Chapter 8: Computer Networking. AIMS The aim of this chapter is to give a brief introduction to computer networking.

Transcription

1 Chapter 8: Computer Networking AIMS The aim of this chapter is to give a brief introduction to computer networking. OBJECTIVES At the end of this chapter you should be able to: Explain the following terms: computer network, LAN, WAN, MAN, internet, protocol, topology, media, peer-to-peer network, server based network Describe the roles a computer can play in a computer network Give an overview of the main types of media used in local area networks Describe how the different devices used to communicate through a network work and in what circumstances they are used. 1. Computer Networks Introduction 1.1 What is a Computer Network? A computer network is an interconnected collection of autonomous computers that communicate with one another using sets of rules known as protocols. Interconnected means that the computers are capable of exchanging information via wired or wireless connections. Autonomous means that the computers must function independently, i.e. there is no master/slave relationship where one computer controls another. Computer networking enables computers to share data, application software and hardware devices. The most basic network consists of two computers connected directly by cable, for example sharing resources, such as printers and modems. Any computer capable of communicating on the network is known as a device or node. 1.2 Categories of Networks A Local Area Network (LAN) is a privately owned group of computers and devices connected, for example, within a single building or campus. A Metropolitan Area Network (MAN) is a larger version of a LAN, covering for example a group of nearby offices or a city. A Wide Area Network (WAN) is a network that covers a large geographical area such as a country or continent. 1.3 What is an internet? A collection of interconnected networks is called an internetwork or internet. For example, a number of LANs can be connected by a WAN. The WAN provides the connecting links between the LANs, for example, via leased telephone lines, satellite or microwave links. COMP1208-Notes08-CompNetworks_PA1.doc Page 1-1

2 2. Network Roles Network architecture refers to the way in which computers participate in a network. There are three roles for computers in a local area network. The first role is a client which uses but does not provide network resources. The second one is a peer which both uses and provides network resources. The last one is a server which provides network resources. The type of architecture chosen by an organisation depends on issues such as cost, size of network required, geographical location, requirements of specific application packages, network support available and what existing systems there are within the organisation. Networks can be divided into three types: Peer-to-Peer Has no dedicated servers or hierarchy among computers. Uses the network to share resources among independent peers. Organized into workgroups, typically of fewer than 10 peers. Client-Server (also called server-based) Has servers on the network that provide security and administration. Clients request services and servers deliver them. Hybrid networks This is a server-based network that also has peers sharing resources. Most networks are actually hybrid networks. 2.1 Client Server Networks Client-Server networks are defined by the presence of servers on a network that provide the security and administration of the network. It consists of many clients and one or more servers. Clients request services, such as file storage and printing, and servers deliver them. In Windows NT or Windows 2000, server-based networks are organised into domains. Domains are collections of networks and clients that share security trust information. Domain security and logon permission are controlled by special servers called domain controllers. There is one master domain controller, called the Primary Domain Controller (PDC) and the secondary domain controllers called Backup Domain Controller (BDC) may assist PDC during busy times or when the PDC is not available for some reason. The Client-Server network have many advantages, including: Central file storage, which allows all users to work form the same set of data and provides easy backup of critical data and keeps data from getting lost among computers Ability of servers to pool available hardware and software, lowering overall costs Optimised dedicated servers, which are faster than peers at sharing network Freeing of users from the task of managing network Easy manageability of a large number of users Ability to share expensive equipment, such as laser printers Less intrusive strong central security, since a single password need to access all shared resources on the network Client-Server networks also have some disadvantages, including: Expensive dedicated hardware Expensive network operating system software and client licenses A dedicated network administrator (usually required) COMP1208-Notes08-CompNetworks_PA1.doc Page 2-2

