The basic concept of communications theory is that an electronic signal can be modified so that at least two different states of that signal can be detected. The two states represent a zero or one, mark or space, on or off. As soon as two or more different states can be detected, the capability for moving information exists. This movement occurs within the electromagnetic spectrum. The electromagnetic spectrum defines the range of different types of electromagnetic radiation, from visible light, to gamma rays, to heat, to audio signals. This energy travels through space at or near the speed of light. The primary way in which the military communicates is over radio frequency (RF) waves, which form a portion or band of the spectrum.
Technically, communications is the process of converting data into a signal, a form of energy that occupies a slice of the electromagnetic spec- trum. Once converted, the information can be passed to a distant point. At its destination, the energy is changed back to its original form.
Where the slice falls in the RF band determines its transmission characteristics.
Frequency
Radio wave energy is identified by its frequency, or the amount of time it takes a wave to complete a sine wave cycle. Hertz, or cycles per second, measures a signal’s frequency. Each frequency has a discrete wavelength, or how far the wave travels in space within a given frequency. Fre- quency and wavelength are inversely propor- tional: the higher the frequency, the shorter the wavelength, and vice versa. This relationship is important because knowing where in the RF band a piece of communications equipment functions allows for an accurate prediction of the behavior of that equipment as well as the different proper- ties that influence to what extent a signal can sup- port the exchange of information. Frequency, more than any other characteristic, determines range; throughput, or ability to carry informa- tion; physical makeup, or size and configuration, of equipment; and susceptibility to interference that may be manmade, electrical, or atmospheric. The RF portion of the spectrum is divided into well defined frequency bands that correspond to a discrete frequency range. These bands are assigned unique designations, such as HF, VHF, UHF, SHF, and EHF. They have their own specific capa- bilities and limitations related to the characteris- tics of those frequency groupings. A typical SCR, for instance, transmits in the VHF range. Radios operating here can be lightweight, can use simple and relatively short wire antennas called whips, and can easily be transported by an individual. On the other hand, a typical multichannel satellite radio transmits in the SHF range, which requires complex and relatively bulky combinations of equipment that must be transported by vehicle.
Three types of electromagnetic propagation are common: ground wave, sky wave, and free space. At lower frequencies, radio ground waves travel great distances along the surface of the earth. Ground waves attenuate, or lose signal strength, as frequency increases. At higher frequencies, losses along the surface become so great that the ground wave is limited to short distances. Sky waves occur at medium to high frequencies, where reflection from the ionosphere permits radio com- munication over great distances. At frequencies above 30 MHz, these reflections are not depend- able. Communication above this band depends upon LOS and tropospheric scatter, also called troposcatter, equipment to reach beyond the hori- zon. Atmospheric interference, however, tends to degrade sky wave propagation. At EHF, for exam- ple, there may be wave attenuation caused by rain- fall or absorption by dust and water vapor.
Finally, as frequency increases, required transmit- ter output power decreases. Transmitter output power is important because it determines how much input power is required for the radio sys- tem to operate properly. A 5-watt UHF radio will transmit and receive for many hours on small dry- cell batteries. An HF radio with a 1-kW transmit power, conversely, requires a relatively high amperage constant power source, such as that provided by tactical generators or commercial power sources. Another consideration is that the higher the output power, the easier it is to find the transmitter using direction-finding techniques.
Signals and Encoding
There are two basic types of signals that are used to send information from one point to another. An analog signal is a continuously varying electro- magnetic wave, typically represented by a sine wave. A digital signal is a sequence of voltage pulses: a positive voltage level may represent a one and a negative voltage may represent a zero. Data, such as the human voice, e-mail, or files, can be electronically represented by the sequenc- ing of these voltage pulses.
Encoding
Encoding is the process by which analog or digi- tal data is impressed onto an analog or digital waveform for the purpose of transmission and utilization at a destination. In other words, encod- ing is how data is put onto a waveform by the originator so that it can be sent to a destination. At the destination, the waveform must be decoded in order for the data to be extracted from the waveform for use. Putting data onto a signal permits it to be transmitted at greater distances than it could go in its original form, such as voice versus phone. The signal allows data to be ampli- fied or manipulated at the received end to over- c o m e d i s t o r t i o n , s u c h a s i n f o r w a r d e r r o r correction. It also lets information move quicker and more efficiently, as in electronically transmit- ting a 4-page file vice reading it over a radio net.
