Download CH5_PhysicalLayer

Babu Ram Dawadi
The Theory
 We pass information by breaking it down into bits (1s
and 0s) that can be passed along to another machine
and then put back together to recreate the message.
 With a physical transmission medium, we must
determine a way to vary the physical property of the
medium to denote which bit is being transferred.
 We have control over both voltage and current.
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Maximum Data Rate
 Theory: A perfect (noiseless) channel will still have a
finite transmission capacity.
 Introducing noise into a channel will further reduce the
capacity of that channel.
 Max Rate (rarely achieved): bandwidth = H and the
signal-to-noise ratio is S/N
 max bits/sec = H log2 (1+S/N)
 Bandwidth & Thruput
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The Media
 Guided Transmission Media
 Magnetic
 Twisted Pair
 Coaxial
 Fiber
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Magnetic Media
 Used to be the most common form of moving data from
one computer to another – the floppy disk
 One of the most efficient ways of moving massive
amounts of data from one machine to another would be
magnetic tapes – they are high volume and low cost.
 One tape can hold approx 200GB of data.
 You can put about 1000 tapes in a 60 x 60 x 60 cm box (that’s
2’ x 2’ x 2’) for a total capacity of 200 terabytes = 1600 terabits
= 1.6 petabits.
 Moving a box of tapes is quite easy and efficient. Unless the
tapes spend days in transit, they will be much faster than any
network.
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Baseband and Broadband
 Baseband: digital signals sent through direct current (DC)
pulses applied to a wire
 Requires exclusive use of wire’s capacity
 Baseband systems can transmit one signal at a time
 Ethernet
 Broadband: signals modulated as radiofrequency (RF)
analog waves that use different frequency ranges
 Does not encode information as digital pulses
Twisted-Pair Cable
 Two wires close together can act like an antenna and cause
interference with the signal. Twisting the wires “cancels
out” the waves and reduces the interference problem. It
should be noted that twisted pair wires are insulated wires,
not bare.
 Twisted pair is often used to run phone lines from the house
to the phone switch.
 Twisted pair can run for kilometres without amplification.
 The twisting not only prevents problems between the two
wires, but it also reduces interference from other wires,
allowing them to be bundled together in large numbers.
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Twisted Pair
 Advantages
 Protect against cross talk & interference
 Easy to add computers to network
 Well understood technology
 Less expensive
 Disadvantages
 Susceptibility to noise
 Least secure
 Distance limitations
 Requires more expensive hubs
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Twisted Pair Types
 Category 1—Used for telephone communications. Not suitable for
transmitting data.
 Category 2—Capable of transmitting data at speeds up to 4 megabits per
second (Mbps).
 Category 3—Used in 10BASE-T networks. Can transmit data at
speeds up to 10 Mbps.
 Category 4—Used in Token Ring networks. Can transmit data at speeds up to
16 Mbps.
 Category 5 – a tighter twist, same number of wires, just less
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crosstalk and higher speeds (100 MHz)
Category 6 (250 MHz) – pairs of 24 American Wire Gauge (AWG)
copper wires. Category 6 cable is currently the fastest standard for
UTP.
Category 7 (600 MHz) is upcoming.
All twisted pair is “unshielded” except for some stuff used by IBM in
the early 80s.
These unshielded wires are referred to as “UTP” or “Unshielded
Twisted Pair”
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Unshielded Twisted Pair
 Pair of wires do not have the
shielding against electrical
interference
 Advantages
 Less expensive
 Easy to install
 Disadvantages
 Vulnerable to electromagnetic
interference & crosswalk
 Subject to attenuation
STP
 Has an added shield of copper braid around all the wire
pairs, typical 100 or 150  imp.
 Heavier and larger size than UTP, may have higher
attenuation, required larger bending radius and grounding
problem.
 No longer widely used, typical application IBM Token
Ring system.
 Reduces electromagnetic interference (EMI)
Comparing STP and UTP
 Throughput: STP and UTP can both transmit data at 10,
100, and 1000 Mbps
 Depending on grade of cabling and transmission method
used
 Cost: STP usually more expensive than UTP
 Connector: Both use RJ-45 and RJ-11
 Noise Immunity: STP more noise-resistant
 Size and scalability: Max segment length for both is 100
m on 10BASE-T and 100BASE-T networks
 Maximum of 1024 nodes
Fibre Optics
 Fibre optics are based on the principle that light will
refract at a particular angle given a specific medium.
 This allows us to “shoot” a light down a fibre “cable”
where all of the light bounces back into the cable when it
hits the edge.
 The result is the ability to send data down a fibre “cable”
at the speed of light for extended distances with no loss.
 We must have a light source in order to send the data. It
sends a ray of light for each 1 bit, and no light for a 0 bit.
 We must have a detector to detect the light signal. The
detector emits an electric pulse for each light ray it
detects.
 The slowest part of the system is the conversion that
happens at either end.
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Optical Fiber
 Uses light rather than voltage to indicate one
and zeros
 Transmission distance 2km for multimode, >
40 km in single mode, (only 100 m maximum
for copper).
 Bandwidth: 1G Hz (multiple mode), > 100
GHz (singlemode).
