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November 2008 | Volume 11/ Number 11
Next Wave Redux

Freenum – Dealing with PSTN Realities

Freenum (News - Alert) is poised to revolutionize VoIP traffic exchange and yet few have heard of it. Let me explain.

Most of the world still uses the PSTN and, even when calls go over VoIP, the endpoints are conventional telephones with 12-key dial pads. Don’t expect this to change. Most of the world’s phones are mobile handsets. Globally we’re adding hundreds of millions of new mobile subscribers each year and, with upgrades and replacements, there are more than a billion new handsets sold each year — almost all with 12-key dial pads.

So we have a basic conflict. The SIP community envisions SIP addresses that look like email addresses, e.g. [email protected]. But you can’t type these addresses on most phones.

Meanwhile we’re wasting time and money. Suppose my company has an IP-PBX (News - Alert) and your company has an IP-PBX. We ought to be able to exchange voice traffic much as we exchange email traffic. Instead we call through the PSTN to an auto-attendant which connects us to an extension at the other company.

ENUM might be a solution. ENUM solves the keypad problem by using regular PSTN telephone numbers. There’s just one catch.

PSTN numbers are tightly controlled. They’re defined by the ITU (in recommendation E.164) which assigns country codes to national governments. National governments control number assignments within their countries. Number blocks go to the local fixed-line monopoly, to mobile operators and perhaps to competitive carriers. It’s not possible to assign yourself a number. You have to purchase services from a carrier. has a better answer — ISN (ITAD Subscriber Numbers). This is a free numbering system, delivered via DNS and compatible with SIP. You obtain an ITAD (Internet Telephony (News - Alert) Administrative Domain) number from IANA (the Internet Assigned Numbers Authority). Then you concatenate local extension numbers with your ITAD, separating the two with an asterisk. For example, subscriber 21234 in ITAD 270 would have ISN: 21234*270. ITADs were defined in RFC 3219, Telephony Routing over IP (TRIP), and ISN trials began in 2006, sponsored by MIT (News - Alert), Internet2, Packet Clearing House and Tello. While ISNs remain in “trial,” more than 200 organizations are participating and what started with just a few US-based Intenet2 institutions is now global and has significant commercial participation (companies like Nortel (News - Alert), Nokia, Comcast, and Apple have acquired ITADs).

Although few have heard of them, ISNs appear poised to overtake ENUM and become the standard for VoIP calling between organizations. Expect to hear a lot more over the next 24 months. IT

Brough Turner (News - Alert) is Senior VP of Technology, CTO and Co-Founder of NMS Communications (

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