COMP 421 /CMPET 401COMP 421 /CMPET 401

COMMUNICATIONS and NETWORKING

Chapter 3 (Continued)

Data Transmission

HistoryHistory

Although electricity has been known toexist for centuries, experiments into its friendlyuse did not begin until the 1700s when scientistssuch as Volta, Ampere, and Watt exploredways to harness its potential. In the 19th century,other scientists used electricity to inventthe telegraph, telephone, and radio. With theadvent of Marconi’s transoceanic wireless andthe inauguration of radiotelegraph service, the20th century saw dramatic breakthroughs incommunications 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, relieson parts of the electromagnetic spectrum tocarry information from origin to destination.

Global Network HierarchyGlobal Network Hierarchy

Transmission MediumsTransmission Mediums

BandwidthBandwidth

Width of the spectrum of frequencies that can be transmitted– if spectrum=300 to 3400Hz,

bandwidth=3100HzGreater bandwidth leads to greater costsLimited bandwidth leads to distortionAnalog measured in Hertz Digital measured in baud or Bps

BandwidthBandwidth

The most common means for measuring path/circuit/channel size is describing the "range" (bandwidth) of radio frequency (RF) spectrum necessary tocarry the information assigned to a particularpath, channel, circuit, etc. The wider the path(the larger the bandwidth), the greater its capacity.

Bandwidth is expressed in hertz (Hz) orcycles per second (CPS) of radio frequency.One Hz equals one CPS. The capacity of pathsthat carry digital data is usually expressed inkilobits or megabits per second (kbps/Mbps) asa more meaningful measure of data throughputcapacity.

Analog and Digital Data TransmissionAnalog and Digital Data Transmission

Data – Entities that convey meaning

Signals– Electric or electromagnetic representations of

dataTransmission

– Communication of data by propagation and processing of signals

Bit rate and Baud rateBit 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

SwitchingSwitching

Switching is the means by which traffic isrouted through a communications or system network.Switches may be manual (operator assisted)or automatic; they may serve local (in acity or on a military base) subscribers or performarea network (tandem [many switches connectedto one another]) functions.

An electrical path established betweenterminals, switches, and/or transmission systemsis commonly referred to as a line, circuit,or channel. The "size" of the path is importantin determining the full capability or capacity.

Switching SystemsSwitching Systems

Electronic Switching Systems: A switch using solid-state switching devices and computer software that provides preprogrammed instructions to accomplish theswitching of calls.

Digital Switches: An electronic switching system that processes all signals to beswitched into a digital mode. The circuit switch can also be used toroute 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 forwardas if it were a telephone call.

Switching TypesSwitching TypesThere 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.

SwitchingSwitching

Circuit SwitchesCircuit 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 SwitchingCircuit Switching

Every time a telephone is used, a circuit switch routes the call. The switching center serves as the focal point for the interconnectionof 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 "subscriberloop." When the caller lifts the handset, a signal sent to the end office indicates a request for service. The end office switch placesa 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 subscriberbeing 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 SwitchingCircuit 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 iscompleted by electrical/mechanical relays underthe direction of a computer-like stored program.

Message SwitchingMessage 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 forwardmessage 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 SwitchingPacket Switching

Packet SwitchingThe packet switched network is used to route digital data traffic, including electronic mail (E-mail). In packet switched networks, thesubscriber 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 deliveryto the addressee.

Packet Switched message FlowPacket Switched message Flow

Packet SwitchingPacket Switching

Packet switching requires highly structured protocols to maintain network status and control of the packets. The preamble to eachpacket 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 pointswithin 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-to-computer exchanges, interactive queries to a computer, andbatch processing, as well as processing narrative traffic, such as E-mail. However, since this technology does not switch whole messages, it is an uneconomical method to use for multi-addressedtraffic.

ModulationModulation

Frequency Division MultiplexingFrequency Division Multiplexing

The oldest form of multiplexing, FDM,divides a circuit into several smaller channelsby frequency for simultaneous transmission.FDM, which is analog only, allows a user tocluster many terminals at a given location andshare the same transmission path. Becausethe bandwidth can be divided into just so manyparts, the number of terminals supported islimited. The speed or transmission rate of eachchannel is reduced due to narrower channelbandwidth. Thus, digital signals, such as data,must first be converted to analog.

FDMFDM

For example, when 12 individual 4-kHzanalog voice channels are fed to an FDM multiplexer(channel bank), each channel is assigneda frequency slot until all 12 channels areallocated. The channel bank output, a composite48-kHz analog signal, is sent to the receiverby some transmission path where the reverseprocess (demultiplexing) occurs, restoring theoriginal 12 channels. FDM can be used for

voice, teletypewriter, analog data, and facsimile.

