Data
Communication Fundamentals
·
Various types of transmission
media - guided transmission media :
magnetic media, twisted pair, coaxial cables, fiber optics
·
Introduction to the concept of modulation,
types of modulation, serial transmission vs. parallel transmission, synchronous
transmission v/s asynchronous transmission, circuit switching, packet switching
·
The concept of multiplexing,
Frequency Division Multiplexing (FDM) vs. Time Division Multiplexing (TDM)
TRANSMISSION
MEDIA:
Any
data communication is in need of three components
1) Media
2) Data
3) Transmitter and the receiver
The data generated by the transmitter is carried by the medium to
the receiver. This medium can be classified into two categories.
1. Guided media
2. Unguided
media
Guided
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:
Twisted Pair
Coaxial Cable
Optical Fiber
Twisted pair:
Twisted pair cable consists of a pair of insulated wires twisted
together. It is a cable type used in telecommunication for very long time.
Cable twisting helps to reduce noise pickup from outside sources and crosstalk
on multi-pair cables.
Shielded Twisted Pair Cable is used to eliminate inductive and
capacitive coupling. Twisting cancels out inductive coupling, while the shield
eliminates capacitive coupling. Most applications for this cable are between
equipment, racks and buildings.
Shielding adds usually some attenuation to the cable (compared to
unshielded), but usually not because in the case of balanced transmission, the
complementing signals will effectively cancel out any shield currents, so
shield current losses are negligible.
The noise pickup characteristics of twisted pair cable is
determined by the following cable characteristics: number of twists per meter
(generally more twists per meter gives better performance), uniform cable
construction, capacitance balance (less capacitance difference to ground, the
better), cable diameter (less are between wires is better) and the amount of
shielding (more shielding, the better).
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 - phase 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 characteristic impedance for the
cable. Typical impedance for UTP is 100 ohm for Ethernet 10BaseT cable
Twisted pair cable is good for transferring balanced differential
signals. The practice of transmitting signals differentially dates back to the
early days of telegraph and radio. The advantages of improved signal-to-noise
ratio, crosstalk, and ground bounce that balanced signal transmission are
particularly valuable in wide bandwidth and high fidelity systems. By
transmitting signals along with a 180 degree out-of-phase complement, emissions
and ground currents are theoretically canceled. This eases the requirements on
the ground and shield compared to single ended transmission and results in
improved EMI performance.
The most commonly used form of twisted pair is unshielded twisted
pair (UTP). It is just two insulated wires twisted together. Any data
communication cables and normal telephone cables are this type. Shielded
twisted pair (STP) differs from UTP in that it has a foil jacket that helps
prevent crosstalk and noise from outside source. In data communications there
is a cable type called FTP (foil shielded pairs) which consists of four twisted
pair inside one common shield (made of aluminum foil).
When cable twisted at constant twist rate over the length of the
cable, a cable with well defined characteristic impedance is formed.
Characteristic impedance of twisted pair is determined by the size and spacing
of the conductors and the type of dielectric used between them. Balanced pair,
or twin lines, has a Zo which depends on the ratio of the wire spacing to wire
diameter and the foregoing remarks still apply. For practical lines, Zo at high
frequencies is very nearly, but not exactly, a pure resistance. Because the
impedance of a cable is actually a function of the spacing of the conductors,
so separating the conductors significantly changes the cable impedance at that
point.
When many twisted pairs are put together to form a multi-pair
cable, individual conductors are twisted into pairs with varying twists to
minimize crosstalk. Specified color combinations to provide pair
identification.
The most commonly used twisted pair cable impedance is 100 ohms.
It is widely used for data communications and telecommunications applications
in structured cabling systems. In most twisted pair cable applications the
cable impedance is between 100 ohms and 150 ohms. When a cable has a long
distance between the conductors, higher impedances are possible. Typical wire
conductor sizes for cables used in elecommunications 26, 24, 22 or 19 AWG.
