July 20, 2005

Proto PhoneGnome

During December 2001, for the umpteenth time I was trying to convince a colleague that we should be focusing our attention on VoIP CPEs rather than softswitches and media gateways. For some reason (known, but not relevant now) the argument clicked and we agreed that I should develop my simple idea. The idea was that a caller will dial the standard phone number on a standard phone, which will be collected by this new device (he called it MaxBox). It will do its own analysis either on its own or by consulting a provider’s database to determine whether the call should be established either as a VoIP call or through the PSTN.

For the scheme to work, the important thing is to build the database that maps the standard telephone number to an URI. As I was devising a way to handle this, I came across the ideas behind ENUM. ENUM is precisely the thing I was looking for and a standard scheme is just good. Further readings on ENUM confirmed a problem that I stumbled upon: how to validate the user’s phone number? Of course it is simple to handle the initial condition. Simply require the user to register using both the interfaces and use the ANI information to confirm the valid telephone number. There are two problems with this approach. If the user is behind a PBX like environment, then we may not be able to get the valid telephone number. Also if the user changes the PSTN number for one reason or another and the old number is subsequently assigned to another user, it is possible that the new user may create a conflict. I concluded that both the cases can be handled only through human intervention and some form of identity confirmation.

There was another noteworthy problem that I had to overcome. To a large extent, the MaxBox is behaving like a PBX. What I mean is that it has two “trunk interfaces” – the PSTN line and the VoIP “line” – and has only one station. So it is natural to offer many of the standard telephony features. But unlike a PBX, the PSTN carrier may offer its own features on the PSTN line, but using only in-band signaling. So I have to be extra careful when I extend these features across both the interface. The example of call waiting will illustrate the feature interaction problem. Human factors considerations require that the user be presented a maximum of only two calls – one in talking state and one in held state. Since the PSTN does not know about the broadband interface, it will deliver a second call on the PSTN interface, irrespective of whether or not a call is taking place on the broadband interface. So it is possible that three calls may be in progress at any given time. I felt that allowing the user to cycle through the three calls will lead to confusion. So I required the Maxbox to intercept the call waiting tone on the PSTN interface if a call is in progress on the broadband interface.

MaxBox and PhoneGnome share many common objectives and features. But it is not clear how they overcome these two issues. (I didn’t plan on including voice mail or call blocking capabilities.)

There was one other feature that was planned for MaxBox that I am not sure whether the current version of PhoneGnome provides. Since the main objective of MaxBox was tariff avoidance, any non-toll call to a PSTN number will always be routed over the PSTN. In this respect, the database has to be more than the standard ENUM database.

If you are interested you can look up documents from that time. The following documents are available:

  • Proposal write-up
  • Proposal presentation
  • Call flows
  • SDLs for call logic (1.2 MB) (after all I am a bell-head).
  • Additionally I had developed a mockup for the call logic. It can run on a Windows XP platform by downloading the zipped file (5 MB), expanding to a directory location. To run the mock up, open a command window, change the directory to the new location and type: tbsystem\tb85run.exe. In the Open File dialog box, select the file HGMockup under the directory HomeVoiceGateway. Enable the speaker to hear some sound effects. The instructions are under the Help menu.

    Alternatively, you can copy the contents of the unzipped files under the Mockup directory to the root directory of a CD. The CD will autoload the program. In either case, no additional files are written to your computer.

    After working on it for some 9 months, the partnership broke and I filed away the work. But PhoneGnome made me dust up the old files. If you would like to develop these ideas further I will be interested.

    Posted by aswath at July 20, 2005 01:23 AM
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    Comments

    Aswath links are not working, at least first two pdf files.

    Posted by: Alberto at July 24, 2005 04:16 PM

    Thanks for the pointer. The link was broken due to case mismatch. I think it is fixed now. Please try again.

    Posted by: Aswath at July 24, 2005 09:09 PM

    Aswath:
    The PDF links are still not working. The word and Zip links do work.
    Kanti

    Posted by: Kanti Purohit at July 29, 2005 12:46 AM

    Kanti:

    I checked with IE and Firefox. In both the cases the PDF links work. Can you please make sure your browser is not reloading old cache by asking it to reload the page. Or you can enter the documents' URLs directly on the browser address window (make sure that the extension is PDF, capitals). If you still have problems, I will mail it to you directly.

    Posted by: Aswath at July 29, 2005 07:03 AM

    Sorry, Aswath - I tried both IE and Firefox. No luck.

    Posted by: Kanti Purohit at July 29, 2005 10:18 AM



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