Transcript COMP 421 /CMPET 401 COMMUNICATIONS and NETWORKING Chapter 3 (Continued) Data Transmission

COMP 421 /CMPET 401
COMMUNICATIONS and NETWORKING
Chapter 3 (Continued)
Data Transmission
History
Although electricity has been known to
exist for centuries, experiments into its friendly
use did not begin until the 1700s when scientists
such as Volta, Ampere, and Watt explored
ways to harness its potential. In the 19th century,
other scientists used electricity to invent
the telegraph, telephone, and radio. With the
advent of Marconi’s transoceanic wireless and
the inauguration of radiotelegraph service, the
20th century saw dramatic breakthroughs in
communications technology. The world of
telecommunications has since exploded to
include such developments as television,
communications satellites, lasers, and fiber optics.
Each new invention, like all of its predecessors, relies
on parts of the electromagnetic spectrum to
carry information from origin to destination.
Global Network Hierarchy
Transmission Mediums
Bandwidth

Width of the spectrum of frequencies that
can be transmitted
– if spectrum=300 to 3400Hz,
bandwidth=3100Hz
Greater bandwidth leads to greater costs
 Limited bandwidth leads to distortion
 Analog measured in Hertz
 Digital measured in baud or Bps

Bandwidth
The most common means for measuring
path/circuit/channel size is describing the "range"
(bandwidth) of radio frequency (RF) spectrum necessary to
carry the information assigned to a particular
path, channel, circuit, etc. The wider the path
(the larger the bandwidth), the greater its capacity.
Bandwidth is expressed in hertz (Hz) or
cycles per second (CPS) of radio frequency.
One Hz equals one CPS. The capacity of paths
that carry digital data is usually expressed in
kilobits or megabits per second (kbps/Mbps) as
a more meaningful measure of data throughput
capacity.
Analog and Digital Data Transmission

Data
– Entities that convey meaning

Signals
– Electric or electromagnetic representations of
data

Transmission
– Communication of data by propagation and
processing of signals
Bit rate and Baud rate

Bit rate number of bits that are transmitted in a second

Baud rate number of line signal changes (variations) per second
If a modem transmits 1 bit for every signal change
bit rate = baud rate
If a signal change represents 2 or more or n bits
bit rate = baud rate *n
Switching
Switching is the means by which traffic is
routed through a communications or system network.
Switches may be manual (operator assisted)
or automatic; they may serve local (in a
city or on a military base) subscribers or perform
area network (tandem [many switches connected
to one another]) functions.
An electrical path established between
terminals, switches, and/or transmission systems
is commonly referred to as a line, circuit,
or channel. The "size" of the path is important
in determining the full capability or capacity.
Switching Systems
Electronic Switching Systems:
A switch using solid-state switching devices and computer software
that provides preprogrammed instructions to accomplish the
switching of calls.
Digital Switches:
An electronic switching system that processes all signals to be
switched into a digital mode. The circuit switch can also be used to
route record and data traffic from a terminal to the nearest message
switch for further processing. This dial-up, or hybrid, switching
method uses the data adapter/RS-232 port feature of a digital
telephone to accommodate a teletypewriter/data facsimile terminal
connection to the circuit switch, which then routes the traffic forward
as if it were a telephone call.
Switching Types
There are three broad types of switches: circuit, message, and packet:
Circuit switching - is the process of interconnecting a specific circuit to
provide a direct connection between calling and called stations. For example,
a local civilian telephone company interconnects telephone calls through its
central office computerized circuit switch.
Message switch = accepts a group of characters called a message, reads
the message’s attached routing information, and stores it in computer
memory. When a circuit path becomes available, the message is forwarded
either to its destination or to another message switch closer to its destination.
Message switches are called “store and forward” because they receive and
store an entire message before sending it on its way.
Packet switching - is a specialized technique of dividing messages into
many standardized transmission blocks (packets), whereby the switching
center does not store the packets, but routes them through a network
independent of each other. At the destination, the packets are reassembled
into the original message. Packet switching is an efficient and relatively
inexpensive method to transfer data between local area networks.
Switching
Circuit Switches
Circuit switches principally route voice telephone
traffic, while message switches route the electrical
form of hard copy messages. Message switches are
further categorized as either store and forward or
packet. A store and forward switch receives and
electrically stores an entire message, retrieves it,
determines where it should go, and routes it to its
destination either directly or through another switch.
Packet switches, which are especially adept at
handling data, receive message segments (packets)
Circuit Switching
Every time a telephone is used, a circuit switch routes the call. The
switching center serves as the focal point for the interconnection
of subscribers, via trunk circuits, to subscribers at other locations.
In a circuit switched network, the calling party is connected to an
end office (private branch exchange, or PBX) via a "subscriber
loop." When the caller lifts the handset, a signal sent to the end
office indicates a request for service. The end office switch places
a dial tone on the loop, which alerts the caller that the switch is
prepared to accept his calling instructions. The caller issues
instructions to the switch by dialing the digits of the subscriber
being called. These digits appear as dial pulses in the case of a
rotary dial or as multiple frequency tones (touch-tones) that
represent discrete digits
Circuit Switching
The switch interprets these digits as an indicator of the destination
of the call, and, through preprogrammed instructions, logically and
sequentially executes the actions to complete the call. Call
processing is never a random process: it adheres to strict
procedural rules established in the preprogrammed instructions.
