Opportunistic SRTP Support in VoIP Devices

The widely used VoIP signalling protocol SIP has a bad reputation. In my opinion it's mostly unwarranted. If you don't stray too far off piste and have a decent NAT traversal behaviour things will generally work fine.

In the ideal world everything would be encrypted, but there's a plethora of end of life and unsupported devices out there, and devices which may support encryption are often configured by end users that may not enable it.

Enter Opportunistic SRTP - a method of encrypting the audio stream if it's supported by the other end, and it's not supported just fall back to plain old RTP

Photo by Markus Spiske on Unsplash


Factory Reseting a Polycom VVX 450

I recently acquired a Polycom VVX 450 off Ebay for £30, which is an absolute bargain for a modern 12 Line SIP phone with OPUS / SILK support. 

There was some risk however, it had previously been used by a VoIP provider that shares its name with a character in the Greek Alphabet and some providers lockdown the configuration to make it challenging to make them work with another provider.

Getting around these locks can occasionally involve intercepting traffic and redirecting it to your own TFTP or HTTP server, however with this phone it didn't seem to be the case.


Yealink T42G attended transfer fix

I recently started using a Yealink T42G - An older (but still very functional) SIP Desk Phone. Weeks passed without any issues, until I attempted an attended transfer. I'd receive a call, chat to them, hit transfer, dial the next number, hit send, and my desk phone would start ringing again displaying "ReCall" on the screen. If you answer the call, the leg to the phone you wish to transfer to will drop.

Fortunately I was eventually able to find a fix. I presume the same fix will work on the other T4X series phones, including the T40, T41 T46 and T48 


Battery Charger Discharger Board - WTF ?

 I am currently building an Amateur Radio project that will run off batteries. In order to stop over-discharging that battery I need a controller to cut the power before damage occurs. I wanted something "off-the-shelf" so I bought a  Battery Charger Discharger Board from Ebay.

The instructions are abysmal so I did some testing and managed to figure out how to operate it.


TimeTools T1000/2 GPS Receiver

I've recently got around to attempting to revive the Time Source project at the London Hackspace that was originally started by russss - a GPS synchronised Lucent/Symmetricom KS-24361 REF-0 reference clock with a 10MHz output (GPSDO). 

Russ was originally using a TimeTools T1000/2 GPS Receiver, a device with very little information available online. I contacted TimeTools for a copy of the manual but got told rather bluntly that it was over 10 years old, no longer supported, and probably didn't work anyway because of the GPS Rollover.

Its a fairly simple device so I figured it wouldn't take too much to figure out, and it turns out it does still work after all. 

TimeTools GPS Reciever
TimeTools GPS Reciever


Did I have Coronavirus in late January?

I tend to blog about technical topics, but I think this is worth sharing so I'm going to put it here for visibility.

I am starting to suspect I may have had Coronavirus at the very end of January, with symptoms starting around the 29th - Coincidentally the date of the first confirmed cases in the UK.

Weird Red Blotches (26/06/20)


Flashing the BlitzWolf BW-RC1 with Tasmota

I recently purchased some BlitzWolf BW-RC1 IR Controllers from Banggood to create an IR Blaster between two rooms so I can control my SkyQ box from the bedroom, and also integrate some dumb devices into my home automation system. 

I tend to prefer local control over dodgy servers in the cloud, and as they're ESP8266 based I was going to flash them with Tasmota - an open source project that adds MQTT and other goodies to enable local control.

I was hoping I could use tuya-convert - an awesome tool that fakes part of the Tuya setup process and allows you to flash your own firmware, however it turns out these devices are being shipped with a newer more secured version of firmware that tuya-convert can no longer take advantage of. 

With little documentation online, I took it apart and attempted to flash it via serial. After a few set backs, it became apparent that the serial pins are broken out via the USB connector (N.B, these are the raw serial pins, there is no real USB interface) so flashing is straightforward and doesn't require any soldering.

Note: I only cover  preparing and connecting the device for flashing.  It's assumed that you already know how to flash Tasmota onto a device once it's connected via serial.