Archive August 2019

SCR-299, 399 and 499

The SCR-399; “Mobile communication unit” was developed for long-distance communication during the second world war. The US Army was looking for improvements in the transmission range, flexibility and durability compared to the then used SCR-197 and SCR-597 transmitters. In 1942 the Hallicrafters HT-4 was selected as the transmitter for the SCR-299, 399 and 499 and received the military identification number BC-610. Until this selection, the Hallicrafters HT-4 was a respected transmitter that was originally built for use by radio amateurs.

The first radio station of this series was used on 8 November 1942 during operation TORCH where a company of the 829th Signal Service Battalion built up a first communication network that could send messages from the landing beaches to the bases in Gibraltar. Despite the problems with unloading the radio stations from the landing ships, the radio stations from this series had been employed until the installation of the “Permanent Army Command and Administrative Network Stations.” General Dwight Eisenhower complemented the work of the SCR-299, 399 and 499 radio stations during the successful reorganization of the US Army and the last victory over the Germans in Kasserine pass in Tunisia. The broadcasting range of the radio stations exceeded all expectations. There are connections made over distances of 3700 km. The SCR-299, 399 and 499 radio stations provided a reliable radio connection with England during the “North African Campain” and during D-day in Normandie where the radio stations were also used by the English Airborn divisions. Also during the invasion in Sicily and Italy almost all communication was via these radio stations. War correspondents made frequent use of these radio stations which were offered to them by the second and third army “Group Communication Teams.” It has been a time that these radio stations were their only link with England for passing on the news.

This radio station existed in three variants; the SCR-299, built into a Dodge panelvan K-51; SCR-399, built into a shelter / container on the back of the GMC truck and the SCR-499, the set can be used separate and installed in buildings.

SCR-299
SCR-399
SCR-499

The radio station is designed for long-distance communication between 1 and 18 MHz. and could, as said before, bridge distances up to approximately 3700 km. Broadly speaking, the radio station consisted of the following components;

  • BC-610; transmitter with a capacity of 350 watts
  • BC-312; receiver operating on 12 volts
  • BC-342; receiver operating on 115 volts
  • BC-614; “Speech amplifier” the pre-amp
  • BC-939; Antenna tuner

BC-610

The BC-610 is a Hallicrafters HT-4 transmitter which was originally built as a transmitter for radio amateurs in America. The station has been adapted for use by the US Army. This tube transmitter was built so solidly that it functioned in almost all circumstances. About 25,000 of these transmitters have been produced for the US Army.

BC-610

BC-312 and BC-342

These receivers were only different in the power supply. one works at 12 volts and the other at 115 volts. These receivers were used extensively in various radio sets during and after the Second World War. The receiver could receive from 1.5 to 18 MHz.

BC-312

BC-614

The Speech amplifier is the pre-amp of this radio set which converted the three input signals into a signal that the transmitter can handle. One could use this radio station for transmitting morse local, remote morse via a field telephone line, local telephony and remote telephony via a field telephone connection. All these options were combined with the Speech amplifier.

BC-614

BC-939

The antenna tuner is actually part of the transmitter, this is always shown together and as far as I know not used in other sets except the successor of this set which was used by the Dutch army until the 1980s.

BC-939

This radio station also includes various accessories packed in three boxes and two reels with telephone cable. There is a chest for the transmitter; the Chest CH-88-A; which is filled with tuning units and coils with which the transmitter could reach its frequencies. There was also a large chest; the Chest CH-119-A; containing various tools and rolls with cable. The third box is also the bank and was filled with mainly spare parts and the antennas.

Transmitter coil
Tunerunits and coils in chest CH-88-A
Chest overview

PE-95

The radio station also comes with a 3.5 KW PE-95 unit.

PE-95

Philips BX-925a receiver

A pure Dutch radio made at Philips Hilversum for ships and land installations. Its a all band receiver from 210 kHz to 32 MHz divided into six bands;
1. 210 – 540 kHz
2. 1,45 – 3,6 MHz
3. 3,5 – 9,1 MHz
4. 9,1 – 13,7 MHz
5. 13,7 – 20,7 MHz
6. 20,7 – 32 Mhz

A really nice feature is the motorized tuning, if you have to tune to the other side of the chosen band you can do this motorized, a real pleasure.

