src/sba.c

/* sba.c
 *
 * Copyright (C) 2021 by sysmocom - s.f.m.c. GmbH <info@sysmocom.de>
 * Author: Pau Espin Pedrol <pespin@sysmocom.de>
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 2
 * of the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 */

#include <sba.h>
#include <gprs_debug.h>
#include <bts.h>
#include <pcu_utils.h>
#include <pdch.h>
#include <errno.h>

#include <osmocom/core/logging.h>
#include <osmocom/core/talloc.h>
#include <osmocom/gsm/protocol/gsm_04_08.h>
#include <osmocom/gsm/gsm_utils.h>

#include "pdch.h"
#include "pdch_ul_controller.h"

/* TBF Starting Time for assigning SBA.
 * See TS 44.060 12.21 "Starting Frame Number Description".
 * According to spec, k=0 (offset=4) "should not be used, so as to leave time
 * for the MS to analyze the message and get ready to receive or transmit".
 * Hence, k=1 (offset=8-9) is theoretically the minimum offset which could be
 * used.
 *
 * However, it was found, empirically, that it takes around 30-40 FNs time for
 * the Immediate Assignment message created with this "TBF Starting Time" to
 * travel from PCU->BTS and be transmitted over CCCH on the Um interface. Hence,
 * we must account for this delay here, otherwise the MS would be receiving eg.
 * a TBF Starting Time of FN=40 while the Imm Assigned containing it is sent in
 * eg. FN=50, which would be too late for the MS to answer to it.
 *
 * The situation described above, can even get worse on high BTS load for AGCH,
 * since the Immediate Assignment will queue in the BTS waiting to be
 * transmitted one after the other, hence increasing the required delay.
 */
#define AGCH_START_OFFSET 52


struct gprs_rlcmac_sba *sba_alloc(void *ctx, struct gprs_rlcmac_pdch *pdch, uint8_t ta)
{
	struct gprs_rlcmac_sba *sba;
	uint32_t start_fn;

	sba = talloc_zero(ctx, struct gprs_rlcmac_sba);
	if (!sba)
		return NULL;

	/* TODO: Increase start_fn dynamically based on AGCH queue load in the BTS: */
	start_fn = next_fn(pdch->last_rts_fn, AGCH_START_OFFSET);

	sba->pdch = pdch;
	sba->ta = ta;
	sba->fn = pdch_ulc_get_next_free_fn(pdch->ulc, start_fn);

	pdch_ulc_reserve_sba(pdch->ulc, sba);
	return sba;
}

/* Internal use */
static void sba_free_norelease(struct gprs_rlcmac_sba *sba)
{
	bts_do_rate_ctr_inc(sba->pdch->trx->bts, CTR_SBA_FREED);
	talloc_free(sba);
}

void sba_free(struct gprs_rlcmac_sba *sba)
{
	if (pdch_ulc_release_fn(sba->pdch->ulc, sba->fn) < 0)
		LOGPDCH(sba->pdch, DRLCMAC, LOGL_NOTICE,
			"Trying to release unregistered SBA (FN=%u, TA=%u)\n",
			sba->fn, sba->ta);
	sba_free_norelease(sba);
}

void sba_timeout(struct gprs_rlcmac_sba *sba)
{
	/* Upon timeout, the UL Controller node is already released */
	sba_free_norelease(sba);
}