Modulation
The impressing of analog or digital data onto an analog waveform is known as modulation. The most common application of modulation is apply- ing sound, such as the human voice, to a radio wave for transmission. The radio wave, onto which the data is impressed, is called a carrier. One of the wave characteristics of the carrier— frequency, amplitude, or phase—is varied to rep- resent the data to be transmitted. A modem (“modulator” and “demodulator”) converts sig- nals from one format to another.
Transmission
Transmission is the process of conveying a sig- nal from point to point along a path. This path, or the transmission medium, is either guided or unguided. Unguided transmission media can be either SCR or MCR.
Guided
Guided transmission media use physical conduc- tors, such as metallic wire, coaxial cable, and fiber optic cable, to guide signals along a specific path.
Two-wire or 4-wire field wire and multipair cable exhibit good transmission qualities over short dis- tances. Field wire generally is used for local dis- tribution systems to connect users to a central facility and to interconnect communications sites located at a single node or installation. Coaxial cable has excellent transmission qualities for sev- eral miles and is suitable for connecting major nodes as a short-hop substitute for multichannel radio. Optical fiber can transmit significantly more data than coaxial cable, is lightweight, and is almost impervious to the electromagnetic jam- ming and interference problems associated with metallic conductors.
Unguided
Unguided transmission media use electromag- netic waves that propagate through the atmo- sphere, but are not guided down a specific path. Radio systems provide this capability and oper- ate point-to-point when signals are transmitted and received between two locations. They broad- cast point-to-multipoint when signals serve a broader community of users with one station functioning as the originator or as net control. These signals can be categorized as either single- channel or multichannel.
Unguided SCR
Single channel radios typically operate at half- duplex, meaning that a user may transmit or receive at any given instant, but may not do both simultaneously. They primarily provide the abil- ity to exchange voice with a potentially broad range of users while on the move (OTM). It can also support the exchange of low bandwidth data, but not at the same time as voice. Typi- cally, SCRs, based on the portion of the RF band in which they operate, are smaller and easier to install than multichannel radios. Table 5-1, on page 5-4, discusses the employment consider- ations and requirements for each SCR fre- quency range.
Because of its great flexibility and quick installa- tion, SCR is the principal means of communica- tion for maneuvering and OTM units, and is normally the first means of communication estab- lished upon occupation of a site. SCRs have both positives (capabilities) and negatives (limitations): Capabilities
• Provides secure voice and limited data exchange, such as files and chat.
• Supports OTM communication.
• Installs and operates faster and easier than other means of communication and is more responsive.
• Spans great distances, overcoming physical obstacles.
Limitations
• Is susceptible to enemy EW and electrical, physical, atmospheric, and space weather interference.
• Has lower data throughput than multichannel radio systems.
• Operates at half-duplex—a user can only transmit or receive at any given moment, not both simultaneously.
Unguided MCR
All MCRs operate at full-duplex; information can be transmitted and received simultaneously. Unlike SCR, however, MCR provides multiple channels over a single pathway, accommodat- ing multiple users and services simultaneously. It is generally used for connecting different nodes within a network, such as a MEF CE with its MSCs.
Because of its high bandwidth capability, MCR systems operate in different portions of the elec- tromagnetic spectrum than SCR, using frequen- cies in the UHF, SHF, and EHF bands. Table 5-2, on page 5-7, discusses the employment consider- ations and requirements for each MCR frequency range. Additionally, MCR pathways are both ter- restrial- and space-based.
Table 5-1. Employment Considerations and
Requirements for SCR HF, VHF, UHF, and Satellite Signals.
Frequency Frequency Range Employment Considerations Requirements HF 2–9.9999 MHz Provides short-range, long-range, or
over-the-horizon, secure voice and limited data exchange.
