Growth of Telecommunications Systems
Books Wright, Michael and Mukul Patel. 2000. Scientific American – How Things Work Today.
London: Marshall Publishing Ltd. B. 1 The world is currently in the middle of a communications revolution as dramatic as the Industrial Revolution that created our modern society two centuries ago. The growth of telecommunications systems, the arrival of the Internet, and the proliferation of computers in every aspect of our lives are transforming both industrial and knowledge-based economies round the world. Higher disposable incomes and increased leisure time are also fuelling demand for luxury electronic goods and new forms of entertainment.
Since the British inventor Alexander Graham Bell patented the telephone in 1877, telephony has become the most important form of distance communication. The telephone network is now truly global, with submarine cables and communications satellites linking every continent. This globe-spanning network handles phone calls, fax transmissions, and internet traffic. The long-distance “backbones” of the telephone network are high capacity optical-fiber cables. Lower-capacity copper cables connect individual phones to the network. The analog signal from an ordinary phone is sampled 4,000 times per second and converted into an 8-bit ddigital signal.
Several conversations can then be transmitted simultaneously down the same cable, using a technique known as multiplexing, which increases the capacity of the network. Routing calls through the telephone network is done automatically be telephone exchanges. In near future, the telephone network may also carry video and music channels, interactive televisions, videophone calls. Analog signal – Analog electric signals from ordinary phones are carried by copper cable to the local exchange. Base station – The base station receiving the sthrongest signal from a cell phone routes the call to the cell phone exchange.
Cell – The cell phone network is divided into hexagonal cells, each with a base station in the middle. Cell phone exchange – Calls are routed to the main exchange or direct to a base station by the cell phone exchange. Cell phone network – Base stations send ddigital information to the cell phone exchange over optical-fiber or copper cable. Communication satellite – Orbiting satellites are used to route calls between places not linked by a cable. Ddigital signal – Ddigital information is multiplexed, allowing multiple signals to be transmitted simultaneously. Fax machine – Fax transmissions are sent over the telephone network.
Line-of-sight microwave link – Digitized call from local exchanges are often routed to the main exchange via terrestrial microwave links. Local exchange – The local exchange digitizes calls for long-distance transmission. Long-distance or international connection – Optical fibers are used to transmit long-distance calls, many optical fiber cables are laid on the seabed. Main exchange – The main exchange handles communications between ordinary phones and the cell phone network and routes long-distance and international calls. Microwaves – Ddigital information is sent from cell phones to base stations using microwave frequencies.
Moving cell phone – Mobility is the prime asset of the cell phone. Optical fibers – Transmitting ddigital information by light pulses enables many calls to be sent down one fiber simultaneously. Satellite uplink – Encrypted ddigital information is sent to satellites using microwave frequencies. Seamless reconnection – As the cell phone moves from one cell to another, the call is rerouted from one base station to the next, without the break in the conversation. Urban cell – Cell are smaller in urban areas, giving the network greater capacity. Weakening Signal – As the cell phone moves farther away from the base station, the signal weakens. —- Tanenbaum, Andrew S. and Maareten van Steen. 2002. Distributed Systems Principles and Paradigms. New Jersey: Prentice-Hall, Inc. B. 2 A process is often defined as a program in execution, that is, a program that is currently being executed on one of the operating system’s virtual pprocessors. An important issue is that the operating system takes great care to ensure that independent processes cannot maliciously or inadvertently affect the correctness of each other’s behavior. In other words, the fact that multiple processes may be concurrently sharing the same CPU and other hardware resources is made transparen’t.
Usually, the operating system requires hardware support to enforce this separation. B. 2 Threads in Distributed Systems – An important property of threads is that they can provide a convenient mearns of allowing blocking system calls without blocking the entire process in which the thread is running. This property makes threads particularly attractive to use in distributed systems as it makes it much easier to express communication in the form of maintaining multiple logical connections at the same time. We illustrate this point by taking a closer look at multithreaded clients and servers, respectively. B. 2
Clients and Servers – In the basic client-server model, processes in a distributed system are divided into two (possibly overlapping) groups. A server is a process implementing a specific service, for example, a file system service or a database service. A client is a process that requests a service from a server by sending it a request and subsequently waiting for the server’s reply. This client-server interaction, also known as request-reply behavior. B. 2Communication between a client and a server can be implemented by mearns of a simple connectionless protocol when the underlying network is fairly reliable as in many local-area networks.
