diff options
Diffstat (limited to 'doc/jitterbuffer.txt')
| -rw-r--r-- | doc/jitterbuffer.txt | 137 |
1 files changed, 137 insertions, 0 deletions
diff --git a/doc/jitterbuffer.txt b/doc/jitterbuffer.txt new file mode 100644 index 000000000..e5cd81ce0 --- /dev/null +++ b/doc/jitterbuffer.txt @@ -0,0 +1,137 @@ +The new Jitterbuffer in Asterisk +-------------------------------- +Steve Kann + + + +The new jitterbuffer, PLC, and the IAX2-integration of the new jitterbuffer +have been integrated into Asterisk. The jitterbuffer is generic and work is +going on to implement it in SIP/RTP as well. + +Also, we've added a feature called "trunktimestamps", which adds individual +timestamps to trunked frames within a trunk frame. + +Here's how to use this stuff: + +1) The new jitterbuffer: +------------------------ +You must add "jitterbuffer=yes" to either the [general] part of +iax.conf, or to a peer or a user. (just like the old jitterbuffer). +Also, you can set "maxjitterbuffer=n", which puts a hard-limit on the size of the +jitterbuffer of "n milliseconds". It is not necessary to have the new jitterbuffer +on both sides of a call; it works on the receive side only. + +2) PLC: +------- +The new jitterbuffer detects packet loss. PLC is done to try to recreate these +lost packets in the codec decoding stage, as the encoded audio is translated to slinear. +PLC is also used to mask jitterbuffer growth. + +This facility is enabled by default in iLBC and speex, as it has no additional cost. +This facility can be enabled in adpcm, alaw, g726, gsm, lpc10, and ulaw by setting +genericplc => true in the [plc] section of codecs.conf. + +3) Trunktimestamps: +------------------- +To use this, both sides must be using Asterisk v1.2. +Setting "trunktimestamps=yes" in iax.conf will cause your box to send 16-bit timestamps +for each trunked frame inside of a trunk frame. This will enable you to use jitterbuffer +for an IAX2 trunk, something that was not possible in the old architecture. + +The other side must also support this functionality, or else, well, bad things will happen. +If you don't use trunktimestamps, there's lots of ways the jitterbuffer can get confused because +timestamps aren't necessarily sent through the trunk correctly. + +4) Communication with Asterisk v1.0.x systems +--------------------------------------------- +You can set up communication with v1.0.x systems with the new jitterbuffer, but +you can't use trunks with trunktimestamps in this communication. + +If you are connecting to an Asterisk server with earlier versions of the software (1.0.x), +do not enable both jitterbuffer and trunking for the involved peers/users +in order to be able to communicate. Earlier systems will not support trunktimestamps. + +You may also compile chan_iax2.c without the new jitterbuffer, enabling the old +backwards compatible architecture. Look in the source code for instructions. + + +5) Testing and monitoring: +-------------------------- +You can test the effectiveness of PLC and the new jitterbuffer's detection of loss by using +the new CLI command "iax2 test losspct <n>". This will simulate n percent packet loss +coming _in_ to chan_iax2. You should find that with PLC and the new JB, 10 percent packet +loss should lead to just a tiny amount of distortion, while without PLC, it would lead to +silent gaps in your audio. + +"iax2 show netstats" shows you statistics for each iax2 call you have up. +The columns are "RTT" which is the round-trip time for the last PING, and then a bunch of s +tats for both the local side (what you're receiving), and the remote side (what the other +end is telling us they are seeing). The remote stats may not be complete if the remote +end isn't using the new jitterbuffer. + +The stats shown are: +* Jit: The jitter we have measured (milliseconds) +* Del: The maximum delay imposed by the jitterbuffer (milliseconds) +* Lost: The number of packets we've detected as lost. +* %: The percentage of packets we've detected as lost recently. +* Drop: The number of packets we've purposely dropped (to lower latency). +* OOO: The number of packets we've received out-of-order +* Kpkts: The number of packets we've received / 1000. + +Reporting problems +================== + +There's a couple of things that can make calls sound bad using the jitterbuffer: + +1) The JB and PLC can make your calls sound better, but they can't fix everything. +If you lost 10 frames in a row, it can't possibly fix that. It really can't help much +more than one or two consecutive frames. + +2) Bad timestamps: If whatever is generating timestamps to be sent to you generates +nonsensical timestamps, it can confuse the jitterbuffer. In particular, discontinuities +in timestamps will really upset it: Things like timestamps sequences which go 0, 20, 40, +60, 80, 34000, 34020, 34040, 34060... It's going to think you've got about 34 seconds +of jitter in this case, etc.. +The right solution to this is to find out what's causing the sender to send us such nonsense, +and fix that. But we should also figure out how to make the receiver more robust in +cases like this. + +chan_iax2 will actually help fix this a bit if it's more than 3 seconds or so, but at +some point we should try to think of a better way to detect this kind of thing and +resynchronize. + +Different clock rates are handled very gracefully though; it will actually deal with a +sender sending 20% faster or slower than you expect just fine. + +3) Really strange network delays: If your network "pauses" for like 5 seconds, and then +when it restarts, you are sent some packets that are 5 seconds old, we are going to see +that as a lot of jitter. We already throw away up to the worst 20 frames like this, +though, and the "maxjitterbuffer" parameter should put a limit on what we do in this case. + +Reporting possible bugs +----------------------- +If you do find bad behaviors, here's the information that will help to diagnose this: + +1) Describe + +a) the source of the timestamps and frames: i.e. if they're coming from another chan_iax2 box, +a bridged RTP-based channel, an IAX2 softphone, etc.. + +b) The network between, in brief (i.e. the internet, a local lan, etc). + +c) What is the problem you're seeing. + + +2) Take a look and see what iax2 show netstats is saying about the call, and if it makes sense. + +3) a tcpdump of the frames, (or, tethereal output from), so we can see the timestamps and delivery +times of the frames you're receiving. You can make such a tcpdump with: + +tcpdump -s 2048 -w /tmp/example.dump udp and port 4569 [and host <other-end>] + +Report bugs in the Asterisk bugtracker, http://bugs.digium.com. +Please read the bug guidelines before you post a bug. + +Have fun! + +-SteveK |