Sem 3 Chapter 2 LAN Switching

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Sem 3 Chapter 2LAN Switching
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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 computers
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.
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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.
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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).
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Attenuation is a signal losing energy to its
surroundings. This means our one bit voltage
signal loses height, or amplitude, as energy is
given from the message to the cable. While
careful choice of materials, such as copper
instead of carbon, and geometry, the shape and
positioning of the wires, can reduce the
electrical attenuation, some loss is always
unavoidable when electrical resistance is
present.
Attenuation also happens to optical signals --
the optical fiber absorbs and scatters some of
the light energy as the light pulse, one bit,
travels down the fiber. This can be minimized
by the wavelength, or color, of the light chosen.
This can also minimized by whether or not
single mode or multimode fiber is used and the
actual glass used for the fiber. Even with
these choices, signal loss is unavoidable.
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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.
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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
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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
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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
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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 - 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.
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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.
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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.
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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.
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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). 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.
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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 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.
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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.
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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.
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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.
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Dispersion, jitter, and latency are actually
three different things that can happen to a bit,
they are grouped together because they each
effect the timing of the 1 bit. Dispersion
is when the pulse broadens in time. It is a
function of the material properties and geometry
of the medium involved. If serious enough, 1 bit
can start to interfere with the next bit and
confuse it with the bits before and after it.
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