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<html>
<head>
<link href="style.css" rel="stylesheet" type="text/css" />
<title>osmocom-analog</title>
</head>
<body>
<center><table><tr><td>

<h2><center>C-Netz</center></h2>

<center><img src="c-netz.jpg"/></center>

<ul>
	<li><a href="#history">History</a>
	<li><a href="#howitworks">How it works</a>
	<li><a href="#basestation">Setup of a base station</a>
</ul>

<p class="toppic">
<a name="history"></a>
History
</p>

<p>
C-Netz was the third mobile telephone network in Germany.
It existed between 1984 (officially Mai 1985) and December 2000.
In 1989, the German Wall came down and C-Netz was quickly deployed in eastern Germany to support the congested fixed network.
It was the first cellular system, so frequencies could be used more efficiently.
It offered handover capability, so moving phones could change cells during a call.
The SIM card was introduced, which is similar to GSM.
This card allowed to use phones by different subscribers with different subscriber numbers.
Authentication was used to prevent fraud, but it was used only with SIM card phones. The magnetic cards could be cloned by receiving 'roaming' messages.
Extra services (e.g. conditional call forwarding, answering machine) were installed in the mobile exchange.
A scrambler inside the phone and base station prevented eavedropping with radio scanners.
Phones became smaller and portable, even handheld phones were available.
All phones were available under a dedicated prefix, so the caller must not know the location of the phone anymore.

</p>

<center><img src="c-netz-c130.jpg"/></center>

<p>
<ul>
	<li>Frequency range: 461.300 - 465.740 MHz (downlink); 451.300 - 465.740 MHz (uplink)
	<li>222 voice channels
	<li>237 voice channels since Mai 1988
	<li>287 voice channels since September 1989
	<li>1 organisation channel (call setup and paging)
	<li>Duplex distance: 10 MHz
	<li>Channel spacing: 10 KHz and optionally 12.5 KHz
	<li>Voice modulation: FM
	<li>Signalling modulation: carrier FSK
	<li>Frequency deviation: 2.4 KHz (FSK)
	<li>Mobile station transmit power: 50 mW up to 15 Watts
	<li>Base station transmit power: 25 Watts
</ul>
</p>


<p class="toppic">
<a name="howitworks"></a>
How it works
</p>

<p>
Like AMPS (Advanced Mobile Phone Service), the C-Netz is a cellular network.
</p>

<center><img src="c-netz_zellstruktur.png"/></center>

<p>
Base stations are grouped into clusters.
Each cluster has arround 7 base stations.
Each base station in a cluster must have different channels.
As depictet, the frequency of a base station marked with "1" can be re-used in other clusters.
To handle more calls with the same amount of channels, the size of a cell must be reduced.
By using small cell structure, it is likely to loose radio link while calling and moving at the same time.
To prevent loss of radio link while moving to far away from a cell, handover to stronger or closer cell is performed.
</p>

<center><img src="c-netz_frequenzen.png"/></center>

<p>
Two frequency bands are used, one for uplink and one downlink.
Only one exception is the control channel "131".
It is the same for all base stations.
To avoid collisions, the channel is divided into time slots.
The time slots are re-used in the same way as channels are re-used in other clusters.
This reduces the number of channels for call setup to only one channel.
The mobile phone does not have to search for a valid control channel first.
Also it is possible to have a dedicated control channel for cells in high traffic areas.
</p>

<center><img src="c-netz_zeitschlitze.png"/></center>

<p>
As depicted, the base station and mobile station use one or more time slots.
Generally every 8th time slot is used.
In this case the base station transmits every 600ms.
The mobile phone transmits two time slots later.
Every time slot contains two message blocks.
One block is used for call setup.
The other block is used for periodic check if the phone is still available.
</p>

<p>
Message blocks contain 70 bits.
The first 6 bits define the message type.
The other 64 bits depend on the message type.
The 70 bits are grouped into 10 words of 7 bits each.
Eeach word gets 8 redudancy bits by using a BCH code.
The redundany allows to correct up to two bit errors in a word.
</p>

