HCS 204 Data Communications and Computer Networks

Transcription

1 HCS 204 Data Communications and Computer Networks Introduction Data Communication When we communicate, we are sharing information. This sharing can be local or remote. Between individuals, local communication usually occurs face to face, while remote communication takes place over distance.data Communication is a process of exchanging data or information. In case of computer networks this exchange is done between two devices over a transmission medium. Data communications means the exchange of data between two devices via some form of transmission medium such as a wire cable. For data communications to occur, the communicating devices must be part of a communication system made up of a combination of hardware (physical equipment) and software (programs). The hardware part involves the sender and receiver devices and the intermediate devices through which the data passes. The software part involves certain rules which specify what is to be communicated, how it is to be communicated and when. It is also called as a Protocol. Data Communication Line Configuration Line configuration refers to the way two or more communication devices attach to a link. A link is a physical communications pathway that transfers data from one device to another. For communication to occur, two devices must be connected in some way to the same link at the same time. There are two possible types of connections: point-to-point and multipoint. Point-to-Point. A point-to-point connection provides a dedicated link between two devices. The entire capacity of the link is reserved for transmission between those two devices. Most point-to-point connections use an actual length of wire or cable to connect the two ends, but other options, such as microwave or satellite links, are also possible.

2 When you change television channels by infrared remote control, you are establishing a point-topoint connection between the remote control and the television's control system. Multipoint A multipoint (also called multidrop) connection is one in which more than two specific devices share a single link. In a multipoint environment, the capacity of the channel is shared. If several devices can use the link simultaneously, it is a spatially shared connection. If users must take turns, it is a timeshared connection. The effectiveness of a data communications system depends on four fundamental characteristics: delivery, accuracy, timeliness, and jitter. 1. Delivery: The system must deliver data to the correct destination. Data must be received by the intended device or user and only by that device or user. 2. Accuracy: The system must deliver the data accurately. Data that have been altered in transmission and left uncorrected are unusable. 3. Timeliness: The system must deliver data in a timely manner. Data delivered late are useless. In the case of video and audio, timely delivery means delivering data as they are produced, in the same order that they are produced, and without significant delay. This kind of delivery is called real-time transmission. 4. Jitter:

3 Jitter refers to the variation in the packet arrival time. It is the uneven delay in the delivery of audio or video packets. For example, let us assume that video packets are sent every 3D ms. If some of the packets arrive with 3D-ms delay and others with 4D-ms delay, an uneven quality in the video is the result. Components of a Data Communication System A data communications system has five components. The different components of Data communication are shown in the following figure. 1. Message: The message is the information (data) to be communicated. Popular forms of information include text, numbers, pictures, audio, and video. 2. Sender: The sender is the device that sends the data message. It can be a computer, workstation, telephone handset, video camera, and so on. 3. Receiver: The receiver is the device that receives the message. It can be a computer, workstation, telephone handset, television, and so on. 4. Transmission medium: The transmission medium is the physical path by which a message travels from sender to receiver. Some examples of transmission media include twisted-pair wire, coaxial cable, fiberoptic cable, and radio waves.

4 5. Protocol: A protocol is a set of rules that govern data communications. It represents an agreement between the communicating devices. Without a protocol, two devices may be connected but not communicating, just as a person speaking French cannot be understood by a person who speaks only Japanese. Data Flow Communication between two devices can be simplex, half-duplex, or full-duplex. Simplex In simplex mode, the communication is unidirectional, as on a one-way street. Only one of the two devices on a link can transmit; the other can only receive. Keyboards and traditional monitors are examples of simplex devices. The keyboard can only introduce input; the monitor can only accept output. The simplex mode can use the entire capacity of the channel to send data in one direction. Half-Duplex In half-duplex mode, each station can both transmit and receive, but not at the same time. : When one device is sending, the other can only receive, and vice versa. The half-duplex mode is like a one-lane road with traffic allowed in both directions. When cars are traveling in one direction, cars going the other way must wait. In a half-duplex transmission, the entire capacity of a channel is taken over by whichever of the two devices is transmitting at the time. Walkietalkies are half-duplex systems. The half-duplex mode is used in cases where there is no need for communication in both directions at the same time; the entire capacity of the channel can be utilized for each direction. Full-Duplex In full-duplex both stations can transmit and receive simultaneously.the full-duplex mode is like a two way street with traffic flowing in both directions at the same time. In full-duplex mode, signals going in one direction share the capacity of the link: with signals going in the other direction. This sharing can occur in two ways: Either the link must contain two physically separate transmission paths, one for sending and the other for receiving; or the capacity of the channel is divided between signals traveling in both directions. One common example of fullduplex communication is the telephone network. When two people are communicating by a telephone line, both can talk and listen at the same time. The full-duplex mode is used when communication in both directions is required all the time. The capacity of the channel, however, must be divided between the two directions.