The following are typical circuit switches that may be used in a
network.
Electromechanical Switches: A long proved application whereby
program control is executed by preset electrical/mechanical relays.
Stored Program Control Switches:
Sometimes a hybrid, where the switching is
completed by electrical/mechanical relays under
the direction of a computer-like stored program.
Message Switching
A message switch is a central routing mechanism for
teletypewriter and low-speed data information. The majority of
switching networks in service employ the store and forward
message switch technique. A switch simply receives and stores a
message, retrieves and determines where it is addressed, and
routes it to the next appropriate node. This process is particularly
valuable in handling multi-addressed messages for near
simultaneous delivery without the need for retransmissions. In
military message switching networks, a precedence system
provides expedited service for critical messages having a higher
priority or urgency than the other messages. Message
precedences from lowest to highest are routine, priority,
immediate, and flash.
Packet Switching
Packet Switching
The packet switched network is used to route digital data traffic,
including electronic mail (E-mail). In packet switched networks, the
subscriber transmits the message to the switching center as a total
message. The message is next divided into discrete packets and
routed over any available transmission path to the next node. Each
node of the packet switched network contains an internal processor
that constantly surveys the traffic loads and conditions throughout the
network. Upon receipt of the packets at the destination node, the
switch reassembles the packets in sequence for delivery
to the addressee.
Packet Switched message Flow
Packet Switching
Packet switching requires highly structured protocols to maintain
network status and control of the packets. The preamble to each
packet must contain identification of the message. A normal
schedule is to limit the message to approximately eight packets of
1000 bits per packet (125-250 characters). The nodal points
within a network do not store the messages except for the very brief
time it takes to “packetize” the message and forward it through the
network. Therefore, if an incomplete message is received by the
addressee, the originator must retransmit the message.
The packet switched network is designed to handle computer-tocomputer exchanges, interactive queries to a computer, and
batch processing, as well as processing narrative traffic, such as Email. However, since this technology does not switch whole
messages, it is an uneconomical method to use for multi-addressed
traffic.
Modulation
Frequency Division Multiplexing
The oldest form of multiplexing, FDM,
divides a circuit into several smaller channels
by frequency for simultaneous transmission.
FDM, which is analog only, allows a user to
cluster many terminals at a given location and
share the same transmission path. Because
the bandwidth can be divided into just so many
parts, the number of terminals supported is
limited. The speed or transmission rate of each
channel is reduced due to narrower channel
bandwidth. Thus, digital signals, such as data,
must first be converted to analog.
FDM
For example, when 12 individual 4-kHz
analog voice channels are fed to an FDM multiplexer
(channel bank), each channel is assigned
a frequency slot until all 12 channels are
allocated. The channel bank output, a composite
48-kHz analog signal, is sent to the receiver
by some transmission path where the reverse
process (demultiplexing) occurs, restoring the
original 12 channels. FDM can be used for
voice, teletypewriter, analog data, and facsimile.
Multiplexing
Time Division Multiplexing
In TDM, a digital multiplexing scheme, each individual
channel, called a subchannel, is allocated the entire
transmission bandwidth for specific regular intervals or
time slots. A time slot is allocated to each subchannel
whether or not information is being transmitted. TDM is
more flexible than FDM and allows the user to vary the
number or duration of the time slots, depending on
network requirements. In slightly more technical terms.
TDM
TDM allows simultaneous transmission of two or more signals by
sampling each approximately 8000 times a second. Each
channel sample is trans-formed into a pulse that is further coded
to represent the incoming signal. The pulses from the channel
sample are multiplexed in time. Each pulse is sequenced in a
serial time slot of the output of the channel bank. TDM can
accept various numbers of low-, medium-, or high-speed data
channels directly and sequence them into a higher capacity
data stream. TDM multiplexing is also referred to as pulse code
modulation. Most FDM tactical multiplex equipment is rapidly
being replaced by newer digital TDM equipment
Digital Multi-channel Characteristics
Terminals
Terminals are the most recognizable
communications components. A telephone,
radio, facsimile, computer, television, and teletypewriter
are all examples of terminals used to
transmit (send) and receive information. Information,
often called traffic, can take the form of
voice, data, message, video, or other means.
Traffic may be secure (encrypted/covered) or
nonsecure (clear). Radios and telephones are
the common terminals associated with voice
communications. Facsimiles transmit and receive
maps, photographs, sketches, and printed
or handwritten text. Teletypewriters or printers
attached to computers are used for messages
and cables. This is often referred to as "record
traffic" because the printer produces a "hard
copy" record of what is received. Data terminals
and computers transmit and receive binary
data, while video terminals communicate imagery
and sound.
Inter-Relationship of Terminals,
Switches and Switching Facilities
Data Rate
The speed of transmission of digital data
is reflected as a "data rate." As data rate
increases, so does the bandwidth of the path,
channel, or circuit carrying the data stream..
A single 4-kHz-wide VF channel can also
be subdivided by bandwidth to accommodate
modes other than voice. For example, one VF
channel can be multiplexed to commonly provide
up to 16 teletypewriter (TTY) channels of
about 200 Hz each.
