Fri, 30 Dec 2005

More fun with the SI-680H

After a lot of digging, I determined what "SIMPLE" messaging referred to. This is actually "SIP Message". Its supported by several handsets and pieces of software (like asterisk) to varying degrees. Currently, asterisk does not properly support reception of text messages, and can only transmit text messages to a handset during calls. This makes text messaging difficult to implement. My definition of "text messaging" is the ability to transparently send SMS-like messages of 160 characters or greater to other handsets, and ultimately, other providers. Which, BTW, can be done economically -- more on this later.

Thankfully, there are several promising hacks to offer such a system on VoIP phones like the Senao SI-680H that offer both MO (Mobile Origination) and MT (Mobile Termination) features via SIP Message.

As an added benefit, SIMPLE/SIP Message is supported during a call too on the handset. This is a very good thing.

Also, I have not been able to get the voicemail indicator to light (MWI) to show up on the SI-680H. I suspect that it does not support it. But then again, even one particular GSM carrier went without this for years, and simply used SMS notifications. Sounds pretty reasonable.

After I finish up a finalized asterisk configuration, which will include inbound/outbound dialing, voicemail, and activation system, it will be time for User Acceptance Testing (UAT). My hypothesis is that it will fail, but I plan on publishing some of the results...I suspect they will be interesting. People expect phone to work, cell phones are irrating, and i suspect that a VoIP phone with a UI designed by an engineer for an engineer will likely throw one heck of a wrench into the works. Maybe someone in Taiwan will read it and take a hint. Please give us good WiFi VoIP phones that end users like grandma can use!

[/voip] permanent link

SIP is Scary (Part 1)

There is nothing worse than an unregistered SIP phone. In this state, numerical dialing, as we know it, does not work. Dial 911 on an unregistered phone? HAH! sucker! Nothing happens. The entire concept of voice over IP is so abstract, there are many layers in the system (At Layer 7 or lower) that need to be put into place in order to permit voice traffic.

VoIP is not circuit-switched. The piece that makes the call is not sitting at the bottom of the OSI like other conventional cellular systems. You can't just dial a random number from an unauthorized phone, and get routed to 911, activation, or told that you are not permitted to make calls. Combinding VoIP with WiFi also introduces a number of new problems.

Lets go through the sequence of an average WiFi VoIP phone as it goes through all sorts of hoops to become a telephone. I will include basic flow and state machines in the same diagram. We will learn how complex the process is, what the breakdown points are, whats wrong with the state of WiFi VoIP today, and maybe some solutions to fix this problem.

Look at this giant state machine, and you can see why a simple concept as WiFi VoIP can be a very challenging ordeal. Being able to properly identify where a breakdown is taking place is absolutely critical in a WiFi VoIP network.

WiFi VoIP State Machine:

Here is more detail on all the processes involved:

Currently, on WiFi VoIP SIP phones today, dialing 911 (or any other emergency/activation/etc number) before State 7 results in no action. This probably goes for just about any other SIP phone out today. I propose the following framework or best practices for WiFi VoIP handling of emergency calls. Handset designers should probably consider incorporating some of these features. This will become more and more important as the FCC pushes E911 requirements on VoIP data networks.

Quick Draft - Best Practices for handling Emergency Calls