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any Of the 20 can go to the center merely by picking up the phone but to talk to another station from that they must first go to the center and ask to be connected. The center switchboard can then merely cross-patch it with a conventional cable plug-in. Such a system can then also provide for a party line conversation merely by asking the center to connect them with the number of stations desired. This system is cheaper than the hot line system and has the advantage that one has only to pick up a phone in order to go into central and does not. have to dial. At the same time, you do not have to have a constant monitor on a speaker as the center will ring you merely by plugging in and you pick up the phone and are in contact. This system is cheaper than the hot line system. Restricted Lines A restricted phone system is simply an unlisted number type system. Only these with a need to know have the numbers of the phones and in each case you must dial. The advantage of the restricted system is that you ..n prevent unauthorized parties from tying up the phone circuit. It is cheaper than either the hot line system or the dedicated system. A typical example would be the installation of an unlisted phone with a distinctive color in the | emergency deparment with only the members of the EMS system having the number It has a disadvantage that you cannot patch a number of phones together into a party line. 36
* Radio Stations Base Stations Base stations are transmitters and receivers located at fixed sites such as hospitals, dispatching centers, and the like. Often the base trans- mitter and its antenna tower are located remotely from the point of their control. For example, a dispatcher may be located in a communications center with his control console connected by landlines (wire) to a transmitter placed on a distant hill. Mobile and Portable Stations Mobile stations are transmitters and receivers intended for use while in motion (e.g. in an ambulance). They may, of course, also be used when the carrying vehicle is stopped. Portable stations can be hand-carried, for instance, from an ambulance to the side of a patient. Repeater Stations Repeater stations are stations, either fixed or mobile, that relay a transmitted signal, generally from a low-powered, nearby transmitter to a distant receiver point. Squelch Systems Audio Squelch and Falsing "" To the listener of a receiver used intermittently (e.g., an ambulance attendant monitoring his dispatching radio), the presence of continual static and background noise between legitimate transmissions can be very annoying. To overcome this problem, an "audio" squelch circuit may be employed in the For further information, see Appendixes A and B which present the basic principles of radio communication and role of the Federal Communications Commission in regulating radio use, respectively. ~ 97.
receiver. This circuit, in effect, keeps the audio (sound) circuits of the receiver turned off when no transmissions on the channel to which the receiver is tuned are taking place. It also automatically turns on the receiver's audio circuit instantaneously when a transmission begins and keeps it on throughout the transmission. The system operates rapidly enough that no loss of voice information occurs at the beginning of the transmission. However, adjacent channel transmissions (15 or 30 kHz either side of the desired frequency) can also actuate a receiver's squelch circuit and turn the audio circuit on when, in fact, no desired transmission is taking place. This is known as "falsing." Tone-Coded Squelch The simple audio squelch circuit responds to any transmissions received, whether or not it is of interest to a particular listener. However, within a particular service area, a single radio frequency may be shared by a number of users. If each receiver responded to every transmission made, most of which were of no interest to the user of one particular receiver (e.g., a dispatcher communicating with a fleet of ambulances), the continual "chatter" x on the channel could be of considerable "botherance." The ''tone-coded" squelch circuit was developed to eliminate botherance. The tone-coded squelch circuit in the receiver is activated by a particular low frequency audio tone incorporated in the transmitted signal. If the Botherance is defined as the effect of undesired signals when no desired signal is being received--undesired signals received simultaneously with desired signals constitute "co-channel" interference. 98