Uses ground-wave propagation for short distances.
Uses sky-wave propagation for long distances and overcoming obstacles (usabillity of certain frequencies are dependent upon ionospheric condi- tions, transitions between day and night, seasonal changes, and solar activity).
Provides automatic link establish- ment equipment to maintain communication during adverse conditions.
Requires deliberate radio operator procedures to mitigate lower voice quality.
Represents hazards to personnel, depending on power output of antenna elements.
Requires antenna site selection to ensure proper antenna separation, remoting from COC, and, depending upon antenna type, large physical space.
Multiple, varied frequencies that span the HF portion of the electromag- netic spectrum.
Propagation analysis and prediction to determine optimum frequencies. COMSEC keys.
Data network interfaces. Proper site reconnaissance and selection.
Field expedient antenna planning and employment.
VHF 30–88 MHz Is the primary means of OTM communication.
Employs frequency hopping to miti- gate effects of enemy jamming. Provides short-range (radio LOS), secure voice and, compared with HF, improved voice quality and data exchange to about 15 miles, depend- ing upon radio type, power output, and environmental conditions. Is susceptible to terrain, particularly foliage.
Offers increased range and reliability through use of retransmission sites. Requires antenna site selection to ensure proper antenna separation and remoting from the COC.
Hop-set data for frequency hopping, ordinarily generated by MSC spec- trum managers and net
identifications.
Propagation analysis to determine LOS coverage as well as locations for retransmission sites.
COMSEC keying material. Data network interfaces. Proper site reconnaissance and selection, particularly for antenna stand-off distance from terrain and obstacles.
UHF 225–400 MHz Is the primary means of ground-to-air communication.
Employs frequency hopping to miti- gate effects of enemy jamming. Provides short-range, secure voice and, compared with HF, improved voice quality and data exchange. Offers critical LOS range with both transmitting and receiving antennas having a clear LOS.
Is susceptible to terrain and obstacles.
Hop-set data for frequency hopping. COMSEC keying material. Proper site reconnaissance and selection.
UHF Tactical Satellite (TACSAT) 225–400 MHz Provides long-range, over-the-hori- zon, secure voice and data exchange.
Uses narrowband or wideband chan- nels, which impact data rate. More narrowband and fewer wideband channels are available. Uses demand assigned multiple access equipment and allows sharing of available channels with multiple users.
Has a limited number of satellites and channels, resulting in competi- tion for access and user prioritization across Services and combatant commands.
Reduces channel congestion, noise, and network saturation impact data rate.
Has critical LOS range with reliability affected by weather, terrain, and location. Shallow look angles, or the angle at which an antenna is posi- tioned above the horizon in order to "see" the satellite, indicate terminal placement outside of or near the edge of the satellite's footprint (occurs as terminals move closer to the earth's poles).
Extensive coordination and planning lead times to ensure access to satel- lite channels.
COMSEC keying material. Look angle analysis. Data network interfaces. Proper site reconnaissance and selection.
Commercial Satellite Terminals Provides worldwide secure voice, with appropriate COMSEC modules, and improved data exchange. Provides a first-in, redundant, or amplifying capability to tactical SCRs. Has a potentially significant cost. Has a limited number of terminals.
COMSEC keying material. Data network interfaces. Contractual vehicle. Table 5-1. Employment Considerations and
Requirements for SCR HF, VHF, UHF, and Satellite Signals. (Continued)
The demands of operating in these frequency bands as well as performing multiplexing func- tions require complex and relatively large pieces of equipment. The MCR systems, therefore, have considerably more logistical and operating requirements than SCR systems. MCRs have both positives (capabilities) and negatives (limitations): Capabilities
• Provides simultaneous access to secure voice, video, and data.
• Has high bandwidth potential. • Interfaces with the GIG.
• Extends high bandwidth connectivity.
• Spans great distances and overcomes physical obstacles.
Limitations
• Is susceptible to enemy EW and to electrical, physical, atmospheric, and space weather interference.
• Does not support OTM communication.
