325 lines
12 KiB
HTML
325 lines
12 KiB
HTML
<html>
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<head>
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<link href="style.css" rel="stylesheet" type="text/css" />
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<title>osmocom-analog</title>
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</head>
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<body>
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<center><table><tr><td>
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<h2><center>A-Netz</center></h2>
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<center><img src="a-netz.jpg"/></center>
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<ul>
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<li><a href="#history">History</a>
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<li><a href="#howitworks">How it works</a>
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<li><a href="#basestation">Setup of a base station</a>
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</ul>
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<p class="toppic">
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<a name="history"></a>
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History
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</p>
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<p>
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A-Netz was the first mobile telephone network in Germany.
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It existed between 1958 and 1977.
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The first mobile units were huge and heavy and consisted of radio tubes.
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Only big cars could carry these units in their trunk.
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It used full duplex radio link.
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The call was placed by an operator.
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After full deployment, there were 136 base stations with a sum of 317 radio transceivers.
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A maximum of 10,784 subscribers were counted 1971. The basic charge was 65 German Marks (A1-Netz), 45 German Marks (A2-Netz / A3-Netz).
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The German Federal Post Office then raised the price to 270 German Marks in 1972. Also the second mobile network (B-Netz) was deployed.
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The number of subscribers decreased to 787 on shutdown in 1977.
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</p>
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<center><img src="a-netz-inside.jpg"/></center>
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<p>
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<ul>
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<li>Frequency range: between 156 MHz and 174 MHz
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<li>17 operator switched channels (30..46) for A1-Netz
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<li>19 operator switched channels (47..63, 86, 87) for A2-Netz
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<li>5 semi-automatic switched channels (65, 66, 68, 72, 75) for A3-Netz
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<li>Duplex distance: 4.5 MHz
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<li>Channel spacing: 50 KHz (20 kHz for A3-Netz)
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<li>Modulation: FM
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<li>Frequency deviation: 11..15 KHz
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<li>Mobile station transmit power: 8..12 Watts
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<li>Base station transmit power: 5..30 Watts
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</ul>
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</p>
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<p class="toppic">
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<a name="howitworks"></a>
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How it works
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</p>
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<p>
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1. Idle state
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<br><br>
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A base station transmits a continuous 2280 Hz idle tone on down-link channel.
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A mobile station detects this tone and indicates a green light.
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If the channel is busy or unused in that area, the green light does not appear.
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The user must switch channel using the dial.
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</p>
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<center><img src="a-netz-green.jpg"/></center>
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<p>
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2. Call from mobile station
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<br><br>
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The mobile station will only seize the channel, if the green light is on, otherwise it is blocked.
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When the user picks up the handset, the phone switches on its transmitter on up-link channel and transmits a continuous 1750 Hz calling tone.
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The base station receives this tone and turns off its 2280 Hz idle tone.
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The mobile station detects the tone is missing and turns off its 1750 calling tone.
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The base station detects the tone is missing and the speech path is connected through to an operator.
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The operator asks for the mobile station number and the number to dial and places the call.
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</p>
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<p>
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3. Call to mobile station
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<br><br>
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A caller calls the operator and tells the number and the location of the mobile station to reach.
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The first 2 digits of the 7-digits mobile station number define on which channel the mobile station has to be paged.
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(The user of the mobile station as well as the operator have a list of the calling channels for all base station.
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This list shows which 2 digits refer to which channel in a particular area.
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The user must select the right channel in order to get paged.)
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<br><br>
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The third digit of the mobile station number defines what groups of frequencies are used for paging the phone. For each of the last four digits there is a group. (Some digits use the same group.)
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</p>
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<p>
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<ul>
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<li>Number xx0xxxx: 2, 2, 3, 3
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<li>Number xx1xxxx: 1, 1, 2, 2
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<li>Number xx2xxxx: 1, 1, 3, 3
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<li>Number xx3xxxx: 1, 1, 2, 3
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<li>Number xx4xxxx: 1, 2, 2, 3
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<li>Number xx5xxxx: 1, 2, 3, 3
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<li>Number xx6xxxx: 1, 1, 1, 2
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<li>Number xx7xxxx: 1, 1, 1, 3
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<li>Number xx8xxxx: 2, 2, 2, 3
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<li>Number xx9xxxx: 1, 2, 2, 2
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</ul>
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</p>
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<p>
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If a number is 2431790, the third digit ('3') tells us use group 1 for digits four and five, group 2 for digit six and group 3 for digit seven.