Coaxial cable:
A coaxial cable is one that consists of two conductors that share
a common axis. The inner conductor is typically a straight wire, either solid
or stranded and the outer conductor is typically a shield that might be braided
or a foil.
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 outer
conductor.
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 forinsulating 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 a cable type used to carry radio signals, video
signals, measurement signals and data signals. Coaxial cables exist because we
can't run open-wire line near metallic objects (such as ducting) or bury it. We
trade signal loss for convenience and flexibility. Coaxial cable consists of an
insulated center conductor which is covered with a shield. The signal is carried
between the cable shield and the center conductor. This arrangement give quite
good shielding against noise from outside cable, keeps the signal well inside
the cable and keeps cable characteristics stable.
Coaxial cables and systems connected to them are not ideal. There
is always some signal radiating from coaxial cable. Hence, the outer conductor
also functions as a shield to reduce coupling of the signal into adjacent
wiring. More shield coverage means less radiation of energy (but it does not
necessarily mean less signal attenuation).
Coaxial cables are typically characterized with the impedance and
cable loss. The length has nothing to do with coaxial cable impedance.
Characteristic impedance is determined by the size and spacing of the
conductors and the type of dielectric used between them. For ordinary coaxial
cable used at reasonable frequency, the characteristic impedance depends on the
dimensions of the inner and outer conductors. The characteristic impedance of a
cable (Zo) is determined by the formula 138 log b/a, where b represents the
inside diameter of the outer conductor (read: shield or braid), and a
represents the outside diameter of the inner conductor.
Most common coaxial cable impedances in use in various
applications are 50 ohms and 75 ohms. 50 ohms cable is used in radio
transmitter antenna connections, many measurement devices and in data
communications (Ethernet). 75 ohms coaxial cable is used to carry video
signals, TV antenna signals and digital audio signals. There are also other
impedances in use in some special applications (for example 93 ohms). It is
possible to build cables at other impedances, but those mentioned earlier are
the standard ones that are easy to get.
It is usually no point in trying to get something very little different for
some marginal benefit, because standard cables are easy to get, cheap and
generally very good. Different impedances have different characteristics. For maximum
power handling, somewhere between 30 and 44 Ohms is the optimum. Impedance
somewhere around 77 Ohms gives the lowest loss in a dielectric filled line. 93
Ohms cable gives low capacitance per foot. It is practically very hard to find
any coaxial cables with impedance much higher than that.
Fiber optics
Optical Fibre
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 (1micron = 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.
Important characteristic of Fibre Optics is Refraction. Refraction
is the characteristic of a material to either pass or reflect light. When light
passes through a medium, it "bends" as it passes from one medium to
the other. An example of this is when we look into a pond of water.
If the angle of incidence is small, the light rays are reflected and
do not pass into the water. If the angle of incident is great, light passes
through the media but is bent or refracted.
Optical Fibres work on the principle that the core refracts the
light and the cladding reflects the light. The core refracts the light and
guides the light along its path. The cladding reflects any light back into the
core and stops light from escaping through it.
Optical
Transmission Modes
There are 3 primary
types of transmission modes using optical fibre. They are
a) Step Index
b) Grade Index
c) Single Mode
Step Index has a large core the light rays tend to bounce around,
reflecting off the cladding, inside the core. This causes some rays to take a
longer or shorted path through the core. Some take the direct path with hardly
any reflections while others bounce back and forth taking a longer path. The
result is that the light rays arrive at the receiver at different times. The
signal becomes longer than the original signal. LED light sources are used.
Typical Core: 62.5 microns.
Step
Index Mode
Grade Index has a
gradual change in the Core's Refractive Index. This causes the light rays to be
gradually bent back into the core path. This is represented by a curved
reflective path in the
attached drawing. The
result is a better receive signal than Step Index. LED light sources are used.
Typical Core: 62.5 microns.