<center><img src="c-netz_interleaving.png"/></center>

<p>
To correct burst errors (several bit errors after each other), the bits are interleaved before transmitted.
As depicted above, the first bit of all words are transmitted first, then the next bit and so on.
</p>

<center><img src="c-netz_frame_k.png"/></center>

<p>
A sync word of 11 bits is added in front of each message three times.
Additional 14 guard bits are used to allow the transmitter to ramp up and down RF power between time slots.
</p>

<center><img src="c-netz_frame_v.png"/></center>

<p>
Also message blocks are transmitted during voice conversation.
The blocks are used to terminate the call, perform handover, measure link or transmit metering information.
4 bits the message is transmitted every 12.5 ms. After 600 ms, the complete frame is transmitted.
To remove the 4 bits from audio stream, the voice is time compressed (speed up) to 10/11th.
The 4 bits + 2 guard bits are transmitted in the 1/11th gap between voice chunks.
On the receiving end, the voice chunks are expanded again.
The phone will keep in sync with the voice chunks by keeping in sync with the message bits.
</p>

<center><img src="c-netz_frame_v2.png"/></center>

<p>
The level at the end of one voice chunk may not match the level of the beginning of the next chunk.
The offset must be removed by the receiver.
</p>

<center><img src="c-netz_lr.png"/></center>

<p>
This is an example of a broadcast message by a base station on the control channel.
The message type 'O' (letter o) is "100111", so this message block contains cell informations and access informations.
</p>

<center><img src="c-netz_symbole.png"/></center>

<p>
The location of parameters inside a message (information elements) are shown by letters.
Each letter is a different parameter as depicted above.
</p>

<center><img src="c-netz_call.png"/></center>

<p>
The first thing a phone does is looking for a strongest cell.
The phone accesses the cell by transmitting a location update message.
The cell acknowledges or rejects the phone.
</p>

<p>
The call setup is depiced above.
The mobile phone transmits a message that it wishes to make a normal call or emergency call.
If the channel is available, the base station requests the dialed number from the phone.
The phone transmits the dialed number.
If the number is ok, the base station acknowledges the dialed number.
After a voice channel is allocated, the phone is requested to change to that channel two time slots later.
On this voice channel, the mobe phone and the base station transmit 8 messages to prove quality of channel.
Messages VH1 and VHK are transmitted while the call is set up.
Then the phone is ordered to switch to voice conversation.
The following messages are transmitted between the voice chunks.
</p>

<p class="toppic">
<a name="basestation"></a>
Setup of a base station
</p>

<p>
When I first started with testing early C-Netz implementation, I had great trouble getting a response from a mobile phone.
The main problem was clock speed of the transmitted signal.
The base stations were all synchronized with a high precision clock.
All base stations transmit on the same control channel 131 ('Orgakanal'), but at different time slots.
The phone expects that the time slots repeat precisely every 2.4 seconds.
The specs say that the phone requires clock speed deviation of +- 1 PPM (Parts Per Million) or less.
To comply with the specs, the base stations needs to have a clock deviation far below 1 PPM.
(I guess that there is a tollerance, so maybe a few PPM deviation would make no big problem.)
We need to calibrate the speed of our transmit signal (and receive signal as well).
</p>

<p>
<font color="red">Importaint: We need calibrate the signal processing clock!</font>
</p>

<p>
First of all we need a reference clock.
Since most computer systems have standard crystals, we cannot use our system clock as reference without any help.
I decided to use the NTP daemon.
After running the system for several days, the clock speed should be accurate enough for our needs.
Be sure to choose a place for your PC with constant temperature.
</p>

<p>
In /etc/ntp.conf define one or more servers to get the time from. I suggest to use the German atom clock of the PTB:
</p>

