Lan

Now that you have a basic understanding of the OSI model and what happens to data packets as they

travel through the layers, it is time for you to start looking at basic networking devices. By working

through the layers of the OSI reference model, you will learn what devices operate at each layer as

data packets travel through them from the source to the destination. The focus of this chapter will be

local area network or LAN devices. As you know, LANs are high-speed, low-error data networks that

cover a relatively small geographic area (up to a few thousand meters). LANs connect workstations,

peripherals, terminals, and other devices in a single building or other geographically limited areas.



In this chapter, you will learn about basic LAN devices and the evolution of networking devices. You

will also learn about the networking devices that operate at each layer of the OSI model and how

packets flow through each device as they go through the layers of the OSI model. Lastly, you will learn

about the basic steps in building LANs. Finally, as you work through this chapter, keep in mind that by

interconnecting networking devices, LANs provide multiple connected desktop devices (usually PCs)

with access to high-bandwidth media.





Basic LAN Devices

The teaching topology



Topology defines the structure of the network. There are two parts to the topology definition: the

physical topology, which is the actual layout of the wire (media), and the logical topology, which

defines how the media is accessed by the hosts. The physical topologies that are commonly used are

the Bus, Ring, Star, Extended Star, Hierarchical, and Mesh. These are shown in the graphic.



 A bus topology uses a single backbone segment (length of cable) that all the hosts connect to

directly.

 A ring topology connects one host to the next and the last host to the first. This creates a

physical ring of cable.

 A star topology connects all cables to a central point of concentration. This point is usually a

hub or switch, which will be described later in the chapter.

 An extended star topology uses the star topology to be created. It links individual stars

together by linking the hubs/switches. This, as you will learn later in the chapter, will extend

the length and size of the network.

 A hierarchical topology is created similar to an extended star but instead of linking the

hubs/switches together, the system is linked to a computer that controls the traffic on the

topology.

 A mesh topology is used when there can be absolutely no break in communications, for

example the control systems of a nuclear power plant. So as you can see in the graphic, each

host has its own connections to all other hosts. This also reflects the design of the

Internet, which has multiple paths to any one location.



The logical topology of a network is how the hosts communicate across the medium. The two most

common types of logical topologies are Broadcast and Token-passing.



Broadcast topology simply means that each host sends its data to all other hosts on the network

medium. There is no order the stations follow to use the network, it is first come, first serve. This is the

way that Ethernet works and you will learn much more about this later in the semester.



The second type is token-passing. Token-passing controls network access by passing an electronic

token sequentially to each host. When a host receives the token, that means that that host 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.



The diagram in the graphic shows many topologies. It shows a LAN of moderate complexity that is

typical of a school or a small business. It has many symbols, and it depicts many networking concepts

that will take time to learn. This LAN is typical of a small campus, and represents most of the devices

that you will study for your CCNA.



LAN devices in a topology



Devices that connect directly to a network segment are referred to as hosts. These hosts include

computers, both clients and servers, printers, scanners, and many other user devices. These devices

provide the users with connection to the network, with which the users share, create, and obtain

information. The host devices can exist without a network, but without the network we have greatly

limited the hosts capabilities. This purpose of a LAN was discussed in Chapter 1.



Host devices are not part of any layer. They have a physical connection to the network media by

having a network interface card (NIC) and the other OSI layers are performed in software inside the

host. This means that 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 databases. For those that are familiar with the inner workings of PCs, the PC itself may

be thought of as a tiny network that connects the bus and expansion slots to the CPU, RAM, and

ROM.



There are not standardized symbols within the networking industry for hosts, but they are usually fairly

obvious to figure out. They bear a resemblance to the real device so that you are constantly reminded

of that device.



The basic function of computers on the LAN is to provide the user with an almost limitless set of

opportunities. Modern software, microelectronics, and a relatively small amount of money, enable you

to run word processing, presentation, spreadsheet, and database programs. They also enable you to

run a web browser, which gives you almost instant access to information via the World Wide Web. You

can send e-mail, edit graphics, save information in databases, play games, and communicate with

other computers around the world. The list of applications grows each day.



NICs

So far in this chapter, we have dealt with layer one devices and concepts. Starting with the network

interface card, the discussion moves to layer two, the data link layer, of the OSI model. In terms of

appearance, a network interface card (NIC card or NIC) is a printed circuit board that fits into the

expansion slot of a bus on a computer’s motherboard or peripheral device. It is also called a 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 code, called a Media Access Control (MAC) address. This address is used to control data

communication for the host on the network. You will learn more about the MAC address later. As the

name implies, the NIC controls the host's access to the medium.



In some cases the type of connector on the NIC does not match the type of media that you need to

connect to. A good example is your Cisco 2500 router. On the router you will see AUI (Attachment Unit

Interface) connectors and you need to connect the router to a UTP Cat5 Ethernet cable. To do this a

transceiver (transmitter/receiver) is used. A transceiver converts one type of signal or connector to

another (e.g. to connect a 15-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.



NICs have no standardized symbol. It is implied that whenever you see networking devices attached

to network media, there is some sort of NIC or NIC-like device present even though it is generally not

shown. Wherever you see a dot on a topology, there is either a NIC or an interface (port), which acts

like at least part of a NIC.





Media

The symbols for media vary. For example: the Ethernet symbol is typically a straight line with

perpendicular lines projecting from it; the token-ring network symbol is a circle with hosts attached to

it; and for FDDI, the symbol is two concentric circles with attached devices.