Grade
Index Mode
Single Mode has separate
distinct Refractive Indexes for the cladding and core. The light ray passes
through the core with relatively few reflections off the cladding. Single Mode is
used for a single source of light (one color) operation. It requires a laser
and
the core is very small: 9
microns.
Single Mode:
When the fiber core is so small that only light ray at 0° incident
angle can stably pass through the length of fiber without much loss, this kind
of fiber is called single mode fiber. The basic requirement for single mode
fiber is that the core be small enough to restrict transmission to a singe
mode. This lowest-order mode can propagate in all fibers with smaller cores (as
long as light can physically enter the fiber).
The most common type of single mode fiber has a core diameter of 8
to 10 µm and is designed for use in the near infrared (the most common are
1310nm and 1550nm). Please note that the mode structure depends on the
wavelength of the light used, so that this fiber actually supports a small
number of additional modes at visible wavelengths. Multi mode fiber, by
comparison, is manufactured with core diameters as small as 50um and as large
as hundreds of microns.
The following picture shows the fiber structure of a single mode
fiber.
Introduction to
the concept of modulation
Modulation is the process of
conveying a message signal, for example a digital bit stream or an analog
audio signal, inside another signal that can be physically transmitted. 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 (from "modulator–demodulator").
The Digital Modulation
is to transfer a digital
bit stream over an analog bandpass channel, for example over the public switched telephone network,
or over a limited radio frequency band.
The Analog modulation
is to transfer an analog baseband (or lowpass) signal, for example an audio signal or TV signal,
over an analog bandpass channel at a different frequency, for
example over a limited radio frequency band or a cable TV network channel.
Analog and digital modulation facilitate frequency division multiplexing
(FDM), where several low pass information signals are transferred
simultaneously over the same shared physical medium, using separate passband
channels (several different carrier frequencies).
The Baseband Modulation
methods, also known as line coding, is to transfer a digital bit stream over a
baseband
channel, typically a non-filtered copper wire such as a serial bus
or a wired local area network.
The Pulse Modulation
methods is to transfer a narrowband analog signal, for example a phone call over a wideband
baseband channel or, in some of the schemes, as a bit stream over another digital transmission system.
•
The Process of converting analog data to analog signal is called
Modulation.
•
Modulation is used to send an information bearing signal over long
distances.
•
Modulation is the process of varying some characteristic of a
periodic wave with an external signal called carrier signal.
•
These carrier signals are high frequency signals and can be
transmitted over the air easily and are capable of traveling long distances.
•
The characteristics (amplitude, frequency, or phase) of the
carrier signal are varied in accordance with the information bearing
signal(analog data).
•
The information bearing signal is also known as the modulating
signal.
•
The modulating signal is a slowly varying – as opposed to the
rapidly varying carrier frequency.
Types
of Modulation
Signal modulation can
be divided into two broad categories:
·
Analog modulation and
·
Digital modulation.
·
Analog or digital refers
to how the data is modulated onto a sine wave.
·
If analog audio data is modulated onto a carrier sine wave, then
this is referred to as analog modulation.
·
Digital modulation is
used to convert digital data to analog signal. Ex ASK, FSK, PSK.
·
Analog Modulation can be accomplished in three ways:
1.
Amplitude modulation (AM)
2.
Frequency modulation (FM)
3.
Phase modulation (PM).
Amplitude
modulation (AM)
•
Amplitude modulation is a type of modulation where the amplitude
of the carrier signal is varied in accordance with modulating signal.
•
The envelope, or boundary, of the amplitude modulated signal
embeds modulating signal.
•
Amplitude Modulation is abbreviated AM.
Frequency
modulation (FM)
•
Frequency modulation is a type of modulation where the frequency
of the carrier is varied in accordance with the modulating signal. The
amplitude of the carrier remains
•
constant.
