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Physical Layer Fundamentals
Pawan Shriniwas Parande
Understanding the first layer of many of the
networking architectures is interesting and
challenging. I am compiling the knowledge,
which I can recollect in this document. This
layer is also called the “Touch-And-Feel” layer
and provides for the real transmission of data.
Popularly known as the “Layer 1” in OSI
model, this layer converts the frames received
from the Datalink layer into electrical, optical
or electromagnetic signals. A frame is a
maximum transferable chunk of data, which is
also known as an IP datagram. A frame
contains both actual data and control
information in it. Many such frames
accumulated by the Physical layer over a
period of time are transmitted as bursts at equal
or unequal intervals of time. This burst of
information, if are transmitted over a wire, is
transmitted as electrical signals, which flow on
the media in a simple harmonic motion. The
concept of harmonic motion is directly related
with “Analog Communication”. Both landline
and mobile phones, modems, fax machines and
many more devices follow analog
communication principles. Analog
communication principles have greatly
influenced voice transmission than the data
transmission.
UNDERSTANDING THE PHYSICS
Analog communication and simple harmonic
motion are closely related. Consider this
physics experiment. Let a weight is suspended
at one end of a spring. Let the other end be
hinged to a ceiling. Now if this weight is
pulled down and released then the spring
begins to oscillate up and down. Under a
friction less environment, these oscillations
will continue forever and will display some
consistent characteristics like frequency,
amplitude and wavelength. This idealized
motion is called “Simple Harmonic Motion”.
Many mechanical oscillators and pendulums
display the principles of simple harmonic
motion. Certain natural phenomena like
oceanic tides, water waves, and climatic
wind cycles display the simple harmonic
motion principles. Alternative current, sound
waves, optical waves and radio waves also
display the simple harmonic motion
principles.
In analog communication, the signals flow
across a wire in the form of electromagnetic
waves. These waves resemble a sine curve
and follow the characteristics of amplitude,
frequency and phase. Amplitude is the level
of voltage on a wire. It is also the intensity of
the light beam in a fiber optic. The definition
of frequency is the number of oscillations per
second. This can also be interpreted as a
specific number of waves generated in a
specific interval of time. Phase can be
described a point of advancement of a wave
in one cycle. The frequency rate of one cycle
per second is called 1 Hertz. Hertz is a
measure of frequency in cycles per second.
The inverse of frequency is called period. A
period indicates the time taken to complete
one cycle.

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These concepts can be better understood by
correlating them like this. On a telephone,
wherever we speak softly, the amplitude
(voice) decreases. Yelling at a person in louder
voice increases the amplitude. In a flute, the
upper octave notes, sound shrill and acute.
Here more waves are generated in a period of
time, hence the frequency is high. But as the
notes are lowered and we come to lower
octave, the sound is flat and coarse. Here lesser
waves are generated in a fixed amount of time
and the frequency is less. In data
communication the data is represented as an
analog wave where the voltage varies
regularly. This is called “Amplitude
Modulation”. By varying frequency regularly
we achieve “Frequency Modulation”. Phase
shifting can vary even the phase. This is called
phase modulations and this phase modulation
is a common working principle adopted by
modems.
Let us analyse the wavelength now. This is the
measure of the distance traveled by a signal in
a cycle. The radio signals are usually
characterized by there wavelengths. Short
wave, medium wave and long radio signals
imply the wavelength of the signals being
short, medium and long. For example a 40-
meter ham band means the wavelength of the
radio signal is approximately 40 meters.
But, in the optical field, wavelength is the
inverse of frequency. As one increases, the
other decreases. This is also formulized as
λ = c/f. Here λ => wavelength. c => speed
of light , 3x108
meters per second. f =>
frequency.
Coming to digital communications, here data
is represented in the form of binary digits.
On a wireless medium achieving digital
communication becomes more difficult as
compared to a wired medium. But the
reliability increases in the case of digital
communication as this form of
communication is less affected by the noise
and other factors. In digital communication
the signals are represented as a discrete
collection of zeros and ones. A low voltage
value say 0 volts represent a binary zero and
an apparently higher value of +5volts
represents a binary one. A typical digital
waveform looks like this.
Amplitude
¼ cycle
(phase)
¾ cycle
(phase)
Period
0 volts
5 volts
time
In a RS-232C digital circuit zero is
represented as –5 to –15 volts and a digital
one is represented as +5 to +15 volts. The
reason for keeping these voltages ranges is
that there are many real world factors that
affect the flow of signals like electrical noise,
cable resistance, difference in ground
potential etc. Hence, if a transmitter sends
+15 volts of signal and the receiver receives
just +7 volts, even then the signal is
interpreted as a digital one. In a RS-232C
circuit there is difference of 10 volts between