The difference beat frequency is 735 kHz with a bandwith of 0.45, 1.3, 3, 6.5 or 4.2 kHz
The selectivity is 5 uV at a signal to noise ratio of 10 dB
The AVG has 4 choices and works really nice.

Later more about this great receiver with photo’s of my own.

BC-314 receiver

The radio receivers BC-314 are the superheterodyne type intended for general field usage. They are built ruggedly and are suitable for vehicular, portable, or fixed operation. The receiver are highly sensitive and selective and are designed for the reception of either continuous-wave (c-w) or amplitude-modulated (a-m) voice or tone signals.  The Radio Receivers BC-314,-C,-D,-E,-F,and G. covering a frequency range from 150 to 1,500 kc in four band. Band A 150 to 260 kc, Band B 260 to 450 kc, Band C 450 to 820 kc, Band D 820 to 1.500 kc. All Radio Receivers BC-314  models are for operation from either storage batteries or alternating current (a-c) power source. All receiver models are identical in size, in general external and internal construction and appearance, and in the fundamental arrangement of their electrical circuits.

Frequency Coverage: 150 – 1.500 KHz in four bands 
IF: 92.5 kc 
Tubes: 2 x 6C5, 1 x 6F6,4 x 6K7, 1 x 6L7,1 x 6R7 
Dimensions: 10 Height x 9 1/16 Depth x 18 1/16 inc. 
Weight: 58 lb. 
Power supply:
12-14V with dynamotor DM-21-A 
24-28V with dynamotor DM-21-CX 
Manual: TM-11-850

AN/GRC-9 or Angry Nine

Nice overview borrowed from Pinterest

The AN/GRC-9, often called the “Angry Nine” because the set does not always does what the operator has in mind, is a 2-12 MHz, AM/CW, low power transportable radio set which can be vehicular mounted. It was released in the late 1940s and used through the Korean War and later in the 1950s by many country’s The AN/GRC-9 replaced the SCR-284, SCR-288 and SCR-694 radios and was in turn replaced by the AN/PRC-62 and AN/PRC-74.

The complete AN/GRC-9 is a set of components for operation on Continuous Wave (CW), modulated CW (MCW) or AM from 2 to 12 MHz across three bands. According to TM 11-263 (1956), range is 30 miles on CW, 20 miles on MCW and 10 miles on AM (voice). Operation in a vehicle had shorter ranges, about half the above.

The main AN/GRC-9 component is the receiver-transmitter unit, RT-77/GRC-9, operating on two pre-set frequencies determined by crystal.

In addition to the electronic components, the complete system includes canvas bags, whip antenna mast with counterpoise 1/2 wave sloping wire (and erecting kit), straps, a T-17 microphone, J-45 telegraph key for CW operation, HS-30 head- set and LS-7 speaker.

Options

PE-237 vibrator power supply6,12, or 24 VDC
DY-88/GRC-9 Dynamotor power supply6,12, or 24 VDC
DY-105/GRC-9 Dynamotor power supply24 VDC
PP-317 power supply115/230 VAC
PP-327 power supply115 VAC

Other options included the GN-58 hand-cranked generator and the PE-162 gasoline engine generator. When operating the receiver only, BA-48 or any dry battery of proper voltage will operate the set.

Manuals
TM 11-263 + france suplement
TM 11-5820-453-20
TM 11-5820-452-20P
TM 11-5820-453-10

BC-614 – Speech amplifier

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Speech amplifier frond view.

The BC-614 Speech amplifier is an audio-frequency amplifier which is designed for use with the BC-610 Radio transmitter or transmitters of a similar type. The basic purpose of the speech amplifier is raise a microphone signal to a suitable level for driving the input of the audio circuits of the transmitter.

The speech amplifie does also provides a sidetone to monitor CW transmissions with a headset.

The speech amplifier has its own plate-circuit and gets its power from the transmitter from the left socket on the front.