•
The information-bearing signal (the modulating signal) changes the
instantaneous frequency of the carrier. Since the amplitude is kept constant,
FM modulation is a
•
low-noise process and provides a high quality modulation technique
which is used for music and speech in hifidelity broadcasts.
•
Frequency Modulation is abbreviated FM.
Phase
modulation (PM).
•
In phase modulation, the instantaneous phase of a carrier wave is
varied from its reference value by an amount proportional to the instantaneous
amplitude of the modulating signal.
•
Phase Modulation is abbreviated PM.
Serial and Parallel Transmission
Digital data transmission can occur
in two basic modes: serial or parallel. Data within a computer system is
transmitted via parallel mode on buseswith the width of the parallel bus
matched to the word size of the computer system. Data between computer systems
is usually transmitted in bit serial mode. Consequently, it is necessary
to make a parallel-to-serial conversion at a computer interface when sending
data from a computer system into a network and a serial-to-parallel conversion
at a computer interface when receiving information from a network. The type of
transmission mode used may also depend upon distance and required data rate.
Serial Data Transmission
|
Parallel Data Transmission
|
In serial
transmission, bits are sent sequentially on the same channel (wire)
which reduces costs for wire but also slows the speed
of transmission.
|
In parallel
transmission, multiple bits (usually 8 bits or a byte/character) are
sent simultaneously on different channels (wires, frequency channels) within
the same cable, or radio path, and synchronized to a clock.
|
Data is sent across
one wire, one data bit at a time
|
Data is sent across
Multiple wire, Multiple data bit at a time, Simultanosly
|
Communication
speed is Slow
|
Communication
speed is Fast
|
Serial
Interface can run at high frequencies
|
Parallel Interface are
hard to run at high frequencies
|
Cabling
cost is Cheap and Easy
|
Cabling
cost is Expensive and Complex
|
Used
for transmitting data at Long Distance
|
Used
for Transmitting data at Short Distance
|
PARALLEL
VERSUS SERIAL DATA TRANSMISSIONS
There
are two methods of transmitting digital data. These methods are parallel and serial
transmissions. In parallel data transmission, all bits of the binary data are transmitted simultaneously. For example, to transmit an 8-bit binary number in parallel from one unit to another, eight transmission lines are required. Each bit requires its own separate data path. All bits of a word are transmitted at the same time. This method of transmission can move a significant amount of data in a given period of time. Its disadvantage is the large number of interconnecting
cables between the two units. For large binary words,
cabling becomes complex and expensive. This is
particularly true if the distance between the two units
is great. Long multiwire cables are not only expensive,
but also require special interfacing to minimize noise
and distortion problems. Serial
data transmission is the process of transmitting binary
words a bit at a time. Since the bits time-share the
transmission medium, only one interconnecting lead is
required. While serial data transmission is much
simpler and less expensive because of the use of a
single interconnecting line, it is a very slow method
of data transmission. Serial data transmission is
useful in systems where high speed is not a
requirement. Serial data transmission techniques are
widely used in transmitting data between a computer and
its peripheral units. While the computer operates at
very high speeds, most peripheral units are slow
because of their electromechanical nature. Slower
serial data transmission is more compatible with such
devices. Since the speed of serial transmission is more
than adequate in such units, the advantages of low cost
and simplicity of the signal interconnecting line
can be obtained
Asynchronous Transmission
When data bits are transmitted
serially, it is necessary for the receiver to properly detect the moment in
time that each bit in a character begins and ends. With asynchronous
transmission, this is performed by framing each character with a Start
and Stop pulse.
The illustration above depicts an
8-bit character, framed with a Start bit and a Stop bit. The Mark (1) state is
negative, the Space (0) state is a positive voltage. Note that in an idle
state, the line is Marking (1). The Start bit immediately takes the line out of
that condition, while the Stop bit returns the line to the idle state.