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-5 volts (highest value to represent a zero) and
+5 volts (lowest value to represent a digital
one). This difference is maintained because the
influence of noise is more in this voltage range.
Thus RS-232C circuit grounds any information
that comes in as less than 15 volts and any
information greater than 15 volts might end up
burning the circuit and resulting in a short
circuit.
BANDWIDTH CONCEPTS
Let me discuss some issues relating to
bandwidth concepts.
In analog communication science, bandwidth
represents the capacity of the channel. It is the
difference between the highest and lowest
frequencies that a communication media can
handle. The media can comprise of a wire, an
optical fiber or even open space. The greater
the bandwidth, the greater is the volume of
data transmission. Usually voice grader
frequency lines transmit data over frequencies
between 300Hz to 3300 Hz. Hence the
bandwidth in this case is 3300Hz-300Hz =
3000 Hz. Thus the bandwidth meant to hear
audio signals is about 3 KHz between 300 Hz
to 3300 Hz.
Quite contrast to analog communication, in
digital communication the bandwidth refers to
the amount of data that can be transferred on a
medium in a period of time. Data rates are
measured in “ bits per second (bps)” here. The
typical bandwidth of a local area network is
between 4 Mbps to 1000 Mbps. The bandwidth
of a dialup modem connection is between 300
Mbps to 3300 Mbps.
Baud Rate: There is an interesting concept of
baud rate and it sounds weird that bit rate and
baud rate are analyzed in the same fashion,
but they are different. Baud rate is the
measure of the variation of the line
conditions in a given amount of time, while
bit rate is the number of bits flowing in a
given amount of time. The line conditions
(frequency, amplitude voltage, or phase)
directly influence the signaling rate. At the
lower speed, i.e. <300bps, both data rate and
baud rate are same but at the higher speeds,
they drift from each other. As the speed
increases the signaling methods become
more complex and thus more bits get
encoded in one baud. Consider an example
where a digital transmitter is communicating
at 1024 baud. If 1 bit of information is
transmitted per baud then 1024 bits are
transmitted per second. Here baud rate and
data rate remain same. However, if a signal
represents 4 bits of information, then the
baud rate remains same as 1024 bauds per
second, but the bit rate would be 1024x4 =
4096 bits per second. This is also the channel
bandwidth at that baud rate.
There is also a subtle, but significant
difference between throughput and
bandwidth. Both of these express the data
rate in terms of bits per second but
bandwidth is more a theoretical concept and
throughput is a practical concept. As an
example consider a modem operating on
dedicated channel with bandwidth of 56
kbps. But because of various factors like,
intra circuitry noise, communication software
delays, present traffic skews, buffering queue
leaks etc. only 48 kbps of data transmission
is achieved. This is a practical value is called
throughput.

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TDM.
MULTIPLEXING CONCEPTS
Let me discuss some issues relating to
multiplexing concepts.
Multiplexer is a device, which is designed to
place many incoming signals on a single
communication channel. A de-multiplexer does
the reverse operations. Thus the action of
multiplexing divides a communication channel
into many channels. With this division, we can
send audio, video, text and other information at
the same time over a same channel.
I have heard of different types of Multiplexing
schemes. They are
1. Time Division Multiplexing (TDM).
2. Frequency Division Multiplexing (FDM).
3. Statistical Multiplexing.
Consider that there are many communicating
nodes that want to send information on a
communication channel. The signals of all
these nodes can be made to share the
communication channel in different schemes.
In the case TDM, the mux assigns all the
nodes with a unique number and allocates
burst timings to each of them. On a
repetitive time basis these nodes send signals
on the medium. The advantage with this
method is that at any moment of time, the
entire channel bandwidth is available for any
nodes perusal.
The amount of time a node acquires for
transmission is a direct function of the
number of nodes competing for the channel.
Also here we see that there is only one node
occupying a channel at any given time
4. Wavelength Division Multiplexing
(WDM).
5. Code Division Multiple Access (CDMA).
6. Demand Access Multiple Access (DAMA).
7. Code Access Multiple Access (CAMA).
Let us understand the above listed schemes.
In the case of FDM, the mux divides the
communication channel into many smaller
channels and at any moment many nodes can
send their data into these smaller channels
simultaneously. Although there is a

