May 10, 2005

Skype PSTN Interconnection

DG Lewis shares his thoughts on Skype PSTN interconnect architecture. He argues that Skype probably uses the media gateways of its carrier partners, instead of deploying their own. He also theorizes that they do not use any specialized signaling gateways as well. Instead he thinks that the Skype clients use SIP signaling for SkypeOut calls. He is left with one open question how does a supernode know which carrier partner to use for a given call. What follows are my thoughts.

Originally I wrote this as a comment at Lewis' site. Since it become longer, I decided to make an entry. That explains why it is written in second person.

I agree with you that Skype has not deployed their own media gateways. Then they have to worry about getting physical connectivity to their partners. Why taken on operational burden when (as you point out) the partners accept IP traffic.

But I have different thoughts on the signaling aspects. My theory is that they treat the media gateways as Skype clients and so they "register" with a supernode, which are Skype's own computers. These supernodes map Skype protocol to SIP. Here SIP is only a "trunking" protocol. So the mapping is not complex. This model covers many other requirements:

  1. Carrier partners will insist on only a few well known interconnection points and they will ask that they be hosted by Skype.
  2. There could be differences in the SIP message formatting between the carrier partners. It is easy to run different SIP variant in this model compare to the one where any supernode can directly interconnect to the carrier partner.
  3. Since money is involved, it is critical that the supernode be reliably present for the duration of the call.
  4. Clients' supernodes need to send the signaling traffic to one of a handful of supernodes, which in turn selects the appropriate carrier partner. This addresses your open question.

This violates the P2P nature and this is your basis for suggesting the alternative. But I am of the opinion that Skype has deployed relay nodes to handle NAT traversal problem. So deploying additional supernodes is not an additional burden.

But then again I am biased in that I think in the final analysis there is architectural difference between Skype and other VoIP service providers.

Posted by aswath at May 10, 2005 08:11 AM
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