S. NO. EXPERIMENT DETAILS REMARKS
Conversion of Analog to digital
Conversion of digital to analog
Conversion of Analog to Analog
Conversion of digital to digital
5. Implementation of multiplexers.
To study FDM modulation
To study TDM modulation
To study WDM modulation
To study various transmission
media like twisted pairs, co-
cables,optical fibers etc.
EXPERIMENT NO 1
AIM: CONVERSION OF ANALOG TO DIGITAL
Analog signals are continuous electrical signals that vary in time as shown in figure 4a. Most
of the time, the variations follow that of the non-electric (original) signal. Therefore, the two
are analogous hence the name analog.
Not all analog signals vary as smoothly as the waveform shown in Figure 4a. Analog signals
represent some physical quantity and they are a ‘MODEL’ of the real quantity.
Telephone voice signal is analog. The intensity of the voice causes electric current
variations. At the receiving end, the signal is reproduced in the same proportion. Hence the
electric current is a ‘MODEL’ but not one’s voice since it is an electrical representation or
analog of one’s voice.
Digital signals are non-continuous, they change in individual steps. They consist of pulses or
digits with discrete levels or values. The value of each pulse is constant, but there is an
abrupt change from one digit to the next. Digital signals have two amplitude levels called
nodes. The value of which are specified as one of two possibilities such as 1 or 0, HIGH or
LOW, TRUE or FALSE and so on. In reality, the values are anywhere within specific ranges
and we define values within a given range.
The Pulse Code Modulation (PCM) Process
Pulse Code Modulation (PCM) converts analog signals to a digital format (signal). This process
has four steps:
Step One: Filtering
Frequencies below 300 Hz and above 3400 Hz (Voice Frequency range) are filtered from the
analog signal. The lower frequencies are filtered out to remove electrical noise induced from the
power lines. The upper frequencies are filtered out because they require additional bits and add
to the cost of a digital transmission system. The actual bandwidth of the filtered signal is 3100
Hz (3400 - 300). It is often referred to as 4 kHz.
Step Two: Sampling
The analog signal is sampled 8000 times per second. The rate at which the analog signal is
sampled is related to the highest frequency present in the signal. This is based on the Nyquist
sampling theorem. In his calculations, Nyquist used a voice frequency range of 4000 Hz (which
represents the voice frequency range that contains "intelligent" speech). Thus, the standard
became a sampling rate of 8000 Hz, or twice the bandwidth. The signal that is the result of the
sampling process contains sufficient information to accurately represent the information
contained in the original signal. The output of this sampling procedure is a Pulse Amplitude
Modulated, or PAM, signal.
Step Three: Quantizing
In the third step of the A/D conversion process, we quantize the amplitude of the incoming
samples to one of 255 amplitudes on a quantizing scale. Thus, in this step the sampled signal is
matched to a segmented scale. The purpose of step three is to measure the amplitude (or height)
of the PAM signal and assign a decimal value that defines the amplitude. Based on the
quantizing scale, each sampled signal is assigned a number between 0 and +127 to define its
Step Four: Encoding In the fourth step of the A/D conversion process, the quantized samples
are encoded into a digital bit stream (series of electrical pulses).
ANALOG TO DIGITAL CONVERSION
EXPERIMENT NO 2
AIM: CONVERSION OF DIGITAL TO ANALOG
Digital-to-analog conversion is a process in which signals having a few (usually two) defined
levels or states (digital) are converted into signals having a theoretically infinite number of states
(analog). A common example is the processing, by a modem,of computer data into audio-
frequency (AF) tones that can be transmitted over a twisted pair telephone line. The circuit that
performs this function is a digital-to-analog converter (DAC).
Basically, digital-to-analog conversion is the opposite of analog-to-digital conversion. In most
cases, if an analog-to-digital converter (ADC) is placed in a communications circuit after a
DAC, the digital signal output is identical to the digital signal input. Also, in most instances
when a DAC is placed after an ADC, the analog signal output is identical to the analog signal
Binary digital impulses, all by themselves, appear as long strings of ones and zeros, and have no
apparent meaning to a human observer. But when a DAC is used to decode the binary digital
signals, meaningful output appears. This might be a voice, a picture, a musical tune, or
This is done by LOW PASS FILTER.
A low-pass filter is a filter that passes low-frequency signals but attenuates (reduces the
amplitude of) signals with frequencies higher than the cutoff frequency. The actual amount of
attenuation for each frequency varies from filter to filter. It is sometimes called a high-cut filter,
or treble cut filter when used in audio applications. A low-pass filter is the opposite of a high-
pass filter, and a band-pass filter is a combination of a low-pass and a high-pass.
EXPERIMENT NO 3
AIM: CONVERSION OF ANALOG TO ANALOG
Analog signals are continuous electrical signals that vary in time as shown in figure 4a. Most
of the time, the variations follow that of the non-electric (original) signal. Therefore, the two
are analogous hence the name analog.
Analog to Analog conversion is done by:
Modulation is the process of varying one waveform in relation to another waveform. In
telecommunications, modulation is used to convey a message, or a musician may modulate the
tone from a musical instrument by varying its volume, timing and pitch. Often a high-frequency
sinusoid waveform is used as carrier signal to convey a lower frequency signal. The three key
parameters of a sine wave are its amplitude ("volume"), its phase ("timing") and its frequency
("pitch"), all of which can be modified in accordance with a low frequency information signal to
obtain the modulated signal.
A device that performs modulation is known as a modulator and a device that performs the
inverse operation of modulation is known as a demodulator (sometimes detector or demod). A
device that can do both operations is a modem (short for "Modulator-Demodulator").
The aim of analog modulation is to transfer an analog baseband (or lowpass) signal, for
example an audio signal or TV signal, over an analog passband channel, for example a limited
radio frequency band or a cable TV network channel.
Phase modulation (PM) is a form of modulation that represents information as variations in the
instantaneous phase of a carrier wave.
Unlike its more popular counterpart, frequency modulation (FM), PM is not very widely used.
This is because it tends to require more complex receiving hardware and there can be ambiguity
problems in determining whether, for example, the signal has changed phase by +180° or -180°.
EXPERIMENT NO 4
AIM: CONVERSION OF DIGITAL TO DIGITAL
In order to transport digital bits of data across carrier waves, encoding techniques have been
developed each with their own pros and cons.
CHARACTER CODING TECHNIQUES
Jean-Maurice-Emile Baudot developed a character set in 1874 that used series of bits to represent
characters that could be sent over a telegraph wire or radio signal. A 5-key keyboard was developed to
implement this Baudot code that was modified by Donald Murray in 1901 and it became the
International Telegraph Alphabet 1 (ITA1) and then developed into ITA2. ITA2 was the coding that was
actually implemented on equipment. Characters such as Line Feed (LF) were given a 5 bit code such as
The problem with using 5 bits for each character is that there is a limitation on the number of characters
that can be generated from them, 25 gives 32 different combinations. This may be fine for 26 letters of
the English alphabet but it is not enough to cover punctuation or control characters. Other coding
techniques were needed.
Binary Coded Decimal (BCD)
BCD uses a series of 4 bits called a nibble to represent a decimal number, as the following table
Decimal 0 1 2 3 4 5 6 7 8 9
BCD 0000 0001 0010 0011 0100 0101 0110 0111 1000 1001
So for example, the number 1456 would be represented by 0001 0100 0101 0110. This makes it easier
to convert numbers and for displays, however the electronics required in calculations is quite complex.