MultiplexingMultiplexing

Time Division MultiplexingTime 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.

TDMTDM

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 capacitydata 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 CharacteristicsDigital Multi-channel Characteristics

TerminalsTerminalsTerminals are the most recognizablecommunications components. A telephone,radio, facsimile, computer, television, and teletypewriterare all examples of terminals used totransmit (send) and receive information. Information,often called traffic, can take the form ofvoice, data, message, video, or other means.Traffic may be secure (encrypted/covered) ornonsecure (clear). Radios and telephones arethe common terminals associated with voicecommunications. Facsimiles transmit and receivemaps, photographs, sketches, and printedor handwritten text. Teletypewriters or printersattached to computers are used for messagesand cables. This is often referred to as "recordtraffic" because the printer produces a "hardcopy" record of what is received. Data terminalsand computers transmit and receive binarydata, while video terminals communicate imageryand sound.

Inter-Relationship of Terminals, Inter-Relationship of Terminals, Switches and Switching FacilitiesSwitches and Switching Facilities

Data RateData Rate

The speed of transmission of digital datais reflected as a "data rate." As data rateincreases, so does the bandwidth of the path,channel, or circuit carrying the data stream..

A single 4-kHz-wide VF channel can alsobe subdivided by bandwidth to accommodatemodes other than voice. For example, one VFchannel can be multiplexed to commonly provideup to 16 teletypewriter (TTY) channels ofabout 200 Hz each.

Voice CircuitsVoice Circuits

A typical voice conversation transmittedover a standard telephone line or circuit requiresa radio frequency of sufficient bandwidthto handle the range of voice variations (calledmodulations) needed to convey information.This voice frequency (VF) range is approximately3-4 kHz wide (1000 hertz = kHz) and isthe standard for defining a single channel or"narrowband" circuit.

Circuits established between switchesare called trunks. Trunks ride transmissionsystems that are normally equipped with a multi-channelcapability and employ modems. Theyuse various protocols (rules and codes) requiredfor processing traffic in a switched network.

Duplex CircuitsDuplex CircuitsCircuits may be established to provide aone-way, one-way reversible, or simultaneoustwo-way traffic capability, depending upon userneed and the availability of assets.

The one-way reversible circuit is commonly referred toas "half duplex," meaning that traffic can bepassed in either direction but in only one direction at a time.

A simultaneous two-way path isreferred to as "full duplex"--traffic can be passedin either or both directions at the same time.

The simple one-way-only path is often referredto as a "receive-only" or "transmit-only" circuit.

ModemModemA modem (a contraction of "modulator"and "demodulator") is a device that convertsincoming and outgoing electronic signals fromone form to another. For example, the digitaloutput of a home computer may be fed to amodem that converts the digital bit stream (digitalsignal) into a series of audio tones (analogsignals) transmitted over an analog-capablestandard-grade public telephone circuit. Thehome computer modem may also convert aninbound analog signal from a telephone line toa digital signal that can be used by the computer..

Analog and Digital TransmissionAnalog signals are represented as continuouswave-like (sine wave) signals, such asthose electrical signals generated by a telephonekeyed to a human voice. The analogsignal varies by frequency in cycles per second,or hertz, to represent different voice sounds.Analog voice signals are transmitted overa channel in one of two ways: at their originalfrequency as a baseband signal or modulatedonto another (carrier) signal and transmitted atthe carrier frequency. Voice is transmitted in theaudio frequency range of about 300 to 3400 Hz,or a voice signal bandwidth of 3100 Hz. Invariably,long-distance calls will be modulated to ahigher frequency for transmission to achievegreater efficiency and lower cost.

Digital SignalsDigital Signals

By contrast, digital signals are discreteand consist of two possible states: the presenceor absence of an electrical signal (on or off), or twodifferent electrical signal levels. Communicationsbetween computers use a digital form,where information is conveyed as binary digitsor bits (1s or 0s). For digital communications, the bandwidthdescribes the amount of data that can betransmitted on a channel over time in bits persecond (bps). High-speed data networks mightoffer bandwidths of 10 to 50 Mbps, while telephonesystems used for digital transmissionoffer a bandwidth of 1.544 Mbps, the standardcommercial T-1 carrier rates.

DigitizationDigitization

Channel CapacityChannel 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 to76.8 kbps. To conserve channel capacity, the multiplexingof 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. Thisconversion 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 AnalogDigitized Analog

There is a bandwidth penalty for digitizinganalog signals. For example, in a voicetransmission, the bandwidth requirement increases16-fold, in comparison to music that is20:1 digital versus analog ratio. By convertingsignals from a digital telephone to analog priorto transmission, however, the output can besent over a standard 4 kHz voice frequencycircuit versus a wideband path.

