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Communication Systems
Index
 Acronyms ……………………………………………………….1
 Network types
 Cables(Coax-Fiber-Twist)
 Duplexing(Half-Full)
 Modulation(Analog-Digital-Pulse)
 Wireless basics(Sight Survay-Standards-SNR)
 DSL types(ADSL & Leased Line)
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Table Of Contents :
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Acronyms …………………………………………………………………………………………..3
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All Type of Networks………………………………………………………………………………5
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Acronyms ……………………………………..1
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Acronyms ……………………………………..1
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Acronyms ……………………………………..1
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Acronyms ……………………………………..1
Acronyms
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IEEE : Institute of Electrical & Electronics Engineers
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LAN : Local Area Network
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WLAN : Wireless Local Area Network
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WAN : Wide Area Network
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MAN : Metropolitan Area Network
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SAN : Storage Area Network, System Area Network, Server Area Network, or sometimes Small Area Network
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CAN : Campus Area Network, Controller Area Network, or sometimes Cluster Area Network
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PAN : Personal Area Network
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DAN : Desk Area Network
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GSM - Global standard for digital mobile communication, common in most countries except South Korea and
Japan
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PCS - Personal communication system - not a single standard, this covers both CDMA and GSM networks
operating at 1900 MHz in North America
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ISM : industrial, scientific and medical
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Mobitex - pager-based network in the USA and Canada, built by Ericsson, now used by PDAs such as the Palm
VII and Research in Motion BlackBerry
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GPRS - General Packet Radio Service, upgraded packet-based service within the GSM framework, gives higher
data rates and always-on service
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UMTS - Universal Mobile Telephone Service (3rd generation cell phone network), based on the W-CDMA radio
access network
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AX.25 - amateur packet radio
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NMT - Nordic Mobile Telephony, analog system originally developed by PTTs in the Nordic countries
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AMPS - Advanced Mobile Phone System introduced in the Americas in about 1984.
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D-AMPS - Digital AMPS, also known as TDMA
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Wi-Fi -Wireless Fidelity, widely used for Wireless LAN, and based on IEEE 802.11 standards.
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Wimax - Worldwide Interoperability for Microwave Access.
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Canopy - A wide-area broadband wireless solution from Motorola.
 All Type of Networks
 Peer to Peer
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 Mesh
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 Star
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 Bus
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Common Cables
 Coaxial
 Fiber Optic
 Ribbon cables
 Twisted Pair (Cat 5…)
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Coaxial Cable
Coaxial cabling has a single copper conductor at its center. A plastic layer provides insulation
between the center conductor and a braided metal shield (See fig. 3). The metal shield helps to
block any outside interference from fluorescent lights, motors, and other computers.
 Although coaxial cabling is difficult to install, it is highly resistant to signal interference. In addition,
it can support greater cable lengths between network devices than twisted pair cable. The two types
of coaxial cabling are thick coaxial and thin coaxial.
 Thin coaxial cable is also referred to as thinnet. 10Base2 refers to the specifications for thin coaxial
cable carrying Ethernet signals. The 2 refers to the approximate maximum segment length being
200 meters. In actual fact the maximum segment length is 185 meters. Thin coaxial cable is popular
in school networks, especially linear bus networks.
 Thick coaxial cable is also referred to as thicknet. 10Base5 refers to the specifications for thick
coaxial cable carrying Ethernet signals. The 5 refers to the maximum segment length being 500
meters. Thick coaxial cable has an extra protective plastic cover that helps keep moisture away from
the center conductor. This makes thick coaxial a great choice when running longer lengths in a
linear bus network. One disadvantage of thick coaxial is that it does not bend easily and is difficult
to install.
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Coaxial Cable Connectors
 The most common type of connector used with coaxial
cables is the Baonye-Neill-Concelman (BNC) connector (See
fig. 4). Different types of adapters are available for BNC
connectors, including a T-connector, barrel connector, and
terminator. Connectors on the cable are the weakest points
in any network. To help avoid problems with your network,
always use the BNC connectors that crimp, rather than screw,
onto the cable.
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Fiber Optic
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Fiber optics (optical fibers) are long, thin strands of very pure glass about the diameter of a human hair. They are
arranged in bundles called optical cables and used to transmit light signals over long distances.Printronix
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If you look closely at a single optical fiber, you will see that it has the following parts:
Core -Thin glass center of the fiber where the light travels
Cladding - Outer optical material surrounding the core that reflects the light back into the core
Buffer coating - Plastic coating that protects the fiber from damage and moisture
Hundreds or thousands of these optical fibers are arranged in bundles in optical cables. The bundles are protected
by the cable's outer covering, called a jacket. Optical fibers come in two types:
Single-mode fibers
Multi-mode fibers
Single-mode fibers have small cores (about 3.5 x 10-4 inches or 9 microns in diameter) and transmit infrared
laser light (wavelength = 1,300 to 1,550 nanometers). Multi-mode fibers have larger cores (about 2.5 x 10-3
inches or 62.5 microns in diameter) and transmit infrared light (wavelength = 850 to 1,300 nm) from lightemitting diodes (LEDs).
Some optical fibers can be made from plastic. These fibers have a large core (0.04 inches or 1 mm diameter) and
transmit visible red light (wavelength = 650 nm) from LEDs.
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Fiber Optic Connectors
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Fiber Optic Sensor
Any device in which variations in the transmitted power or the rate of transmission of light in
optical fiber are the means of measurement or control. Fibers can be used to measure temperature,
pressure, strain, voltage, current, liquid level, rotation and particle velocity. The small size and the
fact that no electrical power is needed at the remote location gives the fiber optic sensor advantages
to conventional electrical sensor in certain applications.
 Fiber Optic Sensor: This illustration shows a fiber optic sensor used for measuring moisure
content (relative humidity - RH) in the ground. Specifically, the fiber is a pore pressure sensor- a
porous-polymer fiber optic probe. It measures soil-water potential in unsaturated dry soils.
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Image courtesy of Sandia National Laboratories.
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Fiber Optic Fusion Splicer Rentals
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Twisted pair
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Category 5
The original specification for category 5 cable was defined in ANSI/TIA/EIA568-A, with clarification in TSB-95. These documents specified performance
characteristics and test requirements for frequencies of up to 100 MHz.
 Category 5 cable includes four twisted pairs in a single cable jacket. This use of
balanced lines helps preserve a high signal-to-noise ratio despite interference
from both external sources and other pairs (this latter form of interference is
called crosstalk). It is most commonly used for 100 Mbit/s networks, such as
100BASE-TX Ethernet, although IEEE 802.3ab defines standards for
1000BASE-T - Gigabit Ethernet over category 5 cable. Cat 5 cable typically has
three twists per inch of each twisted pair of 24 gauge copper wires within the
cable.
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Cat5 Cable Connectors
 RJ-45 connector. RJ-45 is an eight-wire (four-pair) connector used for data
transmission over unshielded twisted-pair (UTP) cable and leased telephone line
connections.
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RJ-11
 RJ-11 A commonly used modular telephone connector. RJ-11 is a four-wire
(twopair) connector most often used for voice communications.
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Type
Use
Category 1
Voice Only (Telephone Wire)
Category 2
Data to 4 Mbps (LocalTalk)
Category 3
Data to 10 Mbps (Ethernet)
Category 4
Data to 20 Mbps (16 Mbps Token Ring)
Category 5
Data to 100 Mbps (Fast Ethernet)
Ethernet Cable Summary
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Specification
Cable Type
Maximum length
10BaseT
Unshielded Twisted Pair
100 meters
10Base2
Thin Coaxial
185 meters
10Base5
Thick Coaxial
500 meters
10BaseF
Fiber Optic
2000 meters
100BaseT
Unshielded Twisted Pair
100 meters
100BaseTX
Unshielded Twisted Pair
220 meters
DB connector
 Any of several types of cable connectors used for parallel or serial cables.The number
following the letters DB (for data bus) indicates the number of pins that the connector
usually has; a DB-25 connector can have a maximum of 25 pins, and a DB-9 connector
can have as many as 9. In practice, not all the pins (and not all the lines in the cable) may
be present in the larger connectors. If your situation demands that all the lines be
present, make sure you buy the right cable.
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Common DB connectors include the following:
 DB-9 Defined by the RS-449 standard as well as the ISO (International
Organization for Standardization).
 DB-25 A standard connector used with RS-232-C wiring, with 25 pins (13 on the
top row and 12 on the bottom).
 DB-37 Defined as the RS-449 primary channel connector.
 DB-15, DB-19, and DB-50 connectors are also available. The accompanying
illustration shows a male and female DB-25 connector.
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DB25