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</p>
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<p>
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Group 1:
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<ul>
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<li>Digit 1: 352.5 Hz
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<li>Digit 2: 367.5 Hz
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<li>Digit 3: 382.5 Hz
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<li>Digit 4: 397.5 Hz
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<li>Digit 5: 412.5 Hz
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<li>Digit 6: 427.5 Hz
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<li>Digit 7: 442.5 Hz
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<li>Digit 8: 457.5 Hz
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<li>Digit 9: 472.5 Hz
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<li>Digit 0: 487.5 Hz
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</ul>
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</p>
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<p>
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Group 2:
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<ul>
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<li>Digit 1: 502.5 Hz
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<li>Digit 2: 517.5 Hz
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<li>Digit 3: 532.5 Hz
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<li>Digit 4: 547.5 Hz
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<li>Digit 5: 562.5 Hz
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<li>Digit 6: 577.5 Hz
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<li>Digit 7: 592.5 Hz
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<li>Digit 8: 607.5 Hz
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<li>Digit 9: 622.5 Hz
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<li>Digit 0: 637.5 Hz
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</ul>
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</p>
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<p>
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Group 3:
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<ul>
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<li>Digit 1: 652.5 Hz
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<li>Digit 2: 667.5 Hz
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<li>Digit 3: 682.5 Hz
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<li>Digit 4: 697.5 Hz
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<li>Digit 5: 712.5 Hz
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<li>Digit 6: 727.5 Hz
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<li>Digit 7: 742.5 Hz
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<li>Digit 8: 757.5 Hz
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<li>Digit 9: 772.5 Hz
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<li>Digit 0: 787.5 Hz
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</ul>
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</p>
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<p>
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Because our 4th digit is 1, we find 352.5 Hz in group 1.
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Because our 5th digit is 7, we find 442.5 Hz in group 1.
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Because our 6th digit is 9, we find 622.5 Hz in group 2.
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Because our 7th digit is 0, we find 787.5 Hz in group 3.
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<br><br>
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All these tones are sent simultaneously from the base station to the mobile station.
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The mobile stations has four detectors for the four frequencies.
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If all four tones are detected by the mobile station, a yellow light and a buzzing sound is indicated.
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There is no ring-back indication from the mobile station to the base station.
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When the user picks up the handset, the phone switches on its transmitter on up-link channel and transmits a continuous 1750 Hz answer tone.
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The base station receives this tone and turns off the four paging tones.
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The mobile station detects the tone is missing and turns off its 1750 answer tone.
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The base station detects the tone is missing and the speech path is connected through to the caller (or operator).
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</p>
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<center><img src="a-netz-yellow.jpg"/></center>
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<p>
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Theoretically all combinations of 30 tones could be assigned to 27405 different subscribers.
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Only 24375 combinations can be encoded with the last 5 digits, because some digit combinations do not define 4 different tones or would generate the same 4 tones as other digit combinations do.
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The actual limitation of subscribers were cause by the limited number of available channels.
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</p>
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<p>
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4. Release by the mobile station
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<br><br>
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If the user hangs up, the mobile station transmits a continuous 1750 Hz release tone.
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The base station receives this tone and disconnects the call. It transmits the continuous 2280 Hz idle tone.
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The mobile station receives this tone and turns off the transmitter.
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</p>
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<p>
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5. Release by the base station
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<br><br>
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If the caller or the operator hangs up, the base station returns to idle. It transmits the continuous 2280 Hz idle tone.
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The mobile station receives this tone and turns off the transmitter.
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</p>
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<p class="toppic">
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<a name="basestation"></a>
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Setup of a base station
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</p>
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<p>
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Now run your base station on channel 30.
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Tune the transmitter to 162.050 MHz and the receiver to 157.550 MHz.
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You should tune the receiver to 164.050 MHz first, to check if you hear the idle signal from the base station.
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Then tune to actually up-link frequency 157.550 MHz.
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The actual level is not yet relevant.
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(You may check the quality using '-l 2' or '--loopback' command line option and build a radio loop by tuning the receiver to the transmitter.)
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</p>
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<pre>
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# src/anetz/anetz -k 30
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...
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anetz.c:170 info : Entering IDLE state, sending 2280 Hz tone.
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Base station ready, please tune transmitter to 162.050 MHz and receiver to 157.550 MHz.
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on-hook: ..... (enter 0..9 or d=dial)
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</pre>
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<center><img src="a-netz-display.jpg"/></center>
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<p>
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Turn on you phone and wait for it to warm up (requires about half a minute).
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Tune the phone to channel 30 and switch on your transmitter of the base station.
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The phone should indicate a (green) light, to show idle channel.