In these cases, when a client requests a service, it simply packages a message for the server, identifying the service it wants, along with the necessary input data. The message in then sent to the server. The latter, in turn, will always wait for an incoming request, subsequently process it, and package the results in a reply message that is then sent to the client. Page 657 Communication – All communication in the Web between clients and servers is based on the Hypertext transfer Protocol (HTTP). HTTP is a relatively simple client-server protocol; a client sends a request message to a server and waits for a response message.
An important property of HTTP is that it is stateless. In other words, it does not have any concept of open connection and does not require a server to maintain information on its clients. The most recent version of HTTP is described in (Fielding et al. , 1999). Page 648 The World Wide Web (WWW) can be viewed as a huge distributed system consisting of millions of clients and servers for accessing linked documents. Servers maintain collections of documents, while clients provide users an easy-to-use interface for presenting and accessing those documents.
The Web started as a project at CERN, the European Particle Physics Laboratory in Geneva, to let its large and geographically dispersed group of researchers provide access to shared documents using a simple hypertext system. A document could be anything that could be displayed on a user’s computer terminal, such as personal notes, reports, figures, blueprints, drawings, and so on. By linking documents to each other, it became easy to iintegrate documents from different projects into a new document without the necessity for centralized changes.
The only thing needed was to construct a document providing links to other relevant documents (see also Berners-Lee et al. , 1994). Since 1994, Web developments are primarily initiated and controlled by the World Wide Web Consortium, a collaboration between CERN and M. I. T. this consortium is responsible for standardizing protocols, improving interoperability, and further enhancing the capabilities of the Web. Its home page can be found at http://www. w3. org/. —– London, Sherry. 2000. Illustrator 9 f/x & Design. Arizona: The Coriolis Group, LLC. Page 427 Image resolution seems to be a tricky and complex topic the most people.
I usually urge folks to work in pixels inside the Photoshop because pixels are fixed: A 900-pixel-wide image contains 900 pixels, regardless of its ppl. However, if the ppl is set to 300, the 900-pixel image prints at 3 inches wide; if the ppl is set to 100, the 900-pixel image prints 9 inches wide – but it still contains only 900 pixel across. Therefore, I find it much easier to think in pixels. —– Harb, M. 1989. Modern Telephony. New Jersey: Prentice-Hall, Inc. Page 9 The Telephone of Today The end user or subscriber is the main concern in any communication link. The subscriber can be either a private part or a business.
In all cases, the telephone set is used to transmit and receive, sequentially or simultaneously. The Telephone Set The telephone set is an instrument used to transmit and receive information or calls sequentially or simultaneously, thus permitting one party to communicate with another. For this communicate to take place, the telephone set must have the following component: a transmitter, a receiver, bell, a dialer, and a switchhook. Page 150-152 Cellular mobile communication is considered to be the breakthrough that could revolutionize the way businesses think about and use the most common of communication tools – the telephone.
The cellular mobile telephone is found most often in cars, using low-power radio-frequency carriers to transmit and receive messages in small geographical units called cells. Before the introduction of cellular technology, most North American cities were using high-power radio-frequency transmitters for mobile telephone transmission. Even if a subscriber was lucky enough to have the service, the number of channels was limited, and consequently the possibility of having access to a free line was very slim. Furthermore, once the subscriber got access to a line, the voice quality was poor and privacy almost nonexistent.
This is not the case with the cellular mobile system. With the new technology, subscribers are guaranteed access to a free line 99. 9% of the time. The quality of the voice is comparable to that of regular telephone transmission, and privacy is guaranteed. Cellular mobiles became available in the United States in 1979, when Ameritech Mobile Communications began serving 2000 subscribers in the city of Chicago. By 1985, over 30,000 subscribers in the United Stated were using cellular mobiles, and this number continues to grow in the United States as well as in Canada, the European nations, Japan, and many more.
Operation of a Cellular Mobile System Since is not feasible to set up a communication link between two moving cars using a standard telephone line, it is necessary to use a wireless system with a radio antenna to send and receive telephone conversation over the air. The term “cellular” is derived from the word “cell. ” Cities or towns are divided into geographic areas called cells, each with its own transmitter and receiver. Each cell has a low-power transmission capability, thus making its signal too weak to interfere with other cells with ssimilar frequencies.