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DB37
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DIP switch(dual in-line package)
 DIP switch A small switch used to select the operating mode of a device, mounted
as a dual in-line package. DIP switches can be either sliding or rocker switches, and they
are often grouped for convenience.
 They are used in printed circuit boards, dot-matrix printers, modems, and many other
peripheral devices.
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BandWidth(BW)
 1. In communications, the difference between the highest and lowest frequencies
available for transmission in any given range.
 2. In networking, the transmission capacity of a computer or a communications
channel, stated in megabits per second (Mbps). For example, FDDI (Fiber Distributed
Data Interface) has a bandwidth of 100Mbps. To relate this to a real-world example,a
complete page of text, in English,is approximately 16,000 bits.
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Duplexing
 A duplex communication system is a system composed of two connected parties or
devices which can communicate with one another in both directions. (The term duplex is
not used when describing communication between more than two parties or devices.)
 Duplex systems are employed in nearly all communications networks, either to allow for
a communication "two-way street" between two connected parties or to provide a
"reverse path" for the monitoring and remote adjustment of equipment in the field.
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Half-Duplex
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A half-duplex system provides for communication in both directions, but only one direction at a time
(not simultaneously). Typically, once a party begins receiving a signal, it must wait for the
transmitter to stop transmitting, before replying.
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An example of a half-duplex system is a two-party system such as a "walkie-talkie" style two-way
radio, wherein one must use "Over" or another previously designated command to indicate the end
of transmission, and ensure that only one party transmits at a time, because both parties transmit on
the same frequency.
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A good analogy for a half-duplex system would be a one lane road with traffic controllers at each
end. Traffic can flow in both directions, but only one direction at a time with this being regulated by
the traffic controllers.
Full-Duplex
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A full-duplex system allows communication in both directions, and unlike half-duplex, allows this to happen
simultaneously. Land-line telephone networks are full-duplex since they allow both callers to speak and be heard
at the same time. A good analogy for a full-duplex system would be a two lane road with one lane for each
direction.
Examples: Telephone, Mobile Phone, etc.
Two way radios can be, for instance, designed as full-duplex systems, which transmit on one frequency and receive
on a different frequency. This is also called frequency-division duplex. Frequency-division-duplex systems can be
extended to farther distances using pairs of simple repeater stations, owing to the fact the communications
transmitted on any one frequency always travels in the same direction.
Full-duplex Ethernet connections work by making simultaneous use of all four physical pair of twisted cable
(which are inside the insulation), where two pair are used for receiving packets and two pair are using for sending
packets to a directly connected device. This effectively makes the cable itself a collision-free environment, and
theoretically doubles the maximum bandwidth that can be supported by the connection.
Conclusion: Full duplex means, connection that allows communication in two directions simultaneously at once.
Benefits of full duplex i) Time is not wasted, no frames need to be retransmitted as there are no collisions. ii) Full
bandwidth is available in both directions, because send and receive functions are separated. Iii) Stations/node do
not have to wait till other operations complete their transmission, coz there is only one transmitter for each
twisted pair.
Emulation of full duplex in shared physical media
Where channel access methods are used in point to multipoint networks such as cellular
networks for dividing forward and reverse communication channels on the same physical
communications medium, they are known as duplexing methods, such as:
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1) Time division duplex
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Time division duplex (TDD) is the application of time-division multiplexing to separate
outward and return signals. It emulates full duplex communication over a half duplex
communication link. Time division duplex has a strong advantage in the case where the asymmetry
of the uplink and downlink data speed is variable. As the amount of uplink data increases, more
bandwidth can dynamically be allocated to that and as it shrinks it can be taken away. Another
advantage is that the uplink and downlink radio paths are likely to be very similar in the case of a
slow moving system. This means that techniques such as beamforming work well with TDD
systems.
Examples of TDD systems are:
The W-CDMA TDD mode (for indoor use)
UMTS-TDD's TD-CDMA air interface
The TD-SCDMA system
DECT
IEEE 802.16 WiMax TDD mode
Half-duplex packet mode networks based on carrier sense multiple access, for example 2-wire or
hubbed Ethernet, Wireless local area networks and Bluetooth, can be considered as TDD systems,
albeit not TDMA with fixed frame length.
2) Frequency division duplex
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frequencies. The term is frequently used in ham radio operation, where an operator is attempting to contact a
repeater station. The station must be able to send and receive a transmission at the same time, and does so by
altering the frequency at which it sends and receives slightly. This mode of operation is referred to as duplex mode
or offset mode.
Uplink and downlink sub-bands are said to be separated by the "frequency offset". Frequency division duplex or
frequency duplex is much more efficient in the case of symmetric traffic. In this case TDD tends to waste
bandwidth during switch over from transmit to receive, has greater inherent latency, and may require more
complex, more power-hungry circuitry.
Another advantage of FDD is that it makes radio planning easier and more efficient since base stations do not
"hear" each other (as they transmit and receive in different sub-bands) and therefore will normally not interfere
each other. Conversely with TDD systems, care must be taken to keep guard bands between neighboring base
stations (which decreases spectral efficiency) or to synchronize base stations so they will transmit and receive at
the same time (which increases network complexity and therefore cost, and reduces bandwidth allocation
flexibility as all base stations and sectors will be forced to use the same uplink/downlink ratio)
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Examples of FDD systems are:
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ADSL and VDSL
Most cellular systems, including the UMTS/WCDMA FDD mode
IEEE 802.16 WiMax FDD mode
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Frequency division duplex (FDD)means that the radio transmitter and receiver operates at different
Echo cancellation
 Echo cancellation can also implement full duplex communications over certain types
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of shared media. In this configuration, both devices send and receive over the same
medium at the same time. When processing the signal it receives, a transceiver removes
the "echo" of the signal it sent, leaving, in theory, the other transceiver's signal only.
Echo cancellation is at the heart of the V.32, V.34, V.56 and V.90 modem standards.
Echo cancellers are available as both software and hardware solutions. They can be
independent components in a communications system or integrated into the
communication system's central processing unit. Devices that do not eliminate echo in
the system will not produce good full duplex performance.
Examples
Telephone networks
Mobile phone networks
CB radio
Internet Relay Chat
Modulation
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In telecommunications, modulation is the process of varying a periodic waveform, i.e. a tone, in order to use
that signal to convey a message, in a similar fashion as a musician may modulate the tone from a musical
instrument by varying its volume, timing and pitch. Normally a high-frequency sinusoid waveform is used as
carrier 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 (a contraction of the two terms).
A simple example: A telephone line is designed for transferring audible sounds, for example tones, and not
digital bits (zeros and ones). Computers may however communicate over a telephone line by means of modems,
which are representing the digital bits by tones, called symbols.You could say that modems play music for each
other. If there are four alternative symbols (corresponding to a musical instrument that can generate four different
tones, one at a time), the first symbol may represent the bit sequence 00, the second 01, the third 10 and the
fourth 11. If the modem plays a melody consisting of 1000 tones per second, the symbol rate is 1000
symbols/second, or baud. Since each tone represents a message consisting of two digital bits in this example, the
bit rate is twice the symbol rate, i.e. 2000 bit per second.
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The aim of digital modulation is to transfer a digital bit stream over an analog bandpass channel,
for example over the public switched telephone network (where a filter limits the frequency range
to between 300 and 3400 Hz) or a limited radio frequency band.
The aim of analog modulation is to transfer an analog lowpass signal, for example an audio
signal or TV signal, over an analog bandpass channel, for example a limited radio frequency band or
a cable TV network channel.
Analog and digital modulation facilitate frequency division multiplex (FDM), where several low
pass information signals are transferred simultaneously over the same shared physical medium,
using separate bandpass channels.
The aim of digital baseband modulation methods, also known as line coding, is to transfer a
digital bit stream over a lowpass channel, typically a non-filtered copper wire such as a serial bus or
a wired local area network.
The aim of pulse modulation methods is to transfer a narrowband analog signal, for example a
phone call over a wideband lowpass channel or, in some of the schemes, as a bit stream over another
digital transmission system.
Analog modulation methods
An audio signal (top) may be carried by an AM or FM radio wave.
 Common analog modulation techniques are:
 Amplitude modulation (AM)
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 Double-sideband modulation (DSB)
 Double-sideband modulation with unsuppressed carrier (DSB-WC)(used on the radio AM band)
 Double-sideband suppressed-carrier transmission (DSB-SC)
 Double-sideband reduced carrier transmission (DSB-RC)
 Single-sideband modulation
 Single-sideband modulation (SSB, or SSB-AM), very similar to