The basic functions of media are to carry a flow of information, in the form of bits and bytes, through a

LAN. Other than wireless LANs (that use the atmosphere, or space, as the medium) and the new

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.



You can build computer networks with many different media types. Each media has advantages and

disadvantages. What is an advantage for one media (category 5 cost) might be a disadvantage for

another (fiber optic cost). Some of the advantages and disadvantages are:



 Cable length

 Cost

 Ease of installation



Coaxial cable, optical fiber, and even free space can carry network signals, however, the principal

medium you will study is called Category 5 unshielded twisted-pair cable (CAT 5 UTP).



Switches

A switch is a Layer 2 device just as a bridge is. In fact a switch is called a multi-port bridge, just like a

hub is called a multi-port repeater. The difference between the hub and switch is that switches make

decisions based on MAC addresses and hubs don't make decisions at all. Because of the decisions

that switches make, they make a LAN much more efficient. They do this by "switching" data only out

the port to which the proper host is connected. In contrast, a hub will send the data out all of its ports

so that all of the hosts have to see and process (accept or reject) all of the data.



Switches at first glance often look like hubs. Both hubs and switches have many connection ports,

since part of their function is connectivity concentration (allowing many devices to be connected to one

point in the network). The difference between a hub and a switch is what happens inside the device.



The purpose of a switch is to concentrate connectivity, while making data transmission more efficient.

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 frames from incoming ports (interfaces) to

outgoing ports, while providing each port with full bandwidth (the transmission speed of data on the

network backbone). You will learn more about this later.



The symbol for a switch is shown in the graphic. The arrows on top represent the separate paths

data can take in a switch, unlike the hub, where all data flows on all paths.



Routers

The router is the first device that you will work with that is at the OSI network layer, or otherwise

known as Layer 3. Working at Layer 3 allows the router to make decisions based on groups of

network addresses (Classes) as opposed to individual Layer 2 MAC addresses. Routers can also

connect different Layer 2 technologies, such as Ethernet, Token-ring, and FDDI. However, because of

their ability to route packets based on Layer 3 information, routers have become the backbone of the

Internet, running the IP protocol.



The purpose of a router is to examine incoming packets (Layer 3 data), 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 to

communicate with any other computer anywhere in the world! While performing these basic functions,

routers can also execute many other tasks that are covered in later chapters.



The symbol for a router (Note the inward- and outward-pointing arrows.) is suggestive of its two

primary purposes - path selection, and switching of packets to the best route. A router can have

many different types of interface ports. Figure shows a serial port which is a WAN connection. The

graphic also shows the console port connection which allows direct connection to the router to be able

to configure it. Figure shows another port interface type. The type shown is an Ethernet port which is

a LAN connection. This particular router has both a 10BASE-T and AUI connector for the Ethernet

connection.



Clouds

The cloud symbol suggests another network, perhaps the entire Internet. It reminds us that there is a

way to connect to that other network (the Internet), but does not supply all the details of either the

connection or the network.



The physical features of the cloud are many. To help you understand, you might think of all of the

devices that connect your computer to some very distant computer, perhaps on another continent.

There is no single picture that could display all of the processes and equipment that would be

involved in making that connection.



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 and any time. Because the cloud is not really a

single device, but a collection of devices that operate at all levels of the OSI model, it is classified as a

Layer 1-7 device.





Evolution of Network Devices

Evolution of network devices



The history of computer networking is complex, involving many people from all over the world over the

past thirty years. What is presented here is a simplified view of how the devices you have been

studying evolved from each other. The processes of invention and commercialization are far more

complicated, but it is helpful to look at the problems that each computer device solved and the

problems that still remain.



In the 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. In the 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.



In the late 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. As the late 1980s began, computer

users - with their stand-alone computers - started to share data (files) and resources (printers). People

asked, why not connect them?



While all of this was happening, telephone systems continued to improve. Especially in the areas of

switching technology and long distance service (because of new technologies like microwaves and

optical fibers), a worldwide, reliable telephone system evolved.



Starting in the 1960s and continuing through the 70s, 80s, and 90s, the Department of Defense (DoD)

developed large, reliable, wide area networks (WANS). Some of their technology was used in the

development of LANs, but more importantly, the DoDs WAN eventually became the Internet.

To help you understand the next technological advancement, consider the following problem.

Somewhere in the world, there were two computers that wanted to communicate with each other. In

order to do so, they both needed some kind of device that could talk to the computers and the media

(the NIC card), and some way for the messages to travel (medium).



Suppose, also, that the computers wanted to communicate with other computers that were a great

distance away. The answer to this problem came in the form of repeaters and hubs. 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.



Soon, work groups wanted to communicate with other work groups. 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. It had lots of ports, but allowed each port to

pretend it had a connection to the other side of the bridge, thus allowing many users and lots of

communications.



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. The DoD

already had an extensive internetwork, but the commercial availability of routers - which carried out

best path selections and switching for data from many protocols - caused the explosive growth of

networks that we are experiencing today. The cloud represents that growth.



With the arrival of the new century, the next step is convergence of computer and communications

technology, specifically, the convergence of voice, video, and data - which have traditionally traveled

via different systems - into one information stream.



Evolution of networking devices and the OSI layers



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.



NICs 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 decision on

whether or not to forward packets. They also operate on Layer 1 in order to allow bits to interact with

the media.



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. Router interfaces operate at Layers 2 and 1 as

well as Layer 3. Clouds, which may include routers, switches, servers, and many devices we have not

yet introduced, involve Layers 1-7.