Transmission Rate Vs. Information Rate

In general, the greater the informationrate (or resolution), the larger the transmissionbandwidth. Required transmission bandwidthvaries directly with desired information speed.A single voice conversation or teletype channelcan easily be accommodated by a nominal 3-kHz-wide voice frequency channel. However,beyond voice, required bandwidth increasesprecipitously. Digital facsimile requires 7.5 to50 kHz, high-speed data more than 100 kHz,and full-motion video 4 MHz.

Information RateInformation Rate

In theory, information rate can equal butnever exceed the transmission rate. In practice,

the actual information rate measured inwpm or bps is almost always slower than thetransmission rate, and is frequently significantlyso. There are several reasons for this. Manytimes, information rate is dictated by the speedof the user’s terminal device, which may bemuch slower than the transmission “pipe.” Thiswill be the case with slow-speed teletypewritersand with the human voice.

Bits and BytesBits and BytesA Bitis 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 Byteis 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.

Bitrate 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 ratesTransmission ratesMany factors determine transmission andinformation rates. The transmission rate of anetwork or link includes the trunk speed in bitsper second of the trunk and switching facilitiesor both. The information rate (traffic throughput),on the other hand, takes into considerationthe entire path from one user terminal (A) toanother (B), and reflects the speed at which thereceive end can accept information from thetransmit end as dictated by the slowest terminalin the loop. For example, if terminal A transmitsdata at 19.2 kbps to terminal B, which canhandle traffic input at only 4.8 kbps, the resultantinformation rate is 4.8 kbps. The differenceis stored/buffered by the system to accommodatethe lower rate of the receive terminal.

Factors Affecting Information Rate

Channel CapacityChannel CapacityThe capacity of a channel may be describedas the maximum rate at which informationcan be sent over the channel without error.Bandwidth is the frequency range of a givenelectrical path or circuit. For data transmissionpurposes, channel capacity is measured in bps.The rate at which data may be transmitted isproportional to the channel bandwidth. Overall,data channel capacity is a function of volume,rate or speed, and the quality of the transmissionpath. These factors are important to theplanner because they dictate how well anygiven transmission link can pass intelligencedata from one point to another.

Relationship of Terminals to Bandwidth and Information Rate

Noise Noise

Transmission SpeedsTransmission Speeds

Transmission SpeedsTransmission Speeds

Different terminal types have varioustransmission speed requirements. Standard4-kHz narrowband telephone channels canhandle data transmission speeds up to 9.6kbps. Data sent faster than 9.6 kbps over thesechannels becomes unintelligible. Consequently,high-speed modems operating at speeds fromover 9.6 kbps to 56 kbps or higher will not workusing this type of transmission channel.

Transmission SpeedsTransmission Speeds

The maximum data transmission speedsachieved over a given bandwidth are considerablylower than the theoretical maximum. Thesespeeds fall into the shaded area on the abovegraphic. The dotted lines are theoretical maximumspeeds in the presence of noise, and arecalculated for an error rate of one error bit inevery 10,000 bits transmitted. One way toincrease the capacity of a channel is to raise thesignal-to-noise ratio. While some types of noisecan partially be controlled, the level of randomnoise is determined by natural phenomena,which are uncontrollable. There is a level belowwhich noise cannot be suppressed.

SpectrumSpectrum

Electromagnetic SpectrumElectromagnetic SpectrumFrom 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 exploiteven higher frequencies up to about 100terahertz (100 trillion hertz, or THz) for laserapplications. As a comparison, the typical humanvoice 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 beyondlaser range. Each frequency has a discrete length of its radiated signal (wavelength). The frequency group from about 30 Hz to near 300GHz is generally referred to as the radio frequency band. Electromagnetic energy within this band is referred to as radio frequency, radiowaves, or radio for short.

RF SpectrumRF Spectrum

In another common frame of reference,the radio frequency spectrum is divided intodiscrete frequency bands. These bands are assigned unique designations (ELF, HF, SHF,etc.) that correspond to a particular frequencyrange. (For example, the high frequency [HF]band includes frequencies from 3 million to 30million hertz [expressed as megahertz, or MHz].)

FrequencyFrequency

Each band or group of frequencies possessesdifferent properties that affect useful signalingrange, ability to carry information or "throughput,"physical attributes of supporting equipment,and resistance to natural phenomenaand human-caused interference. Each frequencyband or slice has its own peculiar capabilitiesand limitations, and each has its advantagesand disadvantages.

