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| -rw-r--r-- | doc/manuals/chapters/bts-models.adoc | 47 |
1 files changed, 45 insertions, 2 deletions
diff --git a/doc/manuals/chapters/bts-models.adoc b/doc/manuals/chapters/bts-models.adoc index 4ae73c0b..e1f8d9a1 100644 --- a/doc/manuals/chapters/bts-models.adoc +++ b/doc/manuals/chapters/bts-models.adoc @@ -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>` |