1 - Introduction to Networks.pdf
- 1. Computer Networks MIS Department Mutah University 2021/2022 1. Introduction
- 2. Defining a Network • A network is nothing more than two or more computers connected by a cable or by a wireless radio connection so that they can exchange information. • You can create a simple computer network by hooking together all the computers in your office with cables and using the computer’s network interface (an electronic circuit that resides inside your computer and has a special jack on the computer’s backside). • Then you tweak a few simple settings in the computer’s operating system (OS) software, and you have a working network!. • If you don’t want to mess with cables, you can create a wireless network instead. • In a wireless network, the computers use wireless network adapters that communicate via radio signals. • All modern laptop computers have built-in wireless network adapters, as do most desktop computers.
- 3. Defining a Network – cont. • Figure next slide shows a typical network with four computers. • You can see that all four computers are connected by a network cable to a central network device: the switch. • You can also see that Ward’s computer has a fancy laser printer attached to it. • Because of the network, June, Wally, and the Beaver can also use this laser printer.
- 5. Why Bother with a Network? - Sharing files • Networks enable you to share information with other computers on the network. • Depending on how you set up your network, you can share files with your network friends in several different ways. • You can send a file from your computer directly to a friend’s computer by attaching the file to an email message and then mailing it. • Or you can let your friend access your computer over the network so that your friend can retrieve the file directly from your hard drive. • Yet another method is to copy the file to a disk on another computer and then tell your friend where you put the file so that your friend can retrieve it later.
- 6. Why Bother with a Network? - Sharing resources • You can set up certain computer resources — such as hard drives or printers — so that all computers on the network can access them. • For example, the laser printer attached to Ward’s computer in the last Figure is a shared resource, which means that anyone on the network can use it. • Without the network, June, Wally, and the Beaver would have to buy their own laser printers. • Hard drives can be shared resources, too. In fact, you must set up a hard drive as a shared resource to share files with other users. • Suppose that Wally wants to share a file with the Beaver, and a shared hard drive has been set up on June’s computer. All Wally has to do is copy his file to the shared hard drive in June’s computer and tell Beaver where he put it. • Then, when Beaver gets around to it, he can copy the file from June’s computer to his own. • You can share other resources, too, such as an Internet connection. In fact, sharing an Internet connection is one of the main reasons why many networks are created.
- 7. Why Bother with a Network? - Sharing programs • Instead of keeping separate copies of programs on each person’s computer, put programs on a drive that everyone shares. • For example, if ten computer users all use a particular program, you can purchase and install ten copies of the program, one for each computer. • Or you can purchase a ten-user license for the program and then install just one copy of the program on a shared drive. • Each of the ten users can then access the program from the shared hard drive. • In most cases, however, running a shared copy of a program over the network is unacceptably slow. • A more common way of using a network to share programs is to copy the program’s installation disks or CDs to a shared network drive. • Then you can use that copy to install a separate copy of the program on each user’s local hard drive. • For example, Microsoft Office enables you to do this if you purchase a license from Microsoft for each computer on which you install Office.
- 8. Servers and Clients • The network computer that contains the hard drives, printers, and other resources that are shared with other network computers is a server. This term comes up repeatedly, so you have to remember it. • Any computer that’s not a server is a client. You have to remember this term, too. • Only two kinds of computers are on a network: servers and clients. • The distinction between servers and clients in a network has parallels in sociology — in effect, a sort of class distinction between the “haves” and “have-nots” of computer resources. • Usually, the most powerful and expensive computers in a network are the servers. • There’s a good technical reason: All users on the network share the server’s resources.
- 9. Servers and Clients – cont. • The cheaper and less-powerful computers in a network are the clients. • Clients are the computers used by individual users for everyday work. • Because clients’ resources don’t have to be shared, they don’t have to be as fancy. • Most networks have more clients than servers. • For example, a network with ten clients can probably get by with one server. • In many networks, a clean line of demarcation exists between servers and clients. In other words, a computer functions as either a server or a client, not both. • For the sake of an efficient network, a server can’t become a client, nor can a client become a server. • Other (usually smaller) networks can be more even-handed by allowing any computer in the network to be a server and allowing any computer to be both a server and a client at the same time.
- 10. Dedicated Servers and Peers • In some networks, a server computer is a server computer and nothing else. • It’s dedicated to the sole task of providing shared resources, such as hard drives and printers, to be accessed by the network client computers. • This type of server is a dedicated server because it can perform no other task than network services. • Some smaller networks take an alternative approach by enabling any computer on the network to function as both a client and a server. • Thus, any computer can share its printers and hard drives with other computers on the network. • And while a computer is working as a server, you can still use that same computer for other functions, such as word processing. • This type of network is a peer-to-peer network because all the computers are thought of as peers, or equals.
- 11. Dedicated Servers and Peers – cont. • Peer-to-peer networking features are built into Windows. • Thus, if your computer runs Windows, you don’t have to buy any additional software to turn your computer into a server. • All you have to do is enable the Windows server features. • The network server features that are built into desktop versions of Windows (such as Windows 7 and 8) aren’t particularly efficient because these versions of Windows weren’t designed primarily to be network servers. • If you dedicate a computer to the task of being a full-time server, use a special server operating system rather than the standard Windows desktop operating system. • A server operating system is specially designed to handle networking functions efficiently.
