1. Sem 1 V2 Chapter 3 Local Area Networks LANS C. Marandola Cisco I Fall 2001 Community College of Rhode Island, Cisco Regional Academy

2. Chapter Three Goals Understand Basic LAN Devices. Understand the Evolution of Network Devices. Understand the Basics of Data Flow Through LANs. Understand the Building of LANs.

3. The concept of a topology. The basic function of computers on the LAN is to provide the user with an almost limitless set of applications.

4. A server (at least from a LAN perspective - which typically has more RAM and storage capacity, if not a faster processor than a typical host PC) provides applications and data to the hosts, and performs network services such as the Domain Name System (DNS) that keeps track of domain names End users often call computers clients if they rely on other special computers, called servers, to keep programs and data that are shared by many other clients. Relational databases reside on servers, and can sort and store incredible amounts of information

5. They operate at all 7 layers of the OSI model. They perform the entire process of encapsulation and decapsulation to do their job of sending e-mails, printing reports, scanning pictures, or accessing databasees.

6. A network interface card (NIC card or NIC) is a small printed circuit board that fits into the expansion slot of a bus on a computer’s motherboard or peripheral device network adapter. On laptop/notebook computers NICs are usually the size of a PCMCIA card. Its function is to adapt the host device to the network medium. NICs are considered Layer 2 devices because each individual NIC throughout the world carries a unique coded name, called a Media Access Control (MAC) address.. Sometimes the NIC card has a separate transceiver (transmitter/receiver) device, however, the modern NIC card has the transceiver (which converts one electrical signal into another electrical or optical signal) built into it. If a transceiver is used by itself (e.g. to connect a 25-pin AUI interface to an RJ-45 jack, or to convert electrical signals to optical signals), it is considered a Layer 1 device, because it only looks at bits, and not at any address information or higher level protocols Attachment unit interface.

7. The symbols for media vary. Ethernet, token-ring FDDI, The basic functions of media are to carry a flow of information, in the form of bits and bytes. wireless LANs use theatmosphere, or space PANs (personal area networks, that use the human body as a networking medium) Networking media confine network signals to a wire, cable, or fiber. Networking media are considered Layer 1 components of LANs. Coaxial cable, optical fiber, and even free space can carry network signals The principal medium you will study in this class is called Category 5 unshielded twisted-pair cable (CAT 5 UTP).

8. Repeater The purpose of a repeater is to amplify and retime network signals at the bit level. Repeaters can be single-port "in" and single-port "out" devices, though more often now, they are stackable (modular), or multi-port repeaters, better known as hubs. Repeaters are classified as Layer 1 devices, in the OSI model, because they act only on the bit level and look at no other information.

9. HUBs The purpose of a hub, also known as a multi-port repeater, is to amplify and re-time network signals, at the bit level, to a large number of users (e.g. 4, 8, or even 24) using a process known as concentration. If you have several devices (e.g. hosts) to connect to one shared device (e.g. a server), and it's only reasonable for the server to have one NIC, then you might used a hub. Some hubs passive devices because they split the signal to multiple users. Most hubs are active; they take energy from a power supply and add it to the network signals. Some hubs have console ports, which means they are managed hubs and can be controlled. Many hubs are called dumb hubs because the simply take an incoming networking signal and repeat it to every port (the reason for the term multi-port repeater). Hubs are considered Layer 1 devices because they only look at bits

10. Bridge The purpose of a bridge is to filter traffic on a LAN, to keep local traffic local, yet allow connectivity to other parts (segments) of the LAN for traffic that has been directed there. It looks at the name and the address. Every networking device has a unique NIC card MAC address, the bridge makes its decisions based on this MAC address. The bridge is a Layer 2 device; it uses Layer 2 m processing to make the decision whether or not to forward information

11. Switches The symbol for a switch suggests a two-way flow of data. Switches at first glance often look like hubs, since part of their function is connectivity (allowing many devices to be connected to one point in the network). The front of a switch has interfaces (ports); the back has an ON/OFF button, a power connection, and a console port for managing the switch. The switch uses the MAC address to make its switching decisions. You might think of each port on a switch as a micro-bridge,which makes it a Layer 2 device The purpose of a switch is to concentrate connectivity, while guaranteeing bandwidth. For now, think of the switch as something that is able to combine the connectivity of a hub with the traffic regulation of a bridge on each port. It switches packets from incoming ports (interfaces) to outgoing ports, while providing each port with full bandwidth