The receiver senses the Space bit,
and counts about 10 bit lengths. From there it can derive the value of each
bit. Since the exact clocks that do the counting may differ between the
transmitter and receiver, there always exists a timing error, called
"bias". Most modern UARTs (Universal Asynchronous
Receiver/Transmitter) will use a 16x clock rate to sample received data with to
work around this problem.
Synchronous Transmission
When synchronous transmission is
used, common bit timing is used between a transmitter and a receiver. For
analog modem systems, the modem recovers timing pulses from the incoming data
stream and presents a continuous stream of "clock pulses" to the
receiving DTE device. For other systems, like DDS or T1, the timing is
recovered from the actual incoming digital pulses.
The receiving DTE device will look
at the negative-going clock transition to sample the data stream with. This
negative clock pulse is aligned in the center of the data bit, eliminating the
"bias" problem with asynchronous systems.
The receive clock ensures
accurate sampling of the data stream, but the receiver can't tell what bits
comprise a character. The transmitter will send characters in a group, called a
block or a frame. Special synchronization characters are
transmitted at the beginning of each block. The receiver will look for these
special characters and, once detected, will know where each character begins
and ends.
Although the
synchronization characters add some overhead, it is not near the 20% overhead
incurred with asynchronous Start/Stop transmission. As blocks of more
characters are transmitted (increased block size), the relative overhead
decreases.
Advantages
and disadvantages
Advantages
|
Disadvantages
|
|
Asynchronous transmission
|
|
|
Synchronous transmission
|
|
|
CIRCUIT-SWITCHED
NETWORKS
•
Circuit switching takes place at the physical layer
•
A circuit switching network is one that establishes a circuit (or
channel) between nodes and terminals before the users may communicate, as if
the nodes were physically connected with an electrical circuit.
•
An important property of circuit switching is the need to set up
an end-to-end path before any data can be sent.
•
Example: A telephone call
•
The switching equipment within the telephone system seeks out a
physical path all the way from sender’s telephone to the receiver’s telephone.
•
The actual communication in a circuit-switched network requires
three phases:
i.
connection setup
ii.
data transfer, and
iii.
connection teardown.
Connection
Setup
•
Before the two parties (or multiple parties in a conference call)
can communicate, a dedicated circuit (combination of channels in links) needs
to be established.
•
The end systems are normally connected through dedicated lines to
the switches, so connection setup means creating dedicated channels between the
switches.
Data
Transfer Phase
•
After the establishment of the dedicated circuit (channels),the
two parties can transfer data.
Teardown
Phase
•
When one of the parties
needs to disconnect, a signal is sent to each switch to release the resources.
PACKET
SWITCHED NETWORKS
•
In packet switching network, there is no call setup hence no
particular path to be followed by the packets i.e different packets can follow
different paths depending upon the network conditions, so the packets may
arrive out of order at the receiver.
•
Packet Switching is more fault tolerant than circuit switching. If
a switch goes down, all of the circuits using it are terminated and no more
traffic can be sent on any of them. With packet switching packets can be routed
around dead switches.
•
Packet switching uses store and forward transmission. A packet is
stored in the routers memory and then sent on to the next router. With circuit
switching bits just flow through the wire continuously. The store and forward
technique adds delay to packet switching networks.
•
Packet Switching network differ from circuit switching network in
the way in which users are charged.
•
With Circuit switching charging is done based on distance and
time. Ex. STD calls are charged more compared to local calls.
•
Packet switching networks charge for the volume of traffic. Ex. BSNL allows 400Mb of internet browsing at
Rs 200/-, After this limit of 400Mb is exceeded every MB is charged
explicitly.
•
In a packet-switched network, there is no resource reservation;
resources are allocated on demand .
There
are two approaches to packet switching
•
Datagram Approach
•
Virtual Circuit Approach
Datagram Switching
•
Using Datagram transmission, each packet is treated as a separate
entity and contains a header with the full information about the intended
recipient.
•
The intermediate nodes examine the header of a packet and select
an appropriate link to an intermediate node which is nearer the destination.
