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Asterisk Hardware Ninjutsu • Chapter 11 355Matthew Broderick) dials every telephone number within an exchange searching for interestingtelephone and computer equipment owned by a fictional company namedProtovision, Inc. In real life, the idea of war dialing is the same as in the movie and can beuseful during security audits. During a security audit, you’re dialing numbers within a particularblock around your target searching for things like modems, fax machines, environmentalcontrol systems, PBXs, and other equipment connected to the PSTN.You’ll need prior permission, and checking with your local laws is advised before wardialing!So, why would you want this behind VoIP when you could hook up a modem on thetraditional PSTN? With VoIP, you are able to mask “where” you are calling from. Unlike thePSTN, our ANI information, which cannot be easily spoofed, won’t be passed. We can“spoof ” things like our Caller ID. It makes it harder to track down where the calls arecoming from.For whatever reason you’ll be using an analog modem with VoIP, several things must beconsidered. First off, you won’t be able to make very high-speed connections.The topspeed you’ll be able to accomplish is about 4800 baud.This is due to how the modemMOdulates and DEModulates (hence the term modem) the signal and network latency. Atvery low speeds, like 300 baud, a simple means of encoding the data is used, known as frequency-shiftkeying (FSK).The originator of the call transmits at 1070Hz or 1270Hz.Theanswering side transmits at 2025Hz or 2225Hz.This is well within the range and type ofencoding we can do over VoIP. A speed of 1200 baud is also achievable and stable. At thatspeed, a simple encoding scheme is used, known as Phase-Shift Keying (PSK). Once youstep into higher speeds like 14.4k, 28.8k, 33.6k, and above, you get into very time-sensitiveencoding techniques, like quadrature amplitude modulation (QAM), which don’t respondwell in a VoIP world.To keep things stable, I generally keep my rates locked at 1200 baud. Not blinding fast,but it’s good enough to detect and look at remote systems.You might be wondering,“Wait aminute! How come things like Fax over VoIP can handle such higher baud rates?!” Goodquestion!As VoIP became more and more popular, the ITU (International TelecommunicationsUnion) created a protocol known as T.38, which is sometimes referred to as FoIP (Fax overIP). Asterisk and many VoIP adapters now support T.38. When you plug in your faxmachine to a VoIP adapter, it may very well auto-detect and support the fax under T.38.What T.38 does is it takes the fax signal and converts it to more data-network-friendlySIP/SDP TCP/UDP packets that get transmitted over the Internet. Since the fax signaldoesn’t actually have to traverse the Internet, greater speeds can be achieved. If your adapteror provider does not support T.38 and the analog fax signal has to transverse the Internet,then you’ll run into similar issues as you would with analog modem.This might make you wonder why there isn’t a Modem over IP protocol. Well… intruth, the ITU has created such a standard, known as V.150.1 (also known as V.MOIP) in