5 Computer Networks Introduction Definition: A computer network can be defined as a collection of nodes. A node can be any device capable of transmitting or receiving data. The communicating nodes have to be connected by communication links. Computer Networks are used for data communications Most networks use distributed processing, in which a task is divided among multiple computers. Instead of one single large machine being responsible for all aspects of a process, separate computers (usually a personal computer or workstation) handle a subset. Advantage of distributed processing include the following: Security/encapsulation: System designer can limit the kinds of interaction that a given user can have with the entire system. Eg a bank can allow users access to their own accounts through an ATM without allowing access to the entire bank s database. Distributed databases: No one system needs to provide storage capacity for the entire database. 3Eg the www give users access to information that may be actually stored and manipulated anywhere on the Internet. Faster problem solving: Multiple computers working on parts of a problem concurrently often can solve a problem faster than a single whole machine working alone. Security through redundancy: Multiple computers running the same program at the same time can provide security through redundancy. Collaborative processing: Both multiple computers and multiple users may interact on a task. Lower Cost: Larger organizations invest in expensive mainframe and supercomputers to function as centralized servers. Each mainframe machine, for example, costs several hundred thousand dollars versus several thousand dollars for a few minicomputers, according to the University of New Mexico. Distributed data processing considerably lowers the cost of data sharing and networking across an organization by comprising several minicomputers that cost significantly less than mainframe machines. Improved Performance and Reduced Processing Time: Single computers are limited in their performance and efficiency. An easy way to increase performance is by adding another computer to a network. Adding yet another computer will further augment performance, and so on. Distributed data processing works on this principle and holds that a job gets done faster if multiple machines are handling it in parallel, or synchronously. Complicated statistical problems, for example, are broken into modules

6 and allocated to different machines where they are processed simultaneously. This significantly reduces processing time and improves performance. Network Criteria A network must be able to meet a certain number of criteria. The most important of these are performance, reliability, and security. Performance Performance can be measured in many ways, including transit time and response time. Transit time is the amount of time required for a message to travel from one device to another. Response time is the elapsed time between an inquiry and a response. The performance of a network depends on a number of factors, including the number of users, the type of transmission medium, the capabilities of the connected hardware, and the efficiency of the software. Performance is often evaluated by two networking metrics: throughput and delay. We often need more throughput and less delay. However, these two criteria are often contradictory. If we try to send more data to the network, we may increase throughput but we increase the delay because of traffic congestion in the network. Reliability In addition to accuracy of delivery, network reliability is measured by the frequency of failure, the time it takes a link to recover from a failure, and the network's robustness in a catastrophe. Security Network security issues include protecting data from unauthorized access, protecting data from damage and development, and implementing policies and procedures for recovery from breaches and data losses. Topology The term topology refers to the physical or logical layout of a network. Two or more devices connect to a link and two or more links form a topology. There are five basic topologies possible: mesh, star, tree, bus, and ring. Two relationships are possible in these topologies: peer-to-peer, where the devices share the link equally, and primary-secondary, where one device controls traffic and the others must transmit through it. Ring and mesh are more convenient for peer-topeer transmission, while star and tree are more convenient for primary-secondary. A bus is equally convenient for either. MESH

7 In a mesh topology, every device has a dedicated point-to-point link to every other device. The term dedicated means that the link carries traffic only between the two devices it connects. A fully connected mesh network therefore has n(n-1)/2 physical channels to link n devices. Every node on the network must have n-1 I/O ports. Advantages - A broken node won t distract the transmission of data in a mesh network. ie it is robust. - When every message travels along a dedicated line, only the intended recipient sees it (security or privacy). - point-to-point links make fault identification and fault isolation easy Disadvantages - Because every device must be connected to every other device, installation and reconnection are difficult. - The wiring can be greater than the available space (in walls, ceilings, or floors) can accommodate. - The hardware required to connect each link (I/O ports and cable) can be prohibitively expensive. One practical example of a mesh topology is the connection of telephone regional offices in which each regional office needs to be connected to every other regional office.