Frequency has a significant impacton antenna size. As frequency increases, wavelengthdecreases proportionally (f=1/W where "f" refers to frequency and "W" refers to wavelength). VLF is propagated by ground waves

FrequencyFrequencySince the optimum length of a transmitterantenna equals one wavelength of transmittedfrequency, the higher the frequency theshorter the antenna. While there are methodsto electrically shorten and lengthen antennas,the optimum antenna is physically one wavelengthlong. For example, an ELF antenna ofexactly one wavelength must be hundreds ofmiles long, whereas an ultra high frequency(UHF) radio antenna is only several inches inlength.

FrequencyFrequency

A typical military battlefield tactical singlechannel 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.

FrequencyFrequency

In short, frequency determines how waves propagate.Three types of electromagnetic propagationare common: ground wave, sky wave,and free space propagation. At lower frequencies(ELF and VLF), radio ground waves travelgreat distances along the surface of the earth.Ground waves experience increasing loss (attenuation,or loss of signal strength) as frequency increases.

FrequencyFrequencyVLF 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.

FrequencyFrequency

FrequencyFrequency

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 waveattenuation (signal degradation) due to rainfall and absorption by dust and water vapor. Finally, as frequency increases, requiredtransmitter 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.

FrequencyFrequency

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 tacticalgenerators 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.

ELFELF

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.

VLFVLFVery 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. Theseaircraft 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 every12 seconds, although slightly higher data

LFLFLow 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 50-100 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 cantransmit 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.

MFMF

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 distancecommunications, radio navigation, and AM broadcasting.

Throughput/Data Rates: Channel availability of MF is greater than for LF andlower 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 supportlow-capacity multi-channel circuits for both voice and teletypewriter, the latter generally limited to 75 baud (100 wpm).

HFHF

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 fordistances 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 TTYmodes 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 RangeAltitude Extends the Range

VHFVHFVery 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 beingretransmitted. VHF usage includes short-range FM combat net radio, radio navigation, wideband LOS multi-channel systems, and television broadcasting. Depending upon use, range, andnumber 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 increaselink 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

UHFUHFUltra High Frequency (UHF) Range: The frequency band for UHF is from 300 to 3000 MHz. Principal methods ofUHF 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 canreach 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.

SHFSHF

Super High Frequency (SHF) Range: The SHF band is from 3 to 30 GHz and is used principally for high data ratemulti-channel LOS, troposcatter, and satellite systems. A portion of this band, the frequency range from about 1000 MHz to just above 1000GHz, comprises the microwave region. LOS communications equipment operating in this frequency range is often referred to as microwavesystems. 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 LOSmicrowave 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 microwaveradio relay systems that provide reliable, high-capacity, long-distance communications.

EHFEHFExtremely 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 ormulti-channel mode of operation. The extensivebandwidths available in the EHF band will permit many hundreds of channels per link. With antijamcapabilities activated, however, throughput capacities will be reduced significantly.

LaserLaser

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

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

CrosstalkCrosstalk 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 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)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 ShiftFrequency 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 DistortionNonlinear 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 PhaseJitter: Amplitude and Phase

Here are the 2 types of Jitter: a.Amplitude Jitterb.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 PhaseJitter: 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 Transients: Impulse Noise, Gain Hits, Dropouts & Phase HitsHits, 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 Noiseii.Gain Hitsiii.Dropoutsiv.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 HitsGain 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.

                                                             

DropoutsDropouts 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 HitsPhase 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-of-phase condition is rectified. The telephone company standard allows no more than 8 phase hits in any 15 minute period.

                                                         

The Telephone CompanyThe Telephone Company

The Telephone NetworkThe Telephone NetworkThe 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 Unitii.Hook Switchiii.Dialer Unitiv.Hybrid/Speech Networkv.Hand Set

POTSPOTS

The connection to the CO (Central Office) comprises only 2 wires: Tip and Ring. This connection is called the "Local Loop."

The Local LoopThe Local Loop

Tip & RingTip & RingThe 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 UnitRinger 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 SwitchThe 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 UnitThe 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 NetworkHybrid/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 HandsetThe HandsetHandset 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 LoopsLocal 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 OfficeThe Central Office

The Central Office (2)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)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 Symbol

1 Regional Center RC

2 Sectional Center SC

3 Primary Center PC

4 Toll Center TC

4b Toll Point TP

5 Central Office CO

An ExampleAn Example

Hierarchical Structure Hierarchical Structure

The Hierarchical portion is seen as follows:

Trunk Long distance telephone cable

Toll Trunk Connects CO (Central Office) to TC (Toll Center)

Intertoll Trunk Everything above TC (Toll Center) and TC to TC

Interoffice Trunk Between CO (Central Office)

Intraoffice Trunk Call between 2 subscribers within the same CO (284-7079 to 284-8181

Call RoutingCall Routing

Call routing: 1.Preferred route2.Second choice3.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