Wednesday, May 11, 2005

SkypeOut and SkypeIn Gateways, Part 3

James Enck reports that Skype "has built and deployed its own-spec gateways," and Aswath is puzzled. As am I, but then, I'm a natural-born skeptic. Aswath's puzzlement is centered on (1) why use G.729a for SkypeOut calls instead of the GIPS iSAC codec if they're your own gateways, and (2) do the calls get handed off to the carrier partner as TDM and then converted back to VoIP?

More conjectures: Media gateways need hardware. I don't see Skype buying DSPs and building boards from scratch; seems more likely they'd be buying the hardware from someone like AudioCodes or Brooktrout. This hardware doesn't support the GIPS iSAC codec. GIPS offers a version of its VoiceEngine(TM) product for embedded devices, which could conceivably be embedded in a third-party media gateway card - but this version doesn't support the iSAC codec. So I can see Skype's inability to use iSAC for SkypeOut/SkypeIn calls. And if you're unable to use the GIPS codecs, then G.729a isn't an unreasonable choice if you want Skype to work reasonably well over low-bandwidth connections (since G.711 is a bandwidth hog, as I noted previously).

If Skype is indeed converting G.729a VoIP to G.711 TDM to handoff to its carrier partners, and the carrier partners are carrying the traffic as VoIP across their networks (pick a codec) and converting back to G.711 TDM to handoff to the PSTN, that would do more to explain the recurring complaints about SkypeOut sound quality. The more codecs you string together, the worse the sound gets.

If James's info is correct, Skype starts looking less and less disruptive and more and more like a telco. (And not only a telco, but a vertically-integrated telco.) And it's not clear to me that being a telco is something Skype is very good at - or even something they should want to be good at.