Voice Circuits
A typical voice conversation transmitted
over a standard telephone line or circuit requires
a radio frequency of sufficient bandwidth
to handle the range of voice variations (called
modulations) needed to convey information.
This voice frequency (VF) range is approximately
3-4 kHz wide (1000 hertz = kHz) and is
the standard for defining a single channel or
"narrowband" circuit.
Circuits established between switches
are called trunks. Trunks ride transmission
systems that are normally equipped with a multi-channel
capability and employ modems. They
use various protocols (rules and codes) required
for processing traffic in a switched network.
Duplex Circuits
Circuits may be established to provide a
one-way, one-way reversible, or simultaneous
two-way traffic capability, depending upon user
need and the availability of assets.
The one-way reversible circuit is commonly referred to
as "half duplex," meaning that traffic can be
passed in either direction but in only one direction at a time.
A simultaneous two-way path is
referred to as "full duplex"--traffic can be passed
in either or both directions at the same time.
The simple one-way-only path is often referred
to as a "receive-only" or "transmit-only" circuit.
Modem
A modem (a contraction of "modulator"
and "demodulator") is a device that converts
incoming and outgoing electronic signals from
one form to another. For example, the digital
output of a home computer may be fed to a
modem that converts the digital bit stream (digital
signal) into a series of audio tones (analog
signals) transmitted over an analog-capable
standard-grade public telephone circuit. The
home computer modem may also convert an
inbound analog signal from a telephone line to
a digital signal that can be used by the computer..
Analog and Digital Transmission
Analog signals are represented as continuous
wave-like (sine wave) signals, such as
those electrical signals generated by a telephone
keyed to a human voice. The analog
signal varies by frequency in cycles per second,
or hertz, to represent different voice sounds.
Analog voice signals are transmitted over
a channel in one of two ways: at their original
frequency as a baseband signal or modulated
onto another (carrier) signal and transmitted at
the carrier frequency. Voice is transmitted in the
audio frequency range of about 300 to 3400 Hz,
or a voice signal bandwidth of 3100 Hz. Invariably,
long-distance calls will be modulated to a
higher frequency for transmission to achieve
greater efficiency and lower cost.
Digital Signals
By contrast, digital signals are discrete
and consist of two possible states: the presence
or absence of an electrical signal (on or off), or two
different electrical signal levels. Communications
between computers use a digital form,
where information is conveyed as binary digits
or bits (1s or 0s). For digital communications, the bandwidth
describes the amount of data that can be
transmitted on a channel over time in bits per
second (bps). High-speed data networks might
offer bandwidths of 10 to 50 Mbps, while telephone
systems used for digital transmission
offer a bandwidth of 1.544 Mbps, the standard
commercial T-1 carrier rates.
Digitization
Channel Capacity
Voice lines used in conjunction with modems to transmit data
typically offer speeds of 2.4 and 19.2 kbps, and wide data
compression yielding asynchronous throughputs up to
76.8 kbps. To conserve channel capacity, the multiplexing
of many signals into one channel can be accomplished
normally using FDM or TDM techniques. The FDM method
requires that digital signals be converted into analog. This
conversion of analog-to-digital (A/D) and digital-to-analog (D/A)
is usually accomplished by a modem, which is often used to
transmit digital data over a telephone (analog) network.
A/D and D/A conversion; and it is incompatible with existing
analog equipment.
Digitized Analog
There is a bandwidth penalty for digitizing
analog signals. For example, in a voice
transmission, the bandwidth requirement increases
16-fold, in comparison to music that is
20:1 digital versus analog ratio. By converting
signals from a digital telephone to analog prior
to transmission, however, the output can be
sent over a standard 4 kHz voice frequency
circuit versus a wideband path.
Transmission Rate Vs.
Information Rate
In general, the greater the information
rate (or resolution), the larger the transmission
bandwidth. Required transmission bandwidth
varies directly with desired information speed.
A single voice conversation or teletype channel
can easily be accommodated by a nominal 3kHz-wide voice frequency channel. However,
beyond voice, required bandwidth increases
precipitously. Digital facsimile requires 7.5 to
50 kHz, high-speed data more than 100 kHz,
and full-motion video 4 MHz.
Information Rate
In theory, information rate can equal but
never exceed the transmission rate. In practice,
the actual information rate measured in
wpm or bps is almost always slower than the
transmission rate, and is frequently significantly
so. There are several reasons for this. Many
times, information rate is dictated by the speed
of the user’s terminal device, which may be
much slower than the transmission “pipe.” This
will be the case with slow-speed teletypewriters
and with the human voice.
Bits and Bytes
A Bit
is actually a contraction for the term "binary digit" and is the smallest unit of
information used in data communications. It is represented by either a zero or a one.
A Byte
is a grouping of bits that may or may not be translatable to the user. Although the
number of bits in a byte depends upon the type of communications equipment in use,
eight is the most common.
Bit
rate is a measure of transmission and speed and is usually expressed in bits per
second (bps); bit rates greater than 1000 are given in kbps, greater than one million in
Mbps, etc.