- 12. Dedicated Servers and Peers – cont. • The most commonly used server operating systems are the server versions of Windows. • As of this time, the current server version of Windows is Windows Server 2016. • However, many companies still use the previous version (Windows Server 2012), and a few even use its predecessor, Windows Server 2008. • Another popular server operating system is Linux. • Linux is popular because it’s free. However, it requires more expertise to set up than Windows Server. • Many networks are both peer-to-peer and dedicated-server networks at the same time. • These networks have: At least one server computer that runs a server operating system such as Windows Server 2016. Client computers that use the server features of Windows to share their resources with the network.
- 13. What Makes a Network Tick? • Network interface: Inside any computer attached to a network is a special electronic circuit called the network interface. • The network interface has either an external jack into which you can plug a network cable — or, in the case of a wireless network interface, an antenna. • Network cable: The network cable physically connects the computers. • It plugs into the network interface card (NIC) on the back of your computer. • The type of network cable most commonly used is twisted-pair cable, so named because it consists of several pairs of wires twisted together in a certain way. • Twisted-pair cable superficially resembles telephone cable. • However, appearances can be deceiving. Most phone systems are wired using a lower grade of cable that doesn’t work for networks.
- 14. What Makes a Network Tick? – cont. • Network switch: Networks built with twisted-pair cabling require one or more switches. • A switch is a box with a bunch of cable connectors. • Each computer on the network is connected by cable to the switch. The switch, in turn, connects all the computers to each other. • In the early days of twisted-pair networking, devices known as hubs were used rather than switches. • The term hub is sometimes used to refer to switches, but true hubs went out of style sometime around the turn of the century. • Network router: A router is used to connect two networks. • Typically, a router is used to connect your network to the Internet. • Figure next slide shows what the Cleaver family network would look like if they added a router to connect to the Internet. • As you can see, the router is connected to the switch and also to the Internet. As a result, any computer that’s connected to the switch can also reach the Internet via the router.
- 15. Connecting to the Internet via a router
- 16. What Makes a Network Tick? – cont. • Wireless networks: In a wireless network, most cables and switches are moot. Radio transmitters and receivers take the place of cables. • The main advantage of wireless networking is its flexibility: No cables to run through walls or ceilings, and client computers can be located anywhere within range of the network broadcast. • There are trade-offs, though. For example, wireless networks are inherently less secure than a cabled network because anyone within range can intercept the radio signals. • In addition, cabled networks are inherently faster and more stable than wireless networks. • Figure next slide shows how the Cleaver’s network might look if they used a single device that combines a wireless router, which also includes a built-in switch. • In this example, Ward’s printer and computer are connected by wires because they’re in the same room as the router. • June’s, Wally’s, and the Beave’s computers are connected wirelessly, so no cables are required.
- 17. Using a wireless router/switch combo.
- 18. What Makes a Network Tick? – cont. • Network software: Of course, the software makes the network work. • To make any network work, a whole bunch of software has to be set up just right. • For peer-to-peer networking with Windows, you have to play with the Control Panel to get networking to work. • And a server operating system such as Windows Server 2016 requires a substantial amount of tweaking to get it to work just right.
- 19. Networks Big and Small • Networks come in all sizes and shapes. In fact, networks are commonly based on the geographical size they cover, as described in the following list: • Local area networks (LANs): In this type of network, computers are relatively close together, such as within the same office or building. • Don’t let the descriptor “local” fool you. A LAN doesn’t imply that a network is small. • A LAN can contain hundreds or even thousands of computers. • What makes a network a LAN is that all its connected computers are located within close proximity. • Usually a LAN is contained within a single building, but a LAN can extend to several buildings on a campus, provided that the buildings are close to each other (typically within 300 feet of each other, although greater distances are possible with special equipment).
- 20. Networks Big and Small – cont. • Wide area networks (WANs): These networks span a large geographic territory, such as an entire city or a region or even a country. • WANs are typically used to connect two or more LANs that are relatively far apart. • For example, a WAN may connect an office in San Francisco with an office in New York. • The geographic distance, not the number of computers involved, makes a network a WAN. • If an office in San Francisco and an office in New York each has only one computer, the WAN will have a grand sum of two computers — but will span more than 3,000 miles. • Metropolitan area networks (MANs): This kind of network is smaller than a typical WAN but larger than a LAN. • Typically, a MAN connects two or more LANs within the same city that are far enough apart that the networks can’t be connected via a simple cable or wireless connection.
- 21. It’s Not a Personal Computer Anymore! • After you hook up your personal computer (PC) to a network, it’s not a “personal” computer anymore. • You’re now part of a network of computers, and in a way, you’ve given up one of the key concepts that made PCs so successful in the first place: independence. • Here are several ways in which a network robs you of your independence: • You can’t just indiscriminately delete files from the network. They may not be yours. • You’re forced to be concerned about network security. • For example, a server computer has to know who you are before it allows you to access its files. • So you have to know your user ID and password to access the network. • This precaution prevents some 15-year-old kid from hacking his way into your office network by using its Internet connection and stealing all your computer games.
