<li><ahref="#basestation">Setup of a base station</a>
<li><ahref="#sms">Sending and receiving SMS</a>
</ul>
<pclass="toppic">
<aname="history"></a>
History
</p>
<p>
NMT was a Scandinavian cellular network, developed by and used in Scandinavian countries.
If was used in the Baltic countries, Switzerland, Netherlands, Hungary, Poland, Bulgaria, Romania, Czech Republic, Slovakia, Slovenia, Serbia, Turkey, Croatia, Bosnia, Russia, Ukraine and in Asia.
The NMT network was opened in Sweden and Norway in 1981, and in Denmark and Finland in 1982. Iceland joined in 1986.
Networks where shut down in 2000 (Estonia), 2002 (Finland), 2004 (Norway), 2007 (Sweden), 2010 (Iceland).
In 1989, the German Wall came down and C-Netz was quickly deployed in eastern Germany to support the congested fixed network.
It was a cellular system, so frequencies could be used efficiently..
It offered handover capability, so moving phones could change cells during a call.
Authentication was introduces (SIS), to prevent fraud, but it was optional.
Older phones without SIS could be cloned by receiving and decoding the roaming update message.
Extra services (e. g. conditional call forwarding, answering machine) were installed in the mobile exchange.
Phones were build in cars but also hand-held phones were available.
All phones were available under a dedicated prefix, so the caller must only dial the phone number to reach the phone anywhere.
The networks offered a data service (DMS) that re-used the signaling modem to transfer data to a terminal or PC.
This data service was used to transfer short messages (SMS) between service center (SMSC) and the mobile phone.
<li><b>The Mobile Telephone Exchange (MTX)</b> is the brain of the system and is technically the most complicated part.
Several MTXs form the interface between the NMT system and the fixed telephone network.
Whereas signaling and other system components of NMT are the same in all countries, their telephone networks differ in some respects.
One of the functions of the MTXs is therefore to compensate for these differences.
<br><br>
<li><b>The Base Station (BS)</b> are intermediary links without switching function between the wire (MTX) and radio path.
It is basically a transmitter, connected to the MTX via leased line.
<br><br>
<li><b>The Mobile Station (MS)</b> is the telephone itself.
It can be vehicle-borne, portable, hand-held or of coin-box type.
A mobile station is owned or leased by the subscriber.
</ul>
<center><imgsrc="nmt_ta.jpg"/></center>
<p>
Each country is divided into a number of traffic areas.
In a traffic ares there are a number of base stations spaced about between 8 and 80 km apart.
Each traffic area belongs to a single exchange (MTX).
This means that all base stations in the area are connected to this exchange and that all traffic in the are is channeled through it.
The exchange communicates with the telephone network via trunk exchanges.
</p>
<p>
Every mobile subscriber is registered in a so-called home MTX, usually the MTX controlling the traffic area in which the subscriber normally resides.
</p>
<p>
Setting up of calls
</p>
<p>
A call from an ordinary telephone subscriber is connected on the basis of the first digits of the mobile subscriber's number to his home MTX.
The latter stores the data of his present location and transmits a call signal over all base stations in that traffic area.
The mobile station answers automatically with a call acknowledgment and the MTX then assigns the mobile station a traffic channel.
The call is set up.
</p>
<p>
A call by a mobile subscriber causes the mobile station to search for a traffic channel in the current traffic area automatically.
The call is set up and forwarded through the current MTX to the destination network.
</p>
<p>
If the mobile subscriber is in a traffic are not belonging to his home MTX, the home MTX hands over the call to the MTX controlling the visited traffic areas, and then latter MTX takes over the subsequent setting up of the call.
</p>
<p>
In this example charging is done in the calling subscriber's local exchange.
If the call had been initiated by the mobile subscriber, it would have been charged in the MTX.
</p>
<p>
Roaming
</p>
<p>
The special function which enables a person to call a mobile subscriber without knowing where he or she is, is called roaming.
One need not even know whether the mobile subscriber has gone to another country.
Roaming is made possible by the fact that a mobile station which leaves a traffic areas and enters a new one automatically notifies the MTX that it has entered the new areas. (location update)
The location of the mobile station is stored in the mobile station's home MTX and used for routing calls to the correct area.
