What is the German BOS radio system?
In Germany all rescue and police organisations (called BOS - authorities and organizations for security) share a common standard of wireless communications to be able to get in contact with each other just by choosing the same channel. Also the used radios are the same models - even radios from different manufacturers must be similar in functionality. All BOS approved radios get a FuG number - equal models from different manufacturers get the same FuG number!
Different frequency ranges had been assigned by the telecommunications authority, and they are used differently.
The VHF 4m band is mainly used for communications between car and base, and for radio traffic between the cars.
The low VHF 2m band is used for directed radio links between 4m relais stations (to build a network to cover a wide area with one channel), for data communications and for digital POCSAG pagers to call the people who are on standby.
The high VHF 2m band is used for the handheld transceivers the people carry when they are outside the car. Also it is used for POCSAG pager systems, like the low VHF 2m band.
The UHF 70cm band is exclusively used for directed radio links between VHF relais stations, no FuG radios are available for this band. It was reported that some very special police units use handheld radios in the 70cm BOS band, but this also could be a rumour...
All these bands are divided in an uplink range and a downlink range. A channel is not a single frequency, but a pair of a downlink and an uplink frequency.
Usually all mobile and base radios transmit on the uplink frequency and receive on the corresponding downlink frequency, while the relais stations work vice versa.
The relais receives the transmission of the mobiles on the uplink channel and sends it out again on the downlink channel, so everyone can talk to everyone when they are just within the coverage area of the relais or relais network.
All BOS channels in a little overview
The 4m band has some channels which are blocked. Due to the VHF aviation beacons channels 376 up to 396 (including) are available only as simplex channels in the downlink band, and channel 510 ist just available in the uplink band as the corresponding downlink frequency was used for a nationwide paging systems, which was shut off a few years ago; so it is possible that the formerly used channels 510 up to 519 (incl.) will be available in the future again.
Channel spacing is 20 KHz for 4m and 2m, 12.5 KHz for 70cm. Duplex shift ist 9.8 MHz for 4m, 4.6 MHz for 2m and 5 MHz for 70cm.
Most important facts about all in Germany used BOS radios
Controls and usage of a modern FuG8 BOS radio
On the left side you can see the speaker. Next to it the buttons "I" and "II". "I" transmits a 1750 Hz tone, "II" a 2135 Hz tone. These tones are used differently - in some cases for calling a base station, in some other cases to activate relais stations. Next to the "I" button you can see the codeplug which contains the FMS (RSS) identity of the radio. Under the codeplug the volume switch is located, and right from the codeplug you see the LEDs for "transmitter active", "channel busy" and "power on".
The "R" button is used to toggle the squelch, and under this button you can find the switch for power and operating mode (relais or "normal" radio, FMS (RSS)) of the transceiver.
Each digit of the display has two corresponding buttons to switch its content up or down. The first three digits show the channel, the next one the operating mode ("G"=duplex, "W"=simplex), and the very right digit shows in which band ("U"=uplink, "O"=downlink) the transmitter works.
The numbers written on the display buttons are used when the radio is in FMS (RSS) mode - then the status (e.g. free, busy, on the way back, emergency) can be transmitted by pressing them.
You wonder what FMS (RSS) is? The answer...
The FMS (RSS) selcall system
FMS (RSS) was developed to achieve less traffic on the radio channel and easier registration of many standard messages with a computer.
The shortcut FMS means "FunkMeldeSystem" - sometimes it is called RSS or "Radio Signalling System" although it seems to be used in .de only; who knows other countries using this system?
It uses digital bursts. The modulation is a coherent subcarrier FFSK a rate of 1200 bit per second. A logical "1" means 2/2 periods of 1200 Hz, "0" 3/2 periods of 1800 Hz.
Each burst consists of 20 bits for synchronisation and 48 bits of data.
An example of a FMS (RSS) telegram: 111111111111 00011010 BBBB SSSS CCCC CCCC IIII IIII IIII IIII SSSS MDTT RRRR RRRR 1
The first block of 12 leading bits "1" and the sync word "00011010" always are the same and use for correct detection of the beginnig of a valid telegram. "B" is the ID of the BOS (police, rescue, firefighters and such), "S" shows the state (e.g. Bavaria, Saxonia...) and "C" identifies the city.
"I" identifies the vehicle which carries the radio. The status "S" is used for the most common messages like "free, busy, out of order, ready for new job...". "M" is for the mode the FMS (RSS) system uses. A "0" means the bursts are acknowledged by an audible tone, "1" means that each burst is automatically answerd with another one. "D" shows the direction of a telegram: "0" = "from mobile to base", "1" = "from base to mobile". The "T" is used for additional information, selectable at the mobile unit with a switch, with different meanings according to local agreements.
"R" is the redundancy, a kind of checksum to avoid errors. The whole burst ends with a tailbit "0" or "1", this varies from network to network.
All parts of the burst which carry a number are BCD coded.
But how is this strange system used? See Lisa and Mike. They are employed at the German Red Cross and are on standby. Suddenly their pager alerts, and a voice says "Traffic accident on the motorway A72, KM 38.5 north, one injured person".
Lisa and Mike enter their car, turn on the radio and - STOP: They DO NOT take the microphone and say smth. like "Received your order, are on our way"; they just press the button 3 which sends out a FMS (RSS) burst signalling that they have received their order. When they arrive at the accident, they press button 4, after having the injured person save in the back of their car they press 7, and after arriving at he hospital the 8 is being pressed. May be the injured person vomited, and their car has to be cleaned before it can be used again...no problem at all, they just press the "not ready" button 6 :-)
At least theoretically not one single spoken word is necessary if no problems occur. Of course very often FMS (RSS) is used and at the same time the rescue staff says what`s going on, but in cases of traffic overload on the channel many standard actions can be told by just pressing the FMS (RSS) buttons.
The FMS (RSS) bursts always identify the their car wit a unique number, so the guys at the regional headquarters do not have to write down every single action - the computer makes an accurate protocol!