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FDM
Total Bandwidth
Individual Banwidths
continues flow of data from all nodes, but they
do not enjoy the entire bandwidth
Statistical multiplexing calculates all the
probable nodes that require a share in the entire
bandwidth. As, at any moment only few nodes
compete for the bandwidth, the mux divides
the communication channel into few sub
channels. If a new request arrives for some
bandwidth then the subdivision again happens.
Wavelength division multiplexing happens
only in fiber optic cable communication. In the
fiber optical technology the electrical signals
are converted into optical signals. Now, we
have a situation where many electrical to
optical converters (EOC), want to send
information on a fiber optic cable. The
information coming out of each EOC will have
a different light intensity. The mux combines
all these light sources and the de-mux filters all
these at the other end.
DAMA is a concept where individual nodes
place a request for bandwidth with the
multiplexing administrator. The administrator
has a pool of frequencies. Even to make a
request to the administrator the administrator
should allocate a frequency channel to the
node. Thus administrator allocates two
channels to make a request. This is the
demand part. Once the demand is noted, the
node is allotted two channels using which the
node can transmit the data. Before acquiring
the data channels the node has to relinquish
the request channels. After the data is sent
the node relinquishes the channel back to the
channel pool of the administrator.
EXTERNAL DISTURBANCES AND
NOISE
Noise is a weed. These are the undesired
electrical signals, which manage to sneak
inside a communication channel along with
the genuine data. These signals not only
occupy the scarce bandwidth but also
degrade the current signals. But these
undesired signals are unavoidable. The noise
gets generated because of various factors and
can be categorized into three forms.
1. Ambient Noise
2. Impulse Noise
3. Intermodulation Noise.
Ambient noise is also called as thermal noise.
The transmission equipments like,
transmitter, receiver, repeaters and

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transponders generate such kind of a noise.
External factors like variation of temperature,
florescent light induction, plasma floating
particles etc also introduce such noise.
Filtering this noise is a hard nut to crack at the
reception end.
Impulse noise changes the characteristics of the
flowing signal. Here we do not see additional
signals in the channel, as in the case of ambient
noise, but the physical characteristics of the
data signals change radically. External
lightning, strong magnetic induction, presence
of photo copying machines near the transmitter
etc introduce this kind of noise.
Intermodulation noise is seen in both wired and
wireless communication channels. At any
moment of time, between two bands of
operation, there is a presence of a guard band.
This isolates the signals from interfering with
each other. If the bandwidth of the guard band
is small or gets disturbed, then the signals
flowing in the communication media interfere
with each other. And a part of the information
becomes the subject of interference. The
frequency of the resulting noise is either the
sum of the interfering frequencies or is the
difference of them. This again depends on the
direction of the communication of the
interfering frequencies and the clock phases of
the frequency generators.
Although some of these noises are always
present, by proper installation of equipments
and proper cabling will help in reduction of
noise. Thus, it is clear that the amount of data
transmitted is always greater than the amount
of data received. Rather, the quality of the
received data deteriorates. Thus in order to
determine the maximum rate of data transfer
we apply the Shannon’s limit theorem.
Initially Nquist’s theorem helped in
determining the maximum data rate, on a
communication channel, but it considered the
ideal case where noise is not taken into
consideration. But Shannon’s limit theorem
takes noise into consideration for calculating
the maximum data rate.
MDR = H * log2(1+ (s/n) )
MDR => Maximum data rate.
H => Bandwidth in Hertz.
(s/n) => Signal to noise ration, measured in
dB.
Here signal to noise ratio describes the
quality of the signal, when subjected to
noise, in decibels. Quality is nothing but the
measure of signal strength. Thus, signal to
noise ratio is the ratio of signal strength over
the background noise.
Consider either 28.8 kbps or 33.6 kbps
modem. Even when the modem operates at
the peak of its speed we rarely achieve the
data transfer of 28.8 kbps and 33.6 kbps
respectively. Provided we know the signal to
noise ratio, we can calculate the real
bandwidth of these modems.
Consider this example.
Let H = 3000Hz (voice bands). Let SNR
(s/n) = 1000. Then
MDR = H * log2(1+(sn))
MDR = 3000 * log2(1001)
As 29
= 512 and 210 = 1024 ,
log2(1001) = 9.9(approx).
MDR = 3000*9.9 = 29700 bps.