12. Routers The symbol for a router is suggestive of its two primary purposes path selection, and switching of router routes, and packets. Each router interface is connected to a different network or network segment, hence it is considered an inter-networking device. The purpose of a router is to examine incoming packets, choose the best path for them through the network, and then switch them to the proper outgoing port. Routers are the most important traffic-regulating devices on large networks. They enable virtually any type of computer (using the appropriate protocols) to communicate with almost any other computer anywhere in (or outside) the world! While performing these basic functions, they can also execute many other tasks. Routers make their path selection decisions based on Layer 3 information (packets) - the network addresses – therefore they are considered Layer 3 devices.Routers can also connect different layer 2 technologies, such as Ethernet, token-ring, and FDDI, but because of their ability to route packets, based on Layer 3 information, routers have become the backbone of the Internet, running the IP protocol.

13. Cloud The purpose of the cloud is to represent a large group of details that are not pertinent to a situation, or description, at a given time. It is important to remember that, at this point in the curriculum, you are only interested in how LANs connect to larger WANs, and to the Internet (the ultimate WAN), so that any computer can talk to any other computer, any place, any time. Because the cloud is not really a device, but a collection of assumed capabilities, it is classified as a Layer 1-7 device.

14. A network diagram that includes several LAN technologies uses different symbols for each. In a way, the network appears to be broken into segments. These segments help control traffic on a network, and occur naturally as different small,networks are linked to make a larger network within a company or school. The segments in the diagram shown can be considered Layer 1 and Layer 2 technologies. While each segment performs operations involving all of the layers, the differences between Ethernet, token-ring, and FDDI,are in their Layer 1 and Layer 2 specifications.

15. The history of computer networking is complex 1940s, computers were huge electromechanical devices that were prone to failure. In 1947, the invention of a semiconductor transistor opened up many possibilities for making smaller, more reliable computers. 1950s, mainframe computers, run by punched card programs, began to be commonly used by large institutions. In the late 1950s, the integrated circuit - that combined several, many, and now millions, of transistors on one small piece of semiconductor - was invented. Through the 1960s, mainframes with terminals were common place, and integrated circuits became more widely used. 60s and 70s, smaller computers, called minicomputers (though still huge by today's standards), came into existence. In 1978, the Apple Computer company introduced the personal computer. In 1981, IBM introduced the open-architecture personal computer. The user friendly Mac, the open architecture IBM PC, and the further micro-miniaturization of integrated circuits lead to widespread use of personal computers in homes and businesses. late 1980s began, computer users - with their stand-alone computers - started to share data (files) and resources (printers).

16. Starting in the 1960s and continuing through the 70s, 80s, and 90s, the Department of Defense (DoD) developed large, reliable, wide area networks (WANS). The repeater (an old device used by telephone networks) was introduced to enable computer data signals to travel farther. The multi-port repeater, or hub, was introduced to enable a group of users to share files, servers and peripherals. You might call this a workgroup network. Because of the functions of hubs (they broadcast all messages to all ports, regardless of destination), as the number of hosts and the number of workgroups grew, there were larger and larger traffic jams. The bridge was invented to segment the network, to introduce some traffic control. The best feature of the hub - concentration /connectivity - and the best feature of the bridge - segmentation - were combined to produce a switch. In the mid-1980s, special purpose computers, called gateways (and then routers) were developed. These devices allowed the interconnection of separate LANs. Internetworks were created.

17. The World Wide Web began in the 1990’s.

18. Hosts and servers operate at Layers 2-7; they perform the encapsulation process. Transceivers, repeaters, and hubs are all considered active Layer 1 devices, because they act only on bits and require energy. Patch cables, patch panels, and other interconnection components are considered passive Layer 1 components because they simply provide some sort of conducting path. NIC cards are considered Layer 2 devices since they are the location of the MAC address; but since they often handle signaling and encoding they are also layer 1 devices. Bridges and switches are considered Layer 2 devices because they use Layer 2 (MAC Address) information to make decisions on whether or not to forward packets. Routers are considered Layer 3 devices because they use Layer 3 (network) addresses to choose best paths and to switch packets to the proper route. Clouds, which may include routers, switches, servers, and many devices we have not yet introduced, involve Layers 1-7.

