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CCNA2: Networking MediaCopper Media
Cable specifications
Coaxial cable
STP cable
UTP cable
Optical Media
Multimode fiberSingle-mode fiber
Other optical components
Signals and noise in optical fibers
Installation, care, and testing of optical fiber
Wireless Media
Wireless LAN organizations and standards
Wireless devices and topologiesHow wireless LANs communicate
Authentication and association
The radio wave and microwave spectrums
Signals and noise on a WLAN
Wireless security
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Cable Specifications
Cables have different specifications and expectations pertaining toperformance:
What speeds for data transmission can be achieved using a particular typeof cable? The speed of bit transmission through the cable is extremelyimportant. The speed of transmission is affected by the kind of conduit
used. What kind of transmission is being considered? Will the transmissions
be digital or will they be analog-based? Digital or baseband transmissionand analog-based or broadband transmission are the two choices.
How far can a signal travel through a particular type of cable beforeattenuation of that signal becomes a concern? In other words, will the signalbecome so degraded that the recipient device might not be able toaccurately receive and interpret the signal by the time the signal reachesthat device? The distance the signal travels through the cable directlyaffects attenuation of the signal. Degradation of the signal is directlyrelated to the distance the signal travels and the type of cable used.
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Coaxial Media
Shielding allows signal to travel 500m
unrepeated
Relatively inexpensive
Medium difficulty to install
Limited to 10Mbps
Relatively unreliable
Only usable in bus topologies
Not the easiest to install
Not the least expensive medium
Coaxial Pros and Cons
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Shielded Twisted Pair
In shielded cable, shielding material protects the data signal from external sources of noise and
From noise generated by electrical signals within the cable.Benefits of shielding
protects the inner conductor from external electrical noise
keeps the transmitted signal confined to the cable
protects each wire pair from noise generated by the other pairs
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Unshielded Twisted Pair
Inexpensive media
Supports speeds up to 1 Gbps
Easiest medium to install
Most widely used for workstation
connections, telephone, etc.
Signal can only travel 100m because of
no shielding (50m if used for Gigabit
Ethernet)
Highly susceptible to outside
interference
UTP Pros and Cons
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Optical Media
Light is one type of electromagnetic
energy
Electromagnetic energy is produced when
an electric charge moves back and forth,or accelerates
This energy in the form of waves can
travel through a vacuum, the air, andthrough some materials like glass
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Ray Model of Light
Wavelengths that are not visible to the human eye are used totransmit data over optical fiber.
These wavelengths are slightly longer than red light and are calledinfrared light
Think of light rays as narrow beams of light like those produced bylasers. In the vacuum of empty space, light travels continuously in astraight line at 300,000 kilometers per second
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Optical Media
Light travels at different, slower speeds
through other materials like air, water, and
glass.
When a light ray called the incident ray,crosses the boundary from one material to
another, some of the light energy in the ray
will be reflected back.
That is why you can see yourself in windowglass. The light that is reflected back is called
the reflected ray
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Refracted ray
Refracted ray- light energy in the incidentray that is not reflected will enter the glass.
The entering ray will be bent at an angle from
its original path. How much the incident light ray is bent
depends on the angle at which the incident
ray strikes the surface of the glass & the
different rates of speed at which light travelsthrough the two substances
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Angle of incidence
Angle of incidence : angle between the incident ray and
a line perpendicular to the surface of the glass at the
point where the incident ray strikes the glass
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Refraction
Index of Refraction - ratio of the speed of light in amaterial to the speed of light in a vacuum
Total Internal Reflection - A light ray that is being turned
on and off to send data (1s and 0s) into an optical fiber
must stay inside the fiber until it reaches the far end
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Total internal reflection
Total Internal Reflection- 2 conditions
Core of the optical fiber has to have a larger index of
refraction (n) than the material that surrounds it.
The angle of incidence of the light ray is greater than the
critical angle for the core and its cladding.
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Multimode Fiber
Multimode Fiber
diameter of the core of the fiber is large
enough so that there are many paths that light
can take through the fiber
Single-mode fiber
smaller core
light rays to travel along one mode
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Fiber optic
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Fiber optic 2 glass fibers encased in separate sheaths
One fiber carries transmitted data from device A to device B.
The second fiber carries data from device B to device A.
The fibers are similar to two one-way streets going in
opposite directions.
Full-duplex communication link
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Multimode fiber (62.5/125)
Five parts make up each fiber-optic cable core
cladding
buffer
strength material
outer jacket
Carry data distances of up to 2000 meters (6,560 ft)
2 types of light source Infrared Light Emitting Diodes (LEDs)
Vertical Cavity Surface Emitting Lasers (VCSELs)
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Single-mode Fiber
Single-mode fiber consists of the same parts as multimode. core fiber has a diameter of9 microns and the surrounding cladding
is 125 microns in diameter.
infrared laser is used as the light source
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Single-mode Fiber
single-mode allows only one mode of light to
propagate through the smaller, fiber-optic
core.
3000 meters
More expensive
Used for inter-building connectivity
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Single-mode Fiber
Warning: The laser light used with single-mode has a
longer wavelength than can be seen. The laser is so
strong that it can seriously damage eyes.
Neverlook at the near end of a fiber that is connected toa device at the far end.
Neverlook into the transmit port on a NIC, switch, or
router. Remember to keep protective covers over the
ends of fiber and inserted into the fiber-optic ports of
switches and routers. Be very careful.