 single-sideband suppressed carrier modulation (SSB-SC)
 Vestigial sideband modulation (VSB, or VSB-AM)
 Quadrature amplitude modulation (QAM)
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Angle modulation
 Frequency modulation (FM)
 Phase modulation (PM)
 Double-sideband suppressed-carrier transmission (DSB-SC)
 (DSB-SC): transmission in which (a) frequencies produced by amplitude modulation are
symmetrically spaced above and below the carrier frequency and (b) the carrier level is
reduced to the lowest practical level, ideally completely suppressed.
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In the double-sideband suppressed-carrier transmission (DSB-SC) modulation, unlike
AM, the wave carrier is not transmitted; thus, a great percentage of power that is
dedicated to it is distributed between the sidebands, which implies an increase of the
cover in DSB-SC, compared to AM, for the same power used.
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DSB-SC transmission is a special case of Double-sideband reduced carrier transmission.
 This is used for RDS (Radio Data System) due to the fact that it is difficult to decouple.
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 Spectrum
This is basically an amplitude modulation wave without the carrier therefore
reducing power wastage, giving it a 50% efficiency rate.
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Generation
DSBSC is generated by a mixer. This consists of an audio source
combined with the frequency carrier.
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Demodulation(DSBSC )
For demodulation the audio frequency and the carrier frequency must be exact
otherwise we get distortion.
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How it works
This is best shown graphically. Below, is a message signal that one may wish
to modulate onto a carrier, consisting of a couple of sinusoidal components.
 The equation for this message signal is
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The carrier, in this case, is a plain 5 kHz (
) sinusoid -- pictured below.
The modulation is performed by multiplication in the time domain, which yields
a 5 kHz carrier signal, whose amplitude varies in the same manner as the
message signal.
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 The name "suppressed carrier" comes about because the carrier signal component is
suppressed -- it does not appear (theoretically) in the output signal. This is apparent
when the spectra of the output signal is viewed:
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 Double-sideband reduced carrier transmission (DSB-RC): transmission in
which (a) the frequencies produced by amplitude modulation are symmetrically spaced
above and below the carrier and (b) the carrier level is reduced for transmission at a
fixed level below that which is provided to the modulator.
 Note: In DSB-RC transmission, the carrier is usually transmitted at a level suitable for use
as a reference by the receiver, except for the case in which it is reduced to the minimum
practical level, i.e. the carrier is suppressed.
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Single-sideband modulation (SSB)
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Single-sideband modulation (SSB) is a refinement of amplitude modulation that more efficiently uses
electrical power and bandwidth. It is closely related to vestigial sideband modulation (VSB) (see below).
Amplitude modulation produces a modulated output signal that has twice the bandwidth of the original baseband
signal. Single-sideband modulation avoids this bandwidth doubling, and the power wasted on a carrier, at the cost
of somewhat increased device complexity.
The first U. S. patent for SSB modulation was applied for on 1 December, 1915 by John Renshaw Carson. Patent
1,449,382, titled "Method and Means for Signaling with High Frequency Waves" was awarded to Carson on
March 27, 1923 and assigned to AT&T.
The U.S. Navy experimented with SSB over its radio circuits prior to World War I. SSB first entered commercial
service in January 7, 1927 on the longwave transatlantic public radiotelephone circuit between New York and
London. The high power SSB transmitters were located at Rocky Point, New York and Rugby, England. The
receivers were in very quiet locations in Houlton, Maine and Cupar Scotland.
SSB was also used over long-distance telephone lines, as part of a technique known as frequency-division
multiplexing. (FDM) was pioneered by telephone companies in the 1930s. This enabled many voice channels to be
sent down a single physical circuit. The use of SSB meant that the channels could be spaced (usually) just 4,000 Hz
apart, while offering a speech bandwidth of nominally 300 – 3,400 Hz.
Amateur radio operators began to seriously experiment with SSB after World War II. It has become a de facto
standard for long-distance voice radio transmissions since then.
Vestigial sideband (VSB)
 A vestigial sideband (in radio communication) is a sideband that has been only partly
cut off or suppressed. Television broadcasts (in NTSC, PAL, or SECAM analog video
format) use this method if the video is transmitted in AM, due to the large bandwidth
used. It may also be used in digital transmission, such as the ATSC standardized 8-VSB.
The Milgo 4400/48 modem (circa 1967) used vestigial sideband and phase-shift keying
to provide 4800-bit/s transmission over a 1600 Hz channel.
 The video baseband signal used in TV in countries that use NTSC or ATSC has a
bandwidth of 6 MHz.To conserve bandwidth, SSB would be desirable, but the video
signal has significant low frequency content (average brightness) and has rectangular
synchronising pulses. The compromise is vestigial sideband modulation. In vestigial
sideband the full upper sideband of bandwidth W2 = 4 MHz is transmitted, but only W1
= 1.25 MHz of the lower sideband is transmitted, along with a carrier. This effectively
makes the system AM at low modulation frequencies and SSB at high modulation
frequencies. The absence of the lower sideband components at high frequencies must be
compensated for, and this is done by the RF and IF filters.
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Frequency modulation(FM)
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In telecommunications, frequency modulation (FM) conveys information over a carrier wave by varying its
frequency (contrast this with amplitude modulation, in which the amplitude of the carrier is varied while its
frequency remains constant). In analog applications, the instantaneous frequency of the carrier is directly
proportional to the instantaneous value of the input signal. Digital data can be sent by shifting the carrier's
frequency among a set of discrete values, a technique known as frequency-shift keying.
FM is commonly used at VHF radio frequencies for high-fidelity broadcasts of music and speech (see FM
broadcasting). Normal (analog) TV sound is also broadcast using FM. A narrowband form is used for voice
communications in commercial and amateur radio settings. The type of FM used in broadcast is generally called
wide-FM, or W-FM. In two-way radio, narrowband narrow-fm (N-FM) is used to conserve bandwidth. In
addition, it is used to send signals into space.
FM is also used at intermediate frequencies by most analog VCR systems, including VHS, to record the luminance
(black and white) portion of the video signal. FM is the only feasible method of recording video to and retrieving
video from magnetic tape without extreme distortion, as video signals have a very large range of frequency
components — from a few hertz to several megahertz, too wide for equalisers to work with due to electronic
noise below -60 dB. FM also keeps the tape at saturation level, and therefore acts as a form of audio noise
reduction, and a simple limiter can mask variations in the playback output, and the FM capture effect removes
print-through and pre-echo. A continuous pilot-tone, if added to the signal — as was done on V2000 and many
Hi-band formats — can keep mechanical jitter under control and assist timebase correction.
FM is also used at audio frequencies to synthesize sound. This technique, known as FM synthesis, was popularized
by early digital synthesizers and became a standard feature for several generations of personal computer sound
cards.
An example of frequency modulation. The top diagram shows the modulating
signal superimposed on the carrier wave. The bottom diagram shows the
resulting frequency-modulated signal.
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Phase modulation(PM)
 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 with determining whether, for example, the signal has
0° phase or 180° phase.
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An example of phase modulation. The top diagram shows the
modulating signal superimposed on the carrier wave. The bottom
diagram shows the resulting phase-modulated signal.
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Digital modulation methods
 In digital modulation, an analog carrier signal is modulated by a digital bit stream of
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either equal length signals or varying length signals. This can be described as a form of
analog-to-digital conversion. The changes in the carrier signal are chosen from a finite
number of alternative symbols (the modulation alphabet).
These are the most fundamental digital modulation techniques:
In the case of CW, groupings of on-off keying of varying length signals are used.
In the case of PSK, a finite number of phases are used.
In the case of FSK, a finite number of frequencies are used.
In the case of ASK, a finite number of amplitudes are used.
In the case of QAM, an inphase signal (the I signal, for example a cosine waveform) and
a quadrature phase signal (the Q signal, for example a sine wave) are amplitude
modulated with a finite number of amplitudes. It can be seen as a two channel system.
The resulting signal is a combination of PSK and ASK, with a finite number of at least
two phases, and a finite number of at least two amplitudes.
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Each of these phases, frequencies or amplitudes are assigned a unique pattern of binary bits. Usually,
each phase, frequency or amplitude encodes an equal number of bits. This number of bits comprises
the symbol that is represented by the particular phase.
If the alphabet consists of M = 2N alternative symbols, each symbol represents a message consisting
of N bits. If the symbol rate (also known as the baud rate) is fS symbols/second (or baud), the data
rate is NfS bit/second.
For example, with an alphabet consisting of 16 alternative symbols, each symbol represents 4 bits.
Thus, the data rate is four times the baud rate.
In the case of PSK, ASK and QAM, the modulation alphabet is often conveniently represented on a
constellation diagram, showing the amplitude of the I signal at the x-axis, and the amplitude of the
Q signal at the y-axis, for each symbol.
PSK and ASK, and sometimes also FSK, can be generated and detected using the principle of QAM.
The I and Q signals can be combined into a complex valued signal called the equivalent lowpass
signal or equivalent baseband signal. This is a representation of the valued modulated physical signal
(the so called passband signal or RF signal).
The most common digital modulation techniques are:
60

