1 Networks & Data Representation Overview l Computer networks »Network connections »LANs »WANs l The internet. »Identifying computers on the internet »The World Wide Web (WWW) »Introduction to HTML l Data Representation »Analog and Binary signals »Advantage Binary over Analog signal »Bits and Bytes

2 Computer Networks l A network is two or more computers connected together so that information and resources can be shared. l Most computers are connected to some kind of network. l Each computer has its own network address, which uniquely identifies it among the others. l A file server is a network computer dedicated to storing programs and data that are shared among network users. l A file server often has a large amount of secondary memory. File Server Client 1 Client N Client 2 Client 3 Client N-1

3 LANs and WANs l A local-area network (LAN) is designed to cover small distances and a small number of computers. l A LAN often connects the machines in a single room or building. l A wide-are network (WAN) connects two or more LANs, often over long distances. l Individual LANs are usually owned by a single organization, but WANs often connect LANs from many different groups in many different countries. Long-distance connection (Wireline or Wireless) LAN 2 LAN 1

4 Internet l The Internet is a WAN which spans the entire planet. l The word Internet comes from the term internetworking, which implies a network of networks. l It started as a United States government project, sponsored by the Advanced Research Projects Agency (ARPA), and was originally called the ARPANET. l The Internet grew quickly throughout the 1980s and 90s. l Less than 600 computers were connected to the Internet in 1983; now there are over 10 million. l The software which manages Internet communication is called TCP/IP. l The programs in the Internet Protocol (IP) formats the information for transfer. l The programs in the Transmission Control Protocol (TCP) reassembles messages and handles lost information. l Each computer on the Internet has a unique IP address, such as:

5 Internet l Most computers also have a unique Internet name, which is also referred to as an Internet address: ccse.kfupm.edu.sa aramco.com.sa l The first part indicates a particular computer (ccse) l The rest is the domain name, indicating the organization (kfupm.edu.sa) l The last section of each domain name usually indicates the type of organization (Things will change with Internet 2!!!) : l edu: Educational institution. l com: Commercial business. l org: Non-profit organization. l Sometimes the suffix indicates the country: l New suffix categories are being considered. l A domain name can have several parts. l Unique domain names mean that multiple sites can have individual computers with the same local name. l When used, an Internet address is translated to an IP address by software called the Domain Name System (DNS).There is not a one-to-one correspondence

6 WWW l The World-Wide Web (WWW)allows many different types of information to be accessed using a common interface. l A browser is a program which accesses and presents information: text, graphics, sound, audio, and programs. l A Web document usually contains links to other Web documents, creating a hypermedia environment. l The term Web comes from the fact that information is not organized in a linear fashion. l Web documents are defined by the HyperText Markup Language (HTML). l Information on the Web is found using a Uniform Resource Locator (URL): l A URL may indicate an HTML document, or some other kind of information.

7 Analog & Binary Signals l Everyone knows that computers are "digital" and that they use "binary" numbers. We need to be clear about their precise meaning and to be clear about why computers are digital. l Binary means "two states." The two states are sometimes called "1" and "0", "true" and "false", or called "on" and "off“. l A bit is a single "on"/"off" value. l A good example is a toggle switch, such as a light switch. You can turn it "on" or "off" but not anything else. A light switch holds one bit of information. l There are many advantages to binary. Here are some (somewhat overlapping) important reasons for using binary: 1.Simple; easy to build. 2.Unambiguous signals (hence noise immunity). 3.Flawless copies can be made. 4.Anything that can be represented with some sort of pattern can be represented with patterns of bits.

8 Analog Signal l An analog signal is (usually) continuously changing in value, its values can be anything within a range of values, and its exact value at any time is important. l If the signal shown below is an audio signal, the exact value at each time is part of the information it contains. For example, the value at time "T" must be measured exactly. l It is obvious that any distortion on the signals could lead to corruption of the information it is carrying.

9 Digital Signal l It is easy and fast (for electronics, as well as humans) to tell if something is above a threshold or below the threshold. l Here is the voltage signal we might send down our wire to transmit the data "off" followed by "on". (The wire is examined at times t1 and t2.) l The above signal is clear enough. However, the wire is long, and someone nearby turns on a vacuum cleaner and the signal get distorted as shown in the next slide.

10 Digital Signal –Contd. l Even though the signal is noisy (at the analog level), the binary values are transmitted perfectly. You (and the electronics) can still tell that at time "t1" the signal represents "off" and that at time "t2" the signal represents "on" The receiving end just needs to get the binary values. l The receiving end of the signal is only interested in the binary values. All it has to do it check if the signal is above or below the threshold. This can be done perfectly (as long as the noise is not too great.). Thus, binary signals are said to be flawless.

11 Clock. l Digital systems are built so that the "on" "off" (binary) value is only tested at certain times, giving the wire (or transistor, or...) a chance to change its state between these times. This is why computer systems have a "clock"---to keep all these times synchronized. l So faster clocks mean wires can be tested more times per second, and the whole system runs faster. l Processor chips (and the computers that contain them) are often described in terms of their clock speed l Clock speed is measured in Hertz, where one Hertz is one clock tick per second. The symbol MHz means mega Hertz, a million clock ticks per second. l A 700 MHz Pentium processor checks binary values 700 million times in each second. In between these times values are allowed to change and settle down. l The faster a processor chip is, the more times per second values can be tested, and the more decisions per second can be made.

12 Bits & Bytes l A bit is a single "on"/"off" value. Only these two values are possible. l One bit of information is so little that usually computer memory is organized into groups of eight bits. Each eight bit group is called a byte. l When more than eight bits are required for some data, a whole number of bytes are used. One byte is about enough memory to hold a single character. l Often very much more than eight bits are required for data, and thousands, millions, or even billions of bytes are needed. These amounts have names, as seen in the table: l Any system of symbols can be translated into bit patterns. An example is how English characters have been translated into eight-bit patterns called ASCII. (You will learn more about this later.) NameNumber of BytesPower 2 Byte12020 kilobyte megabyte1,048, gigabyte1,073,741,