3 2.2 Peer-to-Peer Networks Every computer on peer network is equal and can communicate with any other computer on the network to which it has been granted access rights. So basically, every computer on peer network functions as both a server and a client. The peer network is more common in small business. Peer Network advantages are: No extra investment for server software and hardware are needed No network administrator is needed Easy setup Lower cost Peer Network disadvantages are: Additional load on computers for resource sharing Lack of central organisation, which can make data hard to find Users must administer their own computers Weak and intrusive security 2.3 Server Types There are several tasks for a server in a network. All these tasks could be done by one server, or a separate server for each task. A server is dedicated to performing specific tasks in support of other computers on the network. File Server File servers offer the services, which are the network applications that store, retrieve, and move data. With a file server, users can exchange, read, write, and manage shared files and the data contained in them. There are three ways to store a file on networks. They are online, offline, and near-line storage. Online storage consists of hard drive storage. Hard drive is very fast but expensive so the most current and frequently needed information is stored. The common offline storage devices are data tape and removable optical disks. It is the most disadvantage of offline storage that a person must retrieve the disk or tape and mount it on the server. This type of storage is best for data that is rarely used and for data backup. Nearline storage uses a machine, such as a tape carousel or jukebox, automatically retrieve and mounts the tape or disk. It is faster than offline but still only enough for infrequently used data and applications. File synchronization is to ensure that changes made to a file are organized in the chronological order in which they actually took place and that files are properly updated. But a network operating system cannot synchronize data within files if it is not aware of the file format. So, you need an option for it usually Print Server Print server manages and control printing on a network and also offers fax service. The print server allow multiple and simultaneous access to print and fax services. The network operating system achieves this by using print and fax queues. The queues are special storage areas where printing and faxing jobs are stored and then sent to the printer or fax device in an organized fashion. Application Server Application server allows a client on network to access and use extra computing power and expensive software applications on a shared computer. Application servers are used when efficiency and security requires a program to stay close to the data, and the data stays in one place. Database Server Database server is one of application server. Database server allows a network with powerful database capabilities. So, users of a relatively weak client can enjoy the same power of database servers. COMP1208-Notes08-CompNetworks_PA1.doc Page 3-3

4 3. Network Topology Network topology means the way in which computers or devices (called nodes) are connected. Common topologies include a bus, star, and ring. It is important to use the right topology. Each topology has its own strengths and weakness. Physical topology, which is the actual layout of the wire or media. Logical topology, which defines how the media is accessed by the hosts for sending data. Early networks were Point to Point networks. In a point to point network each computer is connected to each other computer by a dedicated connection eg RS-232 N=2, Connections =2(2-1)/2 = 1 N=3, Connections =3(3-1)/2 = 3 N=4, Connections =4(4-1)/2 = 6. N=10 Connections =10(10-1)/2 = 45 There are advantages to this type of setup; Each link is independent and different hardware can be used for different links. Each connection is dedicated to a particular pair of computers so there is no need for the addressing of the data so it get to the right computer. Each link is private The main and overwhelming disadvantage is the number of links required. For N computers N( N 1) links are required. 2 A more economical approach is the use of Local Area Network (LAN) based on a single shared medium. 3.1 Bus topology A bus topology connects the computers along a single shared bus whose ends are terminated. A network that uses a bus topology is referred to as a "bus network" which was the original form of Ethernet networks. Ethernet 10Base2 (also known as thinnet) is used for bus topology. Bus topology is the cheapest way of connecting computers, but it has the disadvantage that a single loose connection or cable break can bring down the entire LAN Termination is important issue in bus networks. The electrical signal from a transmitting computer is free to travel the entire length of the cable. Without the termination, when the signal reaches the end of the wire, it bounces back and travels back up the wire. When a signal echoes back and forth along an un-terminated bus, it is called ringing. The terminators absorb the electrical energy and stop the reflections. Advantages of the bus are following. Bus is easy to use and understand and inexpensive simple network COMP1208-Notes08-CompNetworks_PA1.doc Page 4-4

5 It is easy to extend a network by adding cable with a repeater that boosts the signal and allows it to travel a longer distance. Disadvantages are following. A bus topology becomes slow by heavy network traffic with a lot of computer because networks do not coordinate with each other to reserve times to transmit. It is difficult to troubleshoot a bus because a cable break or loose connector will cause reflections and bring down the whole network. 3.2 Star topology A star topology links the computers by individual cables to a central unit, for example a hub or a LAN switch. When a computer or other networking component transmits a signal to the network, the signal travels to the hub. Then, the hub forwards the signal simultaneously to all other components connected to the hub. In the case of a LAN switch the signal is only sent to the destination node. Ethernet 10BaseT is a network based on the star topology. Star topology is the most popular way to connect computers in a workgroup network. Advantages of star topology are such as: The failure of a single computer or cable doesn't bring down the entire network. The centralised networking equipment can reduce costs in the long run by making network management much easier. It allows several cable types in same network with a hub that can accommodate multiple cable types. Disadvantages of star topology are such as: Failure of the central hub causes the whole network failure. It is slightly more expensive than using bus topology. 3.3 Ring topology A ring topology connects the computers along a single path whose ends are joined to form a circle. The circle might be logical only but the physical arrangement of the cabling might be similar to star topology, with a hub or concentrator at the centre. The ring topology is commonly used in token ring networks that the ring of a token ring network is concentrated inside a device called a Multi-station Access Unit (MAU) and fiber Distributed Data Interface (FDDI) networks that the ring in this case is both a physical and logical ring and usually runs around a campus or collection of buildings to form a high-speed backbone network. Advantages are following: One computer cannot monopolise the network. It continues to function after capacity is exceeded but the speed will be slow. Disadvantages are following: Failure of one computer can affect the whole network. It is difficult to troubleshoot. Adding and removing computers disrupts the network. COMP1208-Notes08-CompNetworks_PA1.doc Page 5-5