Phase-shift keying (PSK)

Frequency-shift keying (FSK) (see also audio frequency-shift keying (AFSK))

Amplitude-shift keying (ASK) and its most common form, on-off keying (OOK)

Quadrature amplitude modulation (QAM) a combination of PSK and ASK

Polar modulation like QAM a combination of PSK and ASK.

Continuous phase modulation (CPM)
 Minimum-shift keying (MSK)
 Gaussian minimum-shift keying (GMSK)

Orthogonal frequency division multiplexing (OFDM) modulation, also known as discrete
multitone (DMT).

Wavelet modulation

Trellis coded modulation (TCM) also known as trellis modulation
Phase-shift keying (PSK)
Phase-shift keying (PSK) is a digital modulation scheme that conveys data by changing, or
modulating, the phase of a reference signal (the carrier wave).
 Any digital modulation scheme uses a finite number of distinct signals to represent digital data. PSK
uses a finite number of phases, each assigned a unique pattern of binary bits. Usually, each phase
encodes an equal number of bits. Each pattern of bits forms the symbol that is represented by the
particular phase. The demodulator, which is designed specifically for the symbol-set used by the
modulator, determines the phase of the received signal and maps it back to the symbol it represents,
thus recovering the original data. This requires the receiver to be able to compare the phase of the
received signal to a reference signal — such a system is termed coherent.
 Alternatively, instead of using the bit patterns to set the phase of the wave, it can instead be used to
change it by a specified amount. The demodulator then determines the changes in the phase of the
received signal rather than the phase itself. Since this scheme depends on the difference between
successive phases, it is termed differential phase-shift keying (DPSK). DPSK can be
significantly simpler to implement than ordinary PSK since there is no need for the demodulator to
have a copy of the reference signal to determine the exact phase of the received signal (it is a noncoherent scheme). In exchange, it produces more erroneous demodulations. The exact
requirements of the particular scenario under consideration determine which scheme is used.
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61
Applications
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Owing to PSK's simplicity, particularly when compared with its competitor quadrature amplitude modulation, it
is widely used in existing technologies.
The most popular wireless LAN standard, IEEE 802.11b[1][2], uses a variety of different PSKs depending on the
data-rate required. At the basic-rate of 1 Mbit/s, it uses DBPSK. To provide the extended-rate of 2 Mbit/s,
DQPSK is used. In reaching 5.5 Mbit/s and the full-rate of 11 Mbit/s, QPSK is employed, but has to be coupled
with complementary code keying. The higher-speed wireless LAN standard, IEEE 802.11g[1][3] has eight data rates:
6, 9, 12, 18, 24, 36, 48 and 54 Mbit/s. The 6 and 9 Mbit/s modes use BPSK. The 12 and 18 Mbit/s modes use
QPSK. The fastest four modes use forms of quadrature amplitude modulation.
Because of its simplicity BPSK is appropriate for low-cost passive transmitters, and is used in RFID standards such
as ISO 14443 which has been adopted for biometric passports, credit cards such as American Express's
ExpressPay, and many other applications.
Bluetooth 2 will use π / 4-DQPSK at its lower rate (2 Mbit/s) and 8-DPSK at its higher rate (3 Mbit/s) when the
link between the two devices is sufficiently robust. Bluetooth 1 modulates with Gaussian minimum-shift keying, a
binary scheme, so either modulation choice in version 2 will yield a higher data-rate. A similar technology, IEEE
802.15.4 (the wireless standard used by ZigBee) also relies on PSK. IEEE 802.15.4 allows the use of two
frequency bands: 868–915 MHz using BPSK and at 2.4 GHz using OQPSK.
Notably absent from these various schemes is 8-PSK. This is because its error-rate performance is close to that of
16-QAM — it is only about 0.5 dB better[citation needed] — but its data rate is only three-quarters that of 16-QAM.
Thus 8-PSK is often omitted from standards and, as seen above, schemes tend to 'jump' from QPSK to 16-QAM
(8-QAM is possible but difficult to implement).
Binary phase-shift keying (BPSK)
 Constellation diagram for BPSK.
 BPSK is the simplest form of PSK. It uses two phases which are separated by
180° and so can also be termed 2-PSK. It does not particularly matter exactly
where the constellation points are positioned, and in this figure they are shown
on the real axis, at 0° and 180°. This modulation is the most robust of all the
PSKs since it takes serious distortion to make the demodulator reach an
incorrect decision. It is, however, only able to modulate at 1 bit/symbol (as
seen in the figure) and so is unsuitable for high data-rate applications when
bandwidth is limited.
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Frequency-shift keying (FSK)
 Frequency-shift keying (FSK) is a modulation scheme in which digital information
is transmitted through discrete frequency changes of a carrier wave. The most common
form of frequency shift keying is 2-FSK. As suggested by the name, 2-FSK uses two
discrete frequencies to transmit binary (0's and 1's) information. With this scheme, the
"1" is called the mark frequency and the "0" is called the space frequency. The time
domain of an FSK modulated carrier is illustrated in the figures at right.

64
One interesting facet of the 2-FSK (binary FSK) modulation scheme is that when it is
generated using a quadrature modulator (see Quadrature phase), the baseband waveform
consists of two sinusoids which use phase shifting to move from one symbol to the next.
In this scenario, baseband I and Q signals adjust from 90 degrees to 270 degrees out of
phase to mark each each symbol of the FSK baseband waveform. For that reason, the
binary FSK modulation scheme is sometimes referred to as "phase reversal" as well.
An example of binary FSK
65
Audio frequency-shift keying (AFSK)
 Audio frequency-shift keying (AFSK) is a modulation technique by which digital
data is represented as changes in the frequency (pitch) of an audio tone, yielding an
encoded signal suitable for transmission via radio or telephone. Normally, the
transmitted audio alternates between two tones: one, the "mark", represents a binary
one; the other, the "space", represents a binary zero.
 AFSK differs from regular frequency-shift keying in that the modulation is
performed at baseband frequencies. In radio applications, the AFSK-modulated signal is
normally used to modulate an RF carrier (using a conventional technique, such as AM
FM or ACSSB(R)(LM Mode(R)) for transmission.
 AFSK is not generally used for high-speed data communications, as it is much less
efficient in both power and bandwidth than most other modulation modes. In addition to
its simplicity, however, AFSK has the advantage that encoded signals will pass through
AC-coupled links, including most equipment originally designed to carry music or
speech.
66
Applications
 Most early telephone-line modems used audio frequency-shift keying to send and
receive data, up to rates of about 300 bits per second. The common Bell 103
modem used this technique, for example. Some early microcomputers used a
specific form of AFSK modulation, the Kansas City standard, to store data on audio
cassettes. AFSK is still widely used in amateur radio, as it allows data transmission
through unmodified voiceband equipment. Radio control gear uses FSK, but calls it
FM and PPM instead.
 AFSK is also used in the United States' Emergency Alert System to transmit
warning information. It is used at higher bitrates for Weathercopy used on
Weatheradio by NOAA in the U.S., and more extensively by Environment Canada.
 The CHU shortwave radio station in Ottawa, Canada broadcasts an exclusive digital
time signal encoded using AFSK modulation.
67
Amplitude-shift keying (ASK)
 Amplitude-shift keying (ASK) is a form of modulation that represents digital data as
variations in the amplitude of a carrier wave.
 The amplitude of an analog carrier signal varies in accordance with the bit stream
(modulating signal), keeping frequency and phase constant. The level of amplitude can be
used to represent binary logic 0s and 1s. We can think of a carrier signal as an ON or
OFF switch. In the modulated signal, logic 0 is represented by the absence of a carrier,
thus giving OFF/ON keying operation and hence the name given.
 Like AM, ASK is also linear and sensitive to atmospheric noise, distortions, propagation
conditions on different routes in PSTN, etc. Both ASK modulation and demodulation
processes are relatively inexpensive. The ASK technique is also commonly used to
transmit digital data over optical fiber. For LED transmitters, binary 1 is represented by
a short pulse of light and binary 0 by the absence of light. Laser transmitters normally
have a fixed "bias" current that causes the device to emit a low light level. This low level
represents binary 0, while a higher-amplitude lightwave represents binary 1.
68
Minimum frequency-shift keying (MSK)
 Minimum frequency-shift keying or minimum-shift keying (MSK) is a
particularly spectrally efficient form of coherent frequency-shift keying. In MSK the
difference between the higher and lower frequency is identical to half the bit rate. As a
result, the waveforms used to represent a 0 and a 1 bit differ by exactly half a carrier
period. This is the smallest FSK modulation index that can be chosen such that the
waveforms for 0 and 1 are orthogonal. A variant of MSK called GMSK is used in the
GSM mobile phone standard.
 FSK is commonly used in Caller ID and remote metering applications.
69
Gaussian minimum-shift keying(GMSK )
 In digital communication, Gaussian minimum shift keying or GMSK is a
continuous-phase frequency-shift keying modulation scheme. It is similar to
standard minimum-shift keying (MSK); however the digital data stream is first
shaped with a Gaussian filter before being applied to a frequency modulator. This
has the advantage of reducing sideband power, which in turn reduces out-of-band
interference between signal carriers in adjacent frequency channels. However, the
Gaussian filter increases the modulation memory in the system and causes
intersymbol interference, making it more difficult to discriminate between different
transmitted data values and requiring more complex channel equalization
algorithms such as an adaptive equalizer at the receiver.
 GMSK is most notably used in the Global System for Mobile Communications
(GSM).
70
Orthogonal frequency-division multiplexing (OFDM)
Orthogonal Frequency-Division Multiplexing (OFDM) — essentially identical to Coded OFDM
(COFDM) — is a digital multi-carrier modulation scheme, which uses a large number of closelyspaced orthogonal sub-carriers. Each sub-carrier is modulated with a conventional modulation
scheme (such as quadrature amplitude modulation) at a low symbol rate, maintaining data rates
similar to conventional single-carrier modulation schemes in the same bandwidth. In practice,
OFDM signals are generated and detected using the Fast Fourier transform algorithm.
 OFDM has developed into a popular scheme for wideband digital communication, wireless as well
as over copper wires.
 The primary advantage of OFDM over single-carrier schemes is its ability to cope with severe
channel conditions — for example, attenuation of high frequencies in a long copper wire,
narrowband interference and frequency-selective fading due to multipath — without complex
equalization filters. Channel equalization is simplified because OFDM may be viewed as using many
slowly-modulated narrowband signals rather than one rapidly-modulated wideband signal. Low
symbol rate makes the use of a guard interval between symbols affordable, making it possible to
handle time-spreading and eliminate inter-symbol interference (ISI).

