Handle Timing Advance IE properly

Move writing Timing Advance IE and Timing Advance Index into separate
functions to simplify adding PTCCH support. This also fixes previous
incorrect (and unused) code for writing Packet TA IE which has not set
TS for TA.

Change-Id: I786bf7fc999d401cc3d9e7f1e7a1fba953b5d458
Related: OS#1545

src/encoding.cpp

@@ -35,6 +35,43 @@ extern "C" {
 #include <errno.h>
 #include <string.h>
 
+/* { 0 | 1 < TIMING_ADVANCE_INDEX : bit (4) > } */
+static inline bool write_tai(bitvec *dest, unsigned& wp, int8_t tai)
+{
+	if (tai < 0) { /* No TIMING_ADVANCE_INDEX: */
+		bitvec_write_field(dest, wp, 0, 1);
+		return false;
+	}
+	/* TIMING_ADVANCE_INDEX: */
+	bitvec_write_field(dest, wp, 1, 1);
+	bitvec_write_field(dest, wp, tai, 4);
+	return true;
+}
+
+/* { 0 | 1 < TIMING_ADVANCE_VALUE : bit (6) > } */
+static inline void write_ta(bitvec *dest, unsigned& wp, int8_t ta)
+{
+	if (ta < 0) /* No TIMING_ADVANCE_VALUE: */
+		bitvec_write_field(dest, wp, 0, 1);
+	else { /* TIMING_ADVANCE_VALUE: */
+		bitvec_write_field(dest, wp, 1, 1);
+		bitvec_write_field(dest, wp, ta, 6);
+	}
+}
+
+/* 3GPP TS 44.060 § 12.12:
+   { 0 | 1 < TIMING_ADVANCE_VALUE : bit (6) > }
+   { 0 | 1 < TIMING_ADVANCE_INDEX : bit (4) >
+           < TIMING_ADVANCE_TIMESLOT_NUMBER : bit (3) > }
+ */
+static inline void write_ta_ie(bitvec *dest, unsigned& wp,
+			       int8_t ta, int8_t tai, uint8_t ts)
+{
+	write_ta(dest, wp, ta);
+	if (write_tai(dest, wp, tai)) /* TIMING_ADVANCE_TIMESLOT_NUMBER: */
+		bitvec_write_field(dest, wp, ts, 3);
+}
+
 static int write_ia_rest_downlink(
 	gprs_rlcmac_dl_tbf *tbf,
 	bitvec * dest, unsigned& wp,
@@ -62,12 +99,7 @@ static int write_ia_rest_downlink(
 	bitvec_write_field(dest, wp,gamma,5);   // GAMMA power control parameter
 	bitvec_write_field(dest, wp,polling,1);   // Polling Bit
 	bitvec_write_field(dest, wp, ta_valid, 1); // N. B: NOT related to TAI!
-	if (ta_idx < 0) {
+	write_tai(dest, wp, ta_idx);
-		bitvec_write_field(dest, wp,0x0,1);   // switch TIMING_ADVANCE_INDEX = off
-	} else {
-		bitvec_write_field(dest, wp,0x1,1);   // switch TIMING_ADVANCE_INDEX = on
-		bitvec_write_field(dest, wp,ta_idx,4);   // TIMING_ADVANCE_INDEX
-	}
 	if (polling) {
 		bitvec_write_field(dest, wp,0x1,1);   // TBF Starting TIME present
 		bitvec_write_field(dest, wp,(fn / (26 * 51)) % 32,5); // T1'
@@ -110,12 +142,7 @@ static int write_ia_rest_uplink(
 		} else
 			bitvec_write_field(dest, wp,0x0,1);   // ALPHA = not present
 		bitvec_write_field(dest, wp,gamma,5);   // GAMMA power control parameter
-		if (ta_idx < 0) {
+		write_tai(dest, wp, ta_idx);
-			bitvec_write_field(dest, wp,0x0,1);   // switch TIMING_ADVANCE_INDEX = off
-		} else {
-			bitvec_write_field(dest, wp,0x1,1);   // switch TIMING_ADVANCE_INDEX = on
-			bitvec_write_field(dest, wp,ta_idx,4);   // TIMING_ADVANCE_INDEX
-		}
 		bitvec_write_field(dest, wp, 1, 1);    // TBF_STARTING_TIME_FLAG
 		bitvec_write_field(dest, wp,(fn / (26 * 51)) % 32,5); // T1'
 		bitvec_write_field(dest, wp,fn % 51,6);               // T3
@@ -311,6 +338,8 @@ void Encoding::write_packet_uplink_assignment(
 	// TODO We should use our implementation of encode RLC/MAC Control messages.
 	unsigned wp = 0;
 	uint8_t ts;
+	/* timeslot assigned for the Continuous Timing Advance procedure */
+	uint8_t ta_ts = 0; /* FIXME: supply it as parameter from caller */
 
 	bitvec_write_field(dest, wp,0x1,2);  // Payload Type
 	bitvec_write_field(dest, wp,0x0,2);  // Uplink block with TDMA framenumber (N+13)
@@ -334,15 +363,7 @@ void Encoding::write_packet_uplink_assignment(
 		bitvec_write_field(dest, wp,0x0,1); // Message escape
 		bitvec_write_field(dest, wp,tbf->current_cs().to_num()-1, 2); // CHANNEL_CODING_COMMAND 
 		bitvec_write_field(dest, wp,0x1,1); // TLLI_BLOCK_CHANNEL_CODING 
-		bitvec_write_field(dest, wp,0x1,1); // switch TIMING_ADVANCE_VALUE = on
+		write_ta_ie(dest, wp,tbf->ta(), ta_idx, ta_ts);
-		bitvec_write_field(dest, wp,tbf->ta(),6); // TIMING_ADVANCE_VALUE
-		if (ta_idx < 0) {
-			bitvec_write_field(dest, wp,0x0,1);   // switch TIMING_ADVANCE_INDEX = off
-		} else {
-			bitvec_write_field(dest, wp,0x1,1);   // switch TIMING_ADVANCE_INDEX = on
-			bitvec_write_field(dest, wp,ta_idx,4);   // TIMING_ADVANCE_INDEX
-		}
-
 	} else { /* EPGRS */
 		unsigned int ws_enc = (tbf->m_window.ws() - 64) / 32;
 		bitvec_write_field(dest, wp,0x1,1); // Message escape
@@ -357,16 +378,7 @@ void Encoding::write_packet_uplink_assignment(
 		bitvec_write_field(dest, wp,0x0,1); // No ARAC RETRANSMISSION REQUEST
 		bitvec_write_field(dest, wp,0x1,1); // TLLI_BLOCK_CHANNEL_CODING 
 		bitvec_write_field(dest, wp,0x0,1); // No BEP_PERIOD2
-
+		write_ta_ie(dest, wp,tbf->ta(), ta_idx, ta_ts);
-		bitvec_write_field(dest, wp,0x1,1); // switch TIMING_ADVANCE_VALUE = on
-		bitvec_write_field(dest, wp,tbf->ta(),6); // TIMING_ADVANCE_VALUE
-		if (ta_idx < 0) {
-			bitvec_write_field(dest, wp,0x0,1);   // switch TIMING_ADVANCE_INDEX = off
-		} else {
-			bitvec_write_field(dest, wp,0x1,1);   // switch TIMING_ADVANCE_INDEX = on
-			bitvec_write_field(dest, wp,ta_idx,4);   // TIMING_ADVANCE_INDEX
-		}
-
 		bitvec_write_field(dest, wp,0x0,1); // No Packet Extended Timing Advance
 	}