</p>
<p>
Transfer of calls to other base station
</p>
<p>
During a conversion it may happen that a mobile subscriber leaves the coverage area of the base station in which the conversation started.
This results in impaired speech quality.
The conversation should then be conducted over another base station.
</p>
<p>
The speech quality is supervised by means of a supervisory signal (Ø signal) one of four tones around 4 kHz.
The base station sends the Ø signal to the mobile station, which sends it back to the base station.
The quality of the returned Ø signal is measured in the base station and, if it is unsatisfactory, the base station transmits alarm to the MTX.
The latter then orders the base station and neighboring base stations to measure the field-strength of the radio signal from the mobile station.
The base stations transmit the results to the MTX, which then switches the call to the base station with the best reception.
</p>
<center><imgsrc="nmt_freq.jpg"/></center>
<p>
Small cell technique
</p>
<p>
The NMT system has been allotted 180 (and optional 20) radio channels.
In the high-traffic urban areas this number of channels would not suffice the future with conventional cell size (the coverage area of a base station).
To increase the traffic capacity in these areas the small-cell technique will be adopted, i. e. the cell size is considerably reduced, so that this distance between base stations having the same radio channel can be reduced.
The channels are thus used more efficiently.
In small-cell areas both base stations and mobile stations transmit with reduced power.
Change of power output of a mobile station takes place automatically on order from the MTX.
</p>
<center><imgsrc="nmt_routing.jpg"/></center>
<p>
The Danish and Norwegian telephone networks permit a maximum of 7 digits after the trunk prefix P<sub>N</sub>.
With an access code of type P<sub>N</sub> M<sub>1</sub> M<sub>2</sub> there remain 5 digits for the subscriber number.
It was anticipated, however, that the system would be constructed for 6-digit subscriber number.
The problem was solved by adding a sixth digit (X<sub>1</sub>) to the dialed number (X<sub>2</sub> X<sub>3</sub> X<sub>4</sub> X<sub>5</sub> X<sub>6</sub>) in all signaling in the NMT system and between MTXs.
The sixth digit will be added automatically by the mobile subscriber's home MTX.
The system thus provides for 6-digit subscriber number in the countries which cannot introduce this length of number when their systems first come into operation.
</p>
<p>
In the Finnish and Swedish fixed telephone network the setting up of calls is done with the digits P<sub>N</sub> M<sub>1</sub> M<sub>2</sub> X<sub>1</sub> X<sub>2</sub>.
The digits X<sub>1</sub> X<sub>2</sub> are the code for the subscriber's home-traffic area.
P<sub>N</sub> M<sub>1</sub> M<sub>2</sub> X<sub>1</sub> X<sub>2</sub> thus constitutes sufficient information for routing the call to the mobile subscriber's home MTX.
In Denmark and Norway the connection is similarly set up with the digits P<sub>N</sub> M<sub>1</sub> M<sub>2</sub> X<sub>2</sub>.
</p>
<p>
To distinguish between subscribers with the same number in different countries and additional digit, the nationality digit Z, is required.
This number is not dialed by the subscriber but added to the subscriber number by the mobile subscriber's home MTX.
The digit Z then accompanies the other signaling in the NMT system.
On signaling from a mobile subscriber, Z is sent automatically by the mobile station.
The receiving MTX can thus distinguish between calling mobile subscribers from different countries.
</p>
<p>
Charging principles
</p>
<p>
The NMT system's character of joint Nordic mobile telephone network has made it natural to adopt common principles als for charging of call.
An important principle is that the charge should be the same in both directions.
A NMT subscriber will pay three compulsory charges:
</p>
<p>
<ul>
<li><b>Initial charge.</b> This charge shall cover the costs involved in opening a subscription, e. g. correspondence, introduction in the accounting routines, registration in the central equipment, directory listing.
<br><br>
<li><b>Subscription fee.</b> A recurring fixed charge for coverage of traffic unrelated costs, e. g. MTX equipment dependent on number of subscribers, costs relating to roaming, administrative costs for billing, keeping of records and telephone directory.