71
Pulse modulation methods
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72
Pulse modulation schemes aim at transferring a narrowband analog signal over an analog lowpass
channel as a two-level quantized signal, by modulating a pulse train. Some pulse modulation
schemes also allow the narrowband analog signal to be transferred as a digital signal (i.e. as a
quantized discrete-time signal) with a fixed bit rate, which can be transferred over an underlying
digital transmission system, for example some line code. They are not modulation schemes in the
conventional sense since they are not channel coding schemes, but should be considered as source
coding schemes, and in some cases analog-to-digital conversion techniques.
Pulse-code modulation (PCM) (Analog-over-digital)
Pulse-width modulation (PWM) (Analog-over-analog)
Pulse-amplitude modulation (PAM) (Analog-over-analog)
Pulse-position modulation (PPM) (Analog-over-analog)
Pulse-density modulation (PDM) (Analog-over-analog)
Sigma-delta modulation (∑Δ) (Analog-over-digital)
Adaptive-delta modulation (ADM)(Analog-over-digital)
Direct-sequence spread spectrum (DSSS) is based on pulse-amplitude modulation.
Pulse-code modulation (PCM)
 Pulse-code modulation (PCM) is a digital representation of an analog signal
where the magnitude of the signal is sampled regularly at uniform intervals,
then quantized to a series of symbols in a digital (usually binary) code. PCM has
been used in digital telephone systems and is also the standard form for digital
audio in computers and the compact disc red book format. It is also standard in
digital video, for example, using ITU-R BT.601. However, straight PCM is not
typically used for video in consumer applications such as DVD or DVR because
it requires too high a bit rate (PCM audio is supported by the DVD standard but
rarely used). Instead, the data are normally converted to compressed formats
which approximate the PCM with lower bit rates. However, many Blu-ray Disc
and HD-DVD movies use uncompressed PCM for audio. Very frequently, PCM
encoding facilitates digital transmission from one point to another (within a
given system, or geographically) in serial form.
73

Sampling and quantization of a signal (red) for 4-bit PCM
 In the Diagram, a sine wave (red curve) is sampled and quantized for PCM. The sine
wave is sampled at regular intervals, shown as ticks on the x-axis. For each sample, one
of the available values (ticks on the y-axis) is chosen by some algorithm (in this case, the
floor function is used). This produces a fully discrete representation of the input signal
(shaded area) that can be easily encoded as digital data for storage or manipulation. For
the sine wave example at right, we can verify that the quantized values at the sampling
moments are 9, 11, 12, 13, 14, 14, 15, 15, 15, 14, etc. Encoding these values as binary
numbers would result in the following set of nibbles: 1001, 1011, 1100, 1101, 1110,
1110, 1111, 1111, 1111, 1110, etc. These digital values could then be further processed
or analyzed by a purpose-specific digital signal processor or general purpose CPU.
Several Pulse Code Modulation streams could also be multiplexed into a larger aggregate
data stream, generally for transmission of multiple streams over a single physical link.
This technique is called time-division multiplexing, or TDM, and is widely used, notably
in the modern public telephone system.
74
History of PCM
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75
In retrospect, PCM, like many other great inventions, appears to be simple and obvious. In the history of electrical communications,
the earliest reason for sampling a signal was to interlace samples from different telegraphy sources, and convey them over a single
telegraph cable. Telegraph time-division multiplexing (TDM) was conveyed as early as 1853, by the American inventor M.B. Farmer.
The electrical engineer W.M. Miner, in 1903, used an electro-mechanical commutator for time-division multiplex of multiple
telegraph signals, and also applied this technology to telephony. He obtained intelligible speech from channels sampled at a rate
above 3500–4300 Hz: below this was unsatisfactory. This was TDM, but pulse-amplitude modulation (PAM) rather than PCM.
Paul M. Rainey of Western Electric in 1926 patented a facsimile machine using an optical mechanical analog to digital converter.The
machine did not go into production. British engineer Alec Reeves, unaware of previous work, conceived the use of PCM for voice
communication in 1937 while working for International Telephone and Telegraph in France. He described the theory and advantages,
but no practical use resulted. Reeves filed for a French patent in 1938, and his U.S. patent was granted in 1943.
The first transmission of speech by digital techniques was the SIGSALY vocoder encryption equipment used for high-level Allied
communications during World War II from 1943.
It was not until about the middle of 1943 that the Bell Labs people who designed the SIGSALY system, became aware of the use of
PCM binary coding as already proposed by Alec Reeves.
PCM in the 1950s used a cathode-ray coding tube with a grid having encoding perforations. As in an oscilloscope, the beam was
swept horizontally at the sample rate while the vertical deflection was controlled by the input analog signal, causing the beam to pass
through higher or lower portions of the perforated grid. The grid interrupted the beam, producing current variations in binary code.
Rather than natural binary, the grid was perforated to produce Gray code lest a sweep along a transition zone produce glitches.
Trellis Coded Modulation(TCM)
 In telecommunication, trellis modulation (also known as trellis
coded modulation, or simply TCM) is a modulation scheme which
allows highly efficient transmission of information over band-limited
channels such as telephone lines. Trellis modulation was invented by
Gottfried Ungerboeck.
 In the late 1980s, modems operating over plain old telephone service
("POTS") typically achieved 9.6 kbit/s by employing 4 bits per symbol
QAM modulation at 2400 baud (symbols/second). This bit rate ceiling
existed despite the best efforts of many researchers, and some engineers
predicted that without a major upgrade of the public phone
infrastructure, the maximum achievable rate for a POTS modem might
be 9.6k for two-way communication. However, 9.6 kbit/s is only 30%
of the theoretical maximum bit rate predicted by Shannon's Theorem for
POTS lines (approximately 35 kbit/s).
76
A new modulation method
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77
The name "trellis" was coined because a state diagram of the technique, when drawn on paper closely resembles the trellis lattice
used in rose gardens. The scheme is basically a convolutional code of rates (r,r+1). Ungerboeck's unique contribution is to apply the
parity check on a per symbol basis instead of the older technique of applying it to the bit stream then modulating the bits. The key
idea he termed Mapping by Set Partitions. This idea was to group the symbols in a tree like fashion then separate them into two limbs
of equal size. At each limb of the tree, the symbols were further apart. Although in multi-dimensions, it is hard to visualize, a simple
one dimension example illustrates the basic procedure. Suppose the symbols are located at [1,2,3,4,...]. Then take all odd symbols
and place them in one group, and the even symbols in the second group. This is not quite accurate because Ungerboeck was looking
at the two dimensional problem, but the principle is the same, take every other one for each group and repeat the procedure for each
tree limb. He next described a method of assigning the encoded bit stream onto the symbols in a very systematic procedure. Once
this procedure was fully described, his next step was to program the algorithms into a computer and let the computer search for the
best codes. The results were astonishing. Even the most simple code (4 state) produced error rates nearly 1000 times lower than an
equivalent uncoded system. For two years Ungerboeck kept these results private and only conveyed them to close colleagues. Finally,
in 1982, Ungerboeck published a paper describing the principles of trellis modulation.
A flurry of research activity ensued, and by 1990 the International Telecommunication Union had published modem standards for the
first trellis-modulated modem at 14.4 kbit/s (2400 baud and 6 bits per symbol). Over the next several years further advances in
encoding, plus a corresponding symbol rate increase from 2400 to 3429 baud, allowed modems to achieve rates up to 34.3 kbit/s
(limited by maximum power regulations to 33.8k). Today, the most common trellis-modulated V.34 modems use a 4-dimensional set
partition which is achieved by treating two 2-dimensional symbols as a single lattice. This set uses 8, 16, or 32 state convolutional
codes to squeeze the equivalent of 6 to 10 bits into each symbol sent by the modem (example: 2400 baud * 8 bits/symbol ==
19,200 bits per second).
Once manufacturers introduced modems with trellis modulation, transmission rates increased to the point where interactive transfer
of multimedia over the telephone became feasible (a 200 kilobyte image and a 5 megabyte song could be downloaded in less than 1
minute and 30 minutes, respectively). Sharing a floppy disk via a BBS could be done in just a few minutes, instead of an hour. Thus
Ungerboeck's invention played a key role in the Information Age.
Pulse-width modulation (PWM)
 An example of PWM: the supply voltage (blue) modulated as a series of pulses results in
a sine-like flux density waveform (red) in a magnetic circuit of electromagnetic actuator.
The smoothness of the resultant waveform can be controlled by the width and number of
modulated impulses (per given cycle)
 Pulse-width modulation (PWM) of a signal or power source involves the
modulation of its duty cycle, to either convey information over a communications
channel or control the amount of power sent to a load.
78
Pulse-amplitude modulation(PAM)
Principle of PAM; (1) original Signal,
(2) PAM-Signal, (a) Amplitude of Signal,
(b) Time
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79
Pulse-amplitude modulation, acronym PAM, is a form of signal modulation where the
message information is encoded in the amplitude of a series of signal pulses.
Example: A two bit modulator (PAM-4) will take two bits at a time and will map the signal
amplitude to one of four possible levels, for example −3 volts, −1 volt, 1 volt, and 3 volts.
Demodulation is performed by detecting the amplitude level of the carrier at every symbol period.
Pulse-amplitude modulation is widely used in baseband transmission of digital data, with nonbaseband applications having been largely superseded by pulse-code modulation, and, more
recently, by pulse-position modulation.
In particular, all telephone modems faster than 300 bit/s use quadrature amplitude modulation
(QAM). (QAM uses a two-dimensional constellation).
 It should be noted, however, that the widely popular Ethernet communication standard
is a good example of PAM usage. In particular, 100BASE-T2 (running at 100Mb/s)
Ethernet medium is utilizing 5 level PAM modulation running at 25 megapulses/sec
over two wire pairs. A special technique is used to reduce inter-symbol interference
between the unshielded pairs. Later, 1000BASE-T medium raised the bar to use 4 pairs
of wire running each at 125 megapulses/sec to achieve 1000Mb/s data rates, still
utilizing PAM-5 for each pair.
 The IEEE 802.3an standard defines the wire-level modulation for 10GBASE-T as a
Tomlinson-Harashima Precoded (THP) version of pulse-amplitude modulation with 16
discrete levels (PAM-16), encoded in a two-dimensional checkerboard pattern known as
DSQ128. Several proposals were considered for wire-level modulation, including PAM
with 12 discrete levels (PAM-12), 10 levels (PAM-10), or 8 levels (PAM-8), both with
and without Tomlinson-Harashima Precoding (THP).
 To achieve full-duplex operation, parties must ensure that their transmitted pulses do
not coincide in time. This makes use of bus topology (featured by older Ethernet
implementations) practically impossible with these modern Ethernet mediums. This
technique is called Carrier Sense Multiple Access and is used in some home networking
protocols such as HomePlug. More modern protocols use Time Division Multiple Access
instead, which performs better under heavy traffic loading conditions.
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Digital baseband modulation or line coding
 The term digital baseband modulation is synonymous to line codes, which are
methods to transfer a digital bit stream over an analog lowpass channel using a discrete
number of signal levels, by modulating a pulse train (a square wave instead of a sinusoidal
waveform). Common examples are unipolar, non-return-to-zero (NRZ), Manchester
and alternate mark inversion (AMI) coding.
81
modulator
 1) Group the incoming data into codewords;
 2) Map the codewords to attributes, for example amplitudes of the I and Q signals (the
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82
equivalent low pass signal), or frequency or phase values.
3) Adapt pulse shaping or some other filtering to limit the bandwidth and form the
spectrum, typically using digital signal processing
4) Digital-to-analog conversion (DAC) of the I and Q signals (since today all of the above
is normally achieved using digital signal processing, DSP). Sometimes the next step is
also achieved using DSP, and then the DAC should be done after that.
5) Modulate the high-frequency carrier waveform, resulting in that the equivalent low
pas signal is frequency shifted into a modulated passband signal or RF signal
6) Amplification and analog bandpass filtering to avoid harmonic distortion and periodic
spectrum
Demodulator
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83
1)Bandpass filtering
2)Automatic gain control, AGC (to compensate for attenuation)
3)Frequency shifting of the RF signal baseband I and Q signals, or to an intermediate frequency (IF)
signal, or
4)Sampling and analog-to-digital conversion (ADC) (Sometimes before the above point)
5)Equalization filtering
6)Detection of the amplitudes of the I and Q signals, or the frequency or phase of the IF signal;
7)Quantization of the amplitudes, frequencies or phases to the nearest allowed values, using
mapping.
8)Map the quantized amplitudes, frequencies or phases to codewords (bit groups);
9)Parallel-to-serial conversion of the codewords into a bit stream
10)Pass the resultant bit stream on for further processing such as removal of any error-correcting
codes.
Summation
+
84
=
Multiplication
85
Summation
+
86
Wireless Basics
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87
Wireless Lan
One type of wireless network is a WLAN or Wireless Local Area Network. Similar to other wireless devices, it
uses radio instead of wires to transmit data back and forth between computers on the same network.
Screenshots of wireless LAN Networks. Figure 1, left, shows that not all networks are encrypted (locked unless
you have the code), which means anyone can get onto them. Figures 2 and 3, middle and right, however, show that
a lot of networks are encrypted.
Wi-Fi
Wi-Fi is a commonly used wireless network in computer systems which enable connection to the internet or
other machines that have Wi-Fi functionalities. Wi-Fi networks broadcast radio waves that can be picked up by WiFi receivers that are attached to different computers.
Fixed Wireless Data
Fixed wireless data is a type of wireless data network that can be used to connect two or more buildings together
in order to extend or share the network bandwidth without physically wiring the buildings together.
Wireless Man
A type of wireless network that connects several Wireless LANs.
WiMax
WiMAX is the term used to refer to wireless MANs.
ISM band
88