<pre>
server ntp1.ptb.de
server ntp2.ptb.de
server ntp3.ptb.de
</pre>

<p>
This clock provides time base for European radio clocks and most German clocks.
It accurate enough for this project, so please use this clock.
</p>

<pre>

netbook ~ # ntptime
ntp_gettime() returns code 0 (OK)
  time dad971f1.9e1b5c44  Sun, May  8 2016 10:06:09.617, (.617605507),
  maximum error 238951 us, estimated error 199 us, TAI offset 0
ntp_adjtime() returns code 0 (<font color="red">OK</font>)
  modes 0x0 (),
  offset 425.797 us, frequency <font color="red">15.796 ppm</font>, interval 1 s,
  maximum error 238951 us, estimated error 199 us,
  status 0x2001 (PLL,NANO),
  time constant 10, precision 0.001 us, tolerance 500 ppm,

</pre>

<p>
One notices that I have quite an unprecise crystal in my test machine.
Since I have NTP running, the system clock is now far below 1 PPM.
</p>

<p>
The second problem is the clock of our sound adapter.
They use own crystals.
Because they are made for audio signals (and not for high precise base stations), they can't be used without calibration.
Cheap USB adapters may even have different clocks on TX and RX.
I found out that the clock speed changes when the USB voltage changes.
The USB voltage changes, when the current changes, due to different volumes on the output.
To minimize this effect, don't use extension cables.
</p>

<p>
Since we have precise system clock now, we can measure the speed of our sound adapter.
Later we will use the measurement result to adjust our signal processor.
Be sure to have your machine running for at least 5 to 7 days (with your sound adapter attached, in case you have USB sound adapter).
The machine must have network connection to the NTP server.
I suggest to use 'ntp1.ptb.de' and 'ntp2.ptb.de' as reference.
Use command line option '-M' or '--measure' to measure my sound adapter.
Let it run for one hour.
</p>

<pre>

# src/cnetz/cnetz -k 131 -M
...
dsp.c:218 notice : Clock: RX=4.75 TX=36.84; Signal: RX=135.38 TX=-378.58 ppm
dsp.c:218 notice : Clock: RX=10.61 TX=31.09; Signal: RX=135.38 TX=-378.58 ppm
dsp.c:218 notice : Clock: RX=3.01 TX=26.55; Signal: RX=135.38 TX=-378.58 ppm
...
dsp.c:218 notice : Clock: RX=19.46 TX=19.45; Signal: RX=19.39 TX=19.42 ppm
dsp.c:218 notice : Clock: RX=19.45 TX=19.50; Signal: RX=19.39 TX=19.42 ppm
dsp.c:218 notice : Clock: RX=19.48 TX=19.47; Signal: RX=19.39 TX=19.42 ppm
dsp.c:218 notice : Clock: RX=19.44 TX=19.44; Signal: RX=19.35 TX=19.43 ppm
dsp.c:218 notice : Clock: RX=19.44 TX=19.49; Signal: RX=19.35 TX=19.43 ppm

</pre>

<p>
One can see that the measured clock from the sound card (sample rate) differs about +19.45 PPM.
Because I did not do any correction to the signal processing, the same error applies to the signal.
Use command line option '-S 19.45,19.45' or '--clock-speed 19.45,19.45' to compensate the clock error by adjusting the signal processor.
Let it run again for one hour and you will see that clock deviation is below 1 PPM.
</p>

<p>
<font color="red">Importaint: We need to connect to PLL and discriminator!</font>
</p>

<p>
C-Netz does not use a modulated tone to carry bits of information, instead it modulates the carrier directly.
This image shows the demodulated signal from a receiver:
</p>

<center><img src="c-netz-burst_full.png"/></center>

<p>
Generally a receiver will do de-emphasis and filtering after demodulating the signal.
We don't want that.
It may work, but be sure to get a receiver with disciminator output.
Many receivers can be modified, so search the Web for more infos.
</p>