Baud
(or sometimes Baud Rate) is the number of signal transitions per period of time on
the phone line. Bit and baud rates are typically the same at 300 bps, but baud rates
are limited to 2400 (the approximate bandwidth of a phone line). It is possible to
transmit about 2400 bps by coding/increasing the number of bits per baud (e.g., 4800
bps can be transmitted at 2400 baud by encoding two bits per baud).
Comparing Transmission Rate to
Information Rate
Transmission rates
Many factors determine transmission and
information rates. The transmission rate of a
network or link includes the trunk speed in bits
per second of the trunk and switching facilities
or both. The information rate (traffic throughput),
on the other hand, takes into consideration
the entire path from one user terminal (A) to
another (B), and reflects the speed at which the
receive end can accept information from the
transmit end as dictated by the slowest terminal
in the loop. For example, if terminal A transmits
data at 19.2 kbps to terminal B, which can
handle traffic input at only 4.8 kbps, the resultant
information rate is 4.8 kbps. The difference
is stored/buffered by the system to accommodate
the lower rate of the receive terminal.
Factors Affecting Information Rate
Channel Capacity
The capacity of a channel may be described
as the maximum rate at which information
can be sent over the channel without error.
Bandwidth is the frequency range of a given
electrical path or circuit. For data transmission
purposes, channel capacity is measured in bps.
The rate at which data may be transmitted is
proportional to the channel bandwidth. Overall,
data channel capacity is a function of volume,
rate or speed, and the quality of the transmission
path. These factors are important to the
planner because they dictate how well any
given transmission link can pass intelligence
data from one point to another.
Relationship of Terminals to
Bandwidth and Information Rate
Noise
Transmission Speeds
Transmission Speeds
Different terminal types have various
transmission speed requirements. Standard
4-kHz narrowband telephone channels can
handle data transmission speeds up to 9.6
kbps. Data sent faster than 9.6 kbps over these
channels becomes unintelligible. Consequently,
high-speed modems operating at speeds from
over 9.6 kbps to 56 kbps or higher will not work
using this type of transmission channel.
Transmission Speeds
The maximum data transmission speeds
achieved over a given bandwidth are considerably
lower than the theoretical maximum. These
speeds fall into the shaded area on the above
graphic. The dotted lines are theoretical maximum
speeds in the presence of noise, and are
calculated for an error rate of one error bit in
every 10,000 bits transmitted. One way to
increase the capacity of a channel is to raise the
signal-to-noise ratio. While some types of noise
can partially be controlled, the level of random
noise is determined by natural phenomena,
which are uncontrollable. There is a level below
which noise cannot be suppressed.
Spectrum
Electromagnetic Spectrum
From a planner’s viewpoint, usable radio frequencies extend
from about 30 Hz to 300 gigahertz (300 billion hertz,
or GHz). Research is being conducted to exploit
even higher frequencies up to about 100
terahertz (100 trillion hertz, or THz) for laser
applications. As a comparison, the typical human
voice ranges in frequency from 80-8000 Hz.
The electromagnetic spectrum in its broadest context is a
continuum of electromagnetic. energy traveling at the speed of light,
186,000 miles per second. Within that spectrum are frequencies of less
than one hertz to those easily exceeding 1000 THz, well beyond
laser range. Each frequency has a discrete length of its radiated signal
(wavelength). The frequency group from about 30 Hz to near 300
GHz is generally referred to as the radio frequency band. Electromagnetic
energy within this band is referred to as radio frequency, radio
waves, or radio for short.
RF Spectrum
In another common frame of reference,
the radio frequency spectrum is divided into
discrete frequency bands. These bands are
assigned unique designations (ELF, HF, SHF,
etc.) that correspond to a particular frequency
range. (For example, the high frequency [HF]
band includes frequencies from 3 million to 30
million hertz [expressed as megahertz, or MHz].)
Frequency
Each band or group of frequencies possesses
different properties that affect useful signaling
range, ability to carry information or "throughput,"
physical attributes of supporting equipment,
and resistance to natural phenomena
and human-caused interference. Each frequency
band or slice has its own peculiar capabilities
and limitations, and each has its advantages
and disadvantages.
Frequency has a significant impact
on antenna size. As frequency increases, wavelength
decreases proportionally (f=1/W where "f" refers to frequency
and "W" refers to wavelength). VLF is propagated by ground
waves
Frequency
Since the optimum length of a transmitter
antenna equals one wavelength of transmitted
frequency, the higher the frequency the
shorter the antenna. While there are methods
to electrically shorten and lengthen antennas,
the optimum antenna is physically one wavelength
long. For example, an ELF antenna of
exactly one wavelength must be hundreds of
miles long, whereas an ultra high frequency
(UHF) radio antenna is only several inches in
length.
Frequency
A typical military battlefield tactical single
channel radio transmits in the very high frequency
(VHF), 30 to 300 MHz, segment of the radio spectrum.
\Radios using this particular spectrum slice can be
lightweight, can use simple and relatively short wire
antennas called "whips,“ and can easily be carried by a
single soldier. On the other hand, a typical multi-channel
satellite radio system in general use with the military
transmits in the super high frequency (SHF) range. The
demands of this part of the spectrum require a very
complex and relatively bulky transmitter-antenna
combination that is far too large for a infantryman to carry
on his back.