8 Example MSU has a fully connected mesh network consisting of 8 devices, calculate the total number of cable links needed and the number of ports for each device. STAR TOPOLOGY In a star topology, each device has a dedicated point-to-point link only to a central controller, usually called a hub. The devices are not directly linked to one another. Unlike a mesh topology, a star topology does not allow direct traffic between devices. The controller acts as an exchange: If one device wants to send data to another, it sends the data to the controller, which then relays the data to the other connected device A star topology is less expensive than a mesh topology. In a star, each device needs only one link and one I/O port to connect it to any number of others. This factor also makes it easy to install and reconfigure. Far less cabling needs to be housed, and additions, moves, and deletions involve only one connection: between that device and the hub. Other advantages include robustness. If one link fails, only that link is affected. All other links remain active. This factor also lends itself to easy fault identification and fault isolation. As long as the hub is working, it can be used to monitor link problems and bypass defective links. One big disadvantage of a star topology is the dependency of the whole topology on one single point, the hub. If the hub goes down, the whole system is dead.

9 Although a star requires far less cable than a mesh, each node must be linked to a central hub. For this reason, often more cabling is required in a star than in some other topologies (such as ring or bus). The star topology is used in local-area networks (LANs). TREE A tree topology is a variation of a star. As in a star, nodes in a tree are linked to a central hub that controls the traffic to the network. However, not all devices plugs directly into the central hub. The majority of devices connect to the secondary hub that in turn is connected to the central hub. The central hub is an active hub. An active hub contains a repeater, which is a hardware device that regenerates the received bit patterns before sending them out. Repeating strengthens transmissions and increases the distance a signal can travel. The secondary hubs may be active or passive hubs. A passive hub provides a simple physical connection between the attached devices. Advantages and disadvantages of a tree are generally the same as those of a star. The addition of secondary hubs, however, brings two further advantages. 1) it allows more devices to be attached to a single central hub and can therefore increase the distance a signal can travel between devices. 2) It allows the network to isolate and prioritize communication from different computers. Eg. The computers attached to one secondary hub can be given priority over computers attached to another secondary hub. BUS The preceding examples all describe point-to-point connections. A bus topology, on the other hand, is multipoint. One long cable acts as a backbone to link all the devices in a network.

10 Nodes are connected to the bus cable by drop lines and taps. A drop line is a connection running between the device and the main cable. A tap is a connector that either splices into the main cable or punctures the sheathing of a cable to create a contact with the metallic core. As a signal travels along the backbone, some of its energy is transformed into heat. Therefore, it becomes weaker and weaker as it travels farther and farther. For this reason there is a limit on the number of taps a bus can support and on the distance between those taps. Advantages of a bus topology include ease of installation. Backbone cable can be laid along the most efficient path, then connected to the nodes by drop lines of various lengths. In this way, a bus uses less cabling than mesh or star topologies. In a star, for example, four network devices in the same room require four lengths of cable reaching all the way to the hub. In a bus, this redundancy is eliminated. Only the backbone cable stretches through the entire facility. Each drop line has to reach only as far as the nearest point on the backbone. Disadvantages include difficult reconnection and fault isolation. A bus is usually designed to be optimally efficient at installation. It can therefore be difficult to add new devices. Signal reflection at the taps can cause degradation in quality. This degradation can be controlled by limiting the number and spacing of devices connected to a given length of cable. Adding new devices may therefore require modification or replacement of the backbone. In addition, a fault or break in the bus cable stops all transmission, even between devices on the same side of the problem. The damaged area reflects signals back in the direction of origin, creating noise in both directions. Bus topology was the one of the first topologies used in the design of early local area networks. Ethernet LANs can use a bus topology, but they are now less popular. RING In a ring topology, each device has a dedicated point-to-point connection with only the two devices on either side of it. A signal is passed along the ring in one direction, from device to device, until it reaches its destination. Each device in the ring incorporates a repeater. When a device receives a signal intended for another device, its repeater regenerates the bits and passes them along.