<p>
On the transmitter we don't want to have filtering and pre-emphasis.
It may work, but be sure to get a transmitter that allows to disable emphasis.
I took the schematics of my transmitter and connected the sound card
directly to the PLL.
</p>

<p>
<font color="red">Importaint: We need to transmit correct polarity, use auto polarity detection always!</font>
</p>

<center><img src="c-netz-burst_part.png"/></center>

<p>
C-Netz uses FSK (Frequency Shift Keying) by directly modulating the carrier, rather than modulating a tone, which is then transmitted via FM.
The advantage is much higher data rate.
The base station's carrier frequency is raised by 2.5 KHz to transmit a logical "1" and lowered by 2.5 KHz to transmit a logial "0".
(The mobile station transmits reversed polarity.)
The mobile station will only detect the correct polarity.
The three sync words of "11100010010", is shown in the picture above.
In the picture above, the polarity is wrong.
(The logical "1" points down and the logical "0" points up.)
To change polarty, use command line option '-F yes|no|auto' or '--flip-polarity yes|no|auto'.
(Note: don't confuse with '-P'!)
</p>

<p>
The 'auto' option (default) will generate a signal of two base stations.
Since we have time division multiplex on the main control channel, we can transmit a second base station easily.
The second base station uses reversed polarity, so there is one base station with correct and one with wrong polarity.
The two base stations have different time slots and different IDs.
If a first registration message is received from the phone, we check which base station this response belongs to.
Then we found the correct polarity and turn off the base station with the wrong polarity.
</p>

<p>
The decoder itself does not care about what polarity it receives from the mobile station.
It just checks for sync words with both polarities and chooses right polarity automatically.
</p>

<p>
I have a transmitter and a receiver without emphasis, my soundcard has a clock error of about 19.2 PPM.
To run the base station I the following command line options:
</p>

<pre>

# cnetz -k 131 -E -e -S 19.25,19.25
...
cnetz.c:142 notice : You selected channel 131 ('Orga-Kanal') for combined callin
g + traffic channel. Some phones will reject this.
dsp.c:96 info   : Using clock speed of 19.2 ppm (RX) and 19.2 ppm (TX) to correc
t sound card's clock.
cnetz.c:274 info   : Entering IDLE state on channel 131.
Base station on channel 131 ready, please tune transmitter to 465.090 MHz and re
ceiver to 455.090 MHz.

</pre>

<p>
The sofware complains about using control channel 131 for voice traffic too.
This is the easiest way to use a single transmitter and receiver without switching.
But this is not compliant, so especially newer phone will reject this, like the Nokia C130.
Old phones like BSA, Storno or Phillips phones don't care.
When we make a call and another phone is regisered, the phone will loose coverage when we switch from control channel to traffic channel.
After the call, the other phone re-registers again.
</p>

<p>
You should tune the receiver to 465.090 first, to check if you can hear and decode the idle signal from the base station.
Then tune to actually uplink frequency 455.090 MHz.
The actual level is not yet relevant.
(You may check the quality using '-L 2' command line option and build a radio loop by tuning the receiver to the transmitter.)
</p>

<p>
Now power up a phone.
Depening on your card reader, insert chip or magnetic card.
Hopefully you get a response from the phone.
If it responds, you will hear a short burs on receiver.
This bursts is just 37.5 ms long and sounds a bit less noisy than the noise floor.
If the burst is received and correctly decoded, the base station knows the transmitter's polarty and selects it.
</p>

<pre>

telegramm.c:1486 info   : RX Level: 96% Jitter: 0.04 Sync Time: 3968.18 (TS 10.02) POSITIVE
***********************************************
*** Autoselecting positive FSK TX polarity! ***
***********************************************
cnetz.c:895 info   : Received Attachment 'Einbuchen' message from Subscriber '6215784' with chip card's security code 11029
transaction.c:208 info   : Transaction state IDLE -&gt; EM
transaction.c:70 info   : Created transaction for subscriber '6215784'
database.c:117 info   : Adding subscriber '6,2,15784' to database.
database.c:121 info   : Subscriber '6,2,15784' busy now.
cnetz.c:709 notice : Frame sync offset = 800.18, correcting!
cnetz.c:760 info   : Sending acknowledgement 'Einbuchquittung' to Attachment request.
database.c:124 info   : Subscriber '6,2,15784' idle now.
transaction.c:89 info   : Destroying transaction for subscriber '6215784'
transaction.c:208 info   : Transaction state EM -&gt; IDLE

