C network
The C-Netz ( radio telephone network -C ) was an analog , cellular mobile network of the German DeTeMobil (formerly Deutsche Bundespost TELEKOM), operated from 1985 to 2000. It was the third and last analog generation of the mobile radio , which was only available as a system in Germany , Portugal and South Africa - based on the C-450 standard - was used. Other countries in Europe (such as the C-Netz in Austria or the Natel C in Switzerland ) used other standards such as NMT (NMT-450) and TACS . This heterogeneity prevented users from roaming internationally and was one of the main motivations for starting the development of GSM . The C network was primarily designed for telephone communication applications ( car telephone network ) with access to the telephone network and ISDN .
history
The C network was introduced in Germany in 1984 (officially 1985) and replaced the cumbersome handling of the B or B2 network . It was limited to Germany, Portugal and South Africa, but was initially more widespread than the digital networks when they were introduced with the D-Netz in 1991. Due to the initially better accessibility, especially in rural areas, the C-Netz was still used in car phones until the middle of the 90s first choice. For a long time, a C-Netz device on board was more or less standard on seagoing vessels near the coast of Germany. During the time of German reunification in 1990, West German owners of C-network telephones were able to use their telephones while staying in East Berlin and saved themselves the time-consuming assignment of a long-distance call in the GDR landline network.
A comprehensive supply was achieved in large cells (radius about 15–20 km) and small cells (2–3 km) in the metropolitan areas. When the C-Network was introduced, the network consisted of two radio switching centers and 175 radio zones or base stations. The C network could (in the final stage) accommodate around 800,000 subscribers. Activated radio connections were passed on when the radio cell was changed ( handover ). The C-Netz subscriber could be reached in the entire supply area under a uniform access code (0161) and radio call number. On December 31, 1988 there were already 98,762 C-Netz subscribers nationwide and 2,076 in the State of Berlin. In addition to the limited number of participants in the C network, the comparatively low voice quality and the high risk of eavesdropping were also criticisms of the C network. There was the so-called speech obfuscation , but that was only a spectral inversion of the speech band, which could be reversed with little technical effort. In the case of bad connections, the user could switch them off and thus increase the intelligibility.
The C-Netz system was the first to support the separation of subscriber identity and terminal. The participant identity and the access authorization were coded on a magnetic card. This means that any mobile phone was assigned to a user by inserting this card. In 1988 the magnetic stripe was replaced by the TeleKarte with an integrated microcontroller . This concept was the forerunner of the SIM card known today .
Unusual for the time were also the functionally richly equipped earphones, which had all the operating elements, LC display and LEDs . The key set was structured in accordance with the CCITT recommendations and the other human-machine interface was regulated for all manufacturers in accordance with an FTZ guideline, so that the user did not have any device-specific conversion problems, but basically states such as: logged in, connected or voice obfuscation switched on in a familiar Shape got displayed.
Compared to the analog mobile networks known at the time, the C network offered a handover function that was not controlled according to the field strength, but rather the relative distance to the base station. This made handovers possible even under the best radio conditions, which was a very useful feature for network planning and the compression of frequency repetition. This also significantly reduced the probability of co-channel interference. In order to be able to support the relative distance measurement, however, additional technical effort was necessary, namely a time synchronization of all base stations with one another. In order to implement this nationwide and across the network, each base station had specific transmitters and receivers for synchronization signals.
Compared to the A-Netz and B-Netz there were many groundbreaking innovations in the C-Netz, e.g. B .:
- Common area code (0161-) for all mobile subscribers; in contrast to the A and B networks, you no longer needed to know where the subscriber was
- Uninterrupted change from one radio station to the next (handover)
- Concealment of the (analog) radio signal made unauthorized eavesdropping difficult
- In addition to built-in devices, removable or even portable devices ( mobile phones ) are also possible
- Larger capacity of up to 850,000 participants (A network 10,500, B network 27,000)
- Since the end of 1990 answering machine and call diversion as a network feature (until then only as hardware accessories)
The operation of the C-Netz, which started on May 1, 1985, was stopped on December 31, 2000. Until the last few years of the network, Deutsche Telekom was still selling contracts for the system now called C-Tel. The tariffs were usually cheaper than those in digital mobile communications.
Railnet (Deutsche Bahn / T-Mobile)
It was planned and partially implemented around 2010 to use the frequencies of the C network for Railnet (Internet on the train). Telekom, which operated the C network until 2000, is represented by its subsidiary Telekom Deutschland at Railnet. A total of around 150 stations were on the air nationwide in 2010, these are systems from Qualcomm, type RR2000. This guaranteed coverage of the Dortmund - Munich and Frankfurt am Main - Hamburg lines.
This technology has not been in operation since October 2018. Internet access on the trains is ensured by means of the public LTE cellular networks due to the increased bandwidth requirements .
Infrastructure replica
The hobby project Osmocom-Analog enables not only the most common old analog mobile radio standards but also the simulation of the C-network infrastructure with a standard Linux PC and a software-defined radio . This means that old devices from this era can be put back into operation for test purposes. Calls between two devices are thus possible. However, operation on antennas is not permitted in Germany, so only experiments under laboratory conditions on artificial antennas or wired via attenuators are permitted.
Technical specifications
- Frequency range: 70 cm band
- Sub- band (subscribers) 451.30-455.74 MHz
- Upper band radio base station (FuFst), radio concentrator 461.30–465.74 MHz
- Transmission power:
- Fixed station: max. 25 watts
- Participants: max. 15 watts ( vehicle built-in version)
- Both powers can be automatically reduced by up to 35 dB
- 222 radio channels
- 287 radio channels from 1991 by changing the frequency grid from 20 kHz to 12.5 kHz and 10 kHz
- Transmission of signaling data by dividing the audio signal into audio blocks of 12.5 ms each and their 10% temporal compression in order to insert 4- bit data telegrams into the 1.25 ms long gaps
- Audio modulation : phase modulation 14F3
- Data modulation : binary frequency shift keying
literature
- Christel Jörges, Helmut Gold : Telephones 1863 until today . From the collections of the Museums for Communication. Edition Braus, 2001, ISBN 3-926318-89-9 , pp. 286-287
Web links
Individual evidence
- ↑ 30 years of mobile communications in Austria . heise online, May 5, 2004
- ↑ New year: C-Netz switched off . heise online, January 1, 2001
- ↑ http://osmocom-analog.eversberg.eu/