- 22. It’s Not a Personal Computer Anymore! – cont. • You may have to wait for shared resources. Just because Wally sends something to Ward’s printer doesn’t mean that it immediately starts to print. • The Beav may have sent a two-hour print job before that. Wally just has to wait. • You may have to wait for access to documents. You may try to retrieve an Excel spreadsheet file from a network drive, only to discover that someone else is using it. Like Wally, you just have to wait. • You don’t have unlimited storage space. If you copy a 100GB video file to a server’s drive, you may get calls later from angry co-workers complaining that no room is left on the server’s drive for their important files.
- 23. It’s Not a Personal Computer Anymore! – cont. • Your files can become infected from viruses given to you by someone over the network. You may then accidentally infect other network users. • You have to be careful about saving sensitive files on the server. If you write an angry note about your boss and save it on the server’s hard drive, your boss may find the memo and read it. • The server computer must be up and running at all times. For example, if you turn Ward’s computer into a server computer, Ward can’t turn his computer off when he’s out of the office. If he does, you can’t access the files stored on his computer. • If your computer is a server, you can’t just turn it off when you’re finished using it. Someone else may be accessing a file on your hard drive or printing on your printer.
- 24. Understanding Network Topology • The term network topology refers to the shape of how the computers and other network components are connected to each other. • There are several different types of network topologies, each with advantages and disadvantages. • In the following discussion of network topologies, we use two important terms: Node: A node is a device that’s connected to the network. For your purposes here, a node is the same as a computer. Network topology deals with how the nodes of a network are connected to each other. Packet: A packet is a message that’s sent over the network from one node to another node. The packet includes the address of the node that sent the packet, the address of the node the packet is being sent to, and data.
- 25. Bus topology • The first type of network topology is called a bus, in which nodes are strung together in a line, as shown in the figure next slide. • The key to understanding how a bus topology works is to think of the entire network as a single cable, with each node “tapping” into the cable so it can listen in on the packets being sent over that cable. • In a bus topology, every node on the network can see every packet that’s sent on the cable. • Each node looks at each packet to determine whether the packet is intended for it. If so, the node claims the packet. If not, the node ignores the packet. • This way, each computer can respond to data sent to it and ignore data sent to other computers on the network.
- 26. Bus topology – cont.
- 27. Bus topology – cont. • If the cable in a bus network breaks, the entire network is effectively disabled. • Obviously, the nodes on opposite sides of the break can continue to communicate with each other, because data can’t span the gap created by the break. • But even those nodes that are on the same side of the break may not be able to communicate with each other, because the open end of the cable left by the break disrupts the proper transmission of electrical signals. • In the early days of Ethernet networking, bus topology was commonplace. • Although bus topology has given way to star topology (see the next section) for most networks today, many networks today still have elements that rely on bus topology.
- 28. Star topology • In a star topology, each network node is connected to a central device called a hub or a switch, as shown in figure next slide. Star topologies are commonly used with LANs. • If a cable in a star network breaks, only the node connected to that cable is isolated from the network. • The other nodes can continue to operate without interruption — unless, of course, the node that’s isolated because of the break happens to be the file server. • You should be aware of the somewhat technical distinction between a hub and a switch. • Simply put, a hub doesn’t know anything about the computers that are connected to each of its ports. • So when a computer connected to the hub sends a packet to a computer that’s connected to another port, the hub sends a duplicate copy of the packet to all its ports. • In contrast, a switch knows which computer is connected to each of its ports. • As a result, when a switch receives a packet intended for a particular computer, it sends the packet only to the port that the recipient is connected to.
- 29. Star topology – cont.
- 30. Star topology – cont. • Strictly speaking, only networks that use switches have a true star topology. • If the network uses a hub, the network topology has the physical appearance of a star, but it’s actually a bus. • That’s because when a hub is used, each computer on the network sees all the packets sent over the network, just like in a bus topology. • In a true star topology, as when a switch is used, each computer sees only those packets that were sent specifically to it, as well as packets that were specifically sent to all computers on the network (those types of packets are called broadcast packets).
- 31. Ring topology • A third type of network topology is called a ring (see figure). • In a ring topology, packets are sent around the circle from computer to computer. • Each computer looks at each packet to decide whether the packet was intended for it. If not, the packet is passed on to the next computer in the ring.
- 32. Mesh topology • A fourth type of network topology, known as mesh, has multiple connections between each of the nodes on the network, as shown in figure next slide. • The advantage of a mesh topology is that if one cable breaks, the network can use an alternative route to deliver its packets. • Mesh networks aren’t very practical in a LAN setting. • For example, to network eight computers in a mesh topology, each computer would have to have seven network interface cards, and 28 cables would be required to connect each computer to the seven other computers in the network. • Obviously, this scheme isn’t very scalable. • However, mesh networks are common for metropolitan or wide area networks. • These networks use routers to route packets from network to network. • For reliability and performance reasons, routers are usually arranged in a way that provides multiple paths between any two nodes on the network in a mesh-like arrangement.
- 33. Mesh topology