are supported by computer networks and to begin the discussion of the technical issues that underlie their elements.

35. The Internet • The Internet is a vast interconnected collection of computer networks of many different types. • Programs running on the computers connected to it interact by passing messages, employing a common means of communication. • The design and construction of the internet communication mechanisms (the Internet Protocols) is a major technical achievement, enabling a program running anywhere to address messages to programs everywhere else.

36. Internet (Continued) • The internet is a very large distributed system. • You all know the significance and usage of internet in our daily life. • You make use of the services such as www. Email and file transfer etc. • Now these days you are even able to have video streaming etc. • Even it can be extended by use of server computers and new types of services, How life transmission, broadcasting etc.

37. Intranets • An intranet is a portion of the internet that is separately administered and has a boundary that can be configured to enforce local security policies. • An intranet is composed of several local area networks (LANs) linked by a backbone connections. • The network configuration of a particular intranet is the responsibility of the organization. • An intranet is connected to the internet via a device called? • Many organization wants to restrict or have their own protection enabled, what is the name of the device that does it?

38. Role of a Firewall • The role of the firewall is to protect an intranet by preventing unauthorized messages leaving or entering. • A firewall is implemented by filtering incoming and outgoing messages. • An example can be according to the source and destination. • Why you all want to have firewall at your computers? I am just asking you specifically towards the operating system side or towards the internet side.

39. A typical Intranet

40. Main issues • Main issues arising in the design of components for use in internet are; • File services are needed to enable user to share data • Firewalls limits the access of services- when resource sharing between internal and external user is required, firewalls must be complemented by the security mechanisms, will be discussed later in depth. • The cost of software installation and support is an important issue. • The cost can be reduced by effectively and efficiently designing the network architecture.

41. Mobile Computing • Mobile means mobility, OK • Now we do have wireless networks that enable us to have the functionality of mobility • We all know the limitations of wireless systems and wireless links. • The devices that are included in portable distributed systems are; • Laptop computers, smart phones, PDA’s, Video cameras, Digital Cameras, smart watches, washing machines (having embedded system processors controlled by remote control) etc.

42. How devices are connected together? • We all know that each and every technology is different. What makes them to operate on one line? • You all have studied data communication and networking, so you must know the basic concepts. • Batayenapsarepatahaikenahikiski basis per ye mumkinhua? Achakoini ma bata don? Achakoinibatata hon.