Thursday, January 29, 2015

How To Buy A Cordless Phone

I bought a new phone recently. I've moved to VOIP, but still consider this my "land line," despite the fact that there's not POTS connection. I still find that cell phones are not reliable enough to replace that land line. Disagree if you must. I know that in much of the 3rd world the opposite is certainly true due to the lack of infrastructure. However... locally it's a pure reception issue. So after fruitlessly cleaning the push-button contacts on my cordless GE phone with deoxit I gave up and bought a new one. I needed all nine buttons to work, even the 2 and the 0. It was time.

So lets discuss how that jobby works. The base and the handset operate over a pair of different frequencies allowing the handset and the base to communicate simultaneously. This allows you and your friend to talk and listen simultaneously. The base unit receives voice signals from the handset and converts them to frequency Modulated (FM) electric signals that transmit through a phone line (or your ISP) to the other person on the call. The base is wired no differently than an old school corded base. Modern cordless phones operate on frequencies as assigned by our friends at the FCC. There are six primary frequency band options: =
  • 43-50 MHz
  • 900 MHz
  • 1880–1930 MHz (PHS)
  • 1920-1930 MHz (DECT)
  • 1.9 GHz  (DECT 6)
  • 2.4 GHz
  • 5.8 GHz


The frequency your phone uses to send signals to the base is not crucial to call quality. This is not a bandwidth issue. It's actually an interference issue. Today there are oodles of wireless devices that can cause interference with your cordless phone. There are local Wi-Fi networks, cellular networks, wireless microphones, Bluetooth devices, wireless printers, tablets, baby monitors.. I recently read about a wireless microwave. These "smart" appliances will only make the situation worse. More here.

While the oldest cordless phones used 46-49 MHz, higher frequencies are not inherently better. The PHS systems is actually newer than the 5.8 GHz and 2.4 GHz technology. Personal Handy-phone System (PHS) is also marketed as the Personal Access System (PAS) and in China it's branded as Xiaolingtong. This system is used primarily in in Japan, China and some South American nations. It's a good system that uses TDMA but is not available in the USA.

DECT stands for Digital Enhanced Cordless Telecommunications Technology.  So since that lacks a frequency it's worth nothing that DECT and DECT 6 operate on different frequencies as noted above. Also DECT in North America is different from DECT in Euope. In North-America the band (1920-1930 MHz) has ample sources of interference. In Europe DECT was assigned a larger band (1880-1900 MHz) and more power, 4 mW compered to 10 mW. 

DECT 6.0 isn't even permitted in Europe because of  interference with their cellular networks. But in the US system uses both TDMA and FDMA making for pretty good audio. And while those older phones operating between 43-50 MHz were easily snooped on with a police scanner, DECT can provide encryption via the DECT Standard Cipher (DSC). The encryption is fairly weak, using a 35-bit initialization vector and encrypting the voice stream with 64-bit encryption. But it's better than nothing which is probably what you have right now.