<br><br>
<li><b>Call charge.</b> Variable charge covering traffic-related costs, e. g. transmission in telephone network, operation/maintenance of MTS and base stations, lines to base station, data transfer between MTXs.
</ul>
</p>
<p>
The NMT system will offer a number of special facilities, e. g. barring of certain outgoing calls, transfer to other telephone number/announcing macine.
Special charges will be made for these facilities.
</p>
<p>
The technical form of the system imposes certain limitation on the rate structure.
Charging in the NMT system will thus differ in some respects from that in the fixed telephone network:
</p>
<p>
<ul>
<li><b>Domestic traffic.</b> A Call from an ordinary telephone subscriber to a mobile subscriber <i>registered</i> in the same country is charged in the ordinary subscriber's local exchange.
The charge is based on the digits dialed and the call is routed to the mobile subscriber's home MTX.
In this case there is a technical possibility of price differentiation only between calls to different MTXs.
For the eventual transfer to another MTS (possibly abroad) and for the link MTX - base station there are no means for price differentiation.
Bit as all mobile telephone calls are switched over the trunk network, it may very well happen that local calls as well are routed over long distances.
The cost difference for different domestic calls will therefore in fact be small.
It has therefore been considered that a uniform rate should be charged.
<br><br>
For calls from home mobile to ordinary subscriber in the same country there are technical possibilities for price differentiation, but in accordance with the principle of equal charge in both directions a uniform rate is adopted here as well.
<br><br>
<li><b>Inter-Nordic traffic.</b> For calls from an ordinary to a mobile subscriber <i>registered</i> abroad the charge is based on the international prefix.
The following digits, which route the call to a MTX, cannot be analyzed by the caller's local exchange.
The call will therefore be charged as an ordinary international call.
<br><br>
For calls in the other direction the mobile subscriber is charged by toll ticketing in MTX.
In this case price differentiation is possible, but owing to the desire for equal rates, the international rate is used here as well.
The national mobile telephone rate is used, however, if the international rate is lower than the mobile telephone rate.
</ul>
</p>
<p>
Many types of signal are used between the different parts of the NMT system.
The signals have the following main objectives:
</p>
<p>
<b>MTX - MS</b>
</p>
<ul>
<li>Setting up and clearing of calls
<li>Switching of call in progress
<li>Updating
<li>Ordering of power-reduction
</ul>
<p>
<b>MTX - BS</b>
</p>
<ul>
<li>Remote control of base station
<li>Transmission of alarm / measurement
</ul>
<p>
<b>MTX - MTX</b>
</p>
<ul>
<li>Updating of subscriber register and roaming register
<li>Transfer of call to roaming subscriber
</ul>
<p>
<b>MTX - Telephone Network</b>
</p>
<ul>
<li>Setting up of calls
</ul>
<p>
<b>BS - MS</b>
</p>
<ul>
<li>Supervision of transmission quality
</ul>
<p>
Signals are sent in the form of so-called frames.
All frames have the same length.
They are divided into fields, each of which contains a given type of information.
Two frame formats exist depending on whether the frame is sent to or from MTX.
</p>
<p>
From MTX:
<tableclass="nmt">
<tr>
<tdclass="nmt">
N<sub>1</sub>
N<sub>2</sub>
N<sub>3</sub>
</td>
<tdclass="nmt">
P
</td>
<tdclass="nmt">
Y<sub>1</sub>
Y<sub>2</sub>
</td>
<tdclass="nmt">
Z
X<sub>1</sub>
X<sub>2</sub>
X<sub>3</sub>
X<sub>4</sub>
X<sub>5</sub>
X<sub>6</sub>
</td>
<tdclass="nmt">
J<sub>1</sub>
J<sub>2</sub>
J<sub>3</sub>
</td>
</tr>
</table>
</p>
<p>
To MTX:
<tableclass="nmt">
<tr>
<tdclass="nmt">
N<sub>1</sub>
N<sub>2</sub>
N<sub>3</sub>
</td>
<tdclass="nmt">
P
</td>
<tdclass="nmt">
Z
X<sub>1</sub>
X<sub>2</sub>
X<sub>3</sub>
X<sub>4</sub>
X<sub>5</sub>
X<sub>6</sub>
</td>
<tdclass="nmt">
J<sub>1</sub>
J<sub>2</sub>
J<sub>3</sub>
J<sub>4</sub>
J<sub>5</sub>
</td>
</tr>
</table>
</p>
<p>
N = Channel Number<br>
P = Prefix (frame ID)<br>
Y = Traffic area number<br>
Z/X = Subscriber number<br>
J = Information<br>
</p>
<p>
Each character (N, P etc) consists of a hexadecimal digit, i. e. four bits.
