Nordic Mobile Telephone

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For other meanings of the abbreviation, see: NMT.

NMT (Nordisk MobilTelefon or Nordisk MobilTelefon-gruppen, Nordic Mobile Telephony in English) is a mobile phone system that was created in 1981 as a response to the increasing congestion and heavy requirements of the ARP mobile phone network. It 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 1981 in most of the Nordic countries, March 1982 in Finland. However, curiously for a mobile phone standard that has the word "Nordic" in it, the first commercial service was introduced in Saudi Arabia in 1977 to 1200 users.

The NMT network has mainly been used in the Nordic countries, 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 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 6 watts, handsets up to 1 watt. NMT had automatic switching built into the standard from the beginning, which was not the case with some preceding car phone services such as the Finnish ARP. Additionally, the NMT standard specified billing and roaming.

A disadvantage of the original NMT specification is that 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 primitive data transfer mode called DMS or NMT-Text, which used the network's signalling channel for data transfer. Transfer speeds vary between 600 and 1200 bits per second, using FFSK (Fast Frequency Shift Keying) modulation. Another data transfer mode was called NMT Mobidigi with transfer speeds of 380 bits per second. 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 that were uniquely characteristic to NMT sound.pl:NMT