From 788a47fe2377b8eadc7ecfa24cf2bbd0eae5327d Mon Sep 17 00:00:00 2001 From: Neels Hofmeyr Date: Fri, 19 Jan 2024 06:12:27 +0100 Subject: [PATCH] amr_mode_set.txt Change-Id: If6cce37b37018ec0f138bd14fc5d3bdfb455ac04 --- doc/sequence_charts/amr_mode_set.txt | 69 ++++++++++++++++++++++++++++ 1 file changed, 69 insertions(+) create mode 100644 doc/sequence_charts/amr_mode_set.txt diff --git a/doc/sequence_charts/amr_mode_set.txt b/doc/sequence_charts/amr_mode_set.txt new file mode 100644 index 000000000..7f76aab9a --- /dev/null +++ b/doc/sequence_charts/amr_mode_set.txt @@ -0,0 +1,69 @@ +AMR bitrates +============ + +AMR has 8 distinct bitrates that it can switch between. +In GSM, only up to four rates can be selected; they are limited by the S bits. + +In SDP: +------- + + m=audio 16010 RTP/AVP 112 + a=fmtp:112 mode-set=0,2,4,7 + +where + + 7 = 12k20 + 6 = 10k20 + 5 = 7k95 + 4 = 7k40 + 3 = 6k70 + 2 = 5k90 + 1 = 5k15 + 0 = 4k75 + +On A-interface: +--------------- + +"S-bits", S0 to S15, where each Sn defines a set of bitrates that are enabled. +For example, S0 means: allow only 4k75. + +A particular S bit, S1, has a special meaning: when on a half-rate channel, +exclude 12k2 implicitly. + +3GPP TS 28.062, Table 7.11.3.1.3-2 + + S0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 + 12.20 (x) x x x + 10.20 x x x + 7.95 x x x + 7.40 x x x x + 6.70 x x x x x x + 5.90 x x x x x x x x x x + 5.15 + 4.75 x x x x x x x x x x + + OM F F F F F F F F F F F A F A F A + + HR Y Y Y Y Y Y Y Y Y + FR Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y + +The immediate question is, are the S0-15 bits a combinatory bitmask, +or a list of distinct rate selections? + +Explained in an example: + +Say S = 0x18 = S7 | S8 + +If it is combinatory, the SDP equivalent is a single resulting payload type: + + m=audio 1234 RTP/AVP 96 + a=fmtp:96 mode-set=0,2,7 + +If a list of distinct selections, the SDP equivalent is two distinct payload +types: + + m=audio 1234 RTP/AVP 96 97 + a=fmtp:96 mode-set=7 + a=fmtp:97 mode-set=0,2 + +