American Standard Code for Information Interchange (ASCII)
Originally published in 1963, ASCII is based on 7 bits to represent English characters and after a number
of revisions ASCII now supports 95 printable characters and 33 control characters (a total of 2 7 = 128).
ASCII is the americanised vsersion of that defined by CCITT in ISO 646 and is known as the International
Alphabet 5 (IA5).
The first 32 characters are control characters and are represented by the 7-bit codes 000 0000 (null
character) through to 001 1111 (unit separator). The 128th control character is 'delete' represented by
111 1111. The rest of the characters are printable and the coding caters for both lower and uppercase
english letters e.g. the letter 'd' is represented by 110 0100 whereas its upper case equivalent is
represented by 100 0100.
Extended Binary Coded Decimal Interchange Code (EBCDIC)
Around the same time that ASCII was developed, in 1964 IBM produced EBCDIC which is an 8-bit coding
system designed to replace BCD within its computer systems. An EBCDIC byte is divided in two nibbles.
The first four bits is called the zone and this represents the category of the character, the last four bits is
called the digit and this identifies the specific character.
Different countries adapted EBCDIC for their own alphabets. The Chinese had a double byte extension
that allowed them to display Chinese characters. IBM numbered the different character sets with Coded
Character Set Identifier (CCSID) of which there are many around the world.
Originally published in 1991 by the Unicode Consortium as Unicode 1.0 (in 2006 Unicode 5.0 was
released), Unicode aims to provide a means for the traditional character sets around the world to take
part in multilingual computer processing amongst themselves rather than have to translate into a
Roman character set first.
The bit patterns of the 95 printable ASCII characters are sufficient to exchange information in modern
English, however many languages that use the Latin alphabet need additional symbols not covered by
ASCII. ISO/IEC 8859 attempts to address this by utilising the eighth bit in an 8-bit byte in order to allow
positions for another 128 characters. This bit was previously used for data transmission protocol
information, or was left unused. Even more characters were needed than could fit in a single 8-bit
character encoding, so several mappings were developed. ISO/IEC 8859 comes in parts and these are
given a number e.g. ISO 8859-15.
Unicode creates codes for the characters or basic graphical representation of the character (called a
'grapheme'). The first 256 code points have been reserved for ISO 5589-1 in order to make it
straightforward to convert the Roman text. There are two Unicode mapping methods; Unicode
Transformation Format (UTF) and Unicode Character Set (UCS). An encoding maps the range of
Unicode code points to sequences of values in a fixed-size range of code values. The numbers in the
names of the encodings indicate the number of bits in one code value (for UTF encodings) or the
number of bytes per code value (for UCS) encodings. UCS assigns a code per character. UCS-2 uses two
bytes per character, UCS-4 uses 4 bytes per character.
Some Unicode examples:
UTF-7 — a 7-bit encoding, often considered obsolete (not part of Unicode but rather an RFC)
UTF-8 — an 8-bit, variable-width encoding, which maximizes compatibility with ASCII. In
common use and is in fact a superset of ASCII. The IMC and IETF use UTF-8 when determining
standards for supporting email and Internet traffic.
UTF-EBCDIC — an 8-bit variable-width encoding, which maximizes compatibility with EBCDIC.
(not part of Unicode)
UTF-16 — a 16-bit, variable-width encoding. In common use.
UTF-32 — a 32-bit, fixed-width encoding
Manchester Phase Encoding (MPE)
802.3 Ethernet uses Manchester Phase Encoding (MPE). A data bit '1' from the level-encoded signal (i.e.
that from the digital circuitry in the host machine sending data) is represented by a full cycle of the
inverted signal from the master clock which matches with the '0' to '1' rise of the phase-encoded signal
(linked to the phase of the carrier signal which goes out on the wire). i.e. -V in the first half of the signal
and +V in the second half.
The data bit '0' from the level-encoded signal is represented by a full normal cycle of the master clock
which gives the '1' to '0' fall of the phase-encoded signal. i.e. +V in the first half of the signal and -V in
the second half.
The above diagram shows graphically how MPE operates. The example at the bottom of the diagram
indicates how the digital bit stream 10110 is encoded.
A transition in the middle of each bit makes it possible to synchronize the sender and receiver. At any
instant the ether can be in one of three states: transmitting a 0 bit (-0.85v), transmitting a 1 bit (0.85v)
or idle (0 volts). Having a normal clock signal as well as an inverted clock signal leads to regular
transitions which means that synchronisation of clocks is easily achieved even if there are a series of '0's
or '1's. This results in highly reliable data transmission. The master clock speed for Manchester encoding
always matches the data speed and this determines the carrier signal frequency, so for 10Mbps Ethernet
the carrier is 10MHz.
Differential Manchester Encoding (DME)
A '1' bit is indicated by making the first half of the signal, equal to the last half of the previous bit's signal
i.e. no transition at the start of the bit-time. A '0' bit is indicated by making the first half of the signal
opposite to the last half of the previous bit's signal i.e. a zero bit is indicated by a transition at the
beginning of the bit-time. In the middle of the bit-time there is always a transition, whether from high to
low, or low to high. Each bit transmitted means a voltage change always occurs in the middle of the bit-
time to ensure clock synchronisation. Token Ring uses DME and this is why a preamble is not required in
Token Ring, compared to Ethernet which uses Manchester encoding.
Non Return to Zero (NRZ)
NRZ encoding uses 0 volts for a data bit of '0' and a +V volts for a data bit of '1'. The problem with this is
that it is difficult to distinguish a series of '1's or '0's due to clock synchronisation issues. Also, the
average DC voltage is 1/2V so there is high power output. In addition, the bandwidth is large i.e. from
0Hz to half the data rate because for every full signal wave, two bits of data can be transmitted
(remember that with MPE the data rate equals the bit rate which is even more inefficient!) i.e. two bits
of information are transmitted for every cycle (or hertz).
After 50m of cable attenuation the signal amplitude may have been reduced to 100mV giving an
induced noise tolerance of 100mV.
Return to Zero (RZ)
With RZ a '0' bit is represented by 0 volts whereas a '1' data bit is represented by +V volts for half the
cycle and 0 volts for the second half of the cycle. This means that the average DC voltage is reduced to
1/4V plus there is the added benefit of there always being a voltage change even if there are a series of
'1's. Unfortunately, the efficiency of bandwidth usage decreases if there are a series of '1's since now a
'1' uses a whole cycle.
Non Return to Zero Invertive (NRZ-I)
With NRZ-I a '1' bit is represented by 0 volts or +V volts depending on the previous level. If the previous
voltage was 0 volts then the '1' bit will be represented by +V volts, however if the previous voltage was
+V volts then the '1' bit will be represented by 0 volts. A '0' bit is represented by whatever voltage level
was used previously. This means that only a '1' bit can 'invert' the voltage, a '0' bit has no effect on the
voltage, it remains the same as the previous bit whatever that voltage was.
This can be demonstrated in the following examples for the binary patterns 10110 and 11111:
Note how that a '1' inverts the voltage whilst a '0' leaves it where it is. This means that the encoding is
different for the same binary pattern depending on the voltage starting point.
The bandwidth usage is minimised with NRZ-I, plus there are frequent voltage changes required for
With fibre there are no issues with power output so a higher clock frequency is fine whereas with
copper NRZ-I would not be acceptable.