Technical Specifications

Frequency response;
– Carbon microphone – +/-3 db from 200 to 2000 hz
– Dynamic microphone – +/- 3 db from 200 to 3500 hz
Amplifier type – a-f (audio-frequency)
Type of input signals – Voice
Inputs;
– Carbon microphone
– Dynamic microphone
– External telephone (field telephone – EE-8)
Output – 500 ohms, single-ended
Power output – Approximately 2 watts
Power supply – 115 volts. 50/60 hz
Weight – 31 pounds or 14 kilo’s

Manual : TM 11-5054

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Inside view from the top
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Inside view from the bottom

Radio-Air C.I.T. ERB-281

About the history of the ERB-281 is not much to known. It is the radio-set selected by the German Bundeswehr in 1964 for there new Leopart battletank.

The choice for the ERB-281 is made because Germany was forced to import everything thay needed for there defence. Thay were since the war not allowed to built anything that can use for war. The only other choice thay had was the SK010 from R&S.

The ERB-281 is build bij Radio-Air C.I.T. (later Thomson) as a aircraft radio-set. In the catalog of Le Bourget Salon from 1963 is writen the following;

Radio-Air (CIT), 2-4 Avenue de la Marne, 
Asnieres (Seine), France. Stand B.27. 
Manufacturer of radio equipment, including highfrequency SSB-units as the ERB 281 but also theERB 423 made for ground and air use. Flightrecorder ENB 284 for recording of one communication channel for two hours.
Flightrecorder ENB 285 that uses a double tape to record two communication channels or equal signals.

BC-610 or Hallicrafters HT-4

Nice picture i borrowed from Google

In the early 1940s, the U.S military sought a high-powered radio transmitter capable of infallible voice communications over 100 miles (160 km), sturdy enough to work in all conditions, flexible enough to be able to cover a wide range of frequencies, self-powered and able to operate in motion or at fixed locations. The Hallicrafters HT-4 transmitter was chosen from units available from various U.S. radio manufacturers. The HT-4 was designed for amateur radio use and had been commercially available for several years at a price of approximately $700, rivaling the cost of a car. It was considered compact and stable for its era and could deliver in excess of 300 watts of power for voice or MCW communications and 400 watts during morse code operation. As was typical in physically large vacuum tube equipment, the manual cautions power output is less at higher frequencies. It was quartz crystal controlled, but could be used over a wide range of frequencies through use of the master-oscillator power amplifier.

Modifications requested by the Signal Corps were performed by Hallicrafters’ engineers working with U.S. Army technicians at Fort Monmouth. They made a new version of the HT-4, which was known as the BC-610 transmitter, a part of the SCR-299 mobile communications unit, and production began in 1942. General Dwight Eisenhower credited the SCR-299 in the reorganization of U.S. forces, which led to their victory against the Nazis at Kasserine Pass. The SCR-299 was also used in the Invasion of Sicily and later, Italy.

A BC-610 transmitter was used by double agent Juan Pujol García during WWII as part of Operation Fortitude. Clear reception by the Germans of messages transmitted by García, code name GARBO, were so crucial to the Allied deception that use of the relatively high-powered transmitter was deemed necessary.

Over 25,000 units were produced by Hallicrafters and other allied companies. In 1944, a short subject film was produced by the Jam Handy Organization and sponsored by the Hallicrafters Company detailing how the HT-4 transmitter was adapted for military service and dramatizing its use by the U.S. military during World War II.

Specifications:

  • Frequency coverage: 2 to 18 MHz
  • Mode: AM, CW
  • RF power Output: <400 watts CW, <300 watts AM, MCW
  • Vacuum tubes: Eimac 250TH final, pair 100TH modulator, various rectifier and low level tubes
  • Operating frequency determined by plug-in tuning units and final coils (up to three at once) in addition to FT-171B crystals
  • Signal Corps Radio sets: SCR-299, SCR-399, SCR-499, or GRC-38, AM requires BC-614 (or Hallicrafters HT-5) speech amplifier
  • Weight: 390 lb (177 kg) (without BC-614)
  • Manual : TM 11-826 and TM 11-4057

The “A” through “I” models are the same basic unit with relatively minor component and cosmetic differences.