•
In this system packets do not follow a pre-established route, and
the intermediate nodes (usually routers) do not require prior knowledge of the
routes that will be used.
·
Datagram switching is normally done at the network layer
·
In a datagram network as packets are treated as independent
entities they may take different routes to reach the destination hence may
arrive out of sequence at the destination.
·
The datagram networks are sometimes referred to as connectionless
networks.
·
The term connectionless here means that the
switch (packet switch) does not keep information about the connection state.
There are no setup or teardown phases. Each packet is treated the same by a
switch regardless of its source or destination.
·
Routing Table
·
Each switch (or packet switch) has a routing table which is based
on the destination address. The routing tables are dynamic and are updated
periodically.
·
The destination addresses and the corresponding forwarding output
ports are recorded in the tables.
·
This is different from the table of a circuit switched network in
which each entry is created when the setup phase is completed and deleted when
the teardown phase is over.
·
Destination Address
·
Every packet in a datagram network carries a header that contains,
among other information, the destination address of the packet.
·
When the switch receives the packet, this destination address is
examined; the routing table is consulted to find the corresponding port through
which the packet should be forwarded.
Virtual-circuit
networks
·
A virtual-circuit network is a cross between a circuit-switched
network and a datagram network. It has some characteristics of both.
·
As in a circuit-switched
network, there are setup and teardown
phases in addition to the data transfer phase.
·
Resources can be allocated during the setup phase, as in a
circuit-switched network, or on demand, as in a datagram network.
·
As in a datagram network,
data are packetized and each packet carries an address in the header.
·
As in a circuit-switched network, all packets follow the same path
established during the connection.
·
A virtual-circuit network is normally implemented in the data link
layer, while a circuit-switched network is implemented in the physical layer
and a datagram network in the network layer.
MULTIPLEXING
•
The concept of Multiplexing is closely related to bandwidth.
Communication links have a definite bandwidth. A signal may require a bandwidth
that is greater than the communication link which may require combining the
several links together or the signal may require a very less portion of
bandwidth which results in its wastage. It would be wise to share this link
with other devices, which is done using Multiplexing techniques.
•
Multiplexing is a set of techniques. It
allows simultaneous transmission of multiple signals across a single
communication link.
•
In the figure above, 4 low speed links are connected to a
Multiplexer (MUX) which combines them into a single high speed link.
•
A Multiplexer performs a many to one signal conversion (many low
speed signals are converted into one high speed signal).
•
This signal is fed to the Demultiplexer (DEMUX) at the other end
which separates the high speed signal into its original component signals.
•
Also, a channel is portion of a link, 1 link may have multiple (n)
channels.
•
Signals are of two types : Analog and Digital so multiplexing
techniques fall under two categories A.
Analog & B. Digital
FDM
(Frequency Division Multiplexing)
•
FDM is applicable when the bandwidth of a link (in hertz) is more
than the combined bandwidth of individual signals to be transmitted
•
In FDM, signals generated by each sending device modulate
different carrier frequencies. These modulated signals are then combined into a
single composite signal that can be transported by the link.
•
Carrier frequencies are separated by sufficient bandwidth to
accommodate the modulated signal.
•
Signals are prevented from overlapping by using strips of unused
bandwidth called guard bands.
TDM
(Time-division multiplexing)
•
It is a digital process that allows several connections to share
the high bandwidth of a link.
•
Instead of sharing a portion of the bandwidth as in FDM, time is
shared. Each connection occupies a portion of time in the link.
•
The next Figure gives a conceptual view of TDM.
•
The same link is used as in FDM; here, however, the link is shown
sectioned by time rather than by frequency. In the figure, portions of signals
1,2,3, and 4 occupy the link sequentially.
Time-Division Multiplexing
Time-Division Multiplexing (TDM) is a type of digital or analog multiplexing in which two or more signals or bit streams are transferred apparently simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel. The time domain is divided into several recurrent timeslots of fixed length, one for each sub-channel.