11 A ring is relatively easy to install and reconfigure. Each device is linked to only its immediate neighbours (either physically or logically). To add or delete a device requires changing only two connections. The only constraints are media and traffic considerations (maximum ring length and number of devices). In addition, fault isolation is simplified. Generally in a ring, a signal is circulating at all times. If one device does not receive a signal within a specified period, it can issue an alarm. The alarm alerts the network operator to the problem and its location. However, unidirectional traffic can be a disadvantage. In a simple ring, a break in the ring (such as a disabled station) can disable the entire network. This weakness can be solved by using a dual ring or a switch capable of closing off the break. HYBRID Often a network combines several topologies

12 Categories of Networks Generally, networks are distinguished based on their geographical span, ownership, the distance it covers, and its physical architecture. A network can be as small as distance between your mobile phone and its Bluetooth headphone and as large as the internet itself, covering the whole geographical world, Personal Area Network A Personal Area Network (PAN) is smallest network which is very personal to a user. This may include Bluetooth enabled devices or infra-red enabled devices. PAN has connectivity range up to 10 meters. PAN may include wireless computer keyboard and mouse, Bluetooth enabled headphones, wireless printers and TV remotes. For example, Piconet is Bluetooth-enabled Personal Area Network which may contain up to 8 devices connected together in a master-slave fashion. Local Area Network A computer network spanned inside a building and operated under single administrative system is generally termed as Local Area Network (LAN). Usually,LAN covers an organization offices, schools, colleges or universities. Number of systems connected in LAN may vary from as least as two to as much as 16 million. LAN provides a useful way of sharing the resources between end users.the resources such as printers, file servers, scanners, and internet are easily sharable among computers.

13 LANs are composed of inexpensive networking and routing equipment. It may contains local servers serving file storage and other locally shared applications. It mostly operates on private IP addresses and does not involve heavy routing. LAN works under its own local domain and controlled centrally. LAN uses either Ethernet or Token-ring technology. Ethernet is most widely employed LAN technology and uses Star topology, while Token-ring is rarely seen. LAN can be wired, wireless, or in both forms at once. Metropolitan Area Network The Metropolitan Area Network (MAN) generally expands throughout a city such as cable TV network Backbone of MAN is high-capacity and high-speed fiber optics. MAN works in between Local Area Network and Wide Area Network. MAN provides uplink for LANs to WANs or internet. Wide Area Network As the name suggests, the Wide Area Network (WAN) covers a wide area which may span across provinces and even a whole country. Generally, telecommunication networks are Wide Area Network. These networks provide connectivity to MANs and LANs. Since they are equipped with very high speed backbone, WANs use very expensive network equipment.

14 Internetworks An internet (note the lowercase letter i) is two or more networks that can communicate with each other. The most notable internet is called the Internet (uppercase letter I), a collaboration of more than hundreds of thousands of interconnected networks.in modern practice, interconnected networks use the Internet Protocol. There are at least three variants of internetworks, depending on who administers and who participates in them: Internet Intranet Extranet Internet The Internet consists of a worldwide interconnection of governmental, academic, public, and private networks based upon the networking technologies of the Internet Protocol Suite. It is the successor of the Advanced Research Projects Agency Network (ARPANET) developed by DARPA of the U.S. Department of Defense. The Internet has revolutionized many aspects of our daily lives. It has affected the way we do business as well as the way we spend our leisure time A Brief History