Tetra – Man-machine interface (MMI)

Virtual MMI for TETRA radio terminals

Any given TETRA radio terminal using Java (Java ME/CLDC) based technology, provides the end user with the communication rights necessary to fulfil his or her work role on any short duration assignment.

For dexterity, flexibility, and evolution ability, the public transportation radio engineering department, have chosen to use the open sources, Java language specification administered by Sun and the associated work groups in order to produce a transport application tool kit.

Tetra MMI

Service acquisition admits different authorised agents to establish communication channels between different services by calling the service identity, and without possessing the complete knowledge of the ISSI, GSSI, or any other TETRA related communication establishment numbering plan. Service acquisition is administered through a communication rights centralised service or roll allocation server, interfaced into the TETRA core network.

In summary, the TETRA MMI aims are to:

  • Allow any given agent while in exercise, to exploit any given radio terminal without materiel constraint.
  • Provide specific transportation application software to the end-user agents (service acquisition, fraud, and aggression control).

This transport application tool-kit has been produced successfully and with TETRA communication technology and assures for the public transport application requirements for the future mentioned hereafter.

The home (main) menu presents the end user with three possibilities:

  1. Service acquisition,
  2. Status SDS,
  3. End-user parameters.

Service acquisition provides a means of virtually personalising the end user to any given radio terminal and onto TETRA network for the duration the end user conserves the terminal under his possession.

Status SDS provides the end user with a mechanism for generating a 440 Hz repeating tone that signals a fraud occurrence to members within the same (dynamic or static) Group Short Subscriber Identity (GSSI) or to a specific Individual Short Subscriber Identity (ISSI) for the duration of the assignment (an hour, a morning patrol or a given short period allocated to the assignment). The advantage being that each of the end users may attach themselves to any given terminal, and group for short durations without requiring any major reconfiguration by means of radio software programming tools. Similarly, the aggression feature functions, but with a higher tone frequency (880 Hz), and with a quicker repetitious nature, so to highlight the urgency of the alert.

The parameters tab provides an essential means to the terminal end-user allowing them to pre-configure the target (preprogrammed ISSI or GSSI ) destination communication number. With this pre-programmed destination number, the end-user shall liaise with the destination radio terminal or roll allocation server, and may communicate, in the group, or into a dedicated server to which the service acquisition requests are received, preprocessed, and ultimately dispatched though the TETRA core network. This simplifies the reconfiguration or recycling configuration process allowing flexibility on short assignments.

The parameters tab also provides a means of choosing between preselected tones to match the work group requirements for the purposes of fraud and aggression alerts. A possibility of selecting any given key available from the keypad to serve as an aggression or fraud quick key is also made possible though the transport application software tool kit. It is recommend to use the asterisk and the hash keys for the fraud and aggression quick keys respectively. For the fraud and aggression tones, it is also recommend to use 440 Hz slow repeating tone (blank space 500 milli-seconds) and 880 Hz fast repeating tone (blank space 250 milliseconds) respectively. The tone options are as follows: 440 Hz, 620 Hz, 880 Hz, and 1060 Hz.

The parameters page provides an aid or help menu and the last tab within parameters describes briefly the tool kit the version and the history of the transport application tool kit to date.

Tetra Enhanced Data Service (TEDS)

The TETRA Association, working with ETSI, developed the TEDS standard, a wideband data solution, which enhances TETRA with a much higher capacity and throughput for data. In addition to those provided by TETRA, TEDS uses a range of adaptive modulation schemes and a number of different carrier sizes from 25 kHz to 150 kHz. Initial implementations of TEDS will be in the existing TETRA radio spectrum, and will likely employ 50 kHz channel bandwidths as this enables an equivalent coverage footprint for voice and TEDS services. TEDS performance is optimised for wideband data rates, wide area coverage and spectrum efficiency.

Advances in DSP technology have led to the introduction of multi-carrier transmission standards employing QAM modulation. WiMAX, Wi-Fi and TEDS standards are part of this family.