4B/5B encoding is sometimes called 'Block coding'. To get around this problem, an intermediate
encoding takes place before the MLT-3 encoding. Each 4-bit 'nibble' of received data has an extra 5th bit
added. If input data is dealt with in 4-bit nibbles there are 24 = 16 different bit patterns. With 5-bit
'packets' there are 25 = 32 different bit patterns. As a result, the 5-bit patterns can always have two '1's
in them even if the data is all '0's a translation occurs to another of the bit patterns. This enables clock
synchronisations required for reliable data transfer.
Notice that the clock frequency is 125MHz. The reason for this is due to the 4B/5B encoding. A 100MHz
signal would not have been enough to give us 100Mbps, we need a 125MHz clock.
Same idea as 4B/5B but you can have DC balance (3 zero bits and 3 one bits in each group of 6) to
prevent polarisation. 5B/6B Encoding is the process of encoding the scrambled 5-bit data patterns into
predetermined 6-bit symbols. This creates a balanced data pattern, containing equal numbers of 0's and
1's, to provide guaranteed clock transitions synchronization for receiver circuitry, as well as an even
power value on the line.
5B6B encoding also provides an added error-checking capability. Invalid symbols and invalid data
patterns, such as more than three 0's or three 1's in a row, are easily detected
For 100VG-AnyLAN for instance, the clock rate on each wire is 30MHz, therefore 30Mbits per second are
transmitted on each pair giving a total data rate of 120Mbits/sec. Since each 6-bits of data on the line
represents 5 bits of real data due to the 5B/6B encoding, the rate of real data being transmitted is
25Mbits/sec on each pair, giving a total rate of real data of 100Mbits/sec. For 2-pair STP and fiber, the
data rate is 120Mbits/sec on the transmitting pair, for a real data transmission rate of 100Mbits/sec.
8B/6T means send 8 data bits as six ternary (one of three voltage levels) signals. 3/4 (6/8) wave
transitions transitions per bit i.e. the carrier just needs to be running at 3/4 of the speed of the data
The incoming data stream is split into 8-bit patterns. Each 8-bit data pattern with two voltage levels 0
volts and V volts is examined. This 8-bit pattern is then converted into a 6-bit pattern but using three
voltage levels -V, 0 and V volts, so each 8-bit pattern has a unique 6T code. For example the bit pattern
0000 0000 (0x00) uses the code +-00+- and 0000 1110 (0x)E) uses the code -+0-0+. There are 36 = 729
possible patterns (symbols). The rules for the symbols are that there must be at least two voltage
transitions (to maintain clock synchronisation) and the average DC voltage must be zero (this is called
'DC balance' that is the overall DC voltage is summed up to 0v, the +V and -V transitions are evenly
balanced either side of 0V) which stops any polarisation on the cable.
The maximum frequency that the 6T codes could generate on one carrier is 37.5MHz. FCC rules do not
allow anything above 30MHz on cables and Category 3 cable does not allow anything above 16MHz
(which is what 100BaseT4 was designed for). The 100BaseT4 standard uses 8B/6T encoding on three
pairs in a round robin fashion such that the maximum carrier frequency on any single pair is 37.5/3 =
Each octet of data is examined and assigned a 10 bit code group. The data octet is split up into the 3
most significant bits and the 5 least significant bits. This is then represented as two decimal numbers
with the least significant bits first e.g. for the octet 101 00110 we get the decimal 6.5. 10 bits are used to
create this code group and the naming convention follows the format /D6.5/. There are also 12 special
code groups which follow the naming convention /Kx.y/.
The 10 bit code groups must either contain five ones and five zeros, or four ones and six zeros, or six
ones and four zeros. This ensures that not too many consecutive ones and zeros occurs between code
groups thereby maintaining clock synchronisation. Two 'commas' are used to aid in bit synchronisation,
these 'commas' are the 7 bit patterns 0011111 (+comma)and 1100000 (-comma).
In order to maintain a DC balance, a calculation called the Running Disparity calculation is used to try to
keep the number of '0's transmitted the same as the number of '1's transmitted.
This uses 10 bits for each 8 bits of data and therefore drops the data rate speed relative to the line
speed, for instance in order to gain a data rate of 1Gbps the line peed has to be 10/8 x 1 = 1.25Gbps .
This scheme was specified by ANSI X3T9.5 committee. It is used by FDDI and TP-PMD to obtain 100MB/s
out of a 31.25MHz signal.
UTP is low pass in nature, meaning that it hinders high frequency signal (like a low-pass filter). So it is
not feasible to merely increase the clock frequency by 10 to 100MHz and use Manchester encoding to
give us 100Mbps. In addition, the FCC (Federal Communications Commission) have severely curtailed
the power that is allowed to be emitted above 30MHz. We have to use another encoding technique in
order to transmit high data rates across UTP.
If you take an averaging spectrum analyser and look at the output signal of the 10Mbps Ethernet phase-
encoded signal, you will see a power peak at 10MHz where there is a stream of '1's or '0's, you will see a
smaller harmonic at 30MHz and if there is a stream of '1's and '0's, you will see a peak at 5MHz. Now
100BaseT uses a master clock running at 125MHz instead of 10MHz. The equivalent peaks would then
be at 125MHz, 375MHz and 62.5MHz. Transmission electronics designed to work within the FCC rules
will block the frequencies higher than 30MHz.
To get around this issue we need to concentrate the signal power below 30MHz if possible. To do this
the encoding method Multi-Level Transition 3 (MLT-3) is used. This involves using the pattern 1, 0, -1, 0.
If the next data signal is a '1' then the output 'transitions' to the next bit in the pattern e.g. if the last
output bit was a '-1', and the input bit is a '1', then the next output bit is a '0'. If the next data signal is a
'0' then there is no transition which means that the next output bit is the same as last time, in our case a
The cycle length of the output signal is therefore going to be 1/4 that of the MPE method so that instead
of the main signal peak being at 125MHz as measured by the averaging spectrum analyser, it will be at
31.25MHz which is near enough to be OK as far as FCC are concerned. 5 bits are transmitted for every 4
bits of data so that the data bit rate is actually 125Mb/s for 100Mb/s data throughput.
There is an issue with this in that you can end up with a series of '0's or '1's which force the local
circuitry to count the bits using its own free running clock rather than have the check of the clock
synchronisation from the transmit source.
This employs multi-level amplitude signalling. To encode 8 bits, 28 = 256 codes or symbols, are required
since there are 256 possible pattern combinations. A five level signal (e.g. -2v, -1v, 0v, 1v and 2v) called
Pulse Amplitude Modulation 5 is used (This works in a similar manner to MLT-3). Bearing in mind that
there are 4 separate pairs being used for transmission and reception of data, this gives us a possibility of
54 = 625 codes to choose from when using all four pairs. Actually only four levels are used for data, the
fifth level (0v) is used for the 4-dimensional 8-state Trellis Forward Error Correction used to recover the
transmitted signal from the high noise.
If you plot time (nanoseconds) against voltage you will see an 'eye pattern' effect showing the different
signal levels. Comparing a plot for MLT-3 against PAM-5 will demonstrate how that the separate levels
for PAM-5 are less discreet. This is why extra convolution coding is used called Trellis coding, which uses
Viterbi decoding for error detection and correction.