</pre>

<p>
Level adjustment:
We see a receive level of arround 96%.
This is already good, since I did the adjustment previously.
Tune your receiver to the uplink frequency, so you get loopback of base station broadcast.
Use the variable resistor (connecting your transmitter) to adjust the volume until the RX level matches the same level of your previously received bust.
In my case I adjust the transmitter to match arround 96%. (+- 10% is good)
Now, whatever frequency deviation the phone transmits for signalling, so does your base station.
Use the other variable resistor (connecting your receviver) to adjust the volume until the level matches about 100%. (+- 10% is good)
Switch back the receiver to uplink frequency and restart the phone.
</p>

<p>
After some time (1 minute) the base station will check, if the phone is still available.
It transmits a request (MA) to the phone.
The phone answers to the request (MFT):
</p>

<pre>

telegramm.c:1473 info   : Ignoring mysterious unmodulated telegramm (noise from phone's transmitter)
database.c:72 info   : Check, if subscriber '6,2,15784' is still available.
transaction.c:208 info   : Transaction state IDLE -&gt; MA
transaction.c:70 info   : Created transaction for subscriber '6215784'
database.c:121 info   : Subscriber '6,2,15784' busy now.
cnetz.c:863 info   : Sending keepalive request 'Meldeaufruf'
transaction.c:208 info   : Transaction state MA -&gt; MFT
telegramm.c:1486 info   : RX Level: 96% Jitter: 0.02 Sync Time: 6534.01 (TS 16.50) POSITIVE
cnetz.c:969 info   : Received acknowledge 'Meldung Funktelefonteilnehmer' message from Subscriber '6215784'
database.c:124 info   : Subscriber '6,2,15784' idle now.
transaction.c:89 info   : Destroying transaction for subscriber '6215784'
transaction.c:208 info   : Transaction state MFT -&gt; IDLE

</pre>

<p>
To make a call from the phone to the base station, just enter some digits and press the dial button on the phone.
The phone requests a call (VWG).
The base station requests dialed number (WAF).
The phone transmits dialed number (WUE).
The base station acknowledges the call (WBP) and then orders the phone to switch to speech channel (VAG).
Then measurement and through connection of speech takes place:
</p>