15 In the mid-1960s, mainframe computers in research organizations were standalone devices. Computers from different manufacturers were unable to communicate with one another. The Advanced Research Projects Agency (ARPA) in the Department of Defense (DoD) was interested in finding a way to connect computers so that the researchers they funded could share their findings, thereby reducing costs and eliminating duplication of effort. In 1967, at an Association for Computing Machinery (ACM) meeting, ARPA presented its ideas for ARPANET, a small network of connected computers. The idea was that each host computer (not necessarily from the same manufacturer) would be attached to a specialized computer, called an interface message processor (IMP). The IMPs, in tum, would be connected to one another. Each IMP had to be able to communicate with other IMPs as well as with its own attached host. By 1969, ARPANET was a reality. Four nodes, at the University of California at Los Angeles (UCLA), the University of California at Santa Barbara (UCSB), Stanford Research Institute (SRI), and the University of Utah, were connected via the IMPs to form a network. Software called the Network Control Protocol (NCP) provided communication between the hosts. In 1972, Vint Cerf and Bob Kahn, both of whom were part of the core ARPANET group, collaborated on what they called the Internetting Projec1. Cerf and Kahn's landmark 1973 paper outlined the protocols to achieve end-to-end delivery of packets. This paper on Transmission Control Protocol (TCP) included concepts such as encapsulation, the datagram, and the functions of a gateway. Shortly thereafter, authorities made a decision to split TCP into two protocols: Transmission Control Protocol (TCP) and Internetworking Protocol (lp). IP would handle datagram routing while TCP would be responsible for higher-level functions such as segmentation, reassembly, and error detection. The internetworking protocol became known as TCP/IIP. The Internet Today The Internet has come a long way since the 1960s. The Internet today is not a simple hierarchical structure. It is made up of many wide- and local-area networks joined by connecting devices and switching stations. It is difficult to give an accurate representation of the Internet because it is continually changing-new networks are being added, existing networks are adding addresses, and networks of defunct companies are being removed. Today most end users who want Internet connection use the services of Internet service providers (lsps). There are international service providers, national service providers, regional service providers, and local service providers. International Internet Service Providers At the top of the hierarchy are the international service providers that connect nations together. National Internet Service Providers

16 The national Internet service providers are backbone networks created and maintained by specialized companies. There are many national ISPs operating in North America; some of the most well known are SprintLink, PSINet, UUNet Technology, AGIS, and internet Mel. To provide connectivity between the end users, these backbone networks are connected by complex switching stations (normally run by a third party) called network access points (NAPs). Some national ISP networks are also connected to one another by private switching stations called peering points. These normally operate at a high data rate (up to 600 Mbps). Regional Internet Service Providers Regional internet service providers or regional ISPs are smaller ISPs that are connected to one or more national ISPs. They are at the third level of the hierarchy with a smaller data rate. Local Internet Service Providers Local Internet service providers provide direct service to the end users. The local ISPs can be connected to regional ISPs or directly to national ISPs. Most end users are connected to the local ISPs. Note that in this sense, a local ISP can be a company that just provides Internet services, a corporation with a network that supplies services to its own employees, or a nonprofit organization, such as a college or a university that runs its own network. Each of these local ISPs can be connected to a regional or national service provider. Intranet An intranet is a set of networks, using the Internet Protocol and IP-based tools such as web browsers and file transfer applications, that is under the control of a single administrative entity. That administrative entity closes the intranet to all but specific, authorized users. Most commonly, an intranet is the internal network of an organization. A large intranet will typically have at least one web server to provide users with organizational information. Extranet An extranet is a network or internetwork that is limited in scope to a single organization or entity but which also has limited connections to the networks of one or more other usually, but not necessarily, trusted organizations or entities (e.g., a company's customers may be given access to some part of its intranet creating in this way an extranet, while at the same time the customers may not be considered 'trusted' from a security standpoint). Technically, an extranet may also be categorized as a CAN, MAN, WAN, or other type of network, although, by definition, an

17 extranet cannot consist of a single LAN; it must have at least one connection with an external network. Applications of Networks Marketing and sales Financial services Manufacturing Electronic messaging Teleconferencing Cellular telephone Education Networking Devices All but the most basic of networks require devices to provide connectivity and functionality. Understanding how these networking devices operate and identifying the functions they perform are essential skills for any network administrator. The following are the commonly used networking devices. Hubs Hubs are used in networks that use twisted-pair cabling to connect devices. Hubs can also be joined together to create larger networks. Hubs are simple devices that direct data packets to all devices connected to the hub, regardless of whether the data package is destined for the device. This makes them inefficient devices and can create a performance bottleneck on busy networks. In its most basic form, a hub does nothing except provide a pathway for the electrical signals to travel along.a hub does not perform any processing on the data that it forwards, nor does it perform any error checking. Switches Like hubs, switches are the connectivity points of an Ethernet network. Devices connect to switches via twisted-pair cabling, one cable for each device. The difference between hubs and switches is in how the devices deal with the data that they receive. Whereas a hub forwards the data it receives to all of the ports on the device, a switch forwards it only to the port that connects to the destination device. It does this by learning the MAC address of the devices attached to it, and then by matching the destination MAC address in the data it receives.

