Nordic Mobile Telephone
From Wikipedia, the free encyclopedia
| Mobile phone and data standards |
| 0G |
| 1G |
| 2G |
| 3G |
| 4G |
| Frequency bands |
- For other meanings of the abbreviation, see: NMT.
NMT (Nordisk MobilTelefoni or Nordiska MobilTelefoni-gruppen, Nordic Mobile Telephone in English) is the first fully-automatic cellular phone system. It was specified by Nordic telecommunications administrations (PTTs) starting in 1970, and opened for service in 1981 as a response to the increasing congestion and heavy requirements of the manual mobile phone networks: ARP (150 MHz) in Finland and MTD (450 MHz) in Sweden, Norway and Denmark. Östen Mäkitalo is considered as the father of this system, and of the cell phone.
NMT is based on analog technology (first generation or 1G) and two variants exist: NMT-450 and NMT-900. The numbers indicate the frequency bands uses. NMT-900 was introduced in 1986 because it carries more channels than the previous NMT-450 network.
The technical principles of NMT were ready by year 1973 and specifications for base stations were ready in 1977. The NMT specifications were free and open, allowing many companies to produce NMT hardware and pushing the prices down. The success of NMT meant a lot to Nokia (then Mobira) and Ericsson. Initial NMT phones were typical portable phones: one could definitely move them, but they were usually intended for car use. Latter-day models (such as Benefon's) were as small as 100 mm and weighed only about 100 grams.
The network was opened in Sweden and Norway in 1981, and in Denmark and Finland in 1982. Iceland joined in 1986. However, curiously for a mobile phone standard that has the word "Nordic" in it, the first commercial service was introduced in Saudi Arabia on September 1st 1981 to 1200 users, one month before Sweden. By 1985 the network had grown to 110,000 subscribers in Scandinavia, 63,300 in Norway alone, which made it the worlds largest mobile network at the time. [1]
The NMT network has mainly been used in the Nordic countries, Switzerland, The Netherlands, Hungary, Slovakia, Slovenia, Serbia, Croatia, Bosnia, Baltic countries and Russia but also in the Middle East and in Asia. The introduction of digital mobile networks such as GSM has reduced the popularity of NMT and some of the Nordic phone companies have suspended their NMT networks (e.g. Sonera's NMT network was suspended on December 31, 2002 in Finland). The NMT network (450 MHz) however has one big advantage over GSM which is the range; this advantage is valuable in big but sparsely populated countries such as Iceland. In Iceland, the GSM network reaches 98% of the country's population but only a small proportion of its land area. The NMT system however reaches most of the country and a lot of the surrounding waters, thus the network is popular with those traveling in the mountains and fishermen.
The cell sizes in an NMT network range from 2 km to 30 km. With smaller ranges the network can service more simultaneous callers; for example in a city the range can be kept short for better service. NMT used full duplex transmission, allowing for simultaneous receiving and transmission of voice. Car phone versions of NMT used transmission power of up to 15 watt (NMT-450) and 6 watt (NMT-900), handsets up to 1 watt. NMT had automatic switching (dialing) and handover of the call built into the standard from the beginning, which was not the case with most preceding car phone services, such as the Finnish ARP. Additionally, the NMT standard specified billing as well as national and international roaming.
A disadvantage of the original NMT specification is that voice traffic was not encrypted. So anyone willing to listen in would just have to buy a scanner and tune it to the correct frequency. As a result, some scanners have had the NMT bands "deleted" so they could not be accessed. This is not particularly effective as it isn't that hard to obtain a scanner that doesn't have these restrictions; it is also possible to re-program a scanner so that the "deleted" bands can be accessed. Later versions of the NMT specifications defined optional analog scrambling which was based on two-band audio frequency inversion. If both the base station and the mobile station supported scrambling, they could agree upon using it when initiating a phone call. Also, if two users had mobile stations (=mobile phones) supporting scrambling, they could turn it on during conversation even if the base stations didn't support it. In this case audio would be scrambled all the way between the two mobile stations. While the scrambling method was not at all as strong as encryption in newer digital phones, such as GSM, it did prevent casual listening with scanners. Scrambling is defined in NMT Doc 450-1: System Description (1999-03-23) and NMT Doc 450-3 and 900-3: Technical Specification for the Mobile Station (1995-10-04)'s Annex 26 v.1.1: Mobile Station with Speech Scrambling - Split Inversion Method (Optional) (1998-01-27).
NMT also supported a simple but robust integrated data transfer mode called DMS (Data and Messaging Service) or NMT-Text, which used the network's signaling channel for data transfer. Using DMS, also text messaging was possible between two NMT handsets before SMS service started in GSM, but this feature was never commercially available except in Russian and Polish NMT networks. - Another data transfer method was called NMT Mobidigi with transfer speeds of 380 bits per second. It required external equipment.
NMT signaling transfer speeds vary between 600 and 1200 bits per second, using FFSK (Fast Frequency Shift Keying) modulation. Signaling between the base station and the mobile station was implemented using the same RF channel that was used for audio, and using the 1200 bit/s FFSK modem. This caused the periodic short noise bursts, e.g. during handover, that were uniquely characteristic to NMT sound.