<pre>

telegramm.c:1486 info   : RX Level: 96% Jitter: 0.04 Sync Time: 12671.99 (TS 32.00) POSITIVE
cnetz.c:931 info   : Received outgoing Call 'Verbindungswunsch gehend' message from Subscriber '6215784'
transaction.c:208 info   : Transaction state IDLE -&gt; VWG
transaction.c:70 info   : Created transaction for subscriber '6215784'
database.c:121 info   : Subscriber '6,2,15784' busy now.
cnetz.c:854 info   : Sending acknowledgement 'Wahlaufforderung' to outging call
transaction.c:208 info   : Transaction state VWG -&gt; WAF
telegramm.c:1486 info   : RX Level: 95% Jitter: 0.04 Sync Time: 3366.02 (TS 8.50) POSITIVE
cnetz.c:957 info   : Received dialing digits 'Wahluebertragung' message from Subscriber '6215784' to Number '02123839349'
transaction.c:208 info   : Transaction state WAF -&gt; WBP
cnetz.c:782 info   : Sending call accept 'Wahlbestaetigung positiv'.
transaction.c:208 info   : Transaction state WBP -&gt; VAG
cnetz.c:789 info   : Sending channel assignment 'Verbindungsaufbau gehend'.
transaction.c:208 info   : Transaction state VAG -&gt; BQ
cnetz.c:807 info   : Staying on combined calling + traffic channel 131
dsp.c:641 info   : Switching channel (mode)
cnetz.c:1014 info   : Sending 'Belegungsquittung' on traffic channel
cnetz.c:1014 info   : Sending 'Belegungsquittung' on traffic channel
cnetz.c:1014 info   : Sending 'Belegungsquittung' on traffic channel
telegramm.c:1486 info   : RX Level: 96% Jitter: 0.02 Sync Time: 9504.02 (TS 24.00) POSITIVE
cnetz.c:1118 info   : Received allocation 'Belegung' message.
cnetz.c:1014 info   : Sending 'Belegungsquittung' on traffic channel
telegramm.c:1486 info   : RX Level: 96% Jitter: 0.02 Sync Time: 9701.99 (TS 24.50) POSITIVE
cnetz.c:1118 info   : Received allocation 'Belegung' message.
...
transaction.c:208 info   : Transaction state BQ -&gt; VHQ
telegramm.c:1486 info   : RX Level: 96% Jitter: 0.03 Sync Time: 10494.00 (TS 26.50) POSITIVE
cnetz.c:1118 info   : Received allocation 'Belegung' message.
cnetz.c:1024 info   : Sending 'Quittung Verbindung halten' on traffic channel
telegramm.c:1486 info   : RX Level: 96% Jitter: 0.03 Sync Time: 10691.98 (TS 27.00) POSITIVE
cnetz.c:1118 info   : Received allocation 'Belegung' message.
...
call.c:623 info   : Incomming call from '6215784' to '02123839349'
transaction.c:208 info   : Transaction state VHQ -&gt; DS
telegramm.c:1486 info   : RX Level: 96% Jitter: 0.02 Sync Time: 11286.00 (TS 28.50) POSITIVE
cnetz.c:1145 info   : Received connection hold 'Verbindung halten' message.
cnetz.c:1051 info   : Sending 'Durchschalten' on traffic channel
telegramm.c:1486 info   : RX Level: 96% Jitter: 0.03 Sync Time: 11483.99 (TS 29.00) POSITIVE
cnetz.c:1145 info   : Received connection hold 'Verbindung halten' message.
...
telegramm.c:1486 info   : RX Level: 96% Jitter: 0.03 Sync Time: 12276.00 (TS 31.00) POSITIVE
cnetz.c:1131 info   : Received assignment confirm 'Durchschaltung Quittung' message.
cnetz.c:1051 info   : Sending 'Durchschalten' on traffic channel
telegramm.c:1486 info   : RX Level: 97% Jitter: 0.02 Sync Time: 12474.01 (TS 31.50) POSITIVE
cnetz.c:1131 info   : Received assignment confirm 'Durchschaltung Quittung' message.
...
transaction.c:208 info   : Transaction state DS -&gt; VHQ
telegramm.c:1486 info   : RX Level: 96% Jitter: 0.03 Sync Time: 1781.99 (TS 4.50) POSITIVE
cnetz.c:1131 info   : Received assignment confirm 'Durchschaltung Quittung' message.
dsp.c:641 info   : Switching channel (mode)
cnetz.c:1238 info   : Sending 'Quittung Verbindung halten' on traffic channel
cnetz.c:1238 info   : Sending 'Quittung Verbindung halten' on traffic channel
cnetz.c:1238 info   : Sending 'Quittung Verbindung halten' on traffic channel
telegramm.c:1486 info   : RX Level: 97% Jitter: 0.07 Sync Time: 5544.05 (TS 14.00) POSITIVE
cnetz.c:1286 info   : Received supervisory frame 'Verbindung halten' message.
cnetz.c:1238 info   : Sending 'Quittung Verbindung halten' on traffic channel
telegramm.c:1486 info   : RX Level: 97% Jitter: 0.07 Sync Time: 8712.02 (TS 22.00) POSITIVE
cnetz.c:1286 info   : Received supervisory frame 'Verbindung halten' message.
cnetz.c:1238 info   : Sending 'Quittung Verbindung halten' on traffic channel
telegramm.c:1486 info   : RX Level: 97% Jitter: 0.09 Sync Time: 11880.04 (TS 30.00) POSITIVE
cnetz.c:1286 info   : Received supervisory frame 'Verbindung halten' message.
cnetz.c:1238 info   : Sending 'Quittung Verbindung halten' on traffic channel
...

