When to say no to technology.
To understand the role of high technology in industrial security two-way communications, it's important to understand the basics of two-way equipment, frequency, and formats.
One of the simplest, most popular forms of communication is the pager. Pagers are a cost-effective method of contacting someone and leaving a message. Early pagers were simply beepers that signaled the owner to call the office. Later pagers allowed the owner to hear the message so a caller could give more information. Digital paging systems now allow the user to hear the beep and see the message, converted from analog to a digital readout on a display. The message can even be stored for future reference.
Paging equipment can be rented, leased, or purchased. A monthly service fee varies with usage. Pagers operate through a computerized system, usually owned by a second party. An industrial security department could set up its own paging system, but paging offers only one-way communication. The receiver does not enjoy the "talkback" communication a two-way system provides.
Radios offer two-way communication and come as portables, mobiles, and base stations. Portables, or handheld, radios are small, battery-operated transceivers. They are limited in power by battery life and size, which vary depending on the power and frequency used-usually six watts for low band, five watts for VHF, four watts for UHF, and two watts or less for 800-900 MHz (trunking and cellular).
Portable radios vary from shirt-pocket size to the traditional, larger size, about the shape of a brick. These radios can be crystal-controlled or synthesized. They can be as basic or sophisticated as you want, from one-watt, one-channel to five-watt, 200-channel radios.
Portable radios offer the advantage of going with you wherever you go. They can be shared by different shifts, and if a maintenance problem arises, the radio is easily sent in for repair.
The disadvantage of portable radios is that they are battery operated. To ensure continuous communications, users have to carry spare batteries. A standard 600 milliamp-hour battery in most radios provides about eight hours of service; an extended 1,000 milliamp-hour battery provides about 12 hours of service. Service life varies depending on radio design, features, temperatures, working conditions, etc.
Because of their power limitations due to battery size, portables are limited as to how far they can transmit. A good rule of thumb is about one mile per watt under full power and average circumstances. Antenna positions, terrain, and battery charge are major moderating factors.
Portables are more susceptible to blockage (dead spots where signals cannot be heard) by terrain, especially hills, tall buildings, and steel in buildings. Their short, flexible antennas do not always allow for maximum transmission and reception.
A repeater system (a combination receiver/transmitter) and proper base antenna position can help, but with a basic portable, it is important to know where your dead spots are and avoid them when transmitting. Sometimes, moving just a few feet or inches from a dead spot is enough. Always keep your antenna away from your body for maximum communication sensitivity.
Mobile radios, which are mounted on vehicles, offer the advantage of increased power and improved antenna position. Because they get their power from the vehicle, the power range stays constant, regardless of usage. Although blockage can still occur, the higher power and improved antenna position (usually on top of the vehicle) give mobiles a distinct advantage in range.
Like their portable cousins, mobile radios can be simple or sophisticated. Their disadvantage is that if security personnel need communications while away from their vehicle, they have to carry a portable radio. Special systems that combine portable flexibility with mobile power and a portable repeater are also available.
Base stations serve as the dispatch or communications center of the operation. They are typically high-power units that use an antenna mounted on a tower or the roof of a tall building to maximize portable and mobile range. Like mobile radios, base station power supplies remain constant.
Base stations can be simple, freestanding, one-channel radios or multichannel units. They can be built into sophisticated consoles combining the latest innovations for radio communication and video surveillance. Me main purpose of base stations is to provide central communication coordination, whether it's for one radio or a hundred.
When determining communication needs, frequency must also be considered. Business radios are based on FM frequencies in low band (30-50 MHz), VHF (148-174 MHz), UHF (450-520 MHz), and 800-900 MHz (806-821 MHz, 851-866 MHz, and 928-930 MHz).
Each frequency band has advantages and disadvantages. The frequency you use depends on a variety of considerations, including existing communication systems you may want to tie in with or for which you are licensed, availability and performance of product in that frequency band, and your location and radio usage. You or your system operator must be licensed by the FCC to operate on your transmitting frequencies.
Low band is a high-power system that allows long-distance communication. Because of its wavelength, it is not as susceptible to interference from buildings, trees, and other geographic obstacles. Because most radio users are in higher frequency bands, generally less interference occurs due to congestion in the low band.
Low band, however, is susceptible to electronic noise from such sources as spark plugs and atmospheric activity. Because of that frequency's high power requirements, low-band portable radios tend to be bigger and heavier to accommodate larger batteries. They are also limited in repeater usage.