All frames start with the number of the channel on which the frame is sent.
If the number does not match with that of the channel to which the receiver is locked, the frame is ignored.
Faults due to inter-modulation are thus avoided.
The character after the channel number, the prefix P, defines the kind of message to which the frame relates.
The actual message is found in the information field.
On signaling from a mobile station the mobile subscriber's number is always sent.
It is also sent on signaling from MTX to a specific subscriber.
Furthermore the MTX always sends the traffic area number of the base station in use.
The frame is sent on the calling channel N<sub>1</sub>..N<sub>3</sub>.
The mobile station ZX<sub>1</sub>..X<sub>6</sub> to which a connection is to be set up is ordered over to traffic channel number N<sub>a</sub>..N<sub>c</sub>.
</p>
<preclass="doc">
10.b Seizure from ordinary MS and identity on traffic
The frame is sent on traffic channel by the mobile station, either as acknowledegment of an identity request or as seizure signal on a call from the mobile station.
</p>
<p>
As appears from the above, a frame consists of 4 x 16 = 64 bits.
To increase the reliability the frame is sent with an error-correcting code, so that the length of the message will be 140 bits.
An additional 15 + 11 bit are used for synchronization.
The transmitted signal frame will have the following form:
The selected error-correcting code is of convolution type and permits correction of error bursts with up to 6 false bits in sequence for at least 19 correct bits between error bursts.
</p>
<center><imgsrc="nmt_mod.jpg"/></center>
<p>
The binary position in the signal frames are transmitted by FFSK (Fast Frequency Shift Keying) modulation.
Logic "1" is represented by a cycle of the frequency 1200 Hz and a logic "0" by 1.5 cycles of 1800 Hz.
Changes between "1" and "0" takes place at the zero crossing of the signal.
The data speed is thus 1200 bits per second.
</p>
<p>
Call from mobile station:
</p>
<center><imgsrc="nmt_mo_call.jpg"/></center>
<p>
Call to mobile station:
</p>
<center><imgsrc="nmt_mt_call.jpg"/></center>
<p>
Handover to another cell:
</p>
<center><imgsrc="nmt_handover.jpg"/></center>
<p>
Roaming:
</p>
<center><imgsrc="nmt_roaming.jpg"/></center>
<p>
Other features:
</p>
<p>
To reduce radio noise, a syllabic compandor is used on both ends.
This compresses the audio level dynamics by 2.
An audio level of 25% is raised to 50% by the compressor and lowered to 25% by the expander.
A radio noise floor of 10% would be reduced to 1% by the expander.
</p>
<p>
SMS transmitted via a protocol that it similar to the GSM 04.11 standard.
To transport SMS data, the user data facility (DMS) of the NMT network is used.
DMS re-uses the existing signaling hardware of the mobile phone and offers a data link protocol with backward error correction.
</p>
<p>
Caller ID was added to the system.
The caller ID is transmitted before the first ringing order.
It is repeated between ringing orders, in case of lost frames.
Tune your receiver to the donw-link frequency, so you get loop-back of base station broadcast.
Use the variable resistor connecting your transmitter, to adjust the volume until the received level matches the same level you previously received from the phone.
Now, whatever frequency deviation the phone transmits for signaling, so does your base station, because you receive the same level from your base station as you did from the phone.
Use the other variable resistor connecting your receiver, to adjust the volume until the level matches about 100%. (+- 10% is good)
Now, your base station receives the correct frequency deviation as 100%. Also voice levels are now adjusted right.
Switch back the receiver to up-link frequency and change the traffic area to see the result of your adjustment as the phone registers.
If you cannot define the SMSC on your phone, send an SMS and check what number was dialed, then use '-S <number>' or '--smsc-number <number>' to define SMSC at this software.