Frequency
In short, frequency determines how waves propagate.
Three types of electromagnetic propagation
are common: ground wave, sky wave,
and free space propagation. At lower frequencies
(ELF and VLF), radio ground waves travel
great distances along the surface of the earth.
Ground waves experience increasing loss (attenuation,
or loss of signal strength) as frequency increases.
Frequency
VLF is propagated by ground waves with little
attenuation over thousands of kilometers.
At higher frequencies, losses along the surface
become so great that the ground wave is limited to
short distances, usually 50 kilometers or less.
Sky waves occur at medium to high frequencies
(MF and HF) where reflection from the ionosphere
permits radio communication over great distances.
At frequencies above 30 MHz (HF), ionospheric
reflections are not dependable.
Frequency
Frequency
Over an LOS path, transmission is much as through free space.
Atmospheric interference, however, tends to degrade sky wave
propagation.At EHF, for example, there may be wave
attenuation (signal degradation) due to rainfall and absorption by
dust and water vapor. Finally, as frequency increases, required
transmitter output power decreases. To illustrate, normal output
power for HF single channel over-the-horizon radios is between a
few hundred and about 2000 watts (2000 W, or 2 kW). In contrast,
most single channel VHF systems operate at well under 100 W.
Transmitter output power is important because it determines how
much input power is required for the radio system to operate
properly.
Frequency
A 5 watt UHF radio will transmit and receive for many hours
on small dry-cell batteries. An HF radio with a one kilowatt
transmit power requires a relatively high amperage
constant power source such as that delivered by tactical
generators or commercial electric power. Another
consideration for the planner regarding high output power is
that the higher the power, the easier it is to find the
transmitter using direction-finding (DF) techniques.
ELF
Extremely Low Frequency (ELF) Range:
The ELF frequency band is from 30 to 300 Hz and is
characterized by ground wave propagation distances of
more than 5000 miles. ELF amplitude modulated (AM)
waves produce sound at high power and are able to
penetrate vegetation and water to depths approaching 600
feet using broadcast codes. ELF communications systems
require enormous transmit antennas covering thousands of
acres and operate at a very high transmit power, often in the
100 megawatt (MW) range.
VLF
Very Low Frequency (VLF) Range:
The VLF frequency band covers 3 to 30 kHz. Like ELF, VLF
transmissions can span distances of 5000 or more miles and,
to a limited degree, can penetrate vegetation and water. As
such, VLF is used principally for navigation and for low-speed
secure TTY communications to shallowly submerged submarines.
VLF transmitters are normally shore based; however, certain
command and control (C2) aircraft may have a VLF capability. These
aircraft use long trailing wire antennas and transmitter power ranges
from 0.5 to 2 megawatts.
Throughput/Data Rates:
VLF broadcast systems employ minimum shift keying (MSK) and
operate four low-speed 50-baud secure TTY channels. This
equates to an information rate of about three characters every
12 seconds, although slightly higher data
LF
Low Frequency (LF) Range:
The LF frequency range is 30 to 300 kHz and can span distances of
1000 to 5000 miles. LF is used for medium- to long distance
communications, particularly to submarines, surface ships, and, in
some cases, aircraft. LF is also used for radio navigation such as
LORAN C. LF is able to penetrate vegetation and sea water, but less
effectively than ELF/VLF. Current shore-based LF systems use 50100 kW transmitters and employ frequency shift keying (FSK) for
single channel secure teletypewriter, or Morse code continuous
wave (CW) operation for communications with ships at sea.
Throughput/Data Rates:
Using FSK with appropriate COMSEC equipment, LF can
transmit in the secure TTY broadcast mode at 75 baud, which
equates to an information rate of 100 words per minute (wpm).
Receivers are mounted on surface ships, while submarines must
maintain antennas near the surface to receive LF broadcasts.
MF
Medium Frequency (MF) Range:
The MF range spans 300 to 3000 kHz; it propagates by ground
wave and sky wave. MF can span distances of 100-1000 miles via
ground wave and from 1000 to 3000 miles by sky wave,
depending upon transmitter power and atmospheric conditions.
Principal uses of the MF band include medium distance
communications, radio navigation, and AM broadcasting.
Throughput/Data Rates:
Channel availability of MF is greater than for LF and
lower bands. For example, the commercial AM broadcast band
extends from 550 to 1600 kHz with a separation of 10 kHz
between stations providing 105 audio channels. MF can support
low-capacity multi-channel circuits for both voice and
teletypewriter, the latter generally limited to 75 baud (100 wpm).
HF
High Frequency (HF) Range:
The HF frequency band covers 2 to 30 MHz. HF propagates by ground wave,
typically from 30 to 50 miles, or by sky wave for
distances greater than 1200 miles. There is usually a skip zone between
ground wave and sky wave propagation. HF is widely used for long-distance
communications, short wave broadcast, over-the-horizon (OTH) radar, and
amateur radio. HF transmitter power can vary from 3 watts to over 100 kW,
depending upon the use and range intended. In the HF band,
Throughput/Data Rates:
HF can accommodate Morse code, voice, and FSK TTY
modes of operation and can operate in the secure mode when using
COMSEC devices. Smaller HF radio systems are capable of a single VF
channel, which can support a single voice circuit or speech plus half-duplex
TTY up to 75 baud. Larger HF systems can support up to four voice circuits,
or typically a combination of three voice and one multiplexed circuit to
generate up to 16 TTY circuits.