The industrial, scientific and medical (ISM) radio bands were originally reserved internationally for the use
of RF electromagnetic fields for industrial, scientific and medical purposes other than communications. In general,
communications equipment must accept any interference generated by ISM equipment

The ISM bands are defined by the ITU-R in 5.138, 5.150, and 5.280 of the Radio Regulations. Individual
countries' use of the bands designated in these sections may differ due to variations in national radio regulations.
Because communication devices using the ISM bands must tolerate any interference from ISM equipment, these
bands are typically given over to uses intended for unlicensed operation, since unlicensed operation typically needs
to be tolerant of interference from other devices anyway. In the United States of America ISM uses of the ISM
bands are governed by Part 18 of the FCC rules, while Part 15 Subpart B contains the rules for unlicensed
communication devices, even those that use the ISM frequencies. Thus, designers of equipment for use in the
United States in the ISM bands should be familiar with the relevant portions of both Part 18 and Part 15 Subpart B
of the FCC Rules.
ITU regions
89
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The International Telecommunication Union (ITU), in its International Radio Regulations, divides the world into
three ITU regions for the purposes of managing the global radio spectrum. Each region has its own set of
frequency allocations, the main reason for defining the regions.

Region 1 comprises Europe, Greenland, Africa, the Middle East west of the Persian Gulf and including Iraq, the
former Soviet Union and Mongolia.
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Region 2 covers the Americas but Greenland and some of the eastern Pacific Islands.
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Region 3 contains most of non-former-Soviet-Union Asia, east of and including Iran, and most of Oceania.
Beamwidth
 The main lobe, or main beam, of an antenna radiation pattern is the lobe containing
the maximum power. This is the lobe that exhibits the greatest field strength.
 The horizontal radiation pattern, that which is plotted as a function of azimuth about the
antenna, is usually specified. The width of the main lobe is usually specified as the angle
encompassed between the points where the power has fallen 3 dB below the maximum
value. The vertical radiation pattern, that which is plotted as a function of elevation from
a specified azimuth, is also of interest and may be similarly specified.
This article contains material from the Federal Standard 1037C (in support of MIL-STD-188),
which, as a work of the United States Government, is in the public domain
90
Wireless Standars
Standard
Release Date
Operation Frequency
Throughput
Data Rate
Modulation
802.11a
1999
5 GHz
23 Mbit/s
54 Mbit/s
OFDM
802.11b
1999
2.4 GHz
4.3 Mbit/s
11 Mbit/s
DSSS
802.11g
2003
2.4 GHz
74 Mbit/s
54 Mbit/s
OFDM
802.11n
Sept 2008
2.4 GHz & 5 GHz
74 Mbit/s
248 Mbit/s
MIMO
802.11y
March 2008
3.7 GHz
23 Mbit/s
54 Mbit/s
QPSK
802.16
Bluetooth
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2 GHz to 66 GHz
2003-2004
2.4 to 2.485 GHz
OFDMaA
721 Kbps
V1.2 1Mbps
V2.0 3Mbps
GFSK
802.11a
 The 802.11a standard uses the same core protocol as the original standard, operates in 5
GHz band with a maximum raw data rate of 54 Mbit/s, which yields realistic net
achievable throughput in the mid-20 Mbit/s.
 Since the 2.4 GHz band is heavily used to the point of being crowded, using the relatively
un-used 5 GHz band gives 802.11a a significant advantage. However, this high carrier
frequency also brings a slight disadvantage: The effective overall range of 802.11a is
slightly less than that of 802.11b/g; 802.11a signals cannot penetrate as far as those for
802.11b because they are absorbed more readily by walls and other solid objects in their
path.
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802.11b
 802.11b has a maximum raw data rate of 11 Mbit/s and uses the same media access
method defined in the original standard. 802.11b products appeared on the market in
early 2000, since 802.11b is a direct extension of the modulation technique defined in
the original standard. The dramatic increase in throughput of 802.11b (compared to the
original standard) along with simultaneous substantial price reductions led to the rapid
acceptance of 802.11b as the definitive wireless LAN technology.
 802.11b devices suffer interference from other products operating in the 2.4 GHz band.
Devices operating in the 2.4 GHz range include: microwave ovens, Bluetooth devices,
baby monitors and cordless telephones. Interference issues, and user density problems
within the 2.4 GHz band have become a major concern and frustration for users.
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802.11g
 In June 2003, a third modulation standard was ratified: 802.11g. This works in the 2.4
GHz band (like 802.11b) but operates at a maximum raw data rate of 54 Mbit/s, or
about 19 Mbit/s net throughput. 802.11g hardware is fully backwards compatible with
802.11b hardware.
 The then-proposed 802.11g standard was rapidly adopted by consumers starting in
January 2003, well before ratification, due to the desire for higher speeds, and
reductions in manufacturing costs. By summer 2003, most dual-band 802.11a/b
products became dual-band/tri-mode, supporting a and b/g in a single mobile adapter
card or access point. Details of making b and g work well together occupied much of the
lingering technical process; in an 802.11g network, however, the presence of a legacy
802.11b participant will significantly reduce the speed of the overall 802.11g network.
 802.11g devices suffer interference from other products operating in the 2.4 GHz band.
Devices operating in the 2.4 GHz range include: microwave ovens, Bluetooth devices,
baby monitors and cordless telephones. Interference issues, and user density problems
within the 2.4 GHz band have become a major concern and frustration for users.
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802.11n
 802.11n is a proposed amendment which builds on the previous 802.11 standards by
adding multiple-input multiple-output (MIMO). Though there are already many
products on the market based on Draft 2.0 of this proposal, the TGn workgroup is not
expected to finalize the amendment until November 2008.
 An 802.11 access point may operate in one of three modes:
 Legacy (only 802.11a, and b/g)
 Mixed (802.11a, b/g, and n)
 Greenfield (only 802.11n) - maximum performance
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802.11y
 The US 3650MHz rules allow for registered stations to operate at much higher power
than traditional Wi-Fi gear (Up to 20 watts equivalent isotropically radiated power). The
combination of higher power limits and enhancements made to the MAC timing in
802.11-2007, will allow for the development of standards based 802.11 devices that
could operate at distances of 5km or more.
IEEE 802.11y adds three new concepts to 802.11-2007:
 Contention based protocol (CBP)- enhancements have been made to the carrier
sensing and energy detection mechanisms of 802.11 in order to meet the FCC's
requirements for a contention based protocol.
 Extended channel switch announcement (ECSA)- provides a mechanism for an
access point to notify the stations connected to it of its intention to change channels or to
change channel bandwidth. This mechanism will allow for the WLAN to continuously
choose the channel that is the least noisy and the least likely to cause interference. This
mechanism will also be used in 802.11n, which will allow devices to switch between
.11y operation and .11n operation in the 2.4 and 5 GHz bands.
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Dependent station enablement (DSE)- is the mechanism by which an operator extends and retracts
permission to license exempt devices (referred to as dependent STAs in .11y) to use licensed radio spectrum.
Fundamentally, this process satisfies a regulatory requirement that dictates that a dependent STAs operation is
contingent upon its ability to receive periodic messages from a licensees base station, but DSE is extensible to
other purposes in regards to channel management and coordination. Some of the benefits of DSE include: -The
enabling station (aka the licensee's base station) may or may not be the access point that the dependent STA
connects to. In fact, an enabling station may enable both an access point and its clients. Also, although the
dependent STAs are required by regulation to receive information from the enabling station over the air, they are
not required transmit over the air to complete the DSE process. A dependent STA may connect to a nearby Access
Point for a short period of time and use the internet or some other means to complete the channel permissioning
process with the enabling station. This flexibility reduces the likelihood of a dependent STA causing interference
while attempting to connect to a far off enabling station. -The personal privacy and security of end users are
ensured while, at the same time, licensees will have the information necessary to resolve disputes. All .11y devices
transmit a unique identifier for the purpose of resolving interference. The high powered fixed stations and enabling
stations transmit the location that they are operating from as their unique identifier. This location is also registered
in an FCC database that will identify the licensee. The dependent STAs broadcast the location of the station that
enabled it plus a unique string supplied by the enabling station. This ensures that the responsible party, the
licensee, is contacted to resolve disputes. This mechanism also alleviates the problems associated with having the
dependent STA broadcasting its location. Requiring all devices to have GPS or some other means of verifying their
location would increase the cost and complexity of devices, and this solution may be inadequate indoors. This
method also resolves fears that a mobile devices that constantly beacons its location could be used inappropriately
by third parties to track a users location.
802.16
