The update you've been waiting for...

UPDATE: Any updates for this and other SIP/RTP issues can be found here.

In my last post over one month ago, I ranted on and on (big surprise, right) about some issues with Sonus equipment we were experiencing. After learning more I should elaborate on "Sonus equipment".

Like many other manufacturers Sonus has multiple products. We'll be talking about their NBS SBC. Many providers use the NBS SBC in conjunction with GSX gateways and PSX route servers. I have no comments about GSX gateways or PSX route servers; this equipment is largely transparent to us "end users". My gripes are with the NBS SBC.

Providers that use Sonus NBS:

- Level(3) (w/ GSX)
- XO (w/ PSX & GSX)
- Global Crossing
- Broadvox
- Many others

If you are using these carriers for SIP services, be aware.

Last time I was talking about timestamps. This time it's far more insidious...

Apparently (as relayed to me from Level(3) engineers) Sonus has a DSP buffer limitation for RTP packet handling. If there is ever more than a 100ms (my experience has shown it to be much less) gap in RTP Sonus will in technical terms, "freak out".

We have now identified four RTP interop issues with Sonus equipment:

1) Sonus requires all RTP packets (events or voice) to have unique timestamps. The RFCs specifically state that not only is it valid to use the same timestamp for various RTP packets, it is ideal in some cases (like events, for example).

2) The RFC 2833 events generated by Sonus equipment are goofy, to put it lightly. The event duration increments do not match the packetization of the voice stream as stated in RFC 2833 and elaborated on in RFC 4733. Specifically, Sonus equipment increments RFC 2833 duration 80 samples
at a time as if the voice stream is 10ms (regardless of what it actually is). I don't know of any other implementations that do this. Even when the audio stream is *clearly* 20 ms (in the SDP, too) Sonus will continue to increment 80 samples at a time.

3) The most recent (and biggest problem) has been caused by the Sonus (seemingly arbitrary) requirement that there never be greater than 100ms gaps in RTP. This is inherently broken behavior for robustness in IP networks.

4) Sonus has yet another issue with RTP timing and sequencing... If a call is brought up with an endpoint that clocks it's own RTP stream (IVR server, for example) everything will be fine. Until the IVR server (or whatever) bridges that channel to another device that also clocks its own RTP. Sonus (probably related to #3 above) will lose sync and drop audio for up to several seconds while it catches up to the new RTP stream. This requires those of us that work with Sonus equipment to rewrite all timestamps and sequence numbers on our equipment; which has the adverse effect of less than optimal jitter buffering (which should ideally be done at each far endpoint).

Asterisk is largely ok with all of these issues, believe it or not. The one that still causes problems is #3. If you are using Asterisk and Sonus gateways, make DAMN SURE that you are using Packet2Packet bridging and that your devices (whatever they may be) implement RFC 2833 the Sonus way. If not...

NO DTMF FOR YOU!

If you are not using Packet2Packet bridging and your events need to traverse the Asterisk core (for features, fixup, or anything else) there will be a variable length RTP gap that often exceeds the Sonus DSP buffer requirement. With gaps in RTP...

NO DTMF FOR YOU!

FreeSWITCH is also ok as long as you avoid #4. FreeSWITCH provides the configuration option to rewrite timestamps and break jitter buffering. If you are using Sonus gateways you should enable it, otherwise...

NO DTMF FOR YOU!

All of this makes me wish I was around back in the old days when there was one telco and all DTMF was inband!