Altitude Extends the Range
VHF
Very High Frequency (VHF) Range:
The VHF band extends from 30 to 300 MHz. VHF signals are principally
LOS and are useful to about 40 miles without being
retransmitted. VHF usage includes short-range FM combat net radio,
radio navigation, wideband LOS multi-channel systems, and television
broadcasting. Depending upon use, range, and
number of channels, VHF transmitter power ranges from 1/4 watt for a
portable FM radio, to 120 watts for an FM multi-channel (12-/24-channel)
LOS system. Although inherent LOS restrictions of terrain and antenna
height limit single-hop VHF systems to 20-40 miles per link, multiple
repeaters or relay stations can increase
link distance to several hundred miles.
Throughput/Data Rates:
VHF links can provide easily relocatable, reliable, and high-quality
communications comparable to cable systems. VHF systems can be
either analog or digital voice and data transmissions
UHF
Ultra High Frequency (UHF) Range:
The frequency band for UHF is from 300 to 3000 MHz. Principal methods of
UHF propagation are LOS (air-to-air, air-to surface, or surface-to-surface),
tropospheric scatter, and satellite. The UHF band provides great flexibility with
transmission ranges varying significantly. LOS terrestrial systems can
reach from 5-100 miles depending on terrain. Aircraft LOS distance can be 300
miles. Troposcatter radio communication is possible between 80 and 1200 miles.
And, depending on its altitude and antenna configuration, satellite range is
thousands of miles. Transmitter output power is between 10 watts and 100 watts
for LOS and satellite. Troposcatter systems require much higher output power
levels to achieve the desired range and scatter effect.
Throughput/Data Ranges:
Widely used, with excellent quality and reliability, UHF systems operate at data
rates of 2.4 kbps and higher. Modern UHF LOS systems, for example, can
function up to 600 kbps. UHF radios provide secure/non=secure voice, record,
data, and facsimile service in both mobile and fixed configurations. Along with
VHF, UHF is also used for television transmission.
SHF
Super High Frequency (SHF) Range:
The SHF band is from 3 to 30 GHz and is used principally for high data rate
multi-channel LOS, troposcatter, and satellite systems. A portion of this
band, the frequency range from about 1000 MHz to just above 1000
GHz, comprises the microwave region. LOS communications equipment
operating in this frequency range is often referred to as microwave
systems. Tropospheric scatter functions in the frequency range from 350 to
8000 MHz (8GHz), covering most of the UHF band and the lower SHF band.
Nominal terrestrial coverage for SHF systems ranges from 40 miles for LOS
microwave links to more than 300 miles for troposcatter communications.
Throughput/Data Rates:
SHF bandwidths are capable of handling high data rates of 1000+ kbps over
several multiplexed channels. This feature is essential for LOS microwave
radio relay systems that provide reliable, high-capacity, long-distance
communications.
EHF
Extremely High Frequency (EHF) Range:
EHF frequencies span 30 to 300 GHz. Military operational use of the EHF band is still in
its infancy. Research and development into this potentially rich communications band is
ongoing. The recent launch of the first military EHF satellite, Milstar, initiates an up to six
satellite constellation that will provide a global communications network well into the next
century. Milstar will provide worldwide coverage using geosynchronous satellites. The
geographic range of EHF satellite systems with cross-satellite linking is global.
Development of high-capacity millimeter wave (MMW) radios, another application in this
frequency range, continues. Millimeter wave radio is characteristically operated LOS and
is limited to a theoretical planning range of no more than 40 miles. In a promising
application for LOS tactical communications, the range of MMW radio could be held to
less than 10 miles, making it very difficult to detect. EHF signals transiting the
atmosphere are subject to attenuation by rain and other environmental conditions such
as snow, fog, etc.
Throughput/Data Rates:
EHF systems will be capable of transmitting secure voice and high-speed data at rates of
between 75 bps and 1.544 Mbps, depending on single channel or
multi-channel mode of operation. The extensivebandwidths available in the EHF band will
permit many hundreds of channels per link. With antijam
capabilities activated, however, throughput capacities will be reduced significantly.
Laser
Laser Range:
Laser radiation can be produced in the spectral ranges from ultraviolet,
through visible, to infrared radiation from 300 GHz to 100 THz. The radiation is
easily focused. The laser beam’s high coherence makes it a useful tool for
communicating information with privacy and security. Restricted to LOS, the
transmitted beam can be locked on to the receiver; this allows communications
to a mobile platform, such as a ship or aircraft. Due to larger bandwidth
inherent in laser communications, other types of high-volume transmissions,
including data and imagery, are also possible. Communicating by laser has
some drawbacks, especially over long distances. Fog, mist, rain, and smog
attenuate lasers and restrict useful range to a few hundred feet. Beam energy
must be at low enough levels to prevent eye injury, another indirect limitation
on the range of laser systems.