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The first 802.16 standard was approved in December 2001. It delivered a standard for point to multipoint
Broadband Wireless transmission in the 10-66 GHz band, with only a line-of-sight (LOS) capability. It uses a single
carrier (SC) physical (PHY) standard.
802.16a was an amendment to 802.16 and delivered a point to multipoint capability in the 2-11 GHz band. For
this to be of use, it also required a non line-of-sight (NLOS) capability, and the PHY standard was therefore
extended to include Orthogonal Frequency Division Multiplex (OFDM) and Orthogonal Frequency Division
Multiple Access (OFDMA). 802.16a was ratified in January 2003 and was intended to provide "last mile" fixed
broadband access.
802.16c, a further amendment to 802.16, delivered a system profile for the 10-66 GHz 802.16 standard.
In September 2003, a revision project called 802.16d commenced aiming to align the standard with aspects of the
European Telecommunications Standards Institute (ETSI) HIPERMAN standard as well as lay down conformance
and test specifications. This project concluded in 2004 with the release of 802.16-2004 which superseded the
earlier 802.16 documents, including the a/b/c amendments.
An amendment to 802.16-2004, IEEE 802.16e-2005 (formerly known as IEEE 802.16e), addressing mobility, was
concluded in 2005. This implemented a number of enhancements to 802.16-2004, including better support for
Quality of Service and the use of Scalable OFDMA, and is sometimes called “Mobile WiMAX”, after the WIMAX
forum for interoperability.
Amendments in progress
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Active amendments:
802.16e-2005 — Mobile 802.16
802.16f-2005 — Management Information Base
802.16g-2007 — Management Plane Procedures and Services
802.16k-2007 — Bridging of 802.16 (an amendment to 802.1D)
Amendments under development:
802.16h — Improved Coexistence Mechanisms for License-Exempt Operation
802.16i — Mobile Management Information Base
802.16j — Multihop Relay Specification
802.16Rev2 — Consolidate 802.16-2004, 802.16e, 802.16f, 802.16g and possibly 802.16i into a
new document.
Amendments at pre-draft stage:
802.16m — Advanced Air Interface. Data rates of 100 Mbit/s for mobile applications and 1 Gbit/s
for fixed applications, cellular, macro and micro cell coverage, with currently no restrictions on the
RF bandwidth (which is expected to be 20 MHz or higher).[1] The proposed work plan would allow
completion of the standard by Sept 2008 for approval by Dec 2008.
Radio Devices
Some Brands
 Senao
 Micronet
 Teletronics
 Alcon
 Proxim
 Lobometric
 Motorola(Ortogon)
 Dragonwave
 Ericsson
 Siemens
 Nokia
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Sight Survay(propagation)
 Line-of-sight
 Non-line-of-sight
 Near-line-of-sight
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Line-of-sight propagation
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Line-of-sight propagation refers to electro-magnetic radiation or electro-magnetic waves travelling in a straight line. The rays or
waves are deviated or reflected by obstructions and cannot travel over the horizon or behind obstacles. Beyond that, material
disperses the rays respectively the energy of the waves.
Radio signals, like all electromagnetic radiation, travel in straight lines. At low frequencies (below approximately 2 MHz or so)
ground effect transmission causes significant diffraction, allowing photons to partially follow the Earth's curvature along multiply
deflected straight lines, thus enabling AM radio signals in low-noise environments to be received well after the transmitting antenna
has dropped below the horizon. Additionally, frequencies between approximately 1 and 30 MHz, can be reflected by the F1/F2
Layer, thus giving radio transmissions in this range a potentially global reach (see shortwave radio), again along multiply deflected
straight lines. However, electro-magnetic radiation never propagates other but along straight lines. Just the effects of multiple
diffraction or deflection serve for macroscopically "quasi-curved paths".
However, at higher frequencies and in lower levels of the atmosphere, neither of these effects apply. Thus any obstruction between
the transmitting antenna and the receiving antenna will block the signal, just like the light that the eye senses. Therefore, as the ability
to visually sight a transmitting antenna (with regards to the limitations of the eye's resolution) roughly corresponds with the ability to
receive a signal from it, the propagation characteristic of high-frequency radio is called "line-of-sight" as per radio wave propagation
is called as "radio horizon".
In practice, the propagation characteristics of these radio waves vary substantially depending on the exact frequency and the strength
of the transmitted signal (a function of both the transmitter and the antenna characteristics). Broadcast FM radio, at comparatively
low freqencies of around 100 MHz using immensely-powerful transmitters, easily propagates through buildings and forests.
Non-line-of-sight (NLOS) or near-line-of-sight
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Many types of radio transmissions depend, to varying degrees, on line of sight between the transmitter and
receiver. Obstacles that commonly cause NLOS conditions include buildings, trees, hills, mountains, and, in some
cases, high voltage electric power lines. Some of these obstructions reflect certain radio frequencies, while some
simply absorb or garble the signals; but, in either case, they limit the use of many types of radio transmissions,
including most of those used for Wi-Fi.
How to achieve effective NLOS networking has become one of the major questions of modern computer
networking. Currently, the most common method for dealing with NLOS conditions on wireless computer
networks is simply to place relays at additional locations, sending the content of the radio transmission around the
obstructions. Some more advanced NLOS transmission schemes now use multipath signal propagation, bouncing
the radio signal off other nearby objects to get to the receiver.
Non Line-of-Sight (NLOS) is a term often used in radio communications to describe a radio channel or link where
there is no visual line of sight (LOS) between the transmitting antenna and the receiving antenna. In this context
LOS is taken
either as a straight line free of any form of visual obstruction, even if it is actually too distant to see with the
unaided human eye
as a virtual LOS i.e. as a straight line through visually obstructing material, thus leaving sufficient transmission for
radio waves to be detected.
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There are many electrical characteristics of the transmission media that affect the radio wave propagation and
therefore the quality of operation of a radio channel, if it is possible at all, over an NLOS path.
The acronym NLOS has become more popular in the context of wireless local area networks (WLANs) such as
WiFi and WiMax because the capability of such links to provide a reasonable level of NLOS coverage greatly
improves their marketability and versatility in the typical urban environments in which they are most frequently
used. However NLOS contains many other subsets of radio communications.
The influence of a visual obstruction on a NLOS link may be anything from negligible to complete suppression. An
example might apply to a LOS path between a television broadcast antenna and a roof mounted receiving antenna.
If a cloud passed between the antennas the link could actually become NLOS but the quality of the radio channel
could be virtually unaffected. If, instead, a large building was constructed in the path making it NLOS, the channel
may be impossible to receive.
NLOS links may either be simplex (transmission is in one direction only), duplex (transmission is in both
directions simultaneously) or half-duplex (transmission is possible in both directions but not simultaneously).
Under normal conditions all radio links including NLOS are reciprocal which means that the effects of the
propagation conditions on the radio channel are identical whether it operates in simplex, duplex or half-duplex.
Fresnel zone
Fresnel zone. d is the distance between the transmitter and the receiver, b is the radius of the
Fresnel zone.
 In optics and radio communications, a Fresnel zone (pronounced FRA-nel Zone), named for
physicist Augustin-Jean Fresnel, is one of a (theoretically infinite) number of concentric ellipsoids of
revolution which define volumes in the radiation pattern of a (usually) circular aperture. Fresnel
zones result from diffraction by the circular aperture.
 The cross section of the first Fresnel zone is circular. Subsequent Fresnel zones are annular in cross
section, and concentric with the first.
 To maximize receiver strength, one needs to minimize the effect of the out of phase signals by
removing obstacles from the RF Line of Sight (RF LoS). The strongest signals are on the direct line
between transmitter and receiver and always lie in the 1st Fresnel Zone.
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Several examples of how the Fresnel zone can be
disrupted.
where,
Fn = The nth Fresnel Zone radius in metres
d1 = The distance of P from one end in metres
d2 = The distance of P from the other end in metres
λ = The wavelength of the transmitted signal in metres
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The cross section radius of the first Fresnel zone is the highest in the
center of the RF LoS which can be calculated as:
 where
 r = radius in metres
 D = total distance in kilometres
 f = frequency transmitted in gigahertz.
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SNR(Signal-to-noise ratio)
 Signal-to-noise ratio (often abbreviated SNR or S/N) is an electrical engineering
concept, also used in other fields (such as scientific measurements, biological cell
signaling and oral lore), defined as the ratio of a signal power to the noise power
corrupting the signal.
In less technical terms, signal-to-noise ratio compares the level of a desired signal (such
as music) to the level of background noise. The higher the ratio, the less obtrusive the
background noise is.
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Technical sense

