Izaokas Sapiro, 123RF.com

Warding off the rise of VoIP spam

Line Lint

Article from ADMIN 00/2010

By Michael Hirschbichler , By Christoph Egger

Spam over Internet Telephony (SPIT) is regarded as the next generation of spam. In this article, we investigate SPIT and point out countermeasures for admins with VoIP servers.

Although unsolicited automated calling has been around for years, it hasn't really caught on in the way that spam email has. In the first place, automated cold calling is actually illegal in many places. Another factor limiting the use of automated calling is the cost: A primary rate interface with 30 bidirectional channels is not cheap. If a call center operates 24/7 and an unsuccessful call takes 10 seconds to complete, each call will cost the marketer 0.06 cents in line rental and 20 cents per charge unit. Although this might not seem like a large expense, the relative ineffectiveness of spam, which means the advertiser must place a huge number of calls to make a single sale, limits the effectiveness of conventional spam by phone.

The rise of IP-based Internet Telephony, however, has changed the terms of the spam equation. Using the G.723.1 codec, a VoIP call requires 16Kbps bandwidth (including signaling overhead) in each direction. For 30 bidirectional calls, you would need a maximum bandwidth of 480Kbps, which calls for a 512Kbps line.

Assuming a line rental of EUR 30 (US$ 35-40), the basic rate for the marketer would be 0.004 cents per call. In other words, 5,000 VoIP calls costs the same as one conventional POTS (Plain Old Telephone System) call.

Equally alarming is that the costs remain constant no matter where the call center is located, which means a VoIP call center anywhere in the world could theoretically spam your phone system. VoIP spam is relatively new, but many experts believe it will be a significant problem in the near future. In this article, we take a close look at Spam over Internet Telephony (SPIT) and highlight some techniques that will someday be the first line of defense.