cellular-infrastructure
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osmo-e1-recorder
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hdlc: Simplify + Fix HDLC implementation 

* remove the notion of states, as there is really only one state
* implement zero removal / bit stuffing for synchronous links
This commit is contained in:
Harald Welte 2016-11-14 21:26:11 +01:00
parent 90fb785804
commit 6daa71dbd2
2 changed files with 25 additions and 53 deletions
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71
src/hdlc.c
View File

@ -18,27 +18,27 @@
#define DEBUGP(x, args ...) do {} while (0)
#endif
static const ubit_t five_ones[] = { 1,1,1,1,1 };
static const ubit_t flag_octet[] = { 0,1,1,1,1,1,1,0 };
static const ubit_t five_ones_zero[] = { 0,1,1,1,1,1 };
static int append_bit(struct hdlc_proc *hdlc, uint8_t bit, int ignore)
static void append_bit_history(struct hdlc_proc *hdlc, uint8_t bit)
{
/* we always add the bit to the history */
memmove(hdlc->history+1, hdlc->history, sizeof(hdlc->history)-1);
hdlc->history[0] = bit;
}
/* if it is a to-be-discarded bit, we bail out */
if (ignore)
return -1;
static int append_bit_out(struct hdlc_proc *hdlc, uint8_t bit)
{
memmove(hdlc->next_outbyte+1, hdlc->next_outbyte, sizeof(hdlc->next_outbyte)-1);
hdlc->next_outbyte[0] = bit;
hdlc->num_bits++;
if (hdlc->num_bits == 8) {
hdlc->num_bits = 0;
pbit_t out;
/* generate one output byte */
osmo_ubit2pbit_ext(&out, 0, hdlc->next_outbyte, 0, 8, 0);
hdlc->num_bits = 0;
return out;
}
@ -47,50 +47,27 @@ static int append_bit(struct hdlc_proc *hdlc, uint8_t bit, int ignore)
static int process_hdlc_bit(struct hdlc_proc *hdlc, uint8_t bit)
{
int ignore = 0;
int out, flag = 0;
int out = -1, flag = 0;
DEBUGP("bit=%u, history_in = %s, ", bit, osmo_ubit_dump(hdlc->history, sizeof(hdlc->history)));
switch (hdlc->state) {
case STATE_FLAG_WAIT_ZERO:
/* we've received 6 consecutive '1' just before and are
* waiting for the final '0' to end the flag character */
if (bit == 0) {
/* it was a zero, flag character detected */
DEBUGP("F ");
flag = 1;
/* we're now inside the payload state */
hdlc->state = STATE_PAYLOAD;
} else {
/* if we received yet another '1', we re-start
* from the beginning */
hdlc->state = STATE_INIT;
}
ignore = 1;
/* always append bit to history */
append_bit_history(hdlc, bit);
if (!memcmp(flag_octet, hdlc->history, sizeof(flag_octet))) {
hdlc->num_bits = 0;
break;
case STATE_PAYLOAD:
case STATE_INIT:
if (!memcmp(five_ones, hdlc->history, sizeof(five_ones))) {
/* five consecutive ones in the history */
if (bit == 1) {
/* one more '1' was received -> we wait
* for a zero at the end of the flag
* character 0x7E */
hdlc->state = STATE_FLAG_WAIT_ZERO;
/* discard bit */
ignore = 1;
} else {
/* discard bit */
ignore = 1;
}
}
break;
DEBUGP("S ");
flag = 1;
} else if (!memcmp(five_ones_zero, hdlc->history, sizeof(five_ones_zero))) {
/* 4.3.1 Synchronous transmission: receiver shall
* discard any "0" bit after five contiguous ones */
DEBUGP("I ");
} else {
out = append_bit_out(hdlc, bit);
}
out = append_bit(hdlc, bit, ignore);
DEBUGP("history_out = %s", osmo_ubit_dump(hdlc->history, sizeof(hdlc->history)));
if (out > 0)
if (out >= 0)
DEBUGP(", out 0x%02x\n", out);
else
DEBUGP("\n");
@ -107,6 +84,8 @@ int process_raw_hdlc(struct hdlc_proc *hdlc, uint8_t *data, unsigned int len)
int out;
static int last_out;
DEBUGP("process_raw_hdlc(%s)\n", osmo_hexdump(data,len));
for (i = 0; i < len; i ++) {
out = process_hdlc_bit(hdlc, data[i]);
if (out == -123) {
@ -116,7 +95,7 @@ int process_raw_hdlc(struct hdlc_proc *hdlc, uint8_t *data, unsigned int len)
last_out = out;
} else if (out >= 0) {
/* suppress 0xAA and 0x55 bit pattern */
if (out != 0xaa && out != 0x55)
//if (out != 0xaa && out != 0x55)
printf("%02x ", out);
last_out = out;
}

7
src/hdlc.h
View File

@ -2,16 +2,9 @@
#include <osmocom/core/bits.h>
enum hdlc_proc_state {
STATE_INIT,
STATE_FLAG_WAIT_ZERO,
STATE_PAYLOAD,
};
struct hdlc_proc {
ubit_t history[8];
ubit_t next_outbyte[8];
enum hdlc_proc_state state;
uint8_t num_bits;
};