Community College of Rhode Island, Cisco
Regional Academy
Sem 1 V2
Chapter 3
Local Area Networks
LANS
C. Marandola Cisco I Fall
2001
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.
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.
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
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.
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.
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).
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.
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
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
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 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
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
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.
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.
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.
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).
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.
The World Wide Web began in the 1990’s.
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.
The order of encapsulation is Data, Segment, Packet,
Frame and Bits.
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.
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.
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.
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.
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.
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 P passes through the
router's E0 interface, the router’s E1
interface, the repeater, the bridge,
and is finally de-encapsulated at P.
The packet for H passes through the
main switch, router E0 interface,
router T0 interface, around the tokenring, and is finally de-encapsulated by
host H.
The packet bound for the Internet
passes through the F0 interface onto
the FDDI ring.
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.
We use a crossover cable to
connect same
type devices
together.
A physical
topology is an
illustration of
how a network
is connected.
The
End
A hub is used to
connect
multiple
computers.
With five
computers and
one hub we
would need to
use five IP
addresses.