VHF and UHF frequencies provide the maximum versatility at moderate prices. Given equal power ratings, VHF and UHF signals will not carry as far as low-band signals, but they are less susceptible to electronic interference and offer better quality. VHF and UHF are more susceptible to geographic interference.
VHF and UHF frequencies are adaptable to repeater usage. Equipment is readily available for these bands, and the size of portables can be considerably smaller. Feature for feature, UHF equipment costs are generally higher than VHF. Most public service users use VHF and UHF frequencies, so if mutual aid is important, these bands should be considered.
These are also the most heavily used business bands. Because of their popularity, users are less apt to have a clear frequency in VHF and UHF.
The newest frequency band to be opened for business use is 800-900MHz. This band provides an additional usable spectrum to reduce congestion in UHF and VHF.
The power output of radios in the 800-900 MHz range is kept to a minimum to reduce the risk to personal health due to possible RF radiation problems. Because of its low power output, it is used primarily for cellular and other trunked radio or community systems, which use repeater sites to boost and retransmit signals. Generally, 800900 MHz is more expensive than its low- and mid-band counterparts.
How your system is formatted also helps determine the type of equipment and frequency you choose. The three basic formats in two-way communications are simplex, half duplex, and full duplex.
Simplex, as its name suggests, is the most basic of the three and the one most people are familiar with. It uses a single frequency to both transmit and receive signals, regardless of how many portable, mobile, or base radios are used in the system. You cannot talk and listen at the same time. This system is easy to understand and operate since it does not require any extraordinary equipment.
Half duplex uses two frequencies, one to transmit and one to receive. Although it still does not allow for simultaneous transmission and reception, it allows you to make use of a repeater. This is important when your communication range is greater than the power of the radio will allow and for some special options that you may want to include. Using the repeater also increases the cost of your system or the shared system you will be using.
Full duplex systems operate like a telephone and allow simultaneous transmission and reception. Like the half duplex system, the full duplex system requires two frequencies, one to transmit and one to receive. The full duplex system, however, requires more sophisticated and expensive equipment.
If the congestion on the system becomes unbearable, the repeater system can't handle the range that needs to be covered, or special functions such as data transmission need to be performed, the more sophisticated new technology of trunking and cellular may be worth investigating. Trunking is a half duplex system designed to improve use of the frequency spectrum by allowing more users to share common frequencies. Trunking is based on the principle that an individual user accesses the frequency only a small percentage of the time. The rest of the time, instead of sitting idle, that frequency can be shared by other users who also access the frequency only a small percentage of the time.
Using pairs of frequencies and a sophisticated computerized repeater system, incoming transmissions are routed through repeaters and retransmitted on the first available pair of frequencies to another radio. If the system is busy, the computer calls the user back when a frequency opens. Calls can be made to the base station, other radios, or, with a telephone interconnection, land line telephones. With this system, to accomplish normal communications, all you have to do is talk. The computer makes the channel selection.
Besides improved frequency use, trunking offers more privacy between radio users on the system and, because of the repeater, increased range. The system is usually owned by a second party, so users save the FCC licensing expense and the high cost of the computerized system. Users may, however, have to pay a monthly fee in addition to air-time billings to be on the system. Radio equipment can be either leased or purchased. And there still may be times when users want to make a call and the system is busy.
Cellular operates on the same principle as a trunking system except that it is a full duplex system and is automatically tied into telephone land lines. If all the frequencies are busy, your call is put on hold until a frequency opens. Again, users may have to pay a monthly fee and are billed for air time. But, because of the prolific increase in cell sites and roaming capability, coverage and usable service area are greatly enhanced. This system is applicable if security personnel need access to an extended service area or telephone calls.
When it comes to designing a communication system for your security operation, ask for assistance. An authorized two-way radio dealer can help you set up a system to meet your needs. Shop around. Talk to different dealers.
The equipment that best serves your needs may not come from the biggest manufacturer or even just one manufacturer. And a system doesn't have to be complicated or high tech to be efficient. The adage keep it simple" is still the best rule to follow when it comes to a security communication system. n About the Author . . . Mark Thompson is advertising/public relations manager for Relm Communications in Indianapolis, IN. The company manufactures two-way radios.
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|Title Annotation:||two-way communication technology for security personnel|
|Date:||Mar 1, 1991|
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