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Preserving the sounds of the NMT system

NMT (Nordisk MobilTelefon) was an automatic, analog cellular telephone system, primarily designed by Östen Mäkitalo. In Sweden, the system existed on two bands;

NMT 450 (Forward: 463-467.5 MHz, Reverse: 453-457.5 MHz), in operation between late 1981 and January 1, 2008.

NMT 900 (Forward: 935-960 MHz, Reverse: 890-915 MHz), in operation between 1986 and 2000.

This system used FFSK signaling at 1200 bps which had a very characteristic sound. Anyone who has ever used an NMT phone or had a conversation with an NMT subscriber will have heard one part of the signalling: the FFSK chirp which occured at regular intervals during a conversation, and also during handover.

Those of us with scanners have probably heard more components of the signaling. Specifically: control channel, call setup and clear down, and ringing.

The system also had certain peculiarities. For example, sometimes when calling a NMT unit, the system would leak static and/or FFSK signaling back to the caller before ringing began. And also, when an NMT unit hangs up, the base station would sometimes fail to mute outgoing audio from the NMT subscriber which at that point no longer was transmitting, letting the caller, again, hear static interspersed with bursts of FFSK.

In Sweden, NMT 450 was operated by the incumbent telco Telia. The service was scheduled to be decommisioned on December 31, 2007 (In reality the shutdown occured about 24 hours later). Knowing the system was going to be shut down, and because I wanted to preserve as much of these sounds as possible, I began recording as much as I could, at about 1-2 months prior to the shutdown date.

The work consisted of recording the forward frequency range using my scanner (an Icom IC-R10, with the recordings being made through a discriminator tap output modification for maximum clarity), and also making numerous calls to NMT units (010-2, 010-6 prefix) from my home phone line. (Served from an Ericsson AXE10 switch; recording were made using an Edirol R-09 recorder connected to a homemade telephone audio interface).

The purpose of these recordings is historical preservation. Much of my inspiration to do this came from Evan Doorbell, who worked tirelessly to record and preserve the sounds of the North American telephone network of the 70's. If you have the slightest interest in telecommunications history and/or the subject of "phreaking", I highly recommend his presentations. You can find them on the Phone Trips page. I recommend getting the mp3's from ftp://ftp.wideweb.com/GroupBell

My recordings are being presented in an uncompressed form. I decided against using lossy compression, such as mp3, because it would mangle the high frequency components of some of these recordings.

I would like to thank olle at toolcrypt.org for extending the Tronquito application with the ability to decode NMT signalling. I have included the output from his Tronquito branch (snabel) with each of the recordings where it's applicable.

Scanner recordings

Recording Snabel output Comments
30 seconds of control channel CTIND| Calling/Traffic Ch. indication | Ch(112) TA(65) A control channel. It didn't always sound like this however. The rhythm used to be more monotone, but it changed to this sometime around, I'd say, 2002 or 2003. Please correct me if I'm wrong.
Data connection attempt #1 2x: ALLCT| Traffic channel allocation | Ch(103) TA(65) Z( 6) X(743861) N(103)

4x: LINES| Line signal - Clearing channel | Ch(103) TA(65) Z( 6) X(743861) L(15)
A failed modem connection attempt. Sounds V34:ish.
My scanner had data skip enabled so I didn't catch the whole call.
Data connection attempt #2 4x: ALLCT| Traffic channel allocation | Ch(105) TA(65) Z( 6) X(743861) N(143)

CORDR| Channel activation order | Stop sending ø-signal


CORDR| Channel activation order | Idle radio channel
Another failed attempt.
Possible handover request ALLCT| Traffic channel allocation | Ch(161) TA(65) Z( 6) X(735220) N(161) I suspect the mobile unit was asked to move to another frequency but it didn't obey. There was another identical request in the longer version of this recording.
Handover 4x: ALLCT| Traffic channel allocation | Ch(103) TA(65) Z( 6) X(119712) N(176)

CORDR| Channel activation order | Stop sending ø-signal

CORDR| Channel activation order | Idle radio channel
 
Incoming call 8x: LINES| Line signal - Ringing order | Ch(169) TA(65) Z( 6) X(648965)

4x: LINES| Line signal - Clearing channel | Ch(169) TA(65) Z( 6) X(648965) L(15)

CORDR| Channel activation order | Idle radio channel
 
Call origination #1 4x: LINES| Line signal - Address Complete | Ch(121) TA(65) Z( 6) X(703884)

CORDR| Channel activation order | Start sending ø-signal

4x: LINES| Line signal - Clearing channel | Ch(121) TA(65) Z( 6) X(703884) L(15)

CORDR| Channel activation order | Stop sending ø-signal
 
Call origination #2 3x: LINES| Line signal - Address Complete | Ch(112) TA(65) Z( 6) X(732899)

LINES| Line signal - Address Complete | Ch(112) TA(65) Z( 6) X(732900)

4x: LINES| Line signal | Ch(112) TA(65) Z( 6) X(732899) L( 5)

2x: CORDR| Channel activation order | A: 6

4x: LINES| Line signal - Clearing channel | Ch(112) TA(65) Z( 6) X(732899) L(15)

CORDR| Channel activation order | Idle radio channel
A very unusual sounding call setup. I omitted the voice part of this recording. I have no idea where the thumping noise came from, but it is unrelated.
Call origination #3 4x: LINES| Line signal - Address Complete | Ch(112) TA(65) Z( 6) X(735220)

4x: LINES| Line signal - Clearing channel | Ch(112) TA(65) Z( 6) X(735220) L(15)

CORDR| Channel activation order | Stop sending ø-signal

CORDR| Channel activation order | Idle radio channel
A very clear recording of a call setup and clear down.
The end This recording is from January 2, 2008 at 00:46 CET. I had my scanner setup to search the forward frequency range. In this recording we hear two control channels which go away, never to return.

Good bye, NMT 450.

Telephone recordings

Recording Comments
Calling NMT subscriber #1 A good, "noisy" example.
Calling NMT subscriber #2 This one is interesting. The call appears to go through to an NMT unit aboard a ship, which then forwards the call on a PBX to a phone in the machine room.
Calling NMT subscriber #3 This call is to the same number as above, but here the PBX appears to be bypassed.
Calling NMT subscriber #4 Here we hear some signalling leak through before the first ring.

Additional recordings

Recording Comments
TV-ad TV advertisement for the NMT 900 service. Cirka 93/94. FFSK chirps are audible.
Reporter When the Jas 39 Gripen crashed in Stockholm in 1993, a shaken reporter delivered his report over an NMT phone. Unsure if 450 or 900.

Transcript: Ja, det var för några minuter sen bara som den här <chirp> flyguppvisningen i samband med vattenfestivalen skulle avslutas. Det ser ut som att någonting lossnar från vingen, helt plötsligt ser vi <chirp> piloten skjuta ut sig och planet singlar alltså rakt ner i den här parken.

Translation: It was only a few minutes ago that the <chirp> airshow in connection with the Water Festival was about to end. It looks like something comes off from the wing, and then suddenly we see <chirp> the pilot eject and the plane falls straight down into this park.

 

NMT specifications

Document Description
NMT-DOC450_1.pdf System description
NMT-DOC450_3.pdf Technical specification for the mobile station
NMT-SMS_annex24.pdf NMT short message service

 

Programs

Program Description
Snabel v0.8.1 NMT decoding program for Windows

 

Last updated: March 25, 2016.