2 bits are represented per symbol and the symbol rate is 125Mbps in each direction on a pair because
the clock rate is set at 125MHz. This gives 250Mbps data per pair and therefore 1000Mbps for the whole
This type of encoding is used by Gigabit Ethernet. The data signals have distinct and measurable
amplitude and phases relative to a 'marker signal'. Using this two way matrix allows more data bits per
cycle, in the case of Gigabit Ethernet 1000Mbps is squeezed into 125MHz signals. The electronics are
more complex and the technology is more susceptible to noise.
Feedback Shift Register (FSR)
There is an issue with some encoding schemes of the power of the higher frequency harmonics. To
minimise these there is another small step before wave shaping such as MLT-3 encoding. This step uses
a Feedback Shift Register (FSR) to produce a 'pseudo-random' bit pattern which is Exclusive-ORed with
the data stream. This pseudo random stream is a known quantity and is reversed at the other end by
another Excusive-OR operation using the same known pseudo-random bit pattern. The purpose of the
randomness is to reduce the regularity of the signal frequency and consequently the harmonics. The FSR
used in 100BaseT is an 11-bit register that shifts one bit at a time from bit 0 to bit 10 on each clock cycle.
EXPERIMENT NO 5
AIM: IMPLEMENTATION OF MULTIPLEXERS.
In the design of large-scale digital systems, that a single line is required to carry two or more
different digital signals. Of course, only one signal at a time can be placed on the one line. What
is required is a device that will allow us to select, at different instants, the signal we wish to place
on this common line. Such a circuit is referred to as a Multiplexer.
A multiplexer performs the function of selecting the input on any one of 'n' input lines and
feeding this input to one output line.
Multiplexers are used as one method of reducing the number of integrated circuit packages
required by a particular circuit design. This in turn reduces the cost of the system.
Assume that we have four lines, C0, C1, C2 and C3, which are to be multiplexed on a single line,
Output (f). The four input lines are also known as the Data Inputs. Since there are four inputs,
we will need two additional inputs to the multiplexer, known as the Select Inputs, to select
which of the C inputs is to appear at the output. Call these select lines A and B.
The gate implementation of a 4-line to 1-line multiplexer is shown below:
The circuit symbol for the above multiplexer is:
EXPERIMENT NO 6
AIM: TO STUDY FDM MODULATION
Frequency-division multiplexing (FDM) is a form of signal multiplexing which involves assigning
non-overlapping frequency ranges to different signals or to each "user" of a medium.
This technique is also used by television and radio. Computer networks use the principle of
separate channels to permit multiple communications to share a single, physical connection.
Horak (1996) explain that Frequency Division Multiplexing (FDM) can be described
by dividing the single high-capacity channel into several smaller-capacity
channels (sub-channel). Each sub channel transmits data simultaneously using
different frequency so that each sub-channel has its own frequency to use and
is not affecting other subchannels.
A radio is A good example to explain how FDM works. Note, that we
are only using one broad range of radio frequency and there are several radio
stations broadcasting its service using different frequency. All we need to do is
to adjust the radio to catch only certain radio broadcast on certain frequency.
According to Horak (1996) FDM has drawback by dedicating such frequency
to several smaller circuits even though the designated channel is not
Figure how FDM works by dividing one channel into several frequencies including the Guard-
band act as delimiter for each logical sub-channel
so that the interference from other sub-channel using the same physical circuit
can be minimized. For example, in Figure 2, the multiplexed circuit is divided
into 4 frequencies. Channel #1 using 0-800 Hz for its data transfer and delimited
by 200 Hz Guard-band. Channel #2 using 1000-1800 Hz and delimited by
200 Hz too; and so on.
Frequency Division Multiplexing
In regards to speed, we simply need to divide the main circuit amongst
the availabel subchannels. For example, if we have a 64 Kbps physical circuit
and wanted to use 4 sub-channel, each sub-channel will have 16 Kbps. However,
Guard-band is also using this 64 Kbps physical circuit and therefore each channel
will be using only 15 Kbps with 4 Guard-bands (1 Kbps per Guard-band).
This calculation is subject to change while there are many ways to define the
bandwidth for sub channels and Guard-bands.
EXPERIMENT NO 7
AIM: TO STUDY TDM MODULATION
TIME DIVISION MULTIPLEXING
Time-division multiplexing (TDM) is a method of putting multiple data
streams in a single signal by separating the signal into many segments, each having a very short
duration. Each individual data stream is reassembled
at the receiving end based on the timing.
According to Horak (1996) data for each device are sent in a serial fashion
from one end multiplexer to the other end so that device #1 transmit on time
slot #1, device #2 transmit on the next time slot and so on. On the receiver,
multiplexer will try to recognize that the first data time slot is for device #1
and the next data time slot is for the next device which is #2. Therefore, the
multiplexer must have the proper time synchronization with other end multiplexer.
On the other hand, Fitzgerald and Dennis believes that
TDM is more efficient than FDM as TDM does not use Guard-bands anymore.
Therefore, a 64-Kbps circuit can be fully occupied for 4 circuits with each of
the circuit will have 6-Kbps speed.
Horak (1996) also mentioned that the main drawback of TDM is that
one channel is dedicated to its own use even though the channel is broken or
inactive. This means that if the Device #1 uses Channel #1 and Device #1 is
broken, the time slot for data transmission will still be dedicated to Device #1.
Time Division Multiplexing
EXPERIMENT NO 8
AIM: TO STUDY WDM MODULATION
wavelength-division multiplexing (WDM) is a technology which multiplexes multiple optical
carrier signals on a single optical fiber by using different wavelengths (colours) of laser light to
carry different signals. This allows for a multiplication in capacity, in addition to enabling
bidirectional communications over one strand of fiber.
This is a form of frequency division multiplexing (FDM) but is commonly called wavelength
division multiplexing.The term wavelength-division multiplexing is commonly applied to an
optical carrier (which is typically described by its wavelength), whereas frequency-division
multiplexing typically applies to a radio carrier (which is more often described by frequency).
However, since wavelength and frequency are inversely proportional, and since radio and light
are both forms of electromagnetic radiation, the two terms are equivalent in this context.
Wave Division Multiplexing (WDM) technology multiplexes several optical signals into a
composite signal that is transported over a
single fiber. The composite signal is then de-multiplexed at the receiver end and each unique
wavelength (optical signal) is recovered.
Stealth's Dedicated Wavelength Service utilizes the latest WDM technology, support speeds up
to 10 Gbps. The service provides a cost-effective and flexible alternative to dark fiber or private
line service for connecting locations at high data-rates.
EXPERIMENT NO 9
AIM: TO STUDY VARIOUS TRANSMISSION MEDIA
LIKE TWISTED PAIRS, CO-AXIAL CABLES,
OPTICAL FIBERS etc.
THEORY: TRANSMISSION MEDIA
Network Media Types: Network media is the actual path over which an
electrical signal travels as it moves from one component to another.
The common types of network media, including twisted-pair cable, coaxial cable, fiber-
optic cable, and wireless.
There are 2 basic categories of Transmission Media:
Guided Transmission Media uses a "cabling" system that guides the data signals along a
specific path. The data signals are bound by the "cabling" system. Guided Media is also
known as Bound Media. Cabling is meant in a generic sense in the previous sentences
and is not meant to be interpreted as copper wire cabling only.
Unguided Transmission Media consists of a means for the data signals to travel but
nothing to guide them along a specific path. The data signals are not bound to a cabling
media and as such are often called Unbound Media.