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Advantages of Fiber Optics
 High data rate and wide bandwidth
 Immunity to EMI/RFI and lightning damage
 Low attenuation (data loss)
 Longer distance - 2 and 5 km with Multimode fiber or over 25 km with
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Single Mode fiber
Small cable diameter fits anywhere
 Disadvantages
Light weight
 Expensive
 Difficult to install
No sparks if cut
 Require two cables to transmit &
No shock hazard
receive data
 Require special connections
Secure communications
Low system cost
Longer life expectancy than copper or coaxial cable
Cabling of the future
Multimode fibre
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The light rays are transmitted from one end to the other end via
more than one operating mode (i.e the number of total internal
reflections during its transmission).
The time for travelling in different modes are different, due to
the effective distance are different. This is called model
dispersion.
Model dispersion is one of the key factor in limiting the
bandwidth.
Singlemode fibre
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No modal dispersion, (as the light rays are transmitted
only in one mode) hence very high bandwidth
Usually only used in very high bit rate and longdistance (such as inter-exchange trunk lines in
telephone network)
Bending losses should be avoided by using large
bending radius (applied to multimode as well).
Connecting Fibre Cables
 We can terminate a fibre by plugging it into a fibre socket.
We loose about 10-20% of the light.
 We can splice them mechanically by putting the two cut
ends into a sleeve and clamping them. Although they can
be adjusted, they loose about 10%.
 We can fuse them to form a solid construction. There is
still some minimal loss.
 We have two different light sources:
 semiconductor lasers
 light emitting diodes (LEDs)
 Both have their advantages and their disadvantages.
4343 X2
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Coaxial Cable
 Copper center shielded by a plastic insulating
material
 It is shielded better than twisted pair, so you get
much longer distances and higher bandwidth (up
to 1GHz)
 There are two types:
 50-ohm (digital)
 75-ohm (analog and cable tv)
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BNC Connector
Coax Cable
 Advantages
 Transmits up to 10Mbps over 500m
 Easy to install
 Low maintenance
 Good resistance to noise over long distances
 Disadvantages
 Inflexible
 Low security
 Limited distance
Physical Layer Standards
Physical Layer Standards (contd)
T-568A Straight-Through Ethernet Cable
T-568B Straight-Through Ethernet Cable
 A straight-thru cable has identical ends.
 A crossover cable has different ends.
 A straight-thru is used as a patch cord in Ethernet
connections.
 A crossover is used to connect two Ethernet devices
without a hub or for connecting two hubs.
RJ-45 Crossover Ethernet Cable
 A crossover has one end with the Orange set of
wires switched with the Green set.
 Odd numbered pins are always striped, even
numbered pins are always solid colored.
 Brown is always on the right, and pin 1 is on the
left.
Wireless
 One disadvantage of each of the data transmission
methods that we have seen so far is that they are all wired
connections.
 Moving from a wired connection to a wireless connection
results in the ability to connect to a network without
having a physical connection.
 Electromagnetic waves (radio waves) can move through
space.
 The number of oscillations per second is called the
frequency (f) and is measured in Hertz (Hz).
 The distance between two consecutive “waves” is known
as the wavelength (λ)
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The Wireless Range
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DSSS
 The signal is spread over the entire spectrum, not specific
frequencies within that spectrum.
 There are benefits in terms of noise immunity and
efficiency.
 It is used in some wireless LANs and mobile phones with
2.4Ghz ISM band.
 In DSSS, each bit sent is replaced by a sequence of bits
called a chip code.
Chip code for 0: 110011
0
110011
0
110011
Chip code for 1: 000111
1
1
000111
000111
0
110011
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FHSS
 Method for signal generation in a 2.4GHz ISM band, in which
the signal sends on one carrier frequency for a T1 period of
time and another frequency for T2 times, hops again for T3 and
so on. After N hop the cycle repeats (T1=T2=T3=…)
 If the bandwidth of the original signal is B, the allocated spread
spectrum bandwidth is NXB.
F3
F2
F1
T1
T2
T3
T4 ..
Switching
 There are three different types of switching:
 circuit switching
 message switching
 packet switching
 Circuit Switching
 A physical connection is needed for the phone call to go
through.
 This used to be done by a person at a switchboard.
 Now it is done automatically.
 Setting up the circuit can still take time, depending on how far
the call is going and how many switches it passes through.
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Message Switching
 No physical path is set up between the sender and
receiver.
 The whole message (or block of data) is sent to the
switching office.
 Once it has been received, it is inspected for errors and is
then sent to the next switching office.
 This method is not used anymore.
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Packet Switching
 There is no physical connection for packet switching.
 The data is broken up into packets by the sender and
they are sent to the switching office.
 The first packet can easily be sent to the next
switching office before the second packet has arrived.
 This makes packet switching useful for busy
networks.
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Circuit switching
Packet switching
Message switching
Circuit vs Packet Switching
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PHY Devices: Repeaters, Hubs
 Repeaters
 Repeater receives a signal and regenerates the original bit
pattern
 Can extend the physical length of LAN
 Connects segments of a LAN
 Forwards every frames; has no filtering capability
 It is a regenerator; not an amplifier
 Hubs
 Is a multiport repeater
 Normally used between stations in a physical star topology