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Transmission Devices
Optical fiber links use light to send data
To convert light into electricity, you need a
transmitter & receiver
There are 2 types of light sources used to
encode and transmit the data through the
cable
LEDs
LASERS
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Signals & Noise in Optical Fibers
Advantages
not affected by the sources of external noise
no problem with crosstalk
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Signals & Noise in Optical Fibers
Disadvantages
Scattering
Absorption Attenuation
Dispersion
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Scattering
Reflects and scatters some of the light
energy
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Absorption
cause of light energy loss
When a light ray strikes some types of
chemical impurities in a fiber, the impurities
absorb part of the energy.
This light energy is converted to a small
amount of heat energy.
Absorption makes the light signal a littledimmer
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Attenuation
Due to manufacturing irregularities or roughness in the core-to-claddingboundary
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Dispersion
limits transmission distances on a fiber
Dispersion is the technical term for the
spreading of pulses of light as they travel
down the fiber
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Installation, care, and testing of optical fiber
Too much attenuation
improper installation. Fiber stretched or curved too tightly
cause tiny cracks in the core that will scatter the light rays.
Bending the fiber changes the incident angle of light rays striking the core-to-
cladding boundary
Connectors and the ends of the fibers must be kept spotlessly clean.
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Fiber noise
Fiber noise- diminishes the strength of the
light signal
Types of fiber noise
Scattering
absorption
dispersion
improper installation dirty fiber ends
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Wireless LANs
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Wireless Networking
Increasingly important as computing devices
become smaller and more mobile
Laptops
Handhelds
Cell phones
Important Concerns
Bandwidth
Security
Interoperability
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Wireless LAN Standards
802.11 General Standards Number
802.11b 11 Mbps WEP
Security:
64-bit or
128-bit
encryptio
n
Uses same
frequency range as
some cordless
phones (2.4 Ghz)
802.11a 54 Mbps Not interoperablewith otherstandards uses
higher frequency
range (5 GHz)
802.11g 54 Mbps Interoperable with802.11b because it
uses the same
frequency range
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Wireless Network Adapters
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Wireless Access Point (WAP)
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Wireless Access Point
WAP
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Wireless Range
As range from WAP increases, negotiated speed is reduced toensure data integrity.
Typical Range is300-500 feet.
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Authentication & Association
Who has access to your network and where?
WLAN Authentication occurs at Layer 2 and identifiesthe DEVICE not the USER
Authentication keys must match on the WAP and theWireless NIC
ASSOCIATION means that your wNIC communicatesthrough a particular WAP. This assignment can bechanged manually, or automatically as he user movesthrough the WLAN. Association can also be restrictedbased on MAC address.
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WLAN Communications Process
Client probes, finds a WAP
Client transmits authentication key
WAP accepts/rejects key, notifies client ofresult
Client is associated with the WAP
All other network processes begin (i.e.
dynamic IP address assignment, networklogin, etc.)
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Two Types of WLANs: Ad-Hoc
Ad-hoc system
Also known as an Independent Basic Service
Set (IBSS) or Wireless Peer-to-Peer network
All clients communicate directly with one
another via their wireless adapters
No WAP is used, and there is no access to a
wired network infrastructure
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Two Types of WLANS: Infrastructure
Uses a WAP to provide access to a wired
network
Technical Term: Basic Service Set (BSS)
Multiple WAPs can be used to form
microcells with overlapping coverage
areas
Technical Term: Extended Service Set (ESS)
Allows the user to roam
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Roaming and WAPs
As client moves closerto another WAP, it re-authenticates and re-associates.
This process occurs inthe background and istransparent to the user.
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WLAN Roaming
Network Admin configures multiple WAPs with
overlapping coverage, each one set to a
different radio channel
When client probes, it chooses a WAP based onsignal strength and error rates
If signal strength weakens, client will search for
another WAP and re-tune to a new frequency,
then authenticate and associate with the new
WAP
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Antennae
Unidirectional- emits signal
in one direction only.
Omnidirectional- emits signal
equally in all directions.
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NoiseNoise
Noise related to communications refers to undesirable signals.
Noise can originate from natural and technological sources, and
is added to the data signals in communications systems
Here are some possible sources of noise:
Nearby cables which carry data signals Radio frequency interference (RFI), which is noise from
other signals being transmitted nearby
Electromagnetic interference (EMI), which is noise from
nearby sources such as motors and lights
Laser noise at the transmitter or receiver of an optical signal
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AttenuationAttenuation
Attenuation is the decrease in signal amplitude over
the length of a link (cable)
Causes:
Long cable lengths
High signal frequencies
resistance of the copper cable
leaks through the insulation
impedance caused by defective connectors
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Impedance
Impedance is a measurement of the resistance of the cable toalternating current (AC) and is measured in ohms
Cat5 cable is 100 ohms
A connector improperly installed will have a different impedance
value than the cable
This is called an impedance discontinuity or an impedance
mismatch
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Impedance DiscontinuitiesImpedance Discontinuities
Impedance discontinuities cause attenuation because a portion
of a transmitted signal will be reflected back
This effect with other discontinuities create echoes making it
difficult for the receiver to accurately detect the signal.
This is called jitter and results in data errors
The effects of signal attenuation and impedance mismatch on a
communications link is called insertion loss
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Noise on Copper Media
TIA/EIA-568-B certification of a cable now requires testing for a
variety of types of noise.
Crosstalk involves the transmission of signals from one wire to a
nearby wire
Alien crosstalk is caused by a signal on another cable
There are three distinct types of crosstalk (noise):
Near-end Crosstalk (NEXT)
Far-end Crosstalk (FEXT)
Power Sum Near-end Crosstalk (PSNEXT)
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