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This is approximately equal to 30 kbps. Note
this is the maximum data rate that we can
expect under the given conditions but in reality
we never achieve this. This is called the
Shannon’s limit.
THE ACCESS MEDIA
Considering the physical media, the data
transfer happens on either a copper wire or on
a fiber optic cable. Electrical signals propagate
on the former while light travels in the latter. In
case of a copper wire, there are two types. One
is a stranded copper wire while the other is a
solid copper wire. Stranded copper wire is a
bunch of very thin copper wires. This type of
wire is flexible in usage and handling
compared to its counter part. Hence it is more
in use.
Gauge is the measure to determine the
thickness of these wires. As the gauge value
increases the wires becomes thinner. The
copper wires will usually have two layers of
coating to avoid noise and external
interferences. There will a coating of insulation
and an outer jacket made up of polyvinyl
chloride.
The electrical characteristics of a wire are of
more significance than the physical
characteristics. Capacitance, Impedance and
Attenuation are the major electrical
characteristics. The signal qualities are directly
influenced by the electrical characteristics.
Capacitance is the property of a circuit to store
an electrical charge. Capacitance helps in
determining the distance to which a wire can
carry a signal without distorting it. The
waveform of a distorted signal gets rounded off
which makes the receiving node difficult to
recognize its bipolar value. Hence a binary one
can be read as a binary zero and vice versa.
So now the question is, do we need to
support higher capacitance or lower
capacitance? The answer is simple and
logical. More the capacitance, more charges
are accumulated and the more the charges are
accumulated the rounding off effect also
increases. Hence the high quality copper
wires always have lesser capacitance and
hence data can propagate a longer distance
without distortion. The unshielded twisted
pair (UTP) wires and the shielded twisted
pair (STP) wires need a booster and a
repeater circuits every 100 - 250 meters. The
reason is that, as the length increases the
capacitance also builds up.
In an alternating current circuit, there is
always an opposing force that acts one the
communicating signal. This is called the
impedance and it is measured in Ohms. The
Greek character Ω represents the impedance.
Impedance is basically a function of
capacitance, resistance and inductance.
Bundling wires with variable impedance
values will lead in signal distortion.
Attenuation is the phenomena where
electrical signals loose their signal strength.
The length of the cable is directly
proportional to the attenuation factor.
Attenuation is also directly proportional to
the frequency of the signal. A signal is
subjected to more attenuation in a twisted
pair cable than in a coaxial cable.
Attenuation is calculated taking into
consideration the wavelength of the light in a
fiber optic. Attenuation is described in terms
of the decibels of signal loss.
There are a various types of wires and optical
cables that are used in communication. Let
me restrict the discussion about wires at this
point and discuss wireless media now.

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In a wireless media the signals propagate
through the atmosphere and space (vacuum)
rather than on a physical media like wires
discussed above. The wireless communication
happens via radio frequency transmissions
(RF) or via infrared transmissions (IV). RF
transmissions are very popular. The 2G data is
typically communicated over at 800 MHz to
900 MHz range on the spectrum band.
In the Wireless MAN scenario the access
points are addressed as subscriber stations,
which communicate to a base station, which
again communicates to the subscriber
stations on the other end. Wireless MAN
technology presently works on 2-11 GHz
frequency bands.
Microwave communication operates at
Additionally data communication also happens
in the range of 2GHz to 60GHz frequency.
In the Wireless LAN scenarios usually there
will be wireless computational devices, which
communicate the data to an access point. There
will be many such access points, which will be
connected via wires, and thus the data
propagates.
higher frequencies on the electromagnetic
spectrum band. Usually the operational bands
will be between 2-60 GHz. The sub 11 GHz
bands support non line of sight operations
while majority of bands above 11 GHz
support line of sight data transmission. As
even water vapor easily absorbs microwaves
various modulation schemes and varying
transmission powers are used to send