6 Network Media An important part of designing and installing a computer network is to select the appropriate network medium. Network medium is what the data is transmitted over. The network media that are used can be classified as guided or unguided. Guided media provide a physical path along which the signals are propagated, for example unshielded twisted pair (UTP), shielded twisted pair (STP), coaxial cable and optical fibre. Unguided media employ an antenna for transmitting though air, vacuum or water, for example, broadcast radio (30 MHz to 1 GHz), terrestrial microwave (1 GHz to 40 GHz), infrared (3 x to 2 x Hz) and satellite (1 GHz to 10 GHz). The choice of media will depend on many factors, for example, the environment in which the data is to transmitted. In computer networking the medium affects nearly every aspect of communication. Most importantly it determines speed and cost of communication. UTP is easy to install; widely available and widely used, but is susceptible to interference; can cover only a limited distance. STP reduces crosstalk and is more resistant to EMI than UTP, but it is more difficult to work with and can cover only a limited distance. Coaxial cable is less susceptible to EMI interference than other types of copper media but is difficult to work with, has limited bandwidth (Thicknet), limited application (Thinnet) and damage to the cable can bring down entire network. Optical fibre cannot be tapped, so security is better. It can be used over great distances and is not susceptible to EMI. Fibre also has a higher data rate than coaxial and twisted pair cable, however it is difficult to terminate. Infrared provides an effective solution for temporary or hard to cable environments or for portable computers. However, infrared is a line of sight technology which limits its usefulness in office environments. Infrared is not very high-speed compared with copper and fibre media. Also the cost of infrared equipment is high. Radio links can connect nodes without regard for line of sight. Radio is also immune to rain and snow unlike external infrared installations Twisted Pair Cable Advantages 1. Inexpensive 2. Often available in existing phone system 3. Well tested and easy to get Coaxial Cable Advantages 1. Fairly resistant to RFI and EMI 2. Supports faster data rates than twisted pair 3. More durable than TP Fiber Optic Cable Advantages 1. Highly secure 2. Not affected by RFI and EMI 3. Highest bandwidth available Disadvantages 1. Susceptible to RFI and EMI 2. Not as durable as coax 3. Doesn t support as high a speed as other media Disadvantages 1. Can be effected by strong interference 2. More costly than TP 3. Bulkier and more rigid than TP Disadvantages 1. Extremely costly in product and service 2. Sophisticated tools and methods for installation.. COMP1208-Notes08-CompNetworks_PA1.doc Page 6-6

7 4. Communication Protocols 4.1 What is a protocol? Basic communication hardware consists of mechanisms that can transfer bits from one point to another. To aid programmers, computers attached to a network use complex software that provides a convenient, high-level interface for applications. The software handles most low-level communication details and problems automatically, making it possible for applications to communicate easily. Thus most application programs rely on network software to communicate and do not interact with the network hardware directly. So all parties involved in a communication must agree a set of rules to be used when exchanging messages. These set of rules are called Protocols. A simple example might be that that two parties communicating agree to use simple ASCII characters to exchange data or maybe to use complex coded data. So a protocol is a set of rules for communicating. Protocols govern the organisation and structure of the data and the sequence of events required for effective communication. Due to the design complexity of networks, the functionality of the network is divided into a series of layers. Each layer offers services to the higher layer through a clearly defined interface, so the higher layer does not need to see the implementation detail of the layer below. A layer on one machine can communicate with the same layer on another machine by exchanging messages. Protocols within that layer define the format of messages and actions to be taken on receipt of each message. Protocols are usually designed in suites to make interaction between related groups of protocols more efficient. There are many protocols widely used in computer networking, for example: The Transmission Control Protocol/Internet Protocol (TCP/IP) protocol suite, developed by the US Defence Advanced Research Projects Agency (DARPA) for Internet communication, is one of the most widely used today. NetWare IPX/SPX (Internetwork Packet exchange/sequence Packet exchange) protocol suite used by computers over a Novell NetWare based network AppleTalk protocols designed for the Apple Macintosh NetBEUI used in a Microsoft Windows network. 4.2 TCP/IP Protocol Suite COMP1208-Notes08-CompNetworks_PA1.doc Page 7-7