There 4 basic types of Guided Media:
Open Wire is traditionally used to describe the electrical wire strung along power poles. There
is a single wire strung between poles. No shielding or protection from noise interference is
used. We are going to extend the traditional definition of Open Wire to include any data signal
path without shielding or protection from noise interference. This can include multiconductor
cables or single wires. This media is susceptible to a large degree of noise and interference and
consequently not acceptable for data transmission except for short distances under 20 ft.
Twisted pair cabling is a form of wiring in which two conductors are wound together for
the purposes of canceling out electromagnetic interference (EMI) from external sources;
for instance, electromagnetic radiation from unshielded twisted pair (UTP) cables, and
crosstalk between neighboring pairs.
Twisting wires decreases interference because the loop area between the wires (which
determines the magnetic coupling into the signal) is reduced. In balanced pair
operation, the two wires typically carry equal and opposite signals (differential mode)
which are combined by addition at the destination.
The common-mode noise from the two wires (mostly) cancel each other in this addition
because the two wires have similar amounts of EMI that are 180 degrees out of phase.
This results in the same effect as subtraction. Differential mode also reduces
electromagnetic radiation from the cable, along with the attenuation that it causes.
In contrast to FTP (foiled twisted pair) and STP (shielded twisted pair) cabling, UTP
(unshielded twisted pair) cable is not surrounded by any shielding. It is the primary wire
type for telephone usage and is very common for computer networking, especially as
patch cables or temporary network connections due to the high flexibility of the cables.
WHY THESE WIRES ARE TWISTED ?
The wires in Twisted Pair cabling are twisted together in pairs. Each pair would consist of a wire
used for the +ve data signal and a wire used for the -ve data signal. Any noise that appears on 1
wire of the pair would occur on the other wire.
Because the wires are opposite polarities, they are 180 degrees out of phase
(180 degrees - phasor definition of opposite polarity). When the noise
appears on both wires, it cancels or nulls itself out at the receiving end.
Twisted Pair cables are most effectively used in systems that use a balanced
line method of transmission: polar line coding (Manchester Encoding) as
opposed to unipolar line coding (TTL logic).
The degree of reduction in noise interference is determined specifically by
the number of turns per foot. Increasing the number of turns per foot
reduces the noise interference.
To further improve noise rejection, a foil or wire braid shield is woven around
the twisted pairs. This "shield" can be woven around individual pairs or
around a multi-pair conductor (several pairs).
Cables with a shield are called Shielded Twisted Pair and commonly abbreviated STP. Cables
without a shield are called Unshielded Twisted Pair or UTP. Twisting the wires together results
in a characteristic impedance for the cable. A typical impedance for UTP is 100 ohm for
Ethernet 10BaseT cable.
STP or Shielded Twisted Pair is used with the traditional Token Ring cabling or ICS - IBM Cabling
System. It requires a custom connector. IBM STP (Shielded Twisted Pair) has a characteristic
impedance of 150 ohms.
UNSHIELDED TWISTED PAIR (UTP)
UTP or Unshielded Twisted Pair cable is used on Ethernet 10BaseT and can also be used with
Token Ring. It uses the RJ line of connectors (RJ45, RJ11, etc..)
Each pair is twisted to decrease interference.
UTP cable is also the most common cable used in computer networking. UTP cables are often called
ethernet cables after Ethernet, the most common data networking standard that utilizes UTP cables.
Twisted pair cabling is often used in data networks for short and medium length connections because of
its relatively lower costs compared to optical fiber and coaxial cable
S/STP, also known as S/FTP.
Twisted pair cables are often shielded in attempt to prevent electromagnetic
Because the shielding is made of metal, it may also serve as a ground.
However, usually a shielded or a screened twisted pair cable has a special
grounding wire added called a drain wire.
This shielding can be applied to individual pairs, or to the collection of pairs.
When shielding is applied to the collection of pairs, this is referred to as
screening. The shielding must be grounded for the shielding to work.
ED TWISTED PAIR CABLE.
SHIELDED TWISTED PAIR (STP)
STP cabling includes metal shielding over each individual pair of copper wires. This type of
shielding protects cable from external EMI (electromagnetic interferences). e.g. the 150 ohm
shielded twisted pair cables defined by the IBM Cabling System specifications and used with
token ring networks.
SCREENED SHIELDED TWISTED PAIR (S/STP)
S/STP cabling, also known as Screened Fully shielded Twisted Pair (S/FTP),  is both individually
shielded (like STP cabling) and also has an outer metal shielding covering the entire group of
shielded copper pairs (like S/UTP). This type of cabling offers the best protection from
interference from external sources.
SCREENED UNSHIELDED TWISTED PAIR (S/UTP)
S/UTP, also known as Fully shielded (or Foiled) Twisted Pair (FTP), is a screened UTP cable.
It is a thin, flexible cable that is easy to string between walls.
Because UTP is small, it does not quickly fill up wiring ducts.
UTP costs less per foot than any other type of LAN cable.
Twisted pair’s susceptibility to the electromagnetic interference greatly depends on the pair
twisting schemes (usually patented by the manufacturers) staying intact during the installation.
As a result, twisted pair cables usually have stringent requirements for maximum pulling
tension as well as minimum bend radius.
This relative fragility of twisted pair cables makes the installation practices an important part of
ensuring the cable’s performance.
Twisted-pair cable is a type of cabling that is used for telephone communications and
most modern Ethernet networks. A pair of wires forms a circuit that can transmit data.
The pairs are twisted to provide protection against crosstalk, the noise generated by
When electrical current flows through a wire, it creates a small, circular magnetic field
around the wire. When two wires in an electrical circuit are placed close together, their
magnetic fields are the exact opposite of each other. Thus, the two magnetic fields
cancel each other out.
They also cancel out any outside magnetic fields. Twisting the wires can enhance this
cancellation effect. Using cancellation together with twisting the wires, cable designers
can effectively provide self-shielding for wire pairs within the network media.
Two basic types of twisted-pair cable exist: unshielded twisted pair (UTP) and shielded twisted
pair (STP). The following sections discuss UTP and STP cable in more detail.
UTP cable is a medium that is composed of pairs of wires ;UTP cable is used in a variety of
networks. Each of the eight individual copper wires in UTP cable \is covered by an insulating
material. In addition, the wires in each pair are twisted around each other.
UNSHIELDED TWISTED-PAIR CABLE
UTP cable relies solely on the cancellation effect produced by the twisted wire pairs to limit
signal degradation caused by electromagnetic interference (EMI) and radio frequency
interference (RFI). To further reduce crosstalk between the pairs in UTP cable, the number of
twists in the wire pairs varies. UTP cable must follow precise specifications governing how many
twists or braids are permitted per meter (3.28 feet) of cable.
UTP cable often is installed using a Registered Jack 45 (RJ-45) connector . The RJ-45 is an eight-
wire connector used commonly to connect computers onto a local-area network (LAN),
RJ45 is a standard type of connector for network cables. RJ45 connectors are most
commonly seen with Ethernet cables and networks.
RJ45 connectors feature eight pins to which the wire strands of a cable interface
electrically. Standard RJ-45 pinouts define the arrangement of the individual wires
needed when attaching connectors to a cable.
Several other kinds of connectors closely resemble RJ45 and can be easily confused for
each other. The RJ-11 connectors used with telephone cables, for example, are only
slightly smaller (narrower) than RJ-45 connectors.
