dynamic traffic channels

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Neels Hofmeyr nhofmeyr at sysmocom.de
Tue Jun 28 13:25:48 UTC 2016


On Tue, Jun 28, 2016 at 10:05:28AM +0200, Harald Welte wrote:
> [translated from german]
> is it certain that we switch a channel to PDCH only when 
> gprs mode != none?

A TS can be GSM_PCHAN_TCH_F_PDCH; those are the only ones for which we
send a PDCH ACT message.

We send a PDCH ACT message
- during init (CHANNEL OML's state changed to enabled -> send PDCH ACT),
- and upon channel release ack when pchan == GSM_PCHAN_TCH_F_PDCH.

So the question is, when we receive a channel release ack, could that be
the PDCH release and we switch PDCH right back on by accident? No, because
we only receive a chan rel ack when the *TCH/F* is being released.

That is because the PDCH release is initiated "internally" from the PDCH
DEACT, and thus this condition in common/rsl.c rsl_tx_rf_rel_ack() catches
on, which existed before dyn PDCH:

        if (lchan->rel_act_kind != LCHAN_REL_ACT_RSL) {
                LOGP(DRSL, LOGL_NOTICE, "%s not sending REL ACK\n",
                                gsm_lchan_name(lchan));
                return 0;
        }

In rsl_rx_rf_chan_rel() the rel_act_kind is set to LCHAN_REL_ACT_RSL, but
not in the rsl_rx_dyn_pdch().

This is analogous to the ip.access way -- the ip.access nanobts replies to
a PDCH DEACT with a PDCH DEACT ACK and doesn't send a separate channel
release ack.

Maybe we could set rel_act_kind to some new LCHAN_REL_ACT_IPAC_DYN_PDCH
for clarity? (But we shouldn't actually send a release ack, to stay
compatible.)

Even though it works as-is, we should indeed add another flag check:
- We do check the flags that no ACT/DEACT is already pending;
- And we do send a PDCH DEACT only if ts->flags & TS_F_PDCH_ACTIVE;
- But we would send a PDCH ACT despite ts->flags & TS_F_PDCH_ACTIVE.
This should never happen, but it would make sense to ensure that.

~Neels

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