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.

OFDM and COFDM

Engineers know that OFDM (orthogonal frequency-division multiplexing) is a multi-carrier transmission system i.e. it uses a number of narrow band sub-carriers, and is resilient in the face of multi-path interference, and narrow-band interference. Thouse is it sensitive to phase problems. It's used in both wireless and DTV. The problem with OFDM is that people just don't know what the word "orthagonal" means. So lets discuss orthogonality.

In Euclidean geometry (the normal geometry you got in high school) orthogonality is the relation of two lines at right angles to one another. In other words... they are perpendicular. But that is a strictly 2 dimensional understanding. Radio waves are not lines, they radiate outward and can be polarized in any orientation. So successive waves can overlap and not even intersect and share the same frequency. Try to imagine it like a series of overlapping slinkys. But each slinky is carrying redundant data so while this increases it's resistance to interference, is has the downside of reducing the load of data it can carry. If that sounds a lot like FDMA that's because they are similar.

COFDM is coded orthogonal frequency-division multiplexing. COFDM and OFDM only differ in that with COFDM forward error correction is applied to the signal before transmission. (Remember the transmission can be occurring on 1,000 narrow-band frequencies.) In this manner COFDM reduces the number of errors caused by lost carriers from shadow fading, frequency selective fading, channel noise and other propagation effects.

Forward error correction improved it's redundancy by encoding an error-correcting code. This was the receiver can correct a certain number of errors without retransmission. This idea was comes form a single American mathematician, Richard Hamming. In 1950 he created the first error-correcting algorithm: the Hamming (7,4) code. It was such a good idea we even use it in modems. He was a damn genius and died in 1997 after a long and accomplished career.

TETRA and TETRAPOL

Terrestrial Trunked Radio(TETRA) is a type of professional mobile two-way transceiver; aka a walkie-talkie. TETRA is not for little Billy's camping trip. It was specifically designed for use by government agencies and emergency services, ie the military and police. Under TETRA the transmitter uses a block encoder, a convolutional encoder, a re-order interleaver, a scrambler, burst builder, and differential encoder all before the signal is modulated and transmitted. Needless to say it's highly secure, but it can also be configured for group calls. they can communicate via network or like a walkie-talkie. More recently, in the TETRA release 2, it was modified to also provide wide-band high speed data communications.  (Yes that means the internet. )

It is sometimes confused with TETRAPOL. Tetrapol uses Gaussian Minimum Shift Keying (GMSK) modulation instead of the Quadrature Differential Phase Shift Keying (QDPSK) used by TETRA. This is where most people say "huh?" GMSK is a type of digital modulation that uses spectrum efficiently. Efficiently in these case refers to its needed bandwidth.  Other forms of phase shift keying have sidebands that extend outwards from the main carrier and cause interference to other signals. QDPSK is less efficient and uses two encoded BPSK carrier waves. The two carrier waves are out of phase with each other by 180°. So it's highly noise resistant. If that's getting too complicated just remember that TETRAPOL is the one based on an FDMA multiplex using a narrow-band channel.

There are more than a hundred nations using TETRA systems: China, India, Pakistan, Germany, Ireland, Italy, Poland, Sweden, Australia, Mexico, Israel and the UK. (Notice that the USA isn't on that list. The FCC has not officially allocated any frequencies for the use of any TETRA-based standards. TETRA is a European Telecommunications Standards Institute (ETSI) standard. The first version published 1995 and it's fully endorsed by the European Radio Communications Committee (ERC.)  So why don't we have it here?

You might think that maybe the FCC doesn't like open standards, or maybe just not European ones. But that's not the case. Motorola cock-blocked all that technology. Motorola, having invented the walkie-talkie owns a lot of valid US patents.They just refused to license them to the ETSI. More here. They tried to petition the FCC to intervene, but they did nothing since ETSI  has no standing as a broadcaster in the US.

FDMA DAMA CSMA PAMA FAMA

TDMA, FDMA, DAMA, PAMA, FAMA, CSMA! What? . Let's just say it's all about sharing. There turns out to be more than one way to multiplex a cat

There are a lot of ways you can cut a little slice of RF. We can start small. Even Stereo FM radio multipexes. Frequency Division Multiplexing (FDM) is a form of signal multiplexing. In FM, a 38 kHz subcarrier is used to separate the left-right difference signal from the central left-right sum channel. I expalined that once before. So let's consider other multiplexing protocols. I'll start with satillite then get weird.

FDMA for example shares the available radio spectrum by the communications signals that must pass through that spectrum. The terminology for this is “multiple access.” That's the MA in all those acronyms. FDMA is Frequency Division Multiple Access. In that form of multiplexing, individual frequencies are allocated for each communications signal within the band. It requires high-performing filters in the radio hardware.

Contrast that with Time Division Multiple Access or TDMA. With TDMA the signal is divided into frames, each frame into time slots and each user is assigned a slot. A guard period is used to synchronize this data stream so a single receiver can handle the multiple transmitters. This is used in 2G cellular systems for example.

CDMA is Code Division Multiple Access. This also allows multiple transmitters to share a channel. CDMA uses a "spread-spectrum system." This technology is a tad older than the others, first used in communications satellites in the 1970s. CDMA structures the signal with direct sequencing and frequency hopping. Direct sequencing spreads the normally narrow band information over a wider set of frequencies. A FCC rule change in 1981 allowed it's modern use in WiFi and Bluetooth.

So many acronyms today... DAMA assigns communication channels or circuits based on demand hence Demand Assigned Multiple Access. Requests are issued bu users to a network control system. This is usually done with a pair of frequencies- one transmits, one receives. This is not technically a multipel access system for some as the mangement is near-real time not allowing for concurrent use. PAMA is Permanently Assigned Multiple Access which is almost self-explanatory. It's also called FAMA the F standing for Fixed. FAMA/PAMA is a very innefficient sytesm as the channel remains assigned to the single user whether they are using it or not.

All this multiplexing is messy. It sounds brilliant and efficient but like most inventions by mere mortals sometimes it doesn't work. Sometimes things that are supposed to be sharing a data channel try to use it at the same time. It's called a collision. So we come to CSMA, Carrier Sense Multiple Access. It's a contention protocol. This system detects data collisions by monitoring traffic. The Carrier detects a collision and reacts by staging random time intervals to stagger the two colliding users. Multipel collisions will cause greater and greater intervals of waiting. This is happening on the ethernet network that connects to the back of your PC.