Throughput/Data Rates:
Because of the extremely wide bandwidth of laser radiation, up to 100 million
discrete voice conversations can be carried simultaneously. The laser beam
can be modulated by frequency, amplitude, phase, and polarization formats,
using analog or digital transmission modes.
Line Impairments
Line Impairments are faults in the line that occur due to
either improper line terminations or equipment out of
specifications. These cannot be conditioned out, but can
be measured to determine the amount of the impairment
Crosstalk
Crosstalk is when one line induces a signal into another
line. In voice communications, we often hear this as
another conversation going on in the background. In digital
communication, this can cause severe disruption of the
data transfer. Cross talk can be caused by the overlapping
of bands in a multiplexed system, or by poor shielding of
cables running close to one another. There are no specific
communications standards that are applied to the
measurement of crosstalk.
Echo or Signal Return
All media have a preferred termination condition for
perfect transfer of signal power. The signal arriving at
the end of a transmission line should be fully
absorbed, otherwise it will be reflected back down the
line to the sender (and appear as an Echo). Echo
Suppressors are often fitted to transmission lines to
reduce this effect.
Echo or Signal Return (2)
Usually during data transmission, these suppressors must
be disabled or they will prevent return communication in full
duplex mode. Echo suppressors are disabled on the phone
line if they hear carrier for 400ms or more. If the carrier is
absent for 100 mSec, the echo suppressor is re-enabled.
Echo Cancellers are currently used in Modems to replicate
the echo path response. These cancellers then combine the
results to eliminate the echo (thus, no signal interruption is
necessary).
Frequency Shift
Frequency shift is the difference between the transmitted
frequency and the received frequency. This is caused by the
lack of synchronization of the carrier oscillators.
Nonlinear Distortion
Nonlinear distortion changes the wave shape of the
signal. If the signal was transmitted as a sine wave
(and arrived as a square wave), it would be an
example of severe nonlinear distortion. Amplitude
modulated carriers would suffer drastically if the
original wave shape was distorted.
Jitter: Amplitude and Phase
Here are the 2 types of Jitter:
a.Amplitude Jitter
b.Phase Jitter
Amplitude Jitter is the small constantly changing swing in the
amplitude of a signal. It is principally caused by power supply noise
(60 Hz) and ringing tone (20 Hz) on the signal.
Jitter: Amplitude and Phase
Phase Jitter is the small constantly changing swing in the phase
of a signal. It may result in the pulses moving into time slots that
are allocated to other data pulses (when used with Time Domain
Multiplexing).
Telephone company standards call for no more than 10 degrees
between 20 and 300 Hz and no more than 15 degrees between
4 and 20 Hz.
Transients: Impulse Noise, Gain
Hits, Dropouts & Phase Hits
Transients are irregular-timed impairments. They appear
randomly, and are very difficult to troubleshoot. There are
4 basic types of Transients.
i.Impulse Noise
ii.Gain Hits
iii.Dropouts
iv.Phase Hits
Impulse Noise
Impulse noise is a sharp and quick spike on the signal that can
come from many sources: electromagnetic interference, lightning,
sudden power switching, electromechanical switching, etc.. These
appear on the telephone line as clicks and pops: they're not a
problem for voice communication, but can appear as a loss of data
(or even as wrong data bits) during data transfers. Impulse noise
has a duration of less than 1 mSec and their effect is dissipated
within 4 mSec.
Gain Hits
Gain Hits
Gain Hits are sudden increases in amplitude that last
more than 4 mSec. Telephone company standards
allow for no more than 8 gain hits in any 15 minute
interval. A gain hit would be heard on a voice
conversation as if the volume were turned up for just
an instance. Amplitude modulated carriers are
particularly sensitive to Gain Hits.
Dropouts
Dropouts are sudden losses of signal amplitude that are
greater than 12 db, and last longer than 4 mSec. They
cause more errors than any other type of transients.
Telephone company standards allow no more than 1
dropout for every 30 minute interval. Dropouts can be
heard on a voice conversation (similar to call waiting),
where the line goes dead for a 1/2 second. This is a
sufficient loss of signal for some digital transfer protocols
(such as SLIP), where the connection is lost and would
then have to be re-established.
Phase Hits
Phase Hits are either a sudden--and large--change in the
received signal phase (20 degrees), or a frequency that
lasts longer than 4 mSec. Phase Hits generally occur when
switching between Telcos, common carriers, or
transmitters. FSK and PSK are particularly sensitive to
Phase Hits. The data may be incorrect until the out-ofphase condition is rectified. The telephone company
standard allows no more than 8 phase hits in any 15
minute period.
The Telephone Company
The Telephone Network
The telephone network consists of your phone at home that
is connected (by the Local Loop) to the Central Office. The
Central Office is in turn connected to a Hierarchical Phone
Network. Worldwide, there are over 300 million
(300,000,000) telephones - 98% of them interconnected.
POTS - Plain Old Telephone Set
The POTS, or Plain Old Telephone Set, consists of these 5
sections:
i.Ringer Unit
ii.Hook Switch
iii.Dialer Unit
iv.Hybrid/Speech Network
v.Hand Set
POTS
The connection to the CO (Central Office) comprises only 2 wires:
Tip and Ring. This connection is called the "Local Loop."
