Elliot Murdock wrote: > Thank you...do you know if IAX can do this? > > The reason for doing is this is to get over the adsl upload/download > discrepancy. While G711 gives terrific quality, it is not always that > feasible for the upload direction, which has much more limited > bandwidth. Accordingly, it would be possible to use G729 for upload, > but keep the higher quality codec, G711, for download.
I do not know a great deal about IAX so I will defer to the experts for the definitive word on whether it is possible from the point of view of its formal protocol mechanics. However, poking around the various configuration options for IAX peers on voip-info.org and a few other places suggests that there is no option to do that with IAX, either. It's not really something 99.9% of VoIP users want to do. :-) As far as the asymmetric upstream/downstream rates on ADSL, are you sure that you're solving the right problem? How many calls are you trying to run over such a circuit? If it's one or two or three, it's no big deal either way to do with G.711; it's 64 kbps (8 kB/s), or ~80ish kbps with Layer 2 framing overhead (for Ethernet; it may be a little more once PPP(oE) headers + ATM AAL5 encapsulation + other stuff frequently in use in DSL transport and aggregation architecture is factored in). Personally, I'd recommend that you just use some low-bitrate codec in both directions. If it's a good codec, you won't take a tragically significant hit on the quality on the receive side either, and if it's a bad codec, then the feasibility of conversation with the far end is going to be impacted because they can't hear you well. Opinions on which codec to use vary, but the only low-bit rate codec I've ever used that I've been even remotely satisfied with in terms of conversational quality is G.729. It really provides by far the most optimal intersection of bit rate and quality; it trims the call down to 8 kbps (8x less than clear-channel G.711!) and still sounds quite good. It uses some rather advanced CELP (code-excited linear prediction) techniques to refer to forms out of a table to achieve that effect. The downside - and it probably has to do with why G.729 is so good - is that all implementations are subject to royalties on software patents, so you can't have it for free with Asterisk; at least, you can't have it legally. It comes built into most commercial VoIP gear because the G.729 licensing cost is just baked into the retail price of the unit. But with open-source stuff, it costs money. However, your mileage may vary; you may want to try Speex or iLBC or whatever the cool kids are using. The downside there is that not a lot of VoIP trunking providers' gear supports these. For the most part, the commercial part of the ecosystem (i.e. beyond Asterisk) deals in G.711u/A and G.729A, although the G.722 wideband codec (for "high-definition" voice) is starting to get some rather serious attention. Cheers, -- Alex Balashov Evariste Systems Web : http://www.evaristesys.com/ Tel : (+1) (678) 954-0670 Direct : (+1) (678) 954-0671 Mobile : (+1) (678) 237-1775 _______________________________________________ -- Bandwidth and Colocation Provided by http://www.api-digital.com -- AstriCon 2009 - October 13 - 15 Phoenix, Arizona Register Now: http://www.astricon.net asterisk-users mailing list To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users