Wikipedysta:Manveru/Tetra
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TErrestrial Trunked RAdio (TETRA) (poprzednio znana jako Trans European Trunked RAdio) jest specjalistycznym systemem Professional Mobile Radio oraz przenośnemu nadajnikowi-odbiornikowi (potocznie zwanemu walkie talkie lub słuchawka), którego używanie jest zarezerwowane dla agencji rządowych, głównie służbom (publicznym) takim jak Policja, Straż Pożarna, Pogotowie Ratunkowe oraz wojsku.
TETRA jest standardem ETSI, opublikowanym poraz pierwszy w 1995 roku. TETRA została zaaprobowana przez Europejski Komitet Komunikacji Radiowej (European Radio Communications Committee - ERC).
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[edytuj] Opis
Terminale TETRA działają jak telefon komórkowy, z bezpośrednim połączeniem do PSTN. Często też mogą one działać w grupach, kiedy pojedyncze naciśnięcie przycisku nadawania łączy użytkownika z dyspozytorem oraz innymi użytkownikami w grupie. Jest również możliwe, aby terminal działał jako walkie talkie pomiędzy dwiema osobami, jednak bez typowego ograniczenia zasięgu, gdyż w połączeniu uczetniczy cała infrastruktura sieci. Przycisk alarmowy w które wyposażone są terminale, pozwalają użytkownikowi przesyłać sygnały alarmowe, unieważniające (wywłaszczające) inną mniej ważną aktywność w tym samym czasie.
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 or using Trunked infrastructure (Switching and Management Infrastructure or SwMI) made of TETRA Base Stations (TBS). As well as allowing direct communications in situations where network coverage has been lost, Direct Mode or DMO also includes the possibility of using one (or a chain) of TETRA terminals as relays for a signal. This functionality is called DMO gateway (from DMO to TMO) or DMO Repeater (DMO to DMO). In rescue situations this feature could allow 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 Data or Circuit switched data communication uses specifically assigned traffic channels.
All traffic is normally encrypted. TETRA provides both over the air encryption and end-to-end encryption.
[edytuj] Advantages
The main advantages of TETRA over other technologies (such as GSM) are:
- the much lower frequency used, which permits very high levels of geographic coverage with a smaller number of transmitters, cutting infrastructure cost.
- fast call set-up - a one to many group call is generally set-up within 0.5 seconds (typical less than 250 msec for a single node call) compared with the many seconds that are required for a GSM network.
- the fact that its infrastructure can be separated from that of the public mobile phone network, and made substantially more diverse and resilient by the fact that base stations can be some distance from the area served.
- unlike most cellular technologies, TETRA networks typically provide a number of fall-back modes such as the ability for a base station to process local calls in the absence of the rest of the network, and for 'direct mode' where mobiles can continue to share channels directly if the infrastructure fails or is out-of-reach.
- gateway mode - where a single mobile with connection to the network can act as a relay for other nearby mobiles that are out of contact with the infrastructure.
- TETRA also provides a point-to-point function that traditional analogue emergency services radio systems didn't 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 the cellular technologies, which connect one subscriber to one other subscriber (one-to-one) then 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.
[edytuj] Disadvantages
Its main disadvantages are:
- it can only support a much lower teledensity in a given area, compared to GSM and similar technologies (which is not a problem in the applications for which it is used, but mostly limits it to these applications).
- handsets are more expensive (about 750 EUR in 2003, about 600 EUR in 2006), due to the reduced economies of scale, different business model when compared to mass-market mobile phones, need for security evaluation, and robustness.
- data transfer is slow at 7.2 kbit/s per timeslot (3.5 kbit/s net packet data throughput), although up to 4 timeslots can be combined into a single data channel to achieve higher rates, due to need to fit into 25kHz bandwidth channel plans.
- due to the pulsed nature of TDMA employed by the protocol, handsets can interfere with sensitive electronic devices such as heart pacemakers and defibrillators, as can other RF transmitting equipment when used in close proximity (e.g. < 1 metre distance.)
[edytuj] Radio Frequencies
In Europe, TETRA uses frequencies:
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In Britain the public sector TETRA system operates under the name 'Airwave'. In Belgium, the 'A.S.T.R.I.D' system uses TETRA. In the Netherlands, the TETRA system is called 'C2000'. In Sweden the RAKEL system uses TETRA. In Finland the VIRVE (short for VIRanomaisVErkko, loosely translated: "official network") network uses TETRA. VIRVE and C2000 are currently the only TETRA networks that cover an entire country.
[edytuj] Radio aspects
TETRA uses a digital modulation scheme known as π/4 DQPSK which is a form of phase shift keying. TETRA uses TDMA (see above). The symbol rate is 18,000 symbols per second, and each symbol maps to 2 bits. A single slot consists of 255 symbols, a single frame consist of 4 slots, and a multiframe (whose duration is approximately 1 second) consists of 18 frames. As a form of phase shift keying the downlink power is constant. The downlink (i.e. the output of the basestation) is a continuous transmission consisting of either specific communications with mobiles, synchronisation or other general broadcasts. Although the system uses 18 frames per second only 17 of these are used for traffic channel, with the 18th frame reserved for signalling or synchronisation. TETRA does not employ amplitude modulation. However, TETRA has 17.65 frames per second (18000 symbols/sec / 255 symbols/slot / 4 slots/frame), which is the cause of the PERCEIVED "amplitude modulation" at 17Hz.
[edytuj] Cell Re-Selection (or Hand-over) in Images
This first representation demonstrates where the SRT (Slow Reselect Threshold) the FRT (Fast Reselect Threshold) 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.
[edytuj] 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 MS (mobile station) 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 Rx 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.
- Refer to EN 300 392 2 16.3.1 Activation and control of underlying MLE Service
- Note 18.5.12 Minimum RX access level
The minimum RX access level information element shall indicate the minimum received signal level required at the SwMI in a cell, either the serving cell or a neighbour cell as defined in table 18.24.
[edytuj] 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.
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.
[edytuj] 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 greater than the following: (FAST_RESELECT_THRESHOLD+FAST_RESELECT_HYSTERISIS) for a period of 5 seconds, 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.
[edytuj] 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 MM (Mobility Management) 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.
[edytuj] 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 Rx Lev minimum 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.
[edytuj] See also
- P25, a standard used in North America for similar purposes. P25 equipment is considerably more expensive than TETRA.
[edytuj] External links
- TETRA Memorandum of Understanding - TETRA Facts
- The Trower Report and study about TETRA commission for by The Police Federation for England and Wales
- report on health effects of TETRA
- Powerwatch Report on health effects of TETRA
- COST281 - Potential Health Implications from Mobile Communication Systems
Szablon:Trunked radio systems Category:Wireless communications Category:mobile telephony standards