The Local Loop
Tip & Ring
The Tip is +ve and colored green. The Ring is -ve and
colored Red. If you look at a phone jack in your house,
you will see that it is wired for 4 wires: Red, Green, Black
and Yellow. However, black and yellow are not normally
used.
The black and yellow wires can be used for a second
telephone line or they can be used for running a Network
Physical layer protocol called Phonenet (by Farralon).
Phonenet uses the black and yellow for Network
communications. It is for use with Appletalk, and is a
replacement for Localtalk. It runs at the Localtalk speed of
230 Kbps, reasonable for small networks.
Ringer Unit
Ringer Unit
The ringer is a device that alerts you to an incoming call: it
interprets the ringing voltage from the Central Office.
Originally, the ringer was a electromagnetic bell. Today,
though, most ringers are electronic devices.
The Central Office sends the following:
•a 90 to 120 VAC ringing voltage
•Frequency of 20 Hz
•Cadence for North America is 2 sec On/ 4 sec Off
The Hook Switch
Hook Switch
The hook switch is activated by lifting the handset off of the
cradle. The position of the hook switch determines whether the
telephone is waiting for a call, or is actively using the line. The
off-hook position informs the network of a request for use. The
on-hook position releases the use of the network.
The Dialer Unit
Dialer Unit
There are two types of Dialer Units:
Rotary and Touch Tone. Rotary is the
old "put your finger in the hole and
spin" type. The rotary dial operates by
toggling the Hook Switch on and off.
Touch Tone is the modern
method where 2 frequencies
per push button are sent.
Touch Tone is a trade name;
the correct name is DTMF
(Dual Tone Multi Frequency).
Hybrid/Speech Network
Hybrid/Speech Network
The Hybrid/Speech Network performs these functions:
•It converts the Tx/Rx 4 wires from the Handset to the 2 wires
for the Local Loop.
•It interfaces the signals from the Dialer Unit to the telephone
line.
•It provides auto line compensation for line length to keep the
volume constant.
The Handset
Handset
The Handset contains transducers that convert mechanical energy into
electrical energy. The microphone converts speech into electrical energy
while the diaphragm (or speaker) converts electrical signals into audible
signals.
Functions of a Telephone Set are shown below.
i.Request use of network from the CO (Central Office).
ii.Inform you of the network status: Dial-tone, Ringing, Busy, Fast
Busy (Talk Mail)
iii.Informs CO of desired number.
iv.Informs you when a call is incoming (phone rings).
v.Releases use of network when call is complete (hang-up)
vi.Transmit speech on network & receives speech from distant caller.
vii.Adjust power levels and compensates for line length
Local Loops
Local Loops
The Local Loop is the connection between the Central
Office and the home or business. Two wires (1 pair) are run
into every home. The pair does not go directly to the Central
Office. Instead, it goes to those big green boxes--that you
see on the street corners--called "Serving Area Interfaces"
(SIA) . Large multi-conductor bundles of wires then go from
there to the Central Office.
The Central Office
The Central Office (2)
The Central Office provides the following functions:
i.It supplies the battery voltage for the telephone system. The
On-hook voltage is 48 Vdc +/- 2V. Off-hook voltage is -6.5 Vdc.
ii.It supplies the Ringing Generator - 90 to 120 VAC, 20 Hz, 2
sec on/ 4 sec off
iii.It supplies the Busy signal (480 + 620 Hz, 0.5 sec On/ 0.5
sec Off), Dial Tone (350 + 440 Hz) and Fast Busy (480 + 620
Hz, 0.2 sec On/ 0.3 sec Off).
iv.It has the digital switching gear that determines if the
number is an Interoffice call (local) or an Intraoffice call (Toll long distance).
Central Office (3)
A Central Office can have up to 10,000 subscribers (for
example, 284-0000 to 284-9999). Most have 4,000 to 5,000
subscribers. The Central Office bases the loading
requirements on roughly 10% of the phones that will be in
use at any one time. However, the use of Internet dialup
access has drastically changed this statistic
Hierarchical Phone Networks
The PSTN (Public Switch Telephone Network) is divided into a
hierarchical network. Here are the 5 classes of switching
centers in North America:
Center Class
Description
Abbreviation
1
Regional Center
RC
2
Sectional Center
SC
3
Primary Center
PC
4
Toll Center
TC
4b
Toll Point
5
Central Office
TP
CO
Symbol
An Example
Hierarchical Structure
The Hierarchical portion is seen as follows:
Trunk
Long distance telephone cable
Toll Trunk
Connects CO (Central Office) to
TC (Toll Center)
Everything above TC (Toll
Center) and TC to TC
Between CO (Central Office)
Intertoll Trunk
Interoffice Trunk
Intraoffice Trunk
Call between 2 subscribers
within the same CO (284-7079
to 284-8181
Call Routing
Call routing:
1.Preferred route
2.Second choice
3.Third Choice
Call routing is determined by network engineering and physical location.
When all lines are idle, the call routing selects the preferred route. If the
preferred route is busy, then the call is routed to the second choice. Because
the second choice is routed through one toll center, the charge for the call is
greater than the preferred route. The third choice is used when the second
choice is busy. The third choice goes through 2 toll centers, and is the most
expensive route