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authorPhilipp Maier <pmaier@sysmocom.de>2019-02-27 12:29:19 +0100
committerPhilipp Maier <pmaier@sysmocom.de>2019-03-07 10:22:22 +0100
commitfa7df8726001146c902d3a93e2a4b425fa9024b0 (patch)
tree69f5ad07e901d1bdc2f1d74cdd8b668c9da5e6fe /src/amr.c
parent0fe9b3664a8d0b74d5e1c67593b350d67de3f98f (diff)
AMR: add functions to convert between bw-efficient and octet-alignedpmaier/amrmangle
RFC 3267 describes two different AMR frame formats. Octet Aligned and Bandwidth efficient mode. In Bandwith efficient mode the padding bits, which are used to align CMR, TOC and payload on octet boundaries are saved and the fielda are packed directly one after another. - Add functions to convert from one mode to the other and vice versa. - Add function to detect in which mode an AMR frame is encoded. Change-Id: I5b5a0fa644d8dbb1f04f9d7e35312683c7b3d196 Related: SYS#4470
Diffstat (limited to 'src/amr.c')
-rw-r--r--src/amr.c112
1 files changed, 112 insertions, 0 deletions
diff --git a/src/amr.c b/src/amr.c
index 06cf429..d3f7b38 100644
--- a/src/amr.c
+++ b/src/amr.c
@@ -12,6 +12,8 @@
#include <stdint.h>
#include <unistd.h>
+#include <string.h>
+#include <stdbool.h>
#include <osmocom/netif/amr.h>
/* According to TS 26.101:
@@ -62,3 +64,113 @@ int osmo_amr_ft_valid(uint8_t amr_ft)
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;
+}