Network Topologies 99 Figure 2-46 Hierarchical Topology Figure 2-47 Full-Mesh Topology In a partial-mesh topology, at least one device maintains multiple connections to oth- ers without being fully meshed, as shown in Figure 2-48. A partial-mesh topology still provides redundancy by having several alternative routes. If one route cannot be used, the data takes another route, even if it is longer. The partial-mesh topology is used for many telecommunications backbones, as well as the Internet. Logical Topology A network’s logical topology is how the hosts communicate across the medium. The two most common types of logical topology are broadcast and token passing. 1102.book Page 99 Tuesday, May 20, 2003 2:53 PM 100 Chapter 2: Networking Fundamentals Figure 2-48 Partial-Mesh Topology Broadcast topology simply means that each host addresses its data to a particular NIC, to a multicast address, or to a broadcast address on the network medium. There is no order that the stations must follow to use the network. It is first come, first serve. Ethernet also works this way, as explained later in this course. The second logical topology is token passing. Token passing controls network access by passing an electronic token sequentially to each host. When a host receives the token, it can send data on the network. If the host has no data to send, it passes the token to the next host, and the process repeats itself. Two examples of networks that use token passing are Token Ring and FDDI, both of which are examples of token passing on a physical ring topology. Summary In this chapter, you learned the following key points: ■ Historic reasons for the development of LANs and WANs ■ A LAN consists of computers, NICs, peripheral devices, networking media, and network devices. ■ A WAN consists of two or more LANs spanning two or more geographically sep- arate areas. ■ A MAN spans a metropolitan area, such as a city or a suburban area. 1102.book Page 100 Tuesday, May 20, 2003 2:53 PM Summary 101 ■ A SAN provides enhanced system performance, is scalable, and has disaster tolerance built in. ■ A data center is a globally coordinated network of devices designed to accelerate the delivery of information over the Internet infrastructure. ■ Intranets are designed to be accessed by users who have access privileges to an organization’s internal network. Extranets are designed to deliver applications and services that are intranet-based but that employ extended, secured access to external users or enterprises. ■ A VPN is a private network that is constructed within a public network infra- structure. Three main types of VPNs are access, intranet, and extranet. ■ Bandwidth and throughput are measures of a network’s speed or capacity. ■ Breaking a complex problem into smaller units helps you understand the larger problem. ■ Communication is a complex problem that can be broken into a series of layers. Multiple systems can be used for the breakdown. ■ The ISO created and released the OSI model in 1984 to provide vendors with a set of standards to ensure greater compatibility and interoperability among vari- ous types of network technologies. ■ The OSI reference model is a descriptive network scheme whose standards ensure greater compatibility and interoperability between various types of network technologies. ■ Each OSI layer has a specific function and protocols to support it. The seven layers of the OSI model are application, presentation, session, transport, network, data link, and physical. ■ Each layer in a sending device’s communication protocol communicates with its peer layer in the receiving device. ■ The TCP/IP model has four layers: application, transport, Internet, and network access. ■ All communications on a network originate at a source and are sent to a destina- tion. That encapsulation wraps data with the necessary protocol information before network transit. ■ Networking devices are products that connect networks. Hubs, switches, and routers interconnect devices within LANs, MANs, and WANs. Networking devices function at different layers of the OSI model. ■ Repeaters regenerate, amplify, and retime signals before sending them along the network. 1102.book Page 101 Tuesday, May 20, 2003 2:53 PM 102 Chapter 2: Networking Fundamentals ■ The term hub is used instead of repeater when referring to the device that serves as the center of a network. Hubs (also known as multiport repeaters) work at Layer 1 only and make no decisions. A hubs-only environment is a shared-access environment within which collisions occur. As devices are added to the network, more collisions occur, and performance declines dramatically. ■ LAN switches work at Layer 2 and make limited MAC hardware address deci- sions. Ethernet switches provide dedicated LAN connections. ■ Working at Layers 2, 3, and 4, multilayer switches enable the implementation of Layer 3 quality of service and security functionality. Multilayer switches perform many of the same functions as routers do, but they do so in hardware. ■ Routers can make decisions regarding the best path for delivery of data on the network. ■ Various types of servers provide services to computers and users accessing the network. Firewalls, AAA servers, and VPN concentrators provide security to the network. ■ Various types of wireless networking devices provide services to computers and users accessing the network. Common wireless networking devices include wire- less adapters, wireless access points, and wireless bridges. ■ A physical topology describes the plan for wiring the physical devices. A logical topology describes how information flows through a network. ■ In a physical bus topology, a single cable connects all the devices. ■ The most commonly used architecture in Ethernet LANs is the physical star topology. In a star topology, each host in the network is connected to the central device with its own cable. When a star network is expanded to include additional networking devices that are connected to the main networking device, it is called an extended-star topology. ■ In a ring topology, all the hosts are connected in the form of a ring or circle. ■ A full-mesh topology connects all devices to each other. To supplement all you’ve