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trx: Update documentation for fn-advance and rts-advance 

Change-Id: I83eff296bfb7acff9d8560b5346bd9791fe6e7d9
Related: OS#4036
This commit is contained in:
Harald Welte 2019-06-02 16:47:31 +02:00
parent aaca1217be
commit f6b6c48c8b
1 changed files with 45 additions and 2 deletions
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47
doc/manuals/chapters/bts-models.adoc
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@ -245,10 +245,53 @@ local (OsmoBTS) or remote (OsmoTRX) side of the UDP flows.
Set the number of frames to be transmitted to transceiver in advance of
current GSM frame number.
GSM is a TDMA (time division multiple access) system on the radio
interface. OsmoTRX is the "clock master" of that in the Osmocom
implementation. It informs OsmoBTS of the current GSM frame
number. However, as there is non-zero delays (UDP packet trnsmission
delay, operating system scheduler delay on both OsmoTRX and OsmoBTS
side, ...), OsmoBTS must compensate for that delay by "advancing"
the clock a certain amount of time.
In other words, if OsmoTRX informs us that the current frame number is N,
we advance it by `fn-advance` and transmit burst data for
`N + fn-advance` towards OsmoTRX.
The fn-advance should be kept as low as possible to avoid additional
delays to the user voice plane as well as to improve the performance
of the control plane (LAPDm) as well as GPRS.
However, fn-advance must be kept sufficiently high to ensure no
underruns on the OsmoTRX side.
The detailed value will depend on your underlying computer systems,
operating system and related tuning parameters. Running OsmoTRX
on a remote host will inevitably require a higher fn-advance then
running it on the same machine, where the UDP packetes are just passed
over the loopback device.
The default value for `fn-advance` is 20 (corresponding to 92
milliseconds).
===== `osmotrx rts-advance <0-30>`
Set the number of frames to be requested from PCU in advance of current
frame number. Do not change this unless you have a good reason!
Set the number of frames to be requested from L1SAP in advance of current
frame number and fn-advance.
The value specified as `rts-advance` is added to the current GSM frame
number as reported by OsmoTRX *and* the `osmotrx fn-advance` in order
to generate the PH-RTS.ind (ready to send indications) across the L1SAP
interface inside osmo-bts. This will trigger the Layer 2 (LAPDm for
the ocntrol plane, RTP for the voice plane, and OsmoPCU for GPRS) to
generate a MAC block and input it into the osmo-bts-trx TDMA scheduler.
If OsmoTRX reported N as the current frame number, the actual frame number
reported on L1SAP to higher layes will be computed as follows:
N + fn-advance + rts-advance
The default value of `rts-advance` is 5 (corresponding to 23 milliseconds).
Do not change this unless you have a good reason!
===== `osmotrx rx-gain <0-50>`