Also Known As: Registered Jack 45
When used as a networking medium, UTP cable has four pairs of either 22- or 24-gauge copper
wire. UTP used as a networking medium has an impedance of 100 ohms; this differentiates it
from other types of twisted-pair wiring such as that used for telephone wiring, which has
impedance of 600 ohms.
UTP cable offers many advantages. Because UTP has an external diameter of approximately
0.43 cm (0.17 inches), its small size can be advantageous during installation. Because it has such
a small external diameter, UTP does not fill up wiring ducts as rapidly as other types of cable.
This can be an extremely important factor to consider, particularly when installing a network in
an older building. UTP cable is easy to install and is less expensive than other types of
networking media. In fact, UTP costs less per meter than any other type of LAN cabling. And
because UTP can be used with most of the major networking architectures, it continues to grow
DISADVANTAGES ALSO ARE INVOLVED IN USING TWISTED-PAIR
CABLING, however. UTP cable is more prone to electrical noise and interference than other
types of networking media, and the distance between signal boosts is shorter for UTP than it is
for coaxial and fiber-optic cables.
Although UTP was once considered to be slower at transmitting data than other types of cable,
this is no longer true. In fact, UTP is considered the fastest copper-based medium today.
The following summarizes the features of UTP cable:
Speed and throughput—10 to 1000 Mbps
Average cost per node—Least expensive
Media and connector size—Small
Maximum cable length—100 m (short)
COMMONLY USED TYPES OF UTP CABLING ARE AS FOLLOWS:
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—Can transmit data at speeds up to 100 Mbps.
Category 5e —Used in networks running at speeds up to 1000 Mbps (1 gigabit per second
Category 6—Typically, Category 6 cable consists of four pairs of 24 American Wire Gauge
(AWG) copper wires. Category 6 cable is currently the fastest standard for UTP.
Category 1/2/3/4/5/6 – a specification for the type of copper wire (most telephone and
network wire is copper) and jacks. The number (1, 3, 5, etc) refers to the revision of the
specification and in practical terms refers to the number of twists inside the wire (or the quality
of connection in a jack).
SHIELDED TWISTED-PAIR CABLE
Shielded twisted-pair (STP) cable combines the techniques of shielding, cancellation, and wire
twisting. Each pair of wires is wrapped in a metallic foil . The four pairs of wires then are
wrapped in an overall metallic braid or foil, usually 150-ohm cable. As specified for use in
Ethernet network installations, STP reduces electrical noise both within the cable (pair-to-pair
coupling, or crosstalk) and from outside the cable (EMI and RFI). STP usually is installed with STP
data connector, which is created especially for the STP cable. However, STP cabling also can use
the same RJ connectors that UTP uses.
SHIELDED TWISTED-PAIR CABLE
Although STP prevents interference better than UTP, it is more expensive and difficult to install.
In addition, the metallic shielding must be grounded at both ends. If it is improperly grounded,
the shield acts like an antenna and picks up unwanted signals. Because of its cost and difficulty
with termination, STP is rarely used in Ethernet networks. STP is primarily used in Europe.
The following summarizes the features of STP cable:
Speed and throughput—10 to 100 Mbps
Average cost per node—Moderately expensive
Media and connector size—Medium to large
Maximum cable length—100 m (short)
WHEN COMPARING UTP AND STP, KEEP THE
FOLLOWING POINTS IN MIND:
The speed of both types of cable is usually satisfactory for local-area distances.
These are the least-expensive media for data communication. UTP is less expensive than
Because most buildings are already wired with UTP, many transmission standards are
adapted to use it, to avoid costly rewiring with an alternative cable type.
TYPES OF TWISTED PAIR:
STRAIGHT THRU UTP CABLES
Before starting with cableing types we must know about Pin Number Designations for T568B
and Pin Number Designations for T568A
PIN NUMBER DESIGNATIONS FOR T568B
Note that the odd pin numbers are always the white with stripe color (1,3,5,7). The wires
connect to RJ-45 8-pin connectors as shown below:
Color Codes for T568B
PIN COLOR - PAIR NAME
1 white/orange (pair 2) TxData +
2 orange (pair 2) ........ TxData -
3 white/green (pair 3) ..RecvData+
4 blue (pair 1)
5 white/blue (pair 1)
6 green (pair 3) ...........RecvData-
7 white/brown (pair 4)
8 brown (pair 4)
The wall jack may be wired in a different sequence because the wires are often crossed inside
the jack. The jack should either come with a wiring diagram or at least designate pin numbers.
Note that the blue pair is on the centre pins; this pair translates to the red/green pair for
ordinary telephone lines which is also in the centre pair of an RJ-11. (green=wh/blu; red=blu)
PIN NUMBER DESIGNATIONS FOR T568A
The T568A specification reverses the orange and green connections so that pairs 1 and 2 are on
the centre 4 pins, which makes it more compatible with the telco voice connections. (Note that
in the RJ-11 plug at the top, pairs 1 and 2 are on the centre 4 pins.) T568A goes:
COLOR CODES FOR T568A
Pin color - pair name
1 white/green (pair 3) ..RecvData+
2 green (pair 3) ..........RecvData-
3 white/orange (pair 2) TxData +
4 blue (pair 1)
5 white/blue (pair 1)
6 orange (pair 2) .........TxData -
7 white/brown (pair 4)
8 brown (pair 4)
The diagram below shows the 568A and 568B in comparison:
WHERE ARE THEY USED ?
The most common application for a straight through cable is a connection between a PC
and a hub/switch.
In this case the PC is connected directly to the hub/switch which will automatically cross
over the cable internaly, using special circuits. In the case of a CAT1 cable, which is
usually found in telephone lines, only 2 wires are used, these do not require any special
cross over since the phones connect directly to the phone socket.
The picture above shows us a standard CAT5 straight thru cable, used to connect a PC to a HUB.
You might get a bit confused because you might expect the TX+ of one side to connect to the
TX+ of the other side but this is not the case.
When you connect a PC to a HUB, the HUB it will automatically x-over the cable for you by using
its internal circuits, this results Pin 1 from the PC (which is TX+) to connect to Pin 1 of the HUB
(which connects to RX+).This happens for the rest of the pinouts aswell.
If the HUB didn't x-over the pinouts using its internal circuits (this happens when you use the
Uplink port on the hub) then Pin 1 from the PC (which is TX+) would connect to Pin 1 of the HUB
(which would be TX+ in this case). So you notice that no matter what we do with the HUB port
(uplink or normal), the signals assigned to the 8 Pins on the PC side of things, will always remain
the same, the HUB's pinouts though will change depending wether the port is set to normal or
UTP CROSS-OVER CABLE
The cross-over (x-over) CAT5 UTP cable has to be one of the most used cables after the classic
straight-thru cable. The x-over cable allows us to connect two computers without needing a
hub or switch. If you recall, the hub does the x-over for you internally, so you only need to use a
straight thru cable from the PC to the hub. Since now we don't have a hub, we need to
manually do the x-over.
WHY DO WE NEED AN X-OVER ?
When sending or receiving data between two devices, e.g computers, one will be
sending while the other receives.
All this is done via the network cable and if you look at a network cable you will notice
that it contains multiple cables.
Some of these cables are used to send data, while others are used to receive data and
this is exactly what we take into account when creating an x-over cable. We basically
connect the TX (transmit) of one end to the RX (receive) of the other !