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Wireless Metropolitan Area Network
the data across. In some cases the signals are
shot at ionosphere, which get reflected and are
absorbed. Parabolic antennas are used in
Microwave transmission.
The maximum data rates achieved in the
wireless LAN scenario is around 45 Mbps
while in wireless MAN scenario is around 78
Mbps. Though the projected bandwidth is more
the throughput achievement is less in the
wireless scenario. This is because the data is
subjected to interferences from all the
directions. Many turbo coding schemes are
used to avoid data mangling and also for data
protection.
Before discussing the wireless wide area
networking let us discuss some issues about
spread spectrum and Infrared technology.
Spread spectrum has nothing to do with
spreading frequency or using wide frequency
range. It is one of the basic security science
applied in the data on a wireless media. Here
a pseudo-noise (architectured-noise to be
precise) is interleaved between the real
signals. The transmitter transmits the data
with the pseudo-noise pattern here.
The infrared technology is traditionally a line
of sight technology and this operates at
extremely high frequencies. The infrared
communication operates between 100GHz to
100 THz. There are two form of transmission
that can happen with infrared technology
1. Directed transmission, and
2. Diffused transmission.
In the first kind of transmission the
transmitter and the receiver should be aligned
in a line of sight. In the second kind of
transmission the infrared rays reach the
receiver in a multipath mode.
The satellite based communication forms
Subscriber station
Subscriber station
Subscriber station
Subscriber station
Subscriber station
Subscriber station
Subscriber station
Base station
Base station
Base station
Subscriber station

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the bases for the wireless wide area networks.
In the satellite-based communication data is
directed towards the satellite via ground- based
stations housing parabolic transceivers.
As compared to GEO satellites there are
low-earth orbital (LEO) satellites as well.
Motorola’s ambitious Iridium project was
based on this technology. Here there will be
The transponders of the
satellite receive the data
(uplink), amplify it and
then transmit it back to
earth (downlink). The
geostationary satellites
at placed at an altitude
of 22,300 miles. At this
altitude a geo
synchronous orbit is set
up above the equator.
Any satellite placed at
this altitude rotates
along with the earth.
satellites positioned by
about between 300 to
1200 miles above the
earth. There will be
around 48 such
satellites, which will
be orbiting the earth in
a bobbin thread
wounding fashion.
(Iridium has 66 LEO
satellites) Not only
does the vertical
communication
happen here, but also
Hence it is called the geostationary (GEO)
satellite.
Usually a satellite-based communication
appears asynchronous in nature because of the
time delays. A signal has to travel around
44,600 miles (72983 Kilometers). If this is
divided by the speed of light, i.e. 186,000
miles per second then we see that there is delay
of 1/4th
of a second. This ideal values some
times skews even until a second of delay.
Apart from Geostationary satellites there are
polar orbiting satellites as well. These satellites
rotate perpendicularly to the rotation of the
earth.
In order to avoid satellite signal interferences,
there will be a magnum of 4o
separation in
altitude among satellites. Hence at any altitude
there cannot be more than 90 (360/4) satellites
at a particular altitude.
horizontal, inter-satellite data transfer takes
place. LEO satellites also do not suffer from
propagation delays as faced by the GEO
satellites.
There is another set of medium-earth orbital
(MEO) satellites, which are positioned
around an altitude of 6000-12000 miles.
With a propagation delay of about 150
milliseconds around 20 such satellites can
cover the entire earth.
The bandwidth of operation of satellites is
characterized into four bands. These four
bands are varying functions of the frequency
of operation of the satellite. These four
common bands are ;

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1. C-band. (6 GHz uplink and 4 GHz
downlink)
2. Ka-band (28 GHz uplink and 18 GHz
downlink)
3. Ku-band (14 GHz uplink and 12 GHz
downlink)
4. V –band (still under research, > 30Ghz of
downlink expected)
Apart from these a detail wireless study will
involve studying 802.11, 802.16 and 802.20
standards.