signal includes the correct tone frequency for a given receiver, that receiver's audio circuits are turned on permitting the transmission to be heard. About 30 different audio tones can be incorporated into the standard bandwidth of the VHF channels used in mobile radio services. Thus, with proper equipment, one transmitter can selectively call each of thirty groups of one or more receivers although all are tuned to the same frequency. Digital-Encoded or Dial-Encoded Squelch Another technique for the same purpose employs a train of coded pulses in the transmitted signal to activate the selected receiver which contains special decoding circuitry. This digital system is usually controlled at the transmitter by a digital encoder which resembles a standard telephone dial, and is called a "digital-encoded" or "dial-encoded" squelch system. Since two or three numbers are usually dialed, far more than 30 separate receiver "addresses" can be accommodated on a digitally-encoded system. ~ Suah squelch circuits have several trade names such as "Cia... 21 Guard" and "Private Line." It must be noted, however, that they do not provide a private line nor do they guard any channel. While permitting a transmitter to select only certain receivers, thus reducing 'botherance," the fact remains that only one transmission can be made at a time. In this respect, tone-coded and digital squelch systems operate much as a "party line" in rural telephone systems. In fact, improper use of squelch systems can increase interference. Because his tone-encoded or dial-encoded squelch has muted the receiver, an operator may assume he has a clear channel. If he proceeds to transmit without first listening (with the squelch "off"') to determine that the channel is clear, he can be guilty of creating co-channel interference. 99
Transmission Modes Simplex Systems Radio systems having more than âvie transmitter operate in simplex, half-duplex, or full-duplex modes. In a simplex system, all transmitters and receivers are tuned to one frequency. Only one transmission can occur at any given time, but all stations can communicate with each âother (Fig. la). * Half-Duplex Systems The half-duplex system is generally used, as in dispatching, for a base station communicating with a group of mobile stations; the base station transmits on one frequency and all of the mobile stations on a close but different frequency (Fig. 1b). As in a simplex system, the transmitter- receiver sets (transceivers) are so designed in the half-duplex system that lifting a microphone from its cradle (or pressing a "talk" button) disables the receiving portion of the calling transceiver and activates the squelch circuit of the called receiver(s). Thus, although two frequencies are used, only one transmission (either to or from a base station) can occur at a time because the calling receiver is muted. Further, in the half-duplex system mobile units cannot communicate with each other. It has the advantage over a simplex system of reducing "botherance" since the mobile units do not hear each other's transmissions. It also has value in crowded urban areas where many fleets of mobile users (e.g., police, taxi, ambulance, public services) are all using crowded VHF spectrum space. * Sometimes called a "two-frequency simplex" system. , 100
BASE STATION MOBILE UNIT Transmitter (155.340 âIST Receiver (155.340 MHz) Receiver (155.340 MHz) Transmitter (155.340 MHz) (Using an example in the VHF High Band) Figure la - SIMPLEX TRANSMISSION (Only one transmission at a time) BASE STATION . MOBILE UNIT Transmitter (155.340 MHz) r<\? | __| Receiver (155.340 MHz) Receiver â | Transmitter (155.400 MHz)j . Figure lb - HALF-DUPLEX TRANSMISSION (Only one transmission at a time) BASE STATION MOBILE UNIT Transmitter 460.500 MHz) LBS Transmitter (465.500 MHz) Receiver (465.500 MHz) Receiver (460.500 MHz) (Using an example in the UHF Band assigned for the dispatch of telemetry-equipped units) Figure lc - FULL DUPLEX TRANSMISSION (Simultaneous two-way transmission) 101
Full-Duplex Systems In the full-duplex mode (Fig. lc), transmitters and receivers at a given location operate on separate frequencies spaced far enough apart that mutual interference does not result. As opposed to the simplex or half-duplex systems, the full-duplex mode permits both the transmitter and the receiver at a station to operate simultaneously, thus allowing simul- taneous transmissions on both frequencies. In general, it is the costliest * of the three types. Telemetry The telemetry of physiological signals or patient data involves the transmission of such information over a radio link followed by a decoding and presentation (e.g., on an oscilloscope or on a paper tape) at the re- ceiving site. This is accomplished by modulating the radio signal with an analog (continuous) waveform (e.g. electrocardiogram) or by using pulsed » digital signals which can be used, for example, to drive remote teletypewriters. Paging - Other uses of non-voice radio include signaling functions such as those used in paging systems and highway call boxes. In paging, the receiver is a small portable unit selectively addressed by a tone-coded or digital-encoded squelch system; the simplest merely emits an audible tone to notify the wearer of the page. More sophisticated paging systems allow one-way voice communication (from the paging center to the person paged); others incorporate A complete discussion of the three systems, their interference characteristics, and methods for choosing one in any intended application are given in Report of the Advisory Committee for the Land Mobile Radio Services, Federal Communications Commission, Vol. 2, Part 1, U.S. Government Printing Office, Washington, D.C. 20402; pp. 195-279. 102