The diagram below shows this in the simplest way possible:
The pinouts of a typical x-over CAT5 cable:
Only 4 pins are needed for a x-over cable. When you buy a x-over cable, you might find that all
8 pins are used, these cables aren't any different from the above, it's just that there are cables
running to the unsed pins. This won't make any difference in performance, but is just a habit
some people follow.
Here are the pinouts for a x-over cable which has all 8 pins connected:
WHERE ELSE CAN WE USE A X-OVER ?
X-over cables are not just used to connect computers, but a variety of other devices. Prime
example are switches and hubs. If you have two hubs and you need to connect them, you
would usually use the special uplink port which, when activated through a little switch (in most
cases), makes that particular port not cross the tx and rx, but leave them as if they where
straight through. What happens though if you haven't got any uplink ports or they are already
The X-over cable will allow you to connect them and solve your problem. The diagram below
shows a few examples to make it simpler:
As we can see in the above diagram, thanks to the uplink port, there is no need for a x-over
Let's now have have look at how to cope when we don't have an uplink to spare, in which case
we must make a x-over cable to connect the two hubs:
All the above should explain a x-over cable, where we use it and why we need it. I thought it
would be a good idea to include, as a last picture, the pinouts of a straight thru and a x-over
cable so you can compare them side by side:
UTP ROLLOVER CABLE
A crossover cable, as you found, is a cable with the Rx and Tx pairs reversed between the ends:
A rollover cable, however, is used to connect a computer's serial port to the console prot of a
router or managed switch (with a dongle). It is wired with the to ends completely the reverse of
Coaxial Cable consists of 2 conductors. The inner conductor is held inside an insulator with the other
conductor woven around it providing a shield. An insulating protective coating called a jacket covers the
The outer shield protects the inner conductor from outside electrical signals. The distance between the
outer conductor (shield) and inner conductor plus the type of material used for insulating the inner
conductor determine the cable properties or impedance. Typical impedances for coaxial cables are 75
ohms for Cable TV, 50 ohms for Ethernet Thinnet and Thicknet. The excellent control of the impedance
characteristics of the cable allow higher data rates to be transferred than Twisted Pair cable.
Coaxial cable is the kind of copper cable used by cable TV companies between the community
antenna and user homes and businesses. Coaxial cable is sometimes used by telephone
companies from their central office to the telephone poles near users. It is also widely installed
for use in business and corporation Ethernet and other types of local area network.
Coaxial cable is called "coaxial" because it includes one physical
channel that carries the signal surrounded (after a layer of insulation) by
another concentric physical channel, both running along the same axis.
The outer channel serves as a ground. Many of these cables or pairs of
coaxial tubes can be placed in a single outer sheathing and, with repeaters,
can carry information for a great distance.
Coaxial cable was invented in 1929 and first used commercially in 1941. AT&T
established its first cross-continental coaxial transmission system in 1940. Depending on
the carrier technology used and other factors, twisted pair copper wire and optical fiber
are alternatives to coaxial cable.
Coaxial cable supports 10 to 100 Mbps and is relatively inexpensive, although it is more
costly than UTP on a per-unit length. However, coaxial cable can be cheaper for a
physical bus topology because less cable will be needed.
Coaxial cable can be cabled over longer distances than twisted-pair cable. For example,
Ethernet can run approximately 100 meters (328 feet) using twisted-pair cabling. Using
coaxial cable increases this distance to 500m (1640.4 feet).
For LANs, coaxial cable offers several advantages. It can be run with fewer boosts from
repeaters for longer distances between network nodes than either STP or UTP cable.
Repeaters regenerate the signals in a network so that they can cover greater distances.
Coaxial cable is less expensive than fiber-optic cable, and the technology is well known;
it has been used for many years for all types of data communication.
A connection device known as a vampire tap was used to connect network devices to
The vampire tap then was connected to the computers via a more flexible cable called
the attachment unit interface (AUI). Although this 15-pin cable was still thick and tricky
to terminate, it was much easier to work with than Thicknet.
The most common connectors used with Thinnet are BNC, short for British Naval
Connector or Bayonet Neill Concelman, connectors . The basic BNC connector is a male
type mounted at each end of a cable. This connector has a center pin connected to the
center cable conductor and a metal tube connected to the outer cable shield. A rotating
ring outside the tube locks the cable to any female connector. BNC T-connectors are
female devices for connecting two cables to a network interface card (NIC). A BNC barrel
connector facilitates connecting two cables together.
THE FOLLOWING SUMMARIZES THE FEATURES OF COAXIAL CABLES:
Speed and throughput—10 to 100 Mbps
Average cost per node—Inexpensive
Media and connector size—Medium
Maximum cable length—500 m (medium)
FIBRE OPTIC CABLE
Optical Fibre consists of thin glass fibres that can carry information at frequencies in the visible light
spectrum and beyond. The typical optical fibre consists of a very narrow strand of glass called the Core.
Around the Core is a concentric layer of glass called the Cladding. A typical Core diameter is 62.5 microns
(1 micron = 10-6 meters). Typically Cladding has a diameter of 125 microns. Coating the cladding is a
protective coating consisting of plastic, it is called the Jacket.
THE ADVANTAGES OF USING FIBRE OPTICS
Because of the Low loss, high bandwidth properties of fibre cables they can be used
over greater distances than copper cables.
In data networks this can be as much as 2km without the use of repeaters.
Their light weight and small size also make them ideal for applications where running
copper cables would be impractical and, by using multiplexors, one fibre could replace
hundreds of copper cables
This is pretty impressive for a tiny glass filament, but the real benefit in the data
industry is its immunity to Electro Magnetic Interference (EMI), and the fact that glass is
not an electrical conductor.
Because fibre is non-conductive it can be used where electrical isolation is needed, for instance,
between buildings where copper cables would require cross bonding to eliminate differences in
Fibres also pose no threat in dangerous environments such as chemical plants where a spark
could trigger an explosion. Last but not least is the security aspect, it is very, very difficult to tap
into a fibre cable to read the data signals.
There are many different types of fibre cable, but for the purposes of this explanation we will
deal with one of the most common types, 62.5/125 micron loose tube. The numbers represent
the diameters of the fibre core and cladding, these are measured in microns which are
millionths of a metre.
Loose tube fibre cable can be indoor or outdoor, or both, the outdoor cables usually have the
tube filled with gel to act as a moisture barrier to the ingress of water. The number of cores in
one cable can be anywhere from 4 to 144.
WHAT'S THE DIFFERENCE BETWEEN SINGLE-
MODE AND MULTI-MODE?
With copper cables larger size means less resistance and therefore more current, but with fibre
the opposite is true. To explain this we first need to understand how the light propagates within
the fibre core.
Light travels along a fibre cable by a process called 'Total Internal Reflection' (TIR), this is made
possible by using two types of glass which have different refractive indexes. The inner core has
a high refractive index and the outer cladding has a low index. This is the same principle as the
reflection you see when you look into a pond. The water in the pond has a higher refractive
index than the air and if you look at it from a shallow angle you will see a reflection of the
surrounding area, however, if you look straight down at the water you can see the bottom of
At some specific angle between these two view points the light stops reflecting off the surface
of the water and passes through the air/water interface allowing you to see the bottom of the
pond. In multi-mode fibres, as the name suggests, there are multiple modes of propagation for
the rays of light. These range from low order modes, which take the most direct route straight
down the middle, to high order modes, which take the longest route as they bounce from one
side to the other all the way down the fibre.