Refer also to:

  • JSR-118;
  • Mobile Information Device Profile, JSR-37;
  • Wireless Messaging API, JSR120;
  • Connected Limited Device Configuration, JSR-139; and
  • Technology for the Wireless Industry, JTWI-185.

Reference: Wikipedia

Tetra – Technical

For its modulation TETRA, uses ​π4 differential quadrature phase-shift keying. The symbol (baud) rate is 18,000 symbols per second, and each symbol maps to 2 bits, thus resulting in 36,000 bit/s gross.

As a form of phase shift keying is used to transmit data during each burst, it would seem reasonable to expect the transmit power to be constant. However it is not. This is because the sidebands, which are essentially a repetition of the data in the main carrier’s modulation, are filtered off with a sharp filter so that unnecessary spectrum is not used up. This results in an amplitude modulation and is why TETRA requires linear amplifiers. The resulting ratio of peak to mean (RMS) power is 3.65 dB. If non-linear (or not-linear enough) amplifiers are used, the sidebands re-appear and cause interference on adjacent channels. Commonly used techniques for achieving the necessary linearity include Cartesian loops, and adaptive predistortion.

The base stations normally transmit continuously and (simultaneously) receive continuously from various mobiles on different carrier frequencies; hence the TETRA system is a frequency-division duplex (FDD) system. TETRA also uses FDMA/TDMA (see above) like GSM. The mobiles normally only transmit on 1 slot/4 and receive on 1 slot/4 (instead of 1 slot/8 for GSM).

Speech signals in TETRA are sampled at 8 kHz and then compressed with a vocoder using algebraic code-excited linear prediction (ACELP). This creates a data stream of 4.567 kbit/s. This data stream is error-protection encoded before transmission to allow correct decoding even in noisy (erroneous) channels. The data rate after coding is 7.2 kbit/s. The capacity of a single traffic slot when used 17/18 frames.

A single slot consists of 255 usable symbols, the remaining time is used up with synchronisation sequences and turning on/off, etc. A single frame consists of 4 slots, and a multiframe (whose duration is 1.02 seconds) consists of 18 frames. Hyperframes also exist, but are mostly used for providing synchronisation to encryption algorithms.

The downlink (i.e., the output of the base station) is normally a continuous transmission consisting of either specific communications with mobile(s), synchronisation or other general broadcasts. All slots are usually filled with a burst even if idle (continuous mode). Although the system uses 18 frames per second only 17 of these are used for traffic channels, with the 18th frame reserved for signalling, Short Data Service messages (like SMS in GSM) or synchronisation. The frame structure in TETRA (17.65 frames per second), consists of 18,000 symbols/s; 255 symbols/slot; 4 slots/frame, and is the cause of the perceived “amplitude modulation” at 17 Hz and is especially apparent in mobiles/portables which only transmit on one slot/4. They use the remaining three slots to switch frequency to receive a burst from the base station two slots later and then return to their transmit frequency (TDMA).

Air interface encryption

To provide confidentiality the TETRA air interface is encrypted using one of the TETRA Encryption Algorithm (TEA) ciphers. The encryption provides confidentiality (protect against eavesdropping) as well as protection of signalling.

Currently 4 different ciphers are defined. These TEA ciphers should not be confused with the block cipher Tiny Encryption Algorithm. The TEA ciphers have different availability due to export and use restrictions. Few details are published concerning these proprietary ciphers. Riess mentions in early TETRA design documents that encryption should be done with a stream cipher, due to the property of not propagating transmission errors. Parkinson later confirms this and explains that TEA is a stream cipher with 80-bit keys. TEA1 and TEA4 provide basic level security, and are meant for commercial use. The TEA2 cipher is restricted to European public safety organisations. The TEA3 cipher is for situations where TEA2 is suitable but not available.

Cell selection

Cell re-selection (or hand-over)

This first representation demonstrates where the slow reselect threshold (SRT), the fast reselect threshold (FRT), and propagation delay exceed parameters are most likely to be. These are represented in association with the decaying radio carrier as the distance increases from the TETRA base station.

