The Motorola GMRS network

So this is a thing. If you use high-end walkies-talkies you are probably aware of this top-tier pricing option.Following the relative inaction of the FCCs proposed 2010 rule-making, GMRS (General Mobile Radio Service ) repeaters still require a license as previous.  I've written about walkies-talkies a twice before but this is a unique service worth discussing. It's a walkies-talkie network.

Walkies-talkies can communicate directly, one to one with no intermediary devices or services. If you've ever bought one you know that this is the cheap end of the pool. There are problems with this  typically analog service. Large buildings, hills, and sources of interference can obstruct reception. But most of the walkies-talkies in this group are basically toys.  They operate at low power, literally half a watt. In this application walkies-talkies are often referred to as HTs, for handheld transceiver. While this may work well in an open area, there will be issues indoors. There are a number of solutions to this but many of them require an FCC license. There is a great synopsis of features and spec here.

The typical solution is to increase the power, and use a GMRS repeater. This repeater's antenna can be installed at a high elevation point to mitigate areas of null reception. Some vendors insist at this point that the device is no longer a walkie-talkies but is a "commercial 2 way radio." This is marketing bunkum and should be duly ignored.  However, it's worth noting that many users avoid this scenario because that GMRS repeater requires a license. But there remains another option.

Motorola has carefully patched together what I consider to be the first walkies-talkie radio network. Using "commercial partners" i.e. their distributors, as proxies they have made significant progress building a GMRS subscription model. These resellers/network providers include:  Utah Communications, New York Communications, Triangle Communications, Comtronics, Mohre Electronics, Illinois Communications, Maine Radio, MCE Wireless, Telecom Communications,  and many others.  Instead of buying your own GMRS repeater, and having to maintain your own license you more-or-less just subscribe the theirs. Using TDMA and frequecy allocations they can cram a surprising  number of subscribers onto the same network.

Motola intends to expand the service to become a ubiquitous Nextel-like service. The twist here is that because these all-digital networks are transmitting data over two-way radio, these local monopoly networks may eventually find themselves classified as ISPs. Their own documentation notes a data rate of 9600 bps which is not much, but certainly could be all that's available in some rural areas. Old PSTN dial-up networks offered 300 bps, so it's not unthinkable that this may be inadvertently included in the new ISP-as-a-utility legal concepts.

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.

The Philosophy of PCM

In short, PCM (pulse code modulation) is a digital schema for transmitting analog data. It and it's derivations are how most electrical communications work today. According to the National Inventors Hall of Fame three different men invented PCM (pulse code modulation.) A similar patent was filed by John R. Pierce in 1945 under U.S. Patent # 2,437,707. The three of them had worked at Bell Labs, and jointly published a unifying document "The Philosophy of PCM" in 1948, a victory for geek inventors everywhere. So how did we get here?

It starts with the telegraph. TDM (time-division multiplexing) was patented in 1853 by Moses G. Farmer, [pictured] a telegraph operator who was employed as a technician to maintain a network of telegraph lines north of Massachusetts. Farmer was trying to interlace samples from multiple telegraphy sources, and convey them over a single telegraph cable. He at least proved the viability of future duplex and quadruplex systems. This was borne out by the work of W. M. Miner in 1903. He successfully multiplexed multiple telegraph signals. He managed to achieve a sample rate of 3500–4300 Hz.

But this was not PCM. It was PAM (pulse-amplitude modulation) In PAM the data is encoded in the amplitude of a series of signal pulses. PCM reuires an A-to-D conversion (analog to digital) First the analog signal amplitude is sