Editing Nordic Mobile Telephone (section)

== Technology ==
The cell sizes in an NMT network range from 2&nbsp;km to 30&nbsp;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&nbsp;watt (NMT-450) and 6&nbsp;watt (NMT-900), handsets up to 1&nbsp;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 [[Autoradiopuhelin|ARP]]. Additionally, the NMT standard specified billing as well as national and international [[roaming]].

=== Signaling ===
NMT voice channel is transmitted with FM (Frequency Modulation)<ref name="googlebooks-tele-illustrated">{{cite book|title=The telecommunications illustrated dictionary|author=Petersen, Julie K.|year=2002|url=https://books.google.com/books?id=b2mMzS0hCkAC&pg=PA679|publisher=CRC Press|isbn=9781420040678}}</ref> and NMT signaling transfer speeds vary between 600 and 1,200&nbsp;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 1,200&nbsp;bit/s FFSK modem. This caused the periodic short noise bursts, e.g. during handover, that were uniquely characteristic to NMT sound.

=== Security ===
In the original NMT specification the voice traffic was not [[Encryption|encrypted]]; it was possible to listen to calls using e.g. a [[scanner (radio)|scanner]] or a cable ready TV. As a result, some scanners have had the NMT bands blocked so they could not be accessed. Later versions of the NMT specifications defined optional analog [[scrambler|scrambling]] which was based on two-band audio frequency [[Voice inversion|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 (phone) stations 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 2 mobile stations. While the scrambling method was not at all as strong as encryption of current digital phones, such as [[GSM]] or [[CDMA]], 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).

=== Data transfer ===
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, text messaging was also possible between two NMT handsets before SMS service started in GSM, but this feature was never commercially available except in Russian, Polish and Bulgarian NMT networks. Another data transfer method was called NMT Mobidigi with transfer speeds of 380&nbsp;bits per second. It required external equipment.