From this illustration, these SRT and FRT triggering points are associated to the decaying radio signal strength of the respective cell carriers. The thresholds are situated so that the cell reselection procedures occur on time and assure communication continuity for on-going communication calls.

Initial cell selection

The next diagram illustrates where a given TETRA radio cell initial selection. The initial cell selection is performed by procedures located in the MLE and in the MAC. When the cell selection is made, and possible registration is performed, the mobile station (MS) is said to be attached to the cell. The mobile is allowed to initially select any suitable cell that has a positive C1 value; i.e., the received signal level is greater than the minimum receive level for access parameter.

The initial cell selection procedure shall ensure that the MS selects a cell in which it can reliably decode downlink data (i.e., on a main control channel/MCCH), and which has a high probability of uplink communication. The minimum conditions that shall have to be met are that C1 > 0. Access to the network shall be conditional on the successful selection of a cell.

At mobile switch on, the mobile makes its initial cell selection of one of the base stations, which indicates the initial exchanges at activation.

Cell improvable

The next diagram illustrates where a given TETRA radio cell becomes improvable. The serving cell becomes improvable when the following occurs: the C1 of the serving cell is below the value defined in the radio network parameter cell reselection parameters, slow reselect threshold for a period of 5 seconds, and the C1 or C2 of a neighbour cell exceeds the C1 of the serving cell by the value defined in the radio network parameter cell reselection parameters, slow reselect hysteresis for a period of 5 seconds.

Cell usable

The next diagram illustrates where a given TETRA radio cell becomes usable. A neighbour cell becomes radio usable when the cell has a downlink radio connection of sufficient quality.

The following conditions must be met in order to declare a neighbour cell radio usable: The neighbour cell has a path loss parameter C1 or C2 that is, for a period of 5 seconds, greater than the fast reselect threshold plus the fast reselect threshold, and the service level provided by the neighbour cell is higher than that of the serving cell. No successful cell reselection shall have taken place within the previous 15 seconds unless MM requests a cell reselection. The MS-MLE shall check the criterion for serving cell relinquishment as often as one neighbour cell is scanned or monitored.

The following conditions will cause the MS to rate the neighbour cell to have higher service level than the current serving cell:

  • The MS subscriber class is supported on the neighbour cell but not on the serving cell.
  • The neighbour cell is a priority cell and the serving cell is not.
  • The neighbour cell supports a service (that is, TETRA standard speech, packet data, or encryption) that is not supported by the serving cell and the MS requires that service to be available.
  • The cell service level indicates that the neighbour cell is less loaded than the serving cell.

Cell relinquishable (abandonable)

The next diagram illustrates where a given TETRA radio cell becomes relinquishable (abandonable). The serving cell becomes relinquishable when the following occurs: the C1 of the serving cell is below the value defined in the radio network parameter cell reselection parameters, fast reselect threshold, for a period of 5 seconds, and the C1 or C2 of a neighbour cell exceeds the C1 of the serving cell by the value defined in the radio network parameter cell reselection parameters, fast reselect hysteresis, for a period of 5 seconds.

No successful cell reselection shall have taken place within the previous 15 seconds unless Mobility Management (MM) requests a cell reselection. The MS-MLE shall check the criterion for serving cell relinquishment as often as one neighbour cell is scanned or monitored.

Radio down-link failure

When the FRT threshold is breached, the MS is in a situation where it is essential to relinquish (or abandon) the serving cell and obtain another of at least usable quality. That is to say, the mobile station is aware that the radio signal is decaying rapidly, and must cell reselect rapidly, before communications are terminated because of radio link failure. When the mobile station radio-signal breaches the minimum receive level, the radio is no longer in a position to maintain acceptable communications for the user, and the radio link is broken.

Radio link failure: (C1 < 0). Using the suggested values, this would be satisfied with the serving cell level below −105 dBm. Cell reselection procedures are then activated in order to find a suitable radio base station.

Reference: Wikipedia

Tetra – Basics

Terrestrial Trunked Radio (TETRA; formerly known as Trans-European Trunked Radio), a European standard for a trunked radio system, is a professional mobile radio and two-way transceiver specification. TETRA was specifically designed for use by government agencies, emergency services, (police forces, fire departments, ambulance) for public safety networks, rail transport staff for train radios, transport services and the military. TETRA is the European version of trunked radio similar to Project 25.