8 4.2.1 Link Layer The Link Layer also known as network interface layer, provides reliability to upper layers in a network for the physical layer transmission. The TCP/IP suite specifies that the host connects to the network using some protocol so it can send network layer packets. TCP/IP does not define this protocol but uses existing standards such as Ethernet, Token Ring and FDDI (Fibre Distributed Data Interchange), to make the TCP/IP suite hardware independent. Each node on a network has a unique hardware address, the format depending on the kind of network. For example, Ethernet addresses, known as MAC addresses are attached to the physical hardware and usually assigned at manufacture, for example: 00:03:6D:40:00:A2. This is used by link layer protocols to identify individual nodes Network Layer The network layer (also known as internet layer) handles the routing of packets around the network. IP (Internet Protocol) is used for addressing, packet switching, route selection and error control. It is a connectionless, datagram protocol that uses packet switching and performs route selection by using dynamic routing tables. Each packet contains source and destination addresses. IP addresses are unique 32-bit addresses, assigned permanently or dynamically to each node on the internetwork. For example, the IP version 4 address format is: where www, xxx, yyy and zzz range from 0 to 255. For example, a student PC in the School of Electronics & Communications Engineering at DIT has an address of IP version 6 provides a 128-bit address. ICMP (Internet Control Message Protocol) is used for sending control messages between nodes Transport Layer The transport layer provides the flow of data between two end-system hosts for the application layer above. TCP and UDP are the most prominent transport layer protocols. TCP provides a reliable transport layer for applications. UDP provides a simple no guarantee datagram service Application Layer The application layer contains all the higher-layer protocols that deliver data to the user, for example: Virtual terminal (TELNET) ; File Transfer Protocol (FTP); Simple Mail Transfer Protocol (SMTP); Domain Name Services (DNS) are used to allow users to use symbolic names in place of IP addresses, for example when a user enters a domain name, such as, into a web browser then the local DNS server attempts to resolve the domain name to an IP address or interrogate other servers to resolve the domain name. HyperText Transmission Protocol (HTTP) define how web browsers communicate with a web servers COMP1208-Notes08-CompNetworks_PA1.doc Page 8-8

9 5. Network Devices There are many types of network devices, such as repeaters, hubs, bridges, switches and routers used to interconnect nodes in network. However, these devices operate at different protocol layers and thus perform different interconnection functions. Repeaters When a signal travels along a cable, it tends to lose strength. A repeater is a device that boosts a network's signal as it passes through. The repeater does this by electrically amplifying the signal it receives and re-broadcasting it. Repeaters are used when the total length of your network cable exceeds the standards set for the type of cable being used. HUB A hub also called a repeater hub is the basic networking component used in networks to connect network computers to form a local area network (LAN). The hub operates by gathering the signals from individual network devices, optionally amplifying the signals, and then sending them onto all other connected devices. A hub is like a multiport repeater and do not reduce network traffic. BRIDGE Bridge is a networking component used either to extend or to segment networks. They can be used both to join dissimilar media such as unshielded twisted-pair (UTP) cabling and fiber-optic cabling, and to join different network architectures such as Token Ring and Ethernet. Bridges regenerate signals but do not perform any protocol conversion, so the same networking protocol (such as TCP/IP) must be running on both network segments connected to the bridge. Bridges operate by sensing the source MAC addresses of the transmitting nodes on the network and automatically building an internal MAC routing table. This table is used to determine which connected segment to route packets to, and it provides the filtering capability. If the bridge knows which segment a packet is intended for, it forwards the packet directly to that segment. If the bridge doesn't recognize the packet's destination address, it forwards the packet to all connected segments except the one it originated on. And if the destination address is in the same segment as the source address, the bridge drops the packet. Bridges also forward broadcast packets to all segments except the originating one. LAN Switches A switch is a networking component used to connect workgroup hubs to form a larger network or to connect computers that have high bandwidth needs. The switches provide superior performance to hubs but are more expensive. When a signal enters a port of the switch, the switch looks at the destination address of the frame and internally establishes a logical connection with the port connected to the destination node. Other ports on the switch have no part in the connection. The result is that each port on the switch corresponds to an individual collision domain, and network congestion is avoided. Thus, if a 10-Mbps Ethernet switch has 10 ports, each port effectively gets the entire bandwidth of 10 Mbps-to the frame, the switch's port appears to provide a dedicated connection to the destination node. Ethernet switches are capable of establishing multiple internal logical connections simultaneously. Router A Router is a networking device that is used to extend or segment networks by forwarding packets from one logical network to another. Routers work at the network layer (layer 3) forwarding packets between networks using their logical addresses. Routers contain internal tables of information called routing tables that keep track of all known network addresses and possible paths throughout the internetwork, along with the cost of reaching each network. Routers route packets based on the available paths and their costs. Gateway A gateway that connects dissimilar systems or protocols adds expansion and functionality to a LAN because now unlike computer systems can inter-operate. A gateway basically grants a workstation a direct connection to the host computer and acts as a messenger between the two systems. Gateways operate between the OSI Transport layer through the Application Layer. COMP1208-Notes08-CompNetworks_PA1.doc Page 9-9