Aswath Weblog

A Lemma: VoIP is Possible with Dialup

A couple of weeks back Om posted a quote from an executive of Level 3 wherein he indicated that they are developing a IP phone service including emergency 911 service. Also according to that quote, many dialup ISPs are testing it. Om asked: “Isn’t there a certain bandwidth requirement to make VoIP service work.” I posted a comment suggesting that if Skype can declare that they support dial-up customers why not others also offer a similar capability.

A couple of days later, Phoneboy posted an entry on bandwidth requirement by codec, referencing a paper from Grandstream. I sent an email to Phoneboy pointing out the error in the source and that direct application of those numbers is not appropriate for dial-up connection. Today he posted an update.

Even his update does not satisfactorily answer the possibility of possible Level 3 service. I need to establish the feasibility of using PSTN infrastructure to offer VoIP for my next entry where I discuss SBC’s possible refilling of interconnect tariff.

I pointed out to Phoneboy that iLBC codec uses only 32.8 kbps and so can be used in a dial-up connection that uses V.34bis modem. I also pointed out that introduces 60 ms incremental delay compare to a connection based on broadband. Phoneboy points out in his reply that most of the connection is 28.8 kbps and that dialup connection introduces 150-200 ms delay.

I have no empirical data to question either of these claims. If it is indeed true then I suppose we could use IP and RTP header compression to reduce the overhead on the dial-up link. Since this service will be offered by ISPs, this is possible. I am not sure why dial-up connection introduces 200 ms delay. I suspect that it is because the dial-up ISPs have high contention ratio (as I pointed out in my comment to Om’s entry. If this is so, then the ISPs can reduce the contention ratio for this service. So I continue to maintain that ISPs could replicate “Skype quality” even for dial-up users.