aboutsummaryrefslogtreecommitdiffstats
path: root/src/amr.c
blob: d3f7b383d94607cbe62a64667ad2fa9ebf7f4bf0 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
/*
 * (C) 2012 by Pablo Neira Ayuso <pablo@gnumonks.org>
 * (C) 2012 by On Waves ehf <http://www.on-waves.com>
 *
 * SPDX-License-Identifier: GPL-2.0+
 *
 * 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.
 */

#include <stdint.h>
#include <unistd.h>
#include <string.h>
#include <stdbool.h>
#include <osmocom/netif/amr.h>

/* According to TS 26.101:
 *
 * Frame type    AMR code    bits  bytes
 *      0          4.75       95    12
 *      1          5.15      103    13
 *      2          5.90      118    15
 *      3          6.70      134    17
 *      4          7.40      148    19
 *      5          7.95      159    20
 *      6         10.20      204    26
 *      7         12.20      244    31
 */

static size_t amr_ft_to_bytes[AMR_FT_MAX] = {
	[AMR_FT_0]	= AMR_FT_0_LEN,
	[AMR_FT_1]	= AMR_FT_1_LEN,
	[AMR_FT_2]	= AMR_FT_2_LEN,
	[AMR_FT_3]	= AMR_FT_3_LEN,
	[AMR_FT_4]	= AMR_FT_4_LEN,
	[AMR_FT_5]	= AMR_FT_5_LEN,
	[AMR_FT_6]	= AMR_FT_6_LEN,
	[AMR_FT_7]	= AMR_FT_7_LEN,
	[AMR_FT_SID]	= AMR_FT_SID_LEN,
};

size_t osmo_amr_bytes(uint8_t amr_ft)
{
	return amr_ft_to_bytes[amr_ft];
}

int osmo_amr_ft_valid(uint8_t amr_ft)
{
	/*
	 * Extracted from RFC3267:
	 *
	 * "... with a FT value in the range 9-14 for AMR ... the whole packet
	 *  SHOULD be discarded."
	 *
	 * "... packets containing only NO_DATA frames (FT=15) SHOULD NOT be
	 *  transmitted."
	 *
	 * So, let's discard frames with a AMR FT >= 9.
	 */
	if (amr_ft >= AMR_FT_MAX)
		return 0;

	return 1;
}

/*! Check if an AMR frame is octet aligned by looking at the padding bits.
 *  \param[inout] payload user provided memory containing the AMR payload.
 *  \param[in] payload_len overall length of the AMR payload.
 *  \returns true when the payload is octet aligned. */
bool osmo_amr_is_oa(uint8_t *payload, unsigned int payload_len)
{
	/* NOTE: The distinction between octet-aligned and bandwith-efficient
	 * mode normally relys on out of band methods that explicitly select
	 * one of the two modes. (See also RFC 3267, chapter 3.8). However the
	 * A interface in GSM does not provide ways to communicate which mode
	 * is used exactly used. The following functions uses some heuristics
	 * to check if an AMR payload is octet aligned or not. */

	struct amr_hdr *oa_hdr = (struct amr_hdr *)payload;
	unsigned int frame_len;

	/* Broken payload? */
	if (!payload || payload_len < 2)
		return false;

	/* In octet aligned mode, padding bits are specified to be
	 * set to zero. (However, there is a remaining risk that the FT0 or FT1
	 * is selected and the first two bits of the frame are zero as well,
	 * in this case a bandwith-efficient mode payload would look like an
	 * octet-aligned payload, thats why additional checks are required.) */
	if (oa_hdr->pad1 != 0)
		return false;
	if (oa_hdr->pad2 != 0)
		return false;

	/* This implementation is limited to single-frame payloads only and
	 * since multi-frame payloads are not common in GSM anyway, we may
	 * include the final bit of the first header into this check. */
	if (oa_hdr->f != 0)
		return false;

	/* Match the length of the received payload against the expected frame
	 * length that is defined by the frame type. */
	if(!osmo_amr_ft_valid(oa_hdr->ft))
		return false;
	frame_len = osmo_amr_bytes(oa_hdr->ft);
	if (frame_len != payload_len - 2)
		return false;

	return true;
}

/*! Convert an AMR frame from octet-aligned mode to bandwith-efficient mode.
 *  \param[inout] payload user provided memory containing the AMR payload.
 *  \param[in] payload_len overall length of the AMR payload.
 *  \returns resulting payload length, -1 on error. */
int osmo_amr_oa_to_bwe(uint8_t *payload, unsigned int payload_len)
{
	struct amr_hdr *oa_hdr = (struct amr_hdr *)payload;
	unsigned int frame_len = payload_len - 2;
	unsigned int i;

	/* This implementation is not capable to handle multi-frame
	 * packets, so we need to make sure that the frame we operate on
	 * contains only one payload. */
	if (oa_hdr->f != 0)
		return -1;

	/* Move TOC close to CMR */
	payload[0] |= (payload[1] >> 4) & 0x0f;
	payload[1] = (payload[1] << 4) & 0xf0;

	for (i = 0; i < frame_len; i++) {
		payload[i + 1] |= payload[i + 2] >> 2;
		payload[i + 2] = payload[i + 2] << 6;
	}

	/* The overall saving is one byte! */
	return payload_len - 1;
}

/*! Convert an AMR frame from bandwith-efficient mode to octet-aligned mode.
 *  \param[inout] payload user provided memory containing the AMR payload.
 *  \param[in] payload_len overall length of the AMR payload.
 *  \param[in] payload_maxlen maximum length of the user provided memory.
 *  \returns resulting payload length, -1 on error. */
int osmo_amr_bwe_to_oa(uint8_t *payload, unsigned int payload_len,
		       unsigned int payload_maxlen)
{
	uint8_t buf[256];
	unsigned int frame_len = payload_len - 1;
	unsigned int i;

	memset(buf, 0, sizeof(buf));

	if (payload_len + 1 > payload_maxlen)
		return -1;

	if (payload_len > sizeof(buf))
		return -1;

	buf[0] = payload[0] & 0xf0;
	buf[1] = payload[0] << 4;
	buf[1] |= (payload[1] >> 4) & 0x0c;

	for (i = 0; i < frame_len -1; i++) {
		buf[i + 2] = payload[i + 1] << 2;
		buf[i + 2] |= payload[i + 2] >> 6;
	}
	buf[i + 2] = payload[i + 1] << 2;

	memcpy(payload, buf, payload_len + 1);
	return payload_len + 1;
}