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If there is no green light, increase the level of your FM signal to the transmitter.
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Or if you can't increase the audio level on the transmitter, do it with 'alsamixer'.
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</p>
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<pre>
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dsp.c:150 info : Detecting continuous 1750 Hz tone. (level = 137%)
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anetz.c:244 info : Received 1750 Hz calling signal from mobile station, removing idle signal.
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dsp.c:133 info : Lost 1750 Hz tone after 0 ms.
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anetz.c:256 info : 1750 Hz signal from mobile station is gone, setup call.
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call.c:574 info : Incoming call from '' to 'operator'
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... after hangup...
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dsp.c:150 info : Detecting continuous 1750 Hz tone. (level = 141%)
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anetz.c:272 info : Received 1750 Hz release signal from mobile station, sending idle signal.
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anetz.c:201 info : Entering IDLE state, sending 2280 Hz tone.
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call.c:695 info : Call has been released with cause=16
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dsp.c:133 info : Lost 1750 Hz tone after 0 ms.
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call disconnected: hangup (enter h=hangup)
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</pre>
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<p>
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When you pick up the phone, the phone transmits a 1750 Hz calling tone.
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On reception at the base station, the base station removes the 2280 Hz idle tone.
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After that the call is established.
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After establishment, you can use the headset, if present, for speech communication with the phone.
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On hangup, the phone transmit a 1750 Hz hangup tone.
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Then the base station returns to idle again and sends the 2280 Hz idle tone.
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Be sure that the phone turns off the transmitter and indicates the (green) light.
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</p>
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<p>
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Level adjustment:
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</p>
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<p>
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We see a receive level of around 140%.
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Then start the base station using '-l 2' option for loop-back and tune receiver to the transmitter.
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The base station generates a 1750 Hz test signal, just like the mobile phone.
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Use the variable resistor (connecting your transmitter) to adjust the volume until the received level matches the same level of your previously received signal.
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In my case I adjust the transmitter to match around 140%. (+- 10% is good)
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Now, whatever frequency deviation the mobile phone transmits at 1750 Hz, so does your base station.
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Use the other variable resistor (connecting your receiver) to adjust the volume until the level matches about 100%. (+- 10% is good)
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Switch back the receiver to up-link frequency and restart the phone.
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</p>
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<p>
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To call to the phone, be sure that your transmitter transmits 'loud' enough to send the four paging sine waves.
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The frequency deviation should be between 11 and 15 KHz.
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If transmitter limits the deviation, check if the limit can be adjusted up.
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Alternatively a direct connection to the PLL is suggested.
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</p>
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<p>
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Start the base station, if not running already.
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Enter the last 5 digits of the phone's number and press 'd' to dial.
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If you listen to the transmit signal, you should hear 4 low pitched tones at once.
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They are quite loud, if you use the pre-empfasis inside your transmitter. (default)
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The phone should now turn off the green light and indicate an incoming call with a yellow light and a buzzer sound.
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There is no acknowledgment from the phone until you pick up the call.
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If the phone does not indicate an incoming call, increase the volume of the transmit signal, but be sure not to overdrive it.
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Also be sure that you are actually dialing the right number, so the base station generates the correct paging tones for your phone.
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</p>
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<p>
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As there is no emphasis, a transmitter with pre-emphasis will drastically lower the paging tones.
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If you cannot disable pre-emphasis on the transmitter, try to raise the gain of the paging tones.
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Add command "-V 12" to increase gain.
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The frequency deviation of all 4 tones should be around 11 kHz.
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</p>
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<p>
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Instead of transmitting all 4 tones at once, they can be transmitted after each other.
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Each tone is plays for a short time.
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After the last tone has been played, base station starts again with the first tone.
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My phone also responds to a call, even if the tones cycle rather than sent simultaneously.
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In this case the deviation level of each tone is four times higher (+12 dB).
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This may help transmitters with deviation limiters to make the phone ring.)
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Add command line option "-P 100" to send each tone for 100 milliseconds.
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Try something between 50-200 milliseconds, if the phone still doesn't ring.
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Be sure to check: Does your transmitter has enough frequency deviation (15 KHz is suggested)? Do you really send the correct number of your phone, check the frequencies of your phone and use "-D 0" option to see what 4 frequencies the base station actually transmits.
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</p>
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<p>
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Detecting loss of carrier signal:
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</p>
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<p>
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To automatically release the call, when the carrier signal gets lost, look at the B-Netz page.
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It is the same principle.
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</p>
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<hr><center>[<a href="index.html">Back to main page</a>]</center><hr>
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</td></tr></table></center>
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</body>
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</html>
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