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Encoding: ACK/NACK: always encode with length field present 

In most cases the length field was present and this field takes 7
bits of the maximum available 110 rest bits.
The length field was only removed when encoding huge bitmaps usually
only happen on lossy connections with packet lost.
However the cases without length field were encoded incorrect,
because all remaining bits must be used by the uncompressed bitmaps,
but the PCU violates this by encoding always the "release 5" bit.
Rather than fixing the encoding without length field, simply remove it
and always encode with length field. This also reduces the code
complexity.

Change-Id: I7bc2e18d647b72b8f17ba7a5c9c5e421d88275fb
This commit is contained in:
Alexander Couzens 2019-06-16 17:05:40 +02:00
parent 5c3a9880ca
commit 210ccf4a1d
1 changed files with 39 additions and 43 deletions
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82
src/encoding.cpp
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@ -828,6 +828,17 @@ static void write_packet_uplink_ack_gprs(
bitvec_write_field(dest, &wp, 0, 1); // 0: don't have REL 5
};
/* Encode the Ack/Nack for EGPRS. 44.060
* The PCU encodes to receive block bitmap to the following rules:
* - always encode the lenght field
* - use compressed receive block bitmap if it's smaller than uncompressed
* receive block bitmap
* - use the remaining bits for an uncompressed receive block bitmap if needed
*
* Note: The spec also defines an Ack/Nack without length field, but the PCU's
* doesn't support this in UL. It would require a lot more code complexity
* and only saves 7 bit in lossy sitations.
*/
static void write_packet_ack_nack_desc_egprs(
bitvec * dest, unsigned& wp,
gprs_rlc_ul_window *window, bool is_final, unsigned rest_bits)
@ -849,12 +860,12 @@ static void write_packet_ack_nack_desc_egprs(
bitvec ucmp_vec;
bitvec crbb_vec;
uint8_t uclen_crbb = 0;
bool len_coded = true;
uint8_t crbb_start_clr_code;
uint8_t i;
/* static size of 16 bits
..0. .... = ACKNACK: (Union)
..1. .... = ACKNACK: (Union)
0 0000 000 Length
Desc
...0 .... = FINAL_ACK_INDICATION: False
@ -865,8 +876,9 @@ static void write_packet_ack_nack_desc_egprs(
.... ..10 0101 0001 1... .... = STARTING_SEQUENCE_NUMBER: 1187
.0.. .... = CRBB Exist: 0 */
rest_bits -= 16;
.0.. .... = CRBB Exist: 0
minimal size is 24 rest_bits */
rest_bits -= 24;
if (num_blocks > 0)
/* V(Q) is NACK and omitted -> SSN = V(Q) + 1 */
@ -903,47 +915,31 @@ static void write_packet_ack_nack_desc_egprs(
crbb_len = crbb_vec.cur_bit;
}
if (is_compressed == 0) {
/* length field takes 8 bits*/
if (num_blocks > rest_bits - 8) {
eow = false;
urbb_len = rest_bits;
len_coded = false;
} else if (num_blocks == rest_bits) {
urbb_len = rest_bits;
len_coded = false;
} else
urbb_len = num_blocks;
len = urbb_len + 15;
if (is_compressed) {
/* 8 = 7 (CRBBlength) + 1 (CRBB starting color code) */
rest_bits -= 8;
} else {
if (num_blocks > uclen_crbb) {
eow = false;
urbb_len = num_blocks - uclen_crbb;
}
/* Union bit takes 1 bit */
/* Other fields in descr of compresed bitmap takes 23 bits
* -8 = CRBB_STARTING_COLOR_CODE + CRBB_LENGTH */
if (urbb_len > (rest_bits - crbb_len - 8)) {
eow = false;
len_coded = false;
urbb_len = rest_bits - crbb_len - 8;
/* -16 = ACKNACK Dissector length + CRBB_STARTING_COLOR_CODE + CRBB_LENGTH */
} else if (urbb_len > (rest_bits - crbb_len - 16)) {
eow = false;
len_coded = false;
urbb_len = rest_bits - crbb_len - 16;
}
uclen_crbb = 0;
crbb_len = 0;
}
if (num_blocks > uclen_crbb + rest_bits) {
eow = false;
urbb_len = rest_bits - crbb_len;
} else
urbb_len = num_blocks - uclen_crbb;
if (is_compressed)
len = urbb_len + crbb_len + 23;
}
else
len = urbb_len + 15;
/* EGPRS Ack/Nack Description IE */
if (len_coded == false) {
bitvec_write_field(dest, &wp, 0, 1); // 0: don't have length
} else {
bitvec_write_field(dest, &wp, 1, 1); // 1: have length
bitvec_write_field(dest, &wp, len, 8); // length
}
/* EGPRS Ack/Nack Description IE
* do not support Ack/Nack without length */
bitvec_write_field(dest, &wp, 1, 1); // 1: have length
bitvec_write_field(dest, &wp, len, 8); // length
bitvec_write_field(dest, &wp, is_final, 1); // FINAL_ACK_INDICATION
bitvec_write_field(dest, &wp, bow, 1); // BEGINNING_OF_WINDOW
@ -973,9 +969,9 @@ static void write_packet_ack_nack_desc_egprs(
}
LOGP(DRLCMACUL, LOGL_DEBUG,
"EGPRS URBB, urbb len = %d, SSN = %u, ESN_CRBB = %u, "
"len present = %s,desc len = %d, "
"desc len = %d, "
"SNS = %d, WS = %d, V(Q) = %d, V(R) = %d%s%s\n",
urbb_len, ssn, esn_crbb, len_coded ? "yes" : "No" , len,
urbb_len, ssn, esn_crbb, len,
window->sns(), window->ws(), window->v_q(), window->v_r(),
bow ? ", BOW" : "", eow ? ", EOW" : "");