a signalling transmitter in the paging device to allow the recipient of the page to immediately acknowledge receipt by radio. More generally, the page is acknowledged by a call-back on a telephone or by the paged person reporting to a specific location. Call Boxes Non-voice call boxes (for example, those deployed along some highways) cause a digital pulse-coded signal to be transmitted to a central receiving site. Equipment at the center decodes the pulses to identify the specific call box (thus determining location). In some systems, the nature of the emergency (e.g. fire, polic¢, medical) is also incorporated into the coded pulses; the call boxes having several pushbuttons so marked.. Patching Patching is a method whereby a radio and a telephone link, two radio links operating on different frequencies or two telephones are tied together at a central location to permit direct communication between the two endpoints. Telephone-Telephone Patching Telephone-telephone patching is accomplished in the same way that an Operator on a typical telephone switchboard connects a calling party with a called party by plugging in appropriate cables (called "patchcords") or by using other, more sophisticated telephone switching equipment. Such patching might be used when a dispatcher desires to connect a physician at home with a caller on an emergency department hot line. Radio-Telephone Patching Radio-telephone patching is a similar technique whereby a dispatcher can patch together his radio transmitter/receiver and a telephone line to a 103
place the user of a mobile radio unit in direct communication with a party reached by telephone. A central dispatcher, for example, connected by radio to an ambulance and by dedicated hot line telephone to the emergency suite of a hospital can thus place the ambulance attendant in direct contact with the hospital staff. Special equipment commercially available enables this to be done by simple switching or by patchcords. Radio-Radio Patching This is used when it is desirable to place two radio units in contact that are operating on different frequencies, for example, an ambulance radio transmitting and receiving in the UHF band and a police cruiser using the VHF band. Again, this can be simply accomplished with special equipment at a central dispatching point where transmissions from both vehicles are being received. In this case, the dispatching center serves as a two-way repeater station between the vehicles. . » Microwave Trunking Svstems A "trunking" system utilizes several (or many) communications channels between two or more points that are shared by many users. The users connected by such a network outnumber the channels available; trunking systems are used where not all users desire access to a channel simultaneously. As calls - are initiated between users, they are automatically assigned to any available channel. Only when all channels are in use would a user encounter a "busy" condition. Because the broad bandwidths in the microwave region of the spectrum (above 900 MHz) permit many voice channels, microwave can effectively be used for trunking. Equipment for this is readily available; in fact, the telephone companies use it extensively and routinely in their public long-distance telephone operations. 104
Several EMS systems under development use microwave trunking to provide communications between the various elements of the system. A backbone microwave loop, composed of microwave transmitters and receivers, provide a complete trunking circuit covering the entire geographic area served by the EMS system. Hospitals, dispatcher, ambulances and others are connected to the trunking net through telephone patching, dedicated lines to the microwave transmitters, or by intermediate radio links on VHF or UHF frequencies. The latter is accomplished by incorporating, for example, VHF or UHF transmitters and receivers on each microwave antenna tower. Signals to and from mobile units on VHF or UHF channels are automatically fed into the microwave net at the tower nearest the mobile unit, the microwave circuits then being used to carry the transmissions a greater distance. Such systems also can use tone-encoded and/or digital-encoded squelch techniques so that a dispatcher, using the backbone net and the VHF-UHF connecting links, can address particular stations and units in the system. * , For example: San Diego County, and the Ohio Valley Health Services Foundation (Athens, Ohio), and the State of Nebraska. 105
