Skype and cellular networks (II)

| Networking
On the Skype/FCC thread on I-P, Brett Glass writes:
The key thing that one must understand -- and this is a bit technical -- is that Skype works by "robbing" bandwidth from its users and their ISPs. Skype does not buy enough bandwidth to route or connect all of the calls placed via its network. At any time, a Skype user who merely has the software running -- but is not making a call -- may be using bandwidth to connect a call that involves neither the user's ISP nor any of that ISP's customers. This is a moderate concern on a land-based network, but is of GREAT concern on wireless networks, which are severely constrained by tower capacity and the scarcity of radio spectrum.

If Skype, by operating on the wireless provider's network, would in effect be consuming the provider's valuable bandwidth and airtime without compensation (which really does seem to be the case), the cell phone company is perfectly justified in saying, "No." We operate a terrestrial broadband network (not a cell phone network), which has more capacity. Nonetheless, we do find that we're impacted by bandwidth-robbing applications and do find that it is necessary to rein them in (though we do not currently ban them).

OK, this is a little technical, but it's not too complicated to explain. VoIP media requires two-way communication flows over UDP. Unfortunately, your average firewall or NAT is not set up to conveniently allow such flows by default. Skype (like SIP VoIP stacks and other P2P applications) includes fairly extensive NAT traversal techniques to let you punch a hole in the NAT/firewall. Unfortunately, those techniques don't always work, especially when both sides are behind NATs. The last ditch approach in such situations is to tunnel all the traffic through a media relay, a publicy accessible machine which is not behind a NAT. In the IETF VoIP universe, it's generally expected that service providers (either ISPs or the voice service provider) will prove the media relay.

Skype has the unusual feature that not some of the clients themselves act as media relays. At least in theory, the system automatically detects that a client has a public IP address and has it advertise itself as a media relay.1 (Because the traffic is encrypted the people making the call aren't supposed to have to worry about this.) The advantage of this technique is that Skype Inc., doesn't need to use as many servers (or as much bandwidth) to operate their own media relays. Obviously, if you have a client which is acting as a media relay, it consumes bandwidth when you're not making a call, which is what Glass is referring to.

That said it's unlikely that a Skype client on a cellular network would end up being a media relay. A good media relay needs reliable high speed connectivity and unfiltered, public, Internet access. None of these really apply to cellular phones. So, while in principle Skype could chew up cellular radio capacity over and above that required to transmit to/from the calling stations, it seems unlikely in practice.

1. I say in theory because a friend of mine set up a Skype client with good connectivity and a public address and didn't get selected as a relay.

UPDATE: Rewrote the last graf for added clarity.