• Is logistically more intense than SCR and requires generator support, mobility, and more operators.
• Requires longer installation times and more coordination.
• Uses critical low-density equipment items. • Uses satellite resources and loss of link drops
all circuits.
Multiplexing
Multiplexing is the process of combining two or more discrete signals into a single, higher-capac- ity signal. A telephone circuit and a data circuit, for instance, are combined and then transmitted over the same path; this combination helps reduce the overall number of different transmission sys- tems required. At the receiving end, the signal is demultiplexed and the individual circuits are routed to terminating equipment, such as a tele- phone switchboard or a data router. Multiplexed signals can be transmitted over guided (cable) or unguided (MCR) media.
Synchronization and Timing
Synchronization
Synchronization is a networking term that applies to a state where data or information arrives and de- parts from connected devices at coordinated times so that data is neither lost nor jumbled. Synchroni- zation is critical with multiplexing and MCR, where high volumes of information are carried to multiple nodes. Communicating devices must be synchronized to know when to receive or transmit information and on which channel or path.
Timing
Timing is the glue that holds a communications network together. Timing ensures that exchanged information is synchronized across the different layers of transmissions and multiplexing. Timing sources, which are typically based on the decay of radioactive elements as they provide a high degree of accuracy, are either embedded into cer- tain types of equipment such as an MCR. They can also serve as stand-alone devices that inter- face with a network. The most reliable timing scheme employs separate timing sources at each node in a network.
Switching
Users must be capable of communicating with any other user on demand. Because it is impossi- ble to have every user linked directly to every other user, a different means of communicating— of connecting users to one another—is required. Switching provides the ability to connect many users and their terminal devices in a way that per- mits on-demand exchange with other users and terminal devices without having to link them individually. Switching provides the means by which traffic is routed through a communications network. From a tactical perspective, there are two types of switching: circuit and packet.
Table 5-2. Employment Considerations and Requirements for MCR UHF, SHF, and EHF Signals.
Frequency Frequency Range Employment Considerations Requirements UHF (terrestrial) 300–3000 MHz Provides medium bandwidth
connectivity.
Has LOS range to 35 miles depend- ing on terrain, antenna, and power output. Can be extended using repeater systems.
Logistical support. COMSEC keying material. Network interfaces.
Proper site reconnaissance and selection, particularly for antenna stand-off distance from terrain and obstacles.
SHF (terrestrial) 3–8 GHz Provides high bandwidth connectivity.
Offers range to 100 miles dependent upon mode of operation, terrain, antennas, and power output. Can overcome physical obstacles, depending upon mode of operation. Power output can represent hazards to personnel and ammunition and interfere with other types of equipment.
Logistical support. COMSEC keying material. Network interfaces.
Proper site reconnaissance and selection.
SHF (SATCOM) 3–30 GHz Provides high bandwidth connectivity.
Provides LOS operation with reliabil- ity affected by weather, terrain, and location.
Has various configurations such as point-to-point, hub-spoke, or mesh. Uses critical low density equipment and is expensive.
Has limited numbers of satellites and channels, resulting in competition for access and user prioritization across Services and combatant commands.
Extensive coordination and planning lead times to ensure access to satel- lite channels and termination of services.
Logistical support. COMSEC keying material. Look angle analysis.
Proper site reconnaissance and selection.
EHF (SATCOM) 30–300 GHz Provides high bandwidth connectivity.
Is jam-resistant with low probability of interception.
Operates in a relatively noncrowded portion of the electromagnetic spectrum.
Is more susceptible to effects of weather and atmospheric conditions than UHF or SHF.
Extensive coordination and planning lead times to ensure access to satel- lite channels and termination of services.
Logistical support. COMSEC keying material. Look angle analysis. Network interfaces.
Proper site reconnaissance and selection.
Circuit Switching
Circuit switching is the process of interconnect- ing a specific circuit to provide a direct connec- tion between calling and called stations, and is historically used for telephone networks. With this type of switching, a dedicated path is estab- lished whenever a call is initiated. That path remains fixed for the duration of the connection. This constant connection ensures a degree of reli-