18 MSAU In a Token Ring network, a multistation access unit (MSAU) is used in place of the hub that is used on an Ethernet network. The MSAU performs the token circulation inside the device, giving the network a physical star appearance. Each MSAU has a Ring In (RI) port on the device, which is connected to the Ring Out (RO) port on another MSAU. The last MSAU in the ring is then connected to the first to complete the ring. Bridges Bridges are used to divide larger networks into smaller sections. They do this by sitting between two physical network segments and managing the flow of data between the two. Based on the MAC addresses of the devices connected to each segment, bridges can chose to forward the data if they believe that the destination address is on another interface), or block it from crossing if they can verify that it is on the interface from which it came. Routers In a common configuration, routers are used to create larger networks by joining two network segments. A router derives its name from the fact that it can route data it receives from one network onto another. When a router receives a packet of data, it reads the header of the packet to determine the destination address. Once it has determined the address, it looks in its routing table to determine whether it knows how to reach the destination and, if it does, it forwards the packet to the next hop on the route. The next hop might be the final destination, or it might be another router. Gateways Any device that translates one data format to another is called a gateway. Some examples of gateways include a router that translates data from one network protocol to another and a software application that converts between two dissimilar formats. The key point about a gateway is that only the data format is translated, not the data itself. In many cases, the gateway functionality is incorporated into another device. Network Cards Network cards, also called Network Interface Cards, are devices that enable computers to connect to the network. When specifying or installing a NIC, you must consider the following issues: Wireless Access Points Wireless access points (APs) are a transmitter and receiver (transceiver) devices used to create a wireless LAN (WLAN). APs are typically a separate network device with a built-in antenna,

19 transmitter, and adapter. APs use the wireless infrastructure network mode to provide a connection point between WLANs and a wired Ethernet LAN. APs also typically have several ports allowing a way to expand the network to support additional clients. When working on a Token Ring network, you have to ensure that all network cards are set to transmit at the same speeds. NICs on an Ethernet network can operate at different speeds. Depending on the size of the network, one or more APs might be required. Additional APs are used to allow access to more wireless clients and to expand the range of the wireless network. Each AP is limited by a transmissions range i.e the distance a client can be from a AP and still get a useable signal. The actual distance depends on the wireless standard being used and the obstructions and environmental conditions between the client and the AP.A WAP can operate as a bridge connecting a standard wired network to wireless devices or as a router passing data transmissions from one access point to another. Modems A modem, short for modulator/demodulator, is a device that converts the digital signals generated by a computer into analog signals that can travel over conventional phone lines. The modem at the receiving end converts the signal back into a format the computer can understand. Modems can be used as a means to connect to an ISP or as a mechanism for dialing up to a LAN. Modems can be internal add-in expansion cards, external devices that connect to the serial or USB port of a system Review Questions 1. Identify the 5 components of a data communication system 2. What are the advantages of distributed processing? 3. What are the three criteria necessary for an effective and efficient network? 4. What three fundamental characteristics determine the effectiveness of a data communications system? 5. Name the factors that affect the performance of a network. 6. Name the factors that affect the reliability of a network. 7. Name the factors that affect the security of a network. 8. State and explain any five application areas of networks. 9. How is topology related to line configuration? 10. Give advantages of the 5 basic topologies

20 11. What are advantages of a multipoint connection over point to point connections? 12. What are some of the factors that determine whether a communication system is a LAN, MAN or WAN? 13. What are the two types of line configuration? 14. Name the 5 basic topologies 15. Distinguish between peer-to-peer relationship and a primary-secondary relationship 16. Give the formula that finds the number of cable links required for a mesh network topology 17. Categorise the basic 5 topologies in terms of line configuration. 18. What is the difference between a central and a secondary hub? 19. What is the difference between a passive and an active hub? 20. For each topology, discuss the implication of a single cable fault. 21. Identify the access method used by each topology 22. A bridge uses a filtering table; a router uses a routing table. Can you explain thedifference?