In engineering, signal-to-noise ratio is a term for the power ratio between a signal (meaningful
information) and the background noise:
where P is average power and A is RMS amplitude. Both signal and noise power (or amplitude) must
be measured at the same or equivalent points in a system, and within the same system bandwidth.
Because many signals have a very wide dynamic range, SNRs are usually expressed in terms of the
logarithmic decibel scale. In decibels, the SNR is, by definition, 10 times the logarithm of the power
ratio. If the signal and the noise is measured across the same impedance then the SNR can be
obtained by calculating 20 times the base-10 logarithm of the amplitude ratio:
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Antenna types
 Solid(Dish)
 Parabilic
 Grid
 Omni
 Yagi
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 Omni Directional Antenna
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Omni Radiation Pattern
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Parabolic Antenna(Grid)
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Parabolic Antenna(Grid)
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Parabolic Antenna(Solid)
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Parabolic Radiation Pattern
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Yagi Antenna
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Yagi Antenna
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Yagi Radiation Pattern
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Yagi Radiation Pattern
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 RF connector
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122
An RF connector is an electrical connector designed to work at radio frequencies in the multimegahertz range. RF connectors are typically used with coaxial cables and are designed to maintain
the shielding that the coaxial design offers. Better models also minimize the change in transmission
line impedance at the connection. Mechanically they provide a fastening mechanism (thread,
bayonet, braces, push pull) and springs for a low ohmic electric contact while sparing the gold
surface thus allowing above 1000 reconnects and reducing the insertion force. research activity in
the area of radio-frequency (RF) circuit design has surged in the last decade in direct response to
the enormous market demand for inexpensive, high data rate wireless transceivers.
RF Connectors
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N type RF Connector (Pic
Tail)
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 Power
 When referring to measurements of power or intensity, a ratio
can be expressed in decibels by evaluating ten times the base10 logarithm of the ratio of the measured quantity to the
reference level. Thus, XdB is calculated using the formula:
125
Converting X(w) to X(db)
126
 dBm
 dB(mW) — power relative to 1 milliwatt.

dBμ or dBu
 dB(μV/m) — electric field strength relative to 1 microvolt per metre.
 dBf
 dB(fW) — power relative to 1 femtowatt
 dBW
 dB(W) — power relative to 1 watt.
 dBk
 dB(kW) — power relative to 1 kilowatt.
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Commercial Satellites Orbit
 1) Geosynchronous Orbit (GEO)
 2) Medium Earth Orbit (MEO)
 3) Low Earth Orbit (LEO)
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1) Geosynchronous Orbit (GEO): 35,786 km above the earth
 Orbiting at the height of 22,282 miles above the equator (35,786 km), the satellite
travels in the same direction and at the same speed as the Earth's rotation on its axis,
taking 24 hours to complete a full trip around the globe. Thus, as long as a satellite
is positioned over the equator in an assigned orbital location, it will appear to be
"stationary" with respect to a specific location on the Earth.
 A single geostationary satellite can view approximately one third of the Earth's
surface. If three satellites are placed at the proper longitude, the height of this orbit
allows almost all of the Earth's surface to be covered by the satellites.
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Medium Earth Orbit (MEO): 8,000-20,000 km above the earth
 These orbits are primarily reserved for communications satellites that cover the
North and South Pole
 Unlike the circular orbit of the geostationary satellites, MEO's are placed in an
elliptical (oval-shaped) orbit
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Low Earth Orbit (LEO): 500-2,000 km above the earth
 These orbits are much closer to the Earth, requiring satellites to travel at a very
high speed in order to avoid being pulled out of orbit by Earth's gravity
 At LEO, a satellite can circle the Earth in approximately one and a half hours
131
What’s DSL
 DSL or xDSL, is a family of technologies that provide digital data
transmission over the wires of a local telephone network. DSL
originally stood for digital subscriber loop, although in recent
years, many[attribution neededhave adopted digital subscriber line as
a more marketing-friendly term for the most popular version of
consumer-ready DSL, ADSL.
 Typically, the download speed of consumer DSL services ranges
from 256 kilobits per second (kbit/s) to 24,000 kbit/s, depending
on DSL technology, line conditions and service level
implemented. Typically, upload speed is lower than download
speed for Asymmetric Digital Subscriber Line (ADSL) and equal
to download speed for Symmetric Digital Subscriber Line (SDSL).
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ADSL
133
Voice and data
 Some variants of DSL connections, like ADSL and very high speed
DSL (VDSL), typically work by dividing the frequencies used in a
single phone line into two primary 'bands'. The ISP data is carried
over the high frequency band (25 kHz and above) whereas the
voice is carried over the lower frequency band (4 kHz and below).
(See the ADSL article on how the high frequency band is subdivided). The user typically installs a DSL filter on each phone.
This filters out the high frequencies from the phone, so that the
phone only sends or receives the lower frequencies (the human
voice), creating two independent 'bands'. Thus the DSL modem
and the phone can simultaneously use the same phone line without
interfering with each other.
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Transmission methods
 2B1Q: Two-binary, one-quaternary, used for IDSL and HDSL
 CAP: Carrierless Amplitude Phase Modulation - deprecated in 1996 for ADSL,
used for HDSL
 DMT: Discrete Multitone Modulation, the most numerous kind, otherwise
known as OFDM
 OFDM: Orthogonal Frequency-Division Multiplexing
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Type Of DSL
136
DSL Type
Description
IDSL
ISDN Digital Subscriber Line
CDSL
Consumer DSL from Rockwell
HDSL
High bit-rate Digital Subscriber Line
SHDSL
Symmetric High bit-rate Digital Subscriber
Line
ADSL
Asymmetric Digital Subscriber Line
SDSL
Symmetric Digital Subscriber Line
UDSL
Unidirectional DSL proposed by a company in
Europe
VDSL
Very high Digital Subscriber Line
Data Rate
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Difference between ADSL and ADSL2+
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ADSL
ADSL stands for Asymmetric Digital Subscriber Line service. It is called asymmetric because the download and
upload speeds are not symmetrical (download is faster than upload).
ADSL2
ADSL2 (ITU G.992.3 and G.992.4) adds new features and functionality targeted at improving performance and
interoperability and adds support for new applications and services. Among the changes are improvements in
ADSL's data rate, an increase in the distance ADSL can reach from the local telephone exchange, dynamic data rate
adaptation, better resistance to noise, diagnostics, and a stand-by mode to save power. ADSL2 also reduces the
initialisation time from more than 10 seconds (as is required for ADSL) to less than 3 seconds. ADSL2 has the
same signal footprint as ADSL.
ADSL2+
ADSL2+ (ITU G.992.5) doubles the bandwidth used for downstream data transmission, effectively doubling the
maximum downstream data rates, and achieving rates of 20 Mbps on telephone lines as long at 5,000 feet.
ADSL2+ solutions will interoperate with ADSL and ADSL2, as well as with ADSL2+. ADSL2+ will include all
the feature and performance benefits of ADSL2 while maintaining the capability to interoperate with legacy ADSL
equipment.
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Leased Line
A leased line is a symmetric telecommunications line connecting two locations. It is sometimes
known as a 'Private Circuit' or 'Data Line' in the UK. Unlike traditional PSTN lines it does not have
a telephone number, each side of the line being permanently connected to the other. Leased lines
can be used for telephone, data or Internet services. Some are ringdown services, and some connect
two PBXes.
 A permanent telephone connection between two points set up by a telecommunications common
carrier. Typically, leased lines are used by businesses to connect geographically distant offices. Unlike
normal dial-up connections, a leased line is always active. The fee for the connection is a fixed
monthly rate. The primary factors affecting the monthly fee are distance between end points and the
speed of the circuit. Because the connection doesn't carry anybody else's communications, the
carrier can assure a given level of quality. For example, a T-1 channel is a type of leased line that
provides a maximum transmission speed of 1.544 Mbps.You can divide the connection into
different lines for data and voice communication or use the channel for one high speed data circuit.
Dividing the connection is called multiplexing.
 Increasingly, leased lines are being used by companies, and even individuals, for Internet access
because they afford faster data transfer rates and are cost-effective if the Internet is used heavily.
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Leased Line Modems
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Mohammad Zargari
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