Cellular Network Infrastructure » Radio Access Network » OsmoBSC

Note: This Howto is still incomplete and will be finished soon. Please do not try anything described in this howto unless we have checked and verified everything with our own equipment!

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== Abstract ==

In a state of the art telephone network everything is syncronized by a central, very high accuracy clock. The BTS also uses this clock for various things like channel allocation, frame clock, bitclock ect. If the clock gets a little bit incorrect (more than 50Hz) the BTS will transmit on a wrong frequency and there will be lots of other effects that might destabilize the radio transmission.

The BS11 is shipped with a precalibrated internal oscillator. If you are lucky the calibration (that is made over 10 years ago) in your BTS is still correct. If not your setup will not work unless you recalibrate the oscillator.

This document illustrates how you can use a ceap HFC-S card with the public telephone network as frequency standard for your E1 line. 

== Modifieing the HFC-S Card ==

All in all we need 2 Signals from the HFC-S Card: F0IO and C4IO. This signals are provided by the HFC-S Chip on the card and can be taken from pin 55 (F0IO) and 54 (C4IO) at the HFC-S Chip. (See also Datasheet page 8).

[[Image(hfc-s_card.jpg, 20%)]]

All you need to do is to solder a wire to C4IO and F0I0. The following image shows our modified card:

[[Image(hfc-s_card_modified.jpg, 20%)]]

Note: We used an old led-wire from an old PC-Tower case. The green wire is connected to F0IO and the black one is connected to C4IO

After you have modified the card you should verify if the card works as expected. Start isdnsync and measure the signals with an oscillosope.

{{{

$ ./isdnsync 1

DL_INFORMATION (tei 0 sapi 0)

-> activating layer 2

PH_ACTIVATE

*** Sync available from interface :-)

DL_ESTABLISH

}}}

[[Image(hfc-s_C4IO_bitclock.jpg, 20%)]]

[[Image(hfc-s_F0IO_frameclock.jpg, 20%)]]

From left to right:  F0IO (4,096Mhz bitclock), C4I0 (8kHz frameclock)

== Connecting the modified HFC-S card to your E1 Card ==

Your HFC-E1 card has two so called PCM-Connectors. (the two connectors in the upper right corner of the card) The signal inputs for C4IO and F0IO can be found on Pin 9 and Pin 10 of the HFC-E1 card.

!!FIXME PICTURE!!!

The image shows how plug from the HFC-S card is connected to the HFC-E1 card.

Note: You should have received a printed documentation with your HFC-E1 card. You will find a detailed pinout of the PCM connectors there.

== Perform the calibration procedure ==

The calibration of the internal oscillator is an ongoing process. In a commerical telephone network the BTSs are calibrated at all times. In an experimental Setup it should be ok to operate the BTS in standalone mode and syncronize it from time to time just as shown here:

Things to do:

 * Connect the HFC-S Card to the S0 line. 

 * Add type=0x00800 to the modprobe hfcmulti ... commandline

 * start isdynsync

 * Configure your BTS from standalone to E1 locked

 * start bsc_hack

 * use bs11_config to monitor the calibration process. The internal oscillator will now take over the E1 clock. You should see that the PLL values changing from time to time. If you see the PLL values leveling off, the calibration is done.

 * Configure your BTS from E1 locked to standalone to finish the procedure.

Note: If you do this the first time it is higly recomended to check if valid signals are present at C4IO and F0IO.

Note: The process takes some time. We recommend to warm up the BTS before you start for at least one hour. It is important that the calibration takes place with work temperature.

== Sources: ==

 * http://lists.gnumonks.org/pipermail/openbsc/2009-June/000387.html

 * http://lists.gnumonks.org/pipermail/openbsc/2009-July/000627.html