learned in this chapter, refer to the chapter-specific Videos, PhotoZooms, and e-Lab Activities on the CD-ROM accompanying this book. 1102.book Page 102 Tuesday, May 20, 2003 2:53 PM Key Terms 103 Key Terms application layer Layer 7 of the OSI reference model. This layer provides services to application processes (such as e-mail, file transfer, and terminal emulation) that are outside the OSI reference model. bandwidth The amount of information that can flow through a network connection in a given period of time. bridge A Layer 2 device designed to create two or more LAN segments, each of which is a separate collision domain. broadcast A data packets that is sent to all nodes on a network. Broadcasts are identified by a broadcast address. broadcast domain The set of all devices that receive broadcast frames originating from any device in the set. bus topology Commonly called a linear bus, this topology connects all the devices with a single cable. This cable proceeds from one computer to the next like a bus line going through a city. collision In Ethernet, the result of two nodes transmitting simultaneously. The frames from each device impact and are damaged when they meet on the physical medium. collision domain In Ethernet, the network area within which frames that have col- lided are propagated. Repeaters and hubs propagate collisions; LAN switches, bridges, and routers do not. data center A globally coordinated network of devices designed to accelerate the delivery of information over the Internet infrastructure. data link layer Layer 2 of the OSI reference model. Provides transit of data across a physical link. The data link layer is concerned with physical addressing, network topology, line discipline, error notification, ordered delivery of frames, and flow con- trol. de-encapsulation Unwrapping data in a particular protocol header. encapsulation Wrapping data in a particular protocol header. extended-star topology A network in which a star network is expanded to include an additional networking device that is connected to the main networking device. extranet Intranet-based applications and services that employ extended, secure access to external users or enterprises. 1102.book Page 103 Tuesday, May 20, 2003 2:53 PM 104 Chapter 2: Networking Fundamentals firewall A router or access server designated as a buffer between any connected public networks and a private network. flooding A traffic-passing technique used by switches and bridges in which traffic received on an interface is sent out all that device’s interfaces except the interface on which the information was received originally. frame A logical grouping of information sent as a data link layer unit over a trans- mission medium. full-mesh topology Connects all devices (nodes) to each other for redundancy and fault tolerance. hierarchical topology Created similar to an extended-star topology. The primary dif- ference is that it does not use a central node. Instead, it uses a trunk node from which it branches to other nodes. hub A common connection point for devices in a network. Hubs commonly connect segments of a LAN. A hub contains multiple ports. When a packet arrives at one port, it is copied to the other ports so that all the segments of the LAN can see all the packets. intranet A common LAN configuration. Intranets are designed to be accessed by users who have access privileges to an organization’s internal LAN. local-area network (LAN) A high-speed, low-error data network covering a rela- tively small geographic area (up to a few thousand meters). LANs connect worksta- tions, peripherals, terminals, and other devices in a single building or another geographically limited area. Media Access Control (MAC) A hardware address that uniquely identifies each node of a network. This address controls data communication for the host on the network. metropolitan-area network (MAN) A network that spans a metropolitan area. Gen- erally, a MAN spans a larger geographic area than a LAN but a smaller geographic area than a WAN. microsegmentation Allows the creation of private or dedicated segments—one host per segment. Each host receives instant access to the full bandwidth and does not have to compete for available bandwidth with other hosts. network interface card (NIC) A printed circuit board that fits into the expansion slot of a bus on a computer motherboard. Also can be a peripheral device. network layer Layer 3 of the OSI reference model. This layer provides connectivity and path selection between two end systems. The network layer is the layer at which routing occurs. 1102.book Page 104 Tuesday, May 20, 2003 2:53 PM Key Terms 105 Open System Interconnection (OSI) reference model A network architectural model developed by the ISO. This model consists of seven layers, each of which specifies par- ticular network functions, such as addressing, flow control, error control, encapsulation, and reliable message transfer. The OSI reference model is used universally as a method for teaching and understanding network functionality. packet A logical grouping of information that includes a header containing control information and (usually) user data. Packets most often refer to network-layer units of data. partial-mesh topology At least one device maintains multiple connections to others without being fully meshed. A partial-mesh topology still provides redundancy by hav- ing several alternative routes. peer-to-peer communication A form of communication in which each layer of the OSI model at the source must communicate with its peer layer at the destination. physical layer Layer 1 of the OSI reference model. The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, main- taining, and deactivating the physical link between end systems. presentation layer Layer 6 of the OSI reference model. This layer ensures that infor- mation sent by the application layer of one system can be read by the application layer of another. protocol A formal description of a set of rules and conventions that govern how devices on a network exchange information. protocol suite A set of related communications protocols that operate together and, as a group, address communication at some or all of the seven layers of the OSI refer- ence model. Not every protocol stack covers each layer of the model, and often a single protocol in the suite addresses a number of layers at once. TCP/IP is a typical protocol suite. repeater A networking device that exists at Layer 1, the physical layer, of the OSI ref- erence model. The purpose of a repeater is to regenerate and retime network signals at the bit level, allowing them to travel a longer distance on the medium. ring topology A topology in which hosts are connected in the form of a ring or circle. Unlike the physical bus topology, the ring topology has no beginning or end that needs to be terminated. router A type of internetworking device that passes data packets between networks based on Layer 3 addresses. A router can make decisions regarding the best path for delivery of data on the network. 