Proper measurement reporting

[Harald Welte]

Hi Vadim,

following up on this as I was browsing through the list of open issues
and ended up on OS#2988 and https://gerrit.osmocom.org/#/c/osmocom-bb/+/7470/

On Fri, Mar 23, 2018 at 08:20:22PM +0700, Vadim Yanitskiy wrote:
> In short, according to the GSM TS 04.08, section 3.4.1.1 "SACCH
> procedures", Measurement Report messages are sent at each possible
> occasion when nothing else has to be sent. In other words, a dummy
> LAPDm fill frame (0x01, 0x03, 0x01, 0x2b, ...) is not applicable here.

this part is clear so far.

> The Calypso PHY (i.e. the firmware) is sending self-composed
> Measurement Reports if there is nothing in transmit queue:
>
> [...]
>
> I am not sure if this is the correct way. Why?
> 
>   - The Measurement Reports coming from the higher
>     layers may contain additional information, such
>     as the neighbour measurements.
> 
>   - The higher layers may spoof indicated TA value,
>     while this code uses the actual one.
> 
>   - Measurement Reporting is already implemented
>     in the higher layers, so this duplicates...

I agree.

> My current ideas are:
> 
>   1) Create a separate L1CTL message, that would be
>      used to carry Measurement Reports. This way
>      we wouldn't need to extract them from the
>      L1CTL_DATA_REQ messages manually.

As sylvain already stated, there doesn't seem to be a need for yet another
extra primitive.  For the MS side, it is a DATA_REQ on the uplink SACCH SAPI.

>   2) Keep a last Measurement Report somewhere, and
>      transmit it until a new one is arrived from
>      the higher layers.

That makes sense.  However, that cached last measurement report should have
some kind of age attached.  There's no point in tranmitting neighbor measurements
that are 5 minutes old, particularly not for a mobile user.

> Probably, some parts of the Measurement Reporting implementation
> should be moved to L1, i.e. to the firmware, trxcon and VIRT-PHY.
> This way each Measurement Report would always contain the actual
> measurement results at the moment of transmission...

Measurement reports are sent every SACCH multiframe (102 or 104 frames), i.e. about
every 500ms.  Also, the end of the measurement period is defined independently from
the time at which a measurement report is being sent.  There are, AFAICT, about 12 frames
between the end of the measurement period and the start of the transmission of the
measurement report.  That's about 55ms of time.

I think 55ms might be indeed a bit tight to account for
Caylpso->UART->osmocon->mobile->osmocon->UART->Calypso.  At 115200 bps, there's onyly 633
characters in that period.

Hoewver, I think we shouldn't focus too much on the constraints of the slow serial link
of calypso phones here.  In the worst case, our measurement reports will be 480ms old, so be it.

-- 
- Harald Welte <laforge at gnumonks.org>           http://laforge.gnumonks.org/
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