Skype and cellular networks (I)

| Networking
Skype has asked the FCC (þ Robert Berger via Interesting People) to require cellular providers to carry their VoIP traffic:
Skype yesterday petitioned the FCC to lay the smack down on wireless phone carriers who "limit subscribers' right to run software communications applications of their choosing" (read: Skype software). Skype wants the agency to more stringently apply the famous 1968 Carterfone decision that allowed consumers to hook any device up to the phone network, so long as it did not harm the network. In Skype's eyes, that means allowing any software or applications to run on any devices that access the network.

The reason for Skype's interest in the issue is obvious: they want to force network operators to allow Skype-enabled calling across their networks, something currently prohibited on wireless data plans. In its filing, Skype argues that this capability would offer "tremendous new sources of price competition provided by entities such as Skype," and that's exactly why wireless operators will fight the plan tooth and nail.

The standard perception here is that there's a generic network and the providers use QoS to block third-party VoIP applications. There's an element of truth here, especially for conventional wireline Internet services. But for cellular the situation isn't as simple as that.

So, the first thing you need to know is that older digital cellular networks (pretty much everything before 3GPP IP Multimedia Subsystem) aren't generic packet switch networks like the Internet. They're specialized voice channels. They do have some generic Internet functionality, but it's mostly an add-on overlay on the older voice network—the same way that you layer IP traffic over an old voice network with PPP (except that this is digital-on-digital).

Performance of these systems isn't that great. First, the bandwidth is really inconsistent (even when you nominally get 14.4k it's bursty). Second, it consumes a lot of power. The reason for this is a but subtle: voice transmission is extremely predictable and the phones and networks are able to make a bunch of channel control optimizations (slotted scheduling and variable-width channels being big ones) to minimize power consumption. This sort of tight control requires having the part of the system close to the radio having a lot of knowledge of the network dynamics of the protocols being run over top. In my Treo, for instance, you get 3+ days in voice mode and less than a day in Internet mode, even if you're sending and receiving almost no data.

IMS is a little different, but the problems are similar. In principle 3G phones have more or less generic IP service and provide it to the applications on the phone. IMS is VoIP (SIP + RTP + some custom IMS stuff) and runs over that service. That's true, but in order to make this work efficiently there's actually tight integration between the VoIP system and the IP stack in order to efficiently make use of the air interface. While there is a lot of QoS on 3G networks, it's not solely for the purpose of blocking 3rd party applications, but also for making the native applications work well. So, while in principle you could run generic VoIP over the IP network, it's not clear how well it would really work.

So, if all this is right and Skype wouldn't work well anyway over a neutral channel, what's Skype really want? That's not clear. One possible theory is that it's negotiating leverage. Of the four major wireless carriers, Cingular/ATT and Verizon both operate wireline Internet service and T-Mobile operates a network of hotspots. Requiring neutrality through those networks would be valuable and asking for a lot more might be a way to get it.

Ackowledgement: Thanks to Cullen Jennings for discussions and explanations of some of the issues in this post.