Nordic Mobile Telephone
NMT (Nordisk
Mobil Telefon or Nordisk Mobil Telefonkruppen,
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 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.
Compared to the previous ARP network, NMT had
automatic switching built into the standard from
the beginning. Additionally, the NMT standard
specified billing and roaming. 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.
The technical principles of NMT were ready by
year 1973 and specifications for base stations
were ready in 1977. 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.
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. Newer digital networks such as GSM
encrypt traffic using Smart card technology.
Later versions of the NMT specifications defined
optional analog encryption which was based on
two-band audio frequency inversion. If both the
base station and the mobile station supported
encryption, they could agree upon using it when
initiating a phone call. Also, if two users had
mobile stations (=mobile phones) supporting
encryption, they could turn it on during
conversation even if the base stations didn't
support it. In this case audio would be
encrypted all the way between the two mobile
stations. While the encryption method was not at
all as strong as encryption in newer digital
phones, it did prevent casual listening with
scanners. Encryption 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).
An example of a mobile station that supports
encryption is Benefon's Exion, which also is the
smallest NMT mobile station with a length of
only 100 mm and a weight of 109 grams including
battery.
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 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.
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