19. The order of encapsulation is Data, Segment, Packet, Frame and Bits.

20. The packet flow through Layer 1 devices is simple. Physical media are considered Layer 1 components. All they attend to are bits (e.g. voltage or light pulses, wave forms). If the Layer 1 devices are passive (e.g. plugs, connectors, jacks, patch panels, physical media), then the bits simply travel through the passive devices, hopefully with a minimum of distortion. If the Layer 1 devices are active (e.g. repeaters or hubs ), then the bits are actually amplified and re-timed. Transceivers, also active devices, act as adapters (AUI port to RJ-45), as media converters (RJ-45 electrical to ST Optical), or as parts of NIC cards. In all cases the transceivers act as a Layer 1 devices. No Layer 1 device examines any of the headers or data of encapsulated packet. All they care about are bits Many network problems are caused by problems in Layer 1 - poorly terminated cables, broken jacks, improperly installed cable, or repeaters, hubs, or transceivers left unplugged. In addition, Layer 1 devices can be the sources of reflections, near-end crosstalk, short circuits, open circuits, electromagnetic interference, and radio frequency interference (RFI), all of which can corrupt or destroy packets.

21. NIC cards are where the unique MAC address resides; therefore they are shown on the diagram as a Layer 2 device. Bridges work by examining the MAC address (Layer 2) of incoming packets. If the packet is a local packet (with a MAC address on the same network segment as the incoming port of the bridge), then the packet is not forwarded across the bridge. If the packet is non-local (with a MAC address not on the incoming port of the bridge), then it is forwarded to the next network segment.

22. You will not study the details of switching until Semester 3, but for now, consider a switch to be a hub with individual ports that act like individual bridges. The switch takes packets, de-encapsulates them to Layer 2, examines the Layer 2 MAC addresses, and forwards the packets (switches them) to the appropriate ports. Because the switches’ circuitry makes these decisions based on MAC addresses, switches are also considered a Layer 2 device.

23. Certain devices, such as routers, operate at Layers 3, 2, and 1. Packet flow through routers (i.e. selection of best path and actual switching to the proper output port) involves the use of Layer 3 network addresses and Layer 2 MAC addresses (names). Routers are referred to as Layer 3 devices.

24. The graphic shows that certain devices operate at all seven layers. Some devices (e.g. your PC) are Layer 1-7 devices. In other words, they perform processes can be associated with every layer of the OSI model. Encapsulation and de-encapsulation are two examples of this. A device called a gateway (essentially a computer which acts like a router) is also a Layer 7 device. Finally, clouds that may contain several kinds of media, including NIC cards, networking devices, hardware, and software, should also be considered Layer 1-7 devices.

25. You will follow an e-mail packet as it travels through a small LAN. Note that the layer in which the packet is examined depends on the networking device through which it passes. Host I sends an e-mail to host D, host P, host H, and to another host in the internet cloud. The packet starts at the switch and goes to the e-mail server, which sends it out. Follow the packet as it travels to each host. The packet for host D passes through the main switch, the workgroup switch, and arrives at D. The packet for H passes through the main switch, router E0 interface, router T0 interface, around the token- ring, and is finally de-encapsulated by host H. The packet bound for the Internet passes through the F0 interface onto the FDDI ring. The packet for P passes through the router's E0 interface, the router’s E1 interface, the repeater, the bridge, and is finally de-encapsulated at P.

26. Without intelligent devices - which actually examine packets and help guide them through the network - the e-mail message would be seen by every host on every part of every network. Now, imagine two, three, and four computers sending packets and you will understand why there is a need for intelligent Layer 2 and Layer 3 devices that can break up, or segment, the network into regions, and then filter, or screen, the packets based on their physical and logical address information.

27. The End A physical topology is an illustration of how a network is connected. A hub is used to connect multiple computers. With five computers and one hub we would need to use five IP addresses. We use a cross- over cable to connect same type devices together.