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.
Time division multiplexing (TDM) and has many applications, including wireline telephone systems and some cellular telephone systems. The main reason to use TDM is to take advantage of existing transmission lines.
TIME DIVISION MULTIPLEXING (TDM) allows multiple conversations to take place by the sharing of medium or channel in time. A channel is allocated a the whole of the line bandwidth for a specific period of time. This means that each subscriber is allocated a time slot.
Frequency-Division Multiplexing
Frequency-division multiplexing (FDM) is a form of signal multiplexing where multiple baseband signals are modulated on different frequency carrier waves and added together to create a composite signal
In many communication systems, a single, large frequency band is assigned to the system and is shared among a group of users. Examples of this type of system include:
1. A microwave transmission line connecting two sites over a long distance.
2. AM or FM radio broadcast bands, which are divided among many channels or stations. The stations are selected with the radio dial.
The deriving of two or more simultaneous, continuous channels from a transmission medium by assigning a separate portion of the available frequency spectrum to each of the individual channels. (188)
The simultaneous transmission of multiple separate signals through a shared medium at the transmitter, the separate signals into separable frequency bands, and adding those results linearly either before transmission or within the medium. All the signals may be amplified, conducted, translated in frequency and routed toward a destination as a single signal, resulting in economies which are the motivation for multiplexing.
TDM VS FDM
Difference No. 1
TDM: Total available time is divided into several user
FDM: total frequency bands are divided into several users
Difference No. 2
FDM:A multiplex system for transmitting two or more signals over a common path by using a different frequency band for each signal.
TDM: Transmission of two or more signals on the same path, but at different times.
Difference No. 3
TDM:TDM imply partitioning the bandwidth ofthe channel connecting two nodes into finite set of time slots
FDM:The signals multiplexed come from different sources/transmitters.
Time-Division Multiplexing (TDM) is a type of digital or analog multiplexing in which two or more signals or bit streams are transferred apparently simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel. The time domain is divided into several recurrent timeslots of fixed length, one for each sub-channel.
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.
Time division multiplexing (TDM) and has many applications, including wireline telephone systems and some cellular telephone systems. The main reason to use TDM is to take advantage of existing transmission lines.
TIME DIVISION MULTIPLEXING (TDM) allows multiple conversations to take place by the sharing of medium or channel in time. A channel is allocated a the whole of the line bandwidth for a specific period of time. This means that each subscriber is allocated a time slot.
Frequency-Division Multiplexing
Frequency-division multiplexing (FDM) is a form of signal multiplexing where multiple baseband signals are modulated on different frequency carrier waves and added together to create a composite signal
In many communication systems, a single, large frequency band is assigned to the system and is shared among a group of users. Examples of this type of system include:
1. A microwave transmission line connecting two sites over a long distance.
2. AM or FM radio broadcast bands, which are divided among many channels or stations. The stations are selected with the radio dial.
The deriving of two or more simultaneous, continuous channels from a transmission medium by assigning a separate portion of the available frequency spectrum to each of the individual channels. (188)
The simultaneous transmission of multiple separate signals through a shared medium at the transmitter, the separate signals into separable frequency bands, and adding those results linearly either before transmission or within the medium. All the signals may be amplified, conducted, translated in frequency and routed toward a destination as a single signal, resulting in economies which are the motivation for multiplexing.
TDM VS FDM
Difference No. 1
TDM: Total available time is divided into several user
FDM: total frequency bands are divided into several users
Difference No. 2
FDM:A multiplex system for transmitting two or more signals over a common path by using a different frequency band for each signal.
TDM: Transmission of two or more signals on the same path, but at different times.
Difference No. 3
TDM:TDM imply partitioning the bandwidth ofthe channel connecting two nodes into finite set of time slots
FDM:The signals multiplexed come from different sources/transmitters.