1102.book Page 105 Tuesday, May 20, 2003 2:53 PM 106 Chapter 2: Networking Fundamentals segment In the TCP specification, a logical information group at transport layers of the OSI reference model. session layer Layer 5 of the OSI reference model. This layer establishes, manages, and terminates sessions between applications and manages data exchange between presentation layer entities. star topology The most commonly used physical topology in Ethernet LANs. The star topology is made up of a central connection point that is a device such as a hub, switch, or router, where all the cabling segments meet. storage-area network (SAN) A dedicated, high-performance network that moves data between servers and storage resources. switch A device that connects LAN segments, uses a table of MAC addresses to determine the segment on which a frame needs to be transmitted, and reduces traffic. Switches operate at much higher speeds than bridges. throughput The rate of information arriving at or passing through a particular point in a network system. token passing An access method by which network devices access the physical medium in an orderly fashion based on possession of a small frame called a token. transport layer Layer 4 of the OSI reference model. This layer is responsible for reli- able network communication between end nodes. The transport layer provides mecha- nisms to establish, maintain, and terminate virtual circuits, transport fault detection and recovery, and information flow control. virtual private network (VPN) A private network constructed within a public net- work infrastructure such as the global Internet. wide-area network (WAN) A data communications network that serves users across a broad geographic area and often uses transmission devices provided by common carriers. Check Your Understanding Complete all the review questions to test your understanding of the topics and con- cepts in this chapter. Answers are listed in Appendix C, “Check Your Understanding Answer Key.” 1102.book Page 106 Tuesday, May 20, 2003 2:53 PM Check Your Understanding 107 1. What was the first type of microcomputer network to be implemented? A. MAN B. WAN C. LAN D. PAN 2. Using modem connections, how many modems would it take to allow connec- tions from ten computers? A. One B. Five C. Ten D. Fifteen 3. What is a code that is “burned in” to a network interface card? A. NIC B. MAC address C. Hub D. LAN 4. Which topology has all its nodes connected directly to one center point and has no other connections between nodes? A. Bus B. Ring C. Star D. Mesh 5. What do TIA and EIA stand for? A. Television Industry Association, Electronic Industries Association B. Telecommunications Industry Association, Electronic Industries Alliance C. Telecommunications Industry Alliance, Electronic Industries Association D. Téléphonique International Association, Elégraphique Industries Alliance 6. LANs are designed to do which of the following? (Select all that apply.) A. Operate within a limited geographic area B. Allow many users to access high-bandwidth media C. Connect to the Internet D. Provide full-time connectivity to local services 1102.book Page 107 Tuesday, May 20, 2003 2:53 PM 108 Chapter 2: Networking Fundamentals 7. Which of the following statements best describes a WAN? A. It connects LANs that are separated by a large geographic area. B. It connects workstations, terminals, and other devices in a metropolitan area. C. It connects LANs within a large building. D. It connects workstations, terminals, and other devices within a building. 8. Which of the following statements correctly describes a MAN? A. A MAN is a network that connects workstations, peripherals, terminals, and other devices in a single building. B. A MAN is a network that serves users across a broad geographic area. It often uses transmission devices provided by common carriers. C. A MAN is a network that spans a metropolitan area such as a city or subur- ban area. D. A MAN is a network that is interconnected by routers and other devices and that functions as a single network. 9. Which of the following is not one of the features of a SAN? A. SANs enable concurrent access of disk or tape arrays, providing enhanced system performance. B. SANs provide a reliable disaster recovery solution. C. SANs are scalable. D. SANs minimize system and data availability. 10. What service offers secure, reliable connectivity over a shared public network infrastructure? A. Internet B. Virtual private network C. Virtual public network D. WAN 11. What links enterprise customer headquarters, remote offices, and branch offices to an internal network over a shared infrastructure? A. Access VPN B. Intranet VPN C. Extranet VPN D. Internet VPN 1102.book Page 108 Tuesday, May 20, 2003 2:53 PM . model. ■ Repeaters regenerate, amplify, and retime signals before sending them along the network. 11 02. book Page 10 1 Tuesday, May 20 , 20 03 2: 53 PM 10 2 Chapter 2: Networking Fundamentals ■ The term. types of logical topology are broadcast and token passing. 11 02. book Page 99 Tuesday, May 20 , 20 03 2: 53 PM 10 0 Chapter 2: Networking Fundamentals Figure 2- 48 Partial-Mesh Topology Broadcast topology. enterprises. 11 02. book Page 10 3 Tuesday, May 20 , 20 03 2: 53 PM 10 4 Chapter 2: Networking Fundamentals firewall A router or access server designated as a buffer between any connected public networks and