Babu Ram Dawadi

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The TheoryWe 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 RateTheory: 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/Nmax bits/sec = H log2 (1+S/N)

Bandwidth & Thruput

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The MediaGuided Transmission Media

MagneticTwisted PairCoaxialFiber

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Magnetic MediaUsed to be the most common form of moving data from

one computer to another – the floppy diskOne 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.

Baseband and BroadbandBaseband: digital signals sent through direct current (DC)

pulses applied to a wireRequires exclusive use of wire’s capacityBaseband systems can transmit one signal at a timeEthernet

Broadband: signals modulated as radiofrequency (RF) analog waves that use different frequency rangesDoes not encode information as digital pulses

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Twisted-Pair CableTwo 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 PairAdvantages

Protect against cross talk & interferenceEasy to add computers to networkWell understood technologyLess expensive

DisadvantagesSusceptibility to noiseLeast secureDistance limitationsRequires more expensive hubs

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Twisted Pair TypesCategory 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

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”

Unshielded Twisted PairPair of wires do not have the

shielding against electrical interference

AdvantagesLess expensiveEasy to install

DisadvantagesVulnerable to electromagnetic

interference & crosswalkSubject to attenuation

STPHas 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 UTPThroughput: STP and UTP can both transmit data at 10,

100, and 1000 Mbps Depending on grade of cabling and transmission method

usedCost: STP usually more expensive than UTPConnector: Both use RJ-45 and RJ-11Noise Immunity: STP more noise-resistantSize and scalability: Max segment length for both is 100

m on 10BASE-T and 100BASE-T networksMaximum of 1024 nodes

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Fibre OpticsFibre 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.

Optical FiberUses light rather than voltage to indicate one

and zerosTransmission 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 OpticsHigh 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

Single Mode fiber Small cable diameter fits anywhere Light weight No sparks if cut No shock hazard Secure communications Low system cost Longer life expectancy than copper or coaxial cable Cabling of the future

DisadvantagesExpensiveDifficult to installRequire two cables to transmit

& receive dataRequire special connections

Multimode fibre 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 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 long-

distance (such as inter-exchange trunk lines in telephone network)

Bending losses should be avoided by using large bending radius (applied to multimode as well).

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Connecting Fibre CablesWe 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 laserslight emitting diodes (LEDs)

Both have their advantages and their disadvantages.

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Coaxial CableCopper center shielded by a plastic

insulating materialIt 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)

BNC Connector

Coax CableAdvantages

Transmits up to 10Mbps over 500mEasy to installLow maintenanceGood resistance to noise over long distances

DisadvantagesInflexibleLow securityLimited distance

Physical Layer Standards

Physical Layer Standards (contd)

T-568A Straight-Through Ethernet Cable T-568B Straight-Through Ethernet Cable

RJ-45 Crossover 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.

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.

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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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DSSSThe 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.

0 0 1 1 0

110011

Chip code for 0: 110011

000111 000111 110011

Chip code for 1: 000111

110011

FHSSMethod 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.

F1

F2

F3

T1 T2 T3 T4 ..

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SwitchingThere are three different types of switching:

circuit switchingmessage switchingpacket switching

Circuit SwitchingA 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 SwitchingNo 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 SwitchingThere 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.

Circuit switching

Packet switching

Message switching

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Circuit vs Packet Switching

PHY Devices: Repeaters, HubsRepeaters

Repeater receives a signal and regenerates the original bit pattern

Can extend the physical length of LANConnects segments of a LANForwards every frames; has no filtering capabilityIt is a regenerator; not an amplifier

HubsIs a multiport repeaterNormally used between stations in a physical star topology