TETRA uses time-division multiple access (TDMA) with four user channels on one radio carrier and 25 kHz spacing between carriers. Both point-to-point and point-to-multipoint transfer can be used. Digital data transmission is also included in the standard though at a low data rate.

TETRA Mobile Stations (MS) can communicate direct-mode operation (DMO) or using trunked-mode operation (TMO) using switching and management infrastructure (SwMI) made of TETRA base stations (TBS). As well as allowing direct communications in situations where network coverage is not available, DMO also includes the possibility of using a sequence of one or more TETRA terminals as relays. This functionality is called DMO gateway (from DMO to TMO) or DMO repeater (from DMO to DMO). In emergency situations this feature allows direct communications underground or in areas of bad coverage.

In addition to voice and dispatch services, the TETRA system supports several types of data communication. Status messages and short data services (SDS) are provided over the system’s main control channel, while packet-switched data or circuit-switched data communication uses specifically assigned channels.

TETRA provides for authentication of terminals towards infrastructure and vice versa. For protection against eavesdropping, air interface encryption and end-to-end encryption is available.

The common mode of operation is in a group calling mode in which a single button push will connect the user to the users in a selected call group and/or a dispatcher. It is also possible for the terminal to act as a one-to-one walkie talkie but without the normal range limitation since the call still uses the network. TETRA terminals can act as mobile phones (cell phones), with a full-duplex direct connection to other TETRA Users or the PSTN. Emergency buttons, provided on the terminals, enable the users to transmit emergency signals, to the dispatcher, overriding any other activity taking place at the same time.

The main advantages of TETRA over other technologies (such as GSM) are:

  • The much lower frequency used gives longer range, which in turn permits very high levels of geographic coverage with a smaller number of transmitters, thus cutting infrastructure costs.
  • During a voice call, the communications are not interrupted when moving to another network site. This is a unique feature, which dPMR networks typically provide, that allows a number of fall-back modes such as the ability for a base station to process local calls. So called ‘mission critical’ networks can be built with TETRA where all aspects are fail-safe/multiple-redundant.
  • In the absence of a network, mobiles/portables can use ‘direct mode’ whereby they share channels directly (walkie-talkie mode).
  • Gateway mode – where a single mobile with connection to the network can act as a relay for other nearby mobiles that are out of range of the infrastructure.
  • TETRA also provides a point-to-point function that traditional analogue emergency services radio systems did not provide. This enables users to have a one-to-one trunked ‘radio’ link between sets without the need for the direct involvement of a control room operator/dispatcher.
  • Unlike cellular technologies, which connect one subscriber to one other subscriber (one-to-one), TETRA is built to do one-to-one, one-to-many and many-to-many. These operational modes are directly relevant to the public safety and professional users.
  • Security TETRA supports terminal registration, authentication, air-interface encryption and end-to-end encryption.
  • Rapid deployment (transportable) network solutions are available for disaster relief and temporary capacity provision.
  • Network solutions are available in both reliable circuit-switched (telephone like) architectures and flat, IP architectures with soft (software) switches.

Its main disadvantages are:

  • Requires a linear amplifier to meet the stringent RF specifications that allow it to exist alongside other radio services.
  • Data transfer is slow by modern standards.

Up to 7.2 kbit/s per timeslot, in the case of point-to-point connections, and 3.5 kbit/s per timeslot in case of IP encapsulation. Both options permit the use of between one and four timeslots. Different implementations include one of the previous connectivity capabilities, both, or none, and one timeslot or more. These rates are ostensibly faster than the competing technologies DMR, dPMR, and P25 are capable of). Latest version of standard supports 115.2 kbit/s in 25 kHz or up to 691.2 kbit/s in an expanded 150 kHz channel. To overcome the limitations many software vendors have begun to consider hybrid solutions where TETRA is used for critical signalling while large data synchronization and transfer of images and video is done over 3G / LTE.

Reference: Wikipedia