BTS: accept also relative frame numbers with rach requst

The rach request contains a relative frame number (Fn % 42432),
while BTS::rcv_rach() accepts the full frame number only.

Since the BTS is always aware of the full frame number this is
not a problem. But for BSC co-located PCU schemes it is a problem
since the rach request only contains the relative frame number
as mentioned above.

The pcu continusly receives frame number updates with the GSM time
indication message. It is simple to re-calculate the full frame
number from that information.

This patch makes BTS::rcv_rach() compatible with relative frame
numbers, while not breaking the compatibility for full frame
numbers

Change-Id: Iaa182d8d29c6a0f5fa06064c2eb48b21b1ba2775

1 files changed, 15 insertions, 0 deletions

diff --git a/src/bts.cpp b/src/bts.cpp
index 25cbc60f..e1c6dccf 100644
--- a/src/bts.cpp
+++ b/src/bts.cpp
@@ -536,6 +536,21 @@ int BTS::rcv_rach(uint16_t ra, uint32_t Fn, int16_t qta, uint8_t is_11bit,
 	if (is_11bit)
 		rach_frame_11bit();
 
+	/* Note: If a BTS is sending in a rach request it will be fully aware
+	 * of the frame number. If the PCU is used in a BSC-co-located setup.
+	 * The BSC will forward the incoming RACH request. The RACH request
+	 * only contains the relative frame number (Fn % 42432) in its request
+	 * reference. This PCU implementation has to fit both secenarious, so
+	 * we need to assume that Fn is a relative frame number. */
+
+	/* Ensure that all following calculations are performed with the
+	 * relative frame number */
+	Fn = Fn % 42432;
+
+	/* Restore the full frame number
+	 * (See also 3GPP TS 44.018, section 10.5.2.38) */
+	Fn = Fn + m_cur_fn - m_cur_fn % 42432;
+
 	LOGP(DRLCMAC, LOGL_DEBUG, "MS requests UL TBF on RACH, "
 		"so we provide one \n"
 		"ra=0x%02x Fn=%u qta=%d is_11bit=%d:\n", ra, Fn, qta, is_11bit);