This has the effect of scattering the signal because the rays from one pulse of light arrive at the
far end at different times; this is known as Intermodal Dispersion (sometimes referred to as
Differential Mode Delay, DMD). To ease the problem, graded index fibres were developed.
Unlike the examples above which have a definite barrier between core and cladding, these have
a high refractive index at the centre which gradually reduces to a low refractive index at the
circumference. This slows down the lower order modes allowing the rays to arrive at the far
end closer together, thereby reducing intermodal dispersion and improving the shape of the
SO WHAT ABOUT THE SINGLE-MODE FIBRE?
Well, what's the best way to get rid of Intermodal Dispersion?, easy, only allow one mode of
propagation. So a smaller core size means higher bandwidth and greater distances. Simple as
TRANSMISSION MEDIA - UNGUIDED
Unguided Transmission Media is data signals that flow through the air. They are not guided or
bound to a channel to follow. They are classified by the type of wave propagation.
There are 3 types of RF (Radio Frequency) Propagation:
Line of Sight (LOS) Propagation.
Ground Wave Propagation follows the curvature of the Earth. Ground Waves have
carrier frequencies up to 2 MHz. AM radio is an example of Ground Wave Propagation.
Ionospheric Propagation bounces off of the Earths Ionospheric Layer in the upper
atmosphere. It is sometimes called Double Hop Propagation. It operates in the frequency range
of 30 - 85 MHz. Because it depends on the Earth's ionosphere, it changes with weather and
time of day. The signal bounces off of the ionosphere and back to earth. Ham radios operate in
this range. (See image 1 below)
Line of Sight Propagation transmits exactly in the line of sight. The receive station must be
in the view of the transmit station. It is sometimes called Space Waves or Tropospheric
Propagation. It is limited by the curvature of the Earth for ground based stations (100 km:
horizon to horizon). Reflected waves can cause problems. Examples of Line of Sight Propagation
are: FM Radio, Microwave and Satellite.
Radio Frequencies are in the range of 300 kHz to 10 GHz. We are seeing an emerging
technology called wireless LANs. Some use radio frequencies to connect the workstations
together, some use infrared technology.
Microwave transmission is line of sight transmission. The Transmit station must be in visible
contact with the receive station. This sets a limit on the distance between stations depending
on the local geography. Typically the line of sight due to the Earth's curvature is only 50 km to
the horizon! Repeater stations must be placed so the data signal can hop, skip and jump across
The frequency spectrum operates from 0 Hz (DC) to Gamma Rays (1019 Hz).
Name Frequency (Hertz) Examples
Gamma Rays 10^19 +
Ultra-Violet Light 7.5 x 10^15
Visible Light 4.3 x 10^14
Infrared Light 3 x 10^11
EHF - Extremely High
30 GHz (Giga = 10^9) Radar
SHF - Super High Frequencies 3 GHz Satellite & Microwaves
UHF - Ultra High Frequencies 300 MHz (Mega = 10^6) UHF TV (Ch. 14-83)
VHF - Very High Frequencies 30 MHz FM & TV (Ch2 - 13)
HF - High Frequencies 3 MHz2 Short Wave Radio
MF - Medium Frequencies 300 kHz (kilo = 10^3) AM Radio
LF - Low Frequencies 30 kHz Navigation
VLF - Very Low Frequencies 3 kHz Submarine Communications
VF - Voice Frequencies 300 Hz Audio
ELF - Extremely Low Frequencies 30 Hz Power Transmission
Microwaves operate at high operating frequencies of 3 to 10 GHz. This allows them to carry large
quantities of data due to the large bandwidth.
a. They require no right of way acquisition between towers.
b. They can carry high quantities of information due to their high operating frequencies.
c. Low cost land purchase: each tower occupies small area.
d. High frequency/short wavelength signals require small antenna.
a. Attenuation by solid objects: birds, rain, snow and fog.
b. Reflected from flat surfaces like water and metal.
c. Diffracted (split) around solid objects
d. Refracted by atmosphere, thus causing beam to be projected away from receiver.
Wireless communication uses radio frequencies (RF) or infrared (IR) waves to transmit data
between devices on a LAN. For wireless LANs, a key component is the wireless hub, or access
point, used for signal distribution .
To receive the signals from the access point, a PC or laptop must install a wireless adapter card
(wireless NIC). Wireless signals are electromagnetic waves that can travel through the vacuum
of outer space and through a medium such as air. Therefore, no physical medium is necessary
for wireless signals, making them a very versatile way to build a network. Wireless signals use
portions of the RF spectrum to transmit voice, video, and data. Wireless frequencies range from
3 kilohertz (kHz) to 300 gigahertz (GHz). The data-transmission rates range from 9 kilobits per
second (kbps) to as high as 54 Mbps.
The primary difference between electromagnetic waves is their frequency. Low-frequency
electromagnetic waves have a long wavelength (the distance from one peak to the next on the
sine wave), while high-frequency electromagnetic waves have a short wavelength.
Some common applications of wireless data communication include the following:
Accessing the Internet using a cellular phone
Establishing a home or business Internet connection over satellite
Beaming data between two hand-held computing devices
Using a wireless keyboard and mouse for the PC
Another common application of wireless data communication is the wireless LAN (WLAN),
which is built in accordance with Institute of Electrical and Electronics Engineers (IEEE) 802.11
standards. WLANs typically use radio waves (for example, 902 megahertz [MHz]), microwaves
(for example, 2.4 GHz), and IR waves (for example, 820 nanometers [nm]) for communication.
Wireless technologies are a crucial part of the today's networking. See Chapter 28, "Wireless
LANs," for a more detailed discuss on wireless networking.
COMPARING MEDIA TYPES
Presented in Table 8-1 are comparisons of the features of the common network media. This
chart provides an overview of various media that you can use as a reference. The medium is
possibly the single most important long-term investment made in a network. The choice of
media type will affect the type of NICs installed, the speed of the network, and the capability of
the network to meet future needs.
Table 8-1 Media Type Comparison
Media Maximum Speed Cost Advantages Disadvantages
UTP 100 m 10 Mbps to Least Easy to install; Susceptible to
1000 Mbps expensive widely available and interference; can
widely used cover only a limited
STP 100 m 10 Mbps to More Reduced crosstalk; Difficult to work
100 Mbps expensive more resistant to with; can cover
than UTP EMI than Thinnet or only a limited
Coaxial 500 m 10 Mbps to Relatively Less susceptible to Difficult to work
Media Maximum Speed Cost Advantages Disadvantages
(Thicknet) 100 Mbps inexpensive, EMI interference with (Thicknet);
but more than other types of limited bandwidth;
185 m costly than copper media limited application
(Thinnet) UTP (Thinnet); damage
to cable can bring
Fiber- 10 km and 100 Mbps to Expensive Cannot be tapped, Difficult to
Optic farther 100 Gbps so security is better; terminate
(single- (single mode) can be used over
mode) great distances; is
100 Mbps to not susceptible to
2 km and 9.92 Gbps EMI; has a higher
farther (multimode) data rate than
(multimode) coaxial and twisted-
COAXIAL CABLE CONNECTORS
HyperLink stocks hundreds of the most popular types of Coaxial Cable connectors, Coaxial Cable
adapters and crimp tools for today's wireless applications.
VAMPIRE TAP CONNECTOR:
THINNET AND BNC CONNECTOR :