</pre>

<p>
When calling to the phone, enter the mobile station's ID and press 'd' for dial.
If the phone is attached, the base station orders the phone to switch to speech channel (VAK).
The rest is similar to incomming calls, excpet the the phone has to answer before the speech is throughconnected:
</p>

<pre>

call.c:490 info   : Outgoing call to 6215784
cnetz.c:416 info   : Call to mobile station, paging station id '6215784'
transaction.c:208 info   : Transaction state IDLE -&gt; VAK
transaction.c:70 info   : Created transaction for subscriber '6215784'
database.c:121 info   : Subscriber '6,2,15784' busy now.
cnetz.c:792 info   : Sending channel assignment 'Verbindungsaufbau kommend'.
transaction.c:208 info   : Transaction state VAK -&gt; BQ
cnetz.c:807 info   : Staying on combined calling + traffic channel 131
dsp.c:641 info   : Switching channel (mode)
cnetz.c:1014 info   : Sending 'Belegungsquittung' on traffic channel
cnetz.c:1014 info   : Sending 'Belegungsquittung' on traffic channel
cnetz.c:1014 info   : Sending 'Belegungsquittung' on traffic channel
telegramm.c:1486 info   : RX Level: 96% Jitter: 0.03 Sync Time: 6335.99 (TS 16.00) POSITIVE
cnetz.c:1118 info   : Received allocation 'Belegung' message.
cnetz.c:1014 info   : Sending 'Belegungsquittung' on traffic channel
telegramm.c:1486 info   : RX Level: 95% Jitter: 0.03 Sync Time: 6534.06 (TS 16.50) POSITIVE
cnetz.c:1118 info   : Received allocation 'Belegung' message.

</pre>

<p>
When the mobile phon terminates, it sends the AT message.
The base station responds with AF message and switches back to control channel.
</p>

<pre>

telegramm.c:1486 info   : RX Level: 97% Jitter: 0.06 Sync Time: 11880.01 (TS 30.00) POSITIVE
cnetz.c:1286 info   : Received supervisory frame 'Verbindung halten' message.
cnetz.c:1238 info   : Sending 'Quittung Verbindung halten' on traffic channel
telegramm.c:1486 info   : RX Level: 97% Jitter: 0.06 Sync Time: 2375.99 (TS 6.00) POSITIVE
cnetz.c:1297 info   : Received release frame 'Ausloesen durch FuTln' message.
transaction.c:208 info   : Transaction state VHQ -&gt; AT
call.c:744 info   : Call has been released with cause=16
cnetz.c:1253 info   : Sending 'Auslosen durch FuTln' on traffic channel
database.c:124 info   : Subscriber '6,2,15784' idle now.
transaction.c:89 info   : Destroying transaction for subscriber '6215784'
transaction.c:208 info   : Transaction state AT -&gt; IDLE
cnetz.c:274 info   : Entering IDLE state on channel 131.
telegramm.c:1486 info   : RX Level: 96% Jitter: 0.06 Sync Time: 5543.98 (TS 14.00) POSITIVE
dsp.c:641 info   : Switching channel (mode)
telegramm.c:1484 info   : RX Level: 97% Jitter: 0.08 Sync Time: 9333.99 (TS 23.57) Bit errors: 6 POSITIVE
telegramm.c:1525 notice : Received Telegramm that is not used OgK channel signalling, ignoring! (opcode 22 = AT(V))

</pre>


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