Generally, the frequency occupied by two subscribers within the same cell cannot be used by any other subscribers within that cell. The diagram of a cellular system shown in Figure 12. 1 demonstrates the process of relaying messages from car telephone (or portable telephone) to the cell site’s low-powered transmitter, to the Cantel switching office or MTSO (mobile telephone switching office), and to the wireline telephone company’s switching office, to allow interconnection with the existing telephone system. The message is “handed off” from one cell site transmitter to the next as the caller crosses a cell boundary, without interruption.
A master computer keeps track of which cells are using which frequencies (see Figure 12. 1), and when a subscriber wishes to call a subscriber outside his or her cell, the computer may reassign frequencies. As a subscriber moves from one cell to the next, the assigned frequency may change, but the subscriber will at all times be served without interference (Figure 12. 2). —– Meyers, Mike. 2003. Introduction to PC Hardware and Troubleshooting. Asia: McGraw-Hill / Osborne. Page 390 Much of what makes modern computers so powerful is their ability to connection in small or large groups, or networks, and share files and resources.
Every PC tech worth knowing the basics of networking. After all, why get into computing if you can’t while away an afternoon gaming with your buddies, all from the comfort of your computer chair? How Networks Work A network enables two or more computers to share data, hardware such as printers, and even applications. The computers must have some conneactivity, of course: some way for the signal from one machine to reach the other. In addition, the hardware must be compatible and the software set up so that the receiving computer can understand what the sending machine sends.
Think of two kids chatting over walkie-talkie as a network for communication to occur, the hardware has to be in range for the signal, and it has to be from the same set, and the kids need to speak the same language. Networks work ssimilarly. Computers connect in two basic ways: In dial-up networks in Local Area Networks. In dial-up connection, your computer uses a telephone line to connect to an Internet Service Provider (ISP), which then gives you access to other computers, perhaps at your office or somewhere on the Internet.
Computers in a Local Area Network, on the other hand, are connected to a central box – either by cables or by radio waves – through which they can communicate with each other and, if some machine on the network has the appreciate connection, with other computers on the Internet. Let’s look at how both types of network function before we turn to the nuts and bolts of installing and setting up networks. Dial-Up Networks The most common network connection consists of three pieces: a modem, a working telephone line, and an ISP.
The modem enables the computer to communicate via phone lines. The phone line provides the link between the modem and the computers at the ISP. The ISP computers connect to the Big Kahuna of all networks, the Internet. Property installed you to surf, shop, and otherwise explore websites hosted by computers all over the world. Tune in and turn on to dial-up networking. The venerable modem has to recent years been challenged by two new consumer technologies for accessing the Internet so-called “cable-modems” and DSL (Ddigital Subscriber Line) service.
Your computer’s basic need for some kind of pipeline to communicate over hasn’t charged – just the choice of pipeline and the way it’s used. Both technologies take advantage of unused capacity on widely available transmission media. Cable modems use the cables already in place in many homes for receiving cable TV signals as their pipeline to the Internet, rather than the telephone system. The cable TV companies take advantage of the fact that their cable TV signals occupy only a fraction of the capacity of the coaxial cables running into your home.
Adding a cable modem to your computer enables it to use the cable TV connection as its pipeline to the Internet. Cable modems aren’t actually “modems” at all in the sense that the signals they send and receive are entirely ddigital, but because they perform the same function, they go by the same name. you can install an external cable modem to a port (usually USB) on your computer. Page 394 Modem Technology The modem solves one of the problems with the use of analog voice lines to move ddigital data. Modems take incoming analog serial data – in this case, the signal coming over the telephone line- and turn it into ddigital serial data.
Likewise, modems turn the signal flowing out of the PC into analog data than can be transferred over the telephone line. This process – called modulation / demodulation – provides the name for the technology: MOdulation / DEModulation, get it! Phone lines have a speed based on a unit called a baud, which is one cycle per second. The fastest rate that a phone line can achieve is 2,400 baud. Modem can pack multiple bits of data into each baud; a 33. 6 kilobits per second (Kbps) modem, for example, packs 14 bits into every baud: 2,400 x 14 = 33. 6 Kbps.
It is technically incorrect to say, “I have a 56 K baud modem. ” You should instead say, “’I have a 56 Kbps modem. ” However, people use the term baud instead of bps so other that the terms have become functionally synonymous. Page 415 A network enables two or more computers to share data, hardware such as printers, and even applications. Computers connect in two basic ways: dial-up networks and Local Area Networks. A standard dial-up network connection consists of these pieces: a modem, a working telephone line, and an Internet Service Provider (ISP).
To make a LAN work, you need to ensure those things: conneactivity, compatibility, and proper setup of hardware and software. Data is broken up and sent between computers in small chunks called packets and then reassembled. Page 1 Everything in your computer fits into one of two categories: hardware or software. Anything on your computer that you can touch is hardware. However, hardware alone cannot handle all of the PC’s complex activities – it needs the help of software. Software is the technical word for computer programs, the sets of instructions that tell the hardware how to do things.
Computer programs are often compared to cooking rrecipes. The recipe tells you how to use the tools to manipulate the ingredients, and if all goes well, you produce something edible. In the same way, software instruct the hardware has it manipulates data to produce the desired results, whether that’s a memo, a ddigital picture, or an email message. Figure 1. 1 shows a sample of program code. —– Stamper, David A. 2001. Local Area Networks – Third Edition. New Jersey: Prentice-Hall, Inc. Page 135-137 Classes of Software The reason for having a computing system is to solve problems and accomplish the business’ work.
The software that does this is called application software. Ordinarily, application software operates in an environment that makes writing and using the application software easier. The environment-creating software can be separated into network management, development and network access. Application Software As we mentioned at the beginning of this section, the main reason we use computing system is to solve business or scientific problems. Thus, the computer and its extension, the network, are simply problem-solving tools.
Tthroughout history, humankind has constantly built new tools and improved on those already invented. At the beginning of the computer era, computers were quite primitive tools (at least by today’s standards). Programmers at the dawn of the computing age needed to know not only the nuances of the business problem they were solving but also many of the intricacies of the hardware their solution would be running on. In today’s application environment, we have a supporting cast of software that helps to create an application environment that is mostly hardware independent.
Operating System Software Today, we are so need to using OSs to create the system environment that we take them for granted. However, we were into the second generation of computers before OSs appeared and into the third generation of computers before OSs became common. You may correctly infer from this that OSs are not absolutely necessary and, in the early years of personal computing, a vvariety of applications run without using the services of an OS. The IBM Personal Computer (PC) came equipped with a BASIC interpreter in a read-only memory.
Thus, if you did not have the disk operating system (DOS), you could still use your computer by writing and running BASIC programs. Without DOS, BASIC was the operating environment. Furthermore, a few early programs functioned in a stand-alone mode. These programs where on a diskette and were loaded when the computer was loaded. In this mode, if you wanted to run another program, you swapped disk and rebooted. Without an OS, the application program is responsible for accomplishing many hardware-oriented functions such as input/output (I/O) and memory management.
Because these task are common to all applications, software engineers developed OS software. The OS manages the resources of the computer and creates an application environment in which it is easier to develop and use application software because the OS takes care of a vvariety of functions formerly done by applications. Some of these functions are: • Memory management • File management • User interface • I/O interfaces • Resource allocation • Accounting • Protection/security Today, systems running on a LAN are more sophisticated that the early systems that ran without an OS; a LAN node without an OS is unthinkable.
Memory Management – When the computer is started, the OS is loaded into memory a certain portion of the available memory is constantly occupied by the resident portion of the OS. The OS manages the remaining memory and allocates it among itself and the requesting processes according to a memory management scheme adopted by the OS designers. Most current OSs use a memory management algorithm called virtual memory. With virtual memory, the disk is used as an extension of real memory. A process may be thought of as consisting of pages of data and code.
File Management – A disk is a raw storage device. It has the ability to store bits of data but inherently does not have the ability to organize those bits into files, files into directories, and so on. The OS provides this level of disk organization. it establish the data structure that allows users to create partitions, directories volumes, and other disk subdivisions. Some file management systems allow multiple disks to be combines into one logical disk. The file management system allows users to create, delete, and access files.
The file management system maintains the directory structure and store directory and file information such as the date and time last modified, end-of-file pointer, and the file or directories’ locations on disk. Obviously, there are fundamental aspects of using a system. User Interface – When a programmer begins to write a program, an environment is created for that user. If you are a programmer, you may need to use a text editor, compiler, link editor, and application program interface (API), which allows you to use OS procedures to carry out activities such as creating a new file, or starting a new process.
Sometimes a compiler or interpreter includes the API interfaces. Protection Security – Today, nearly everyone is aware that security is an important aspect of computer usage. The OS provides the base on which security is established. We expect the OS to prevent user programs from crashing the system and to eliminate the intrusion of one program into the memory occupied by another program’s data. In shared systems, we also expect the OS and the file management system to provide certain levels of file security. Commonly, an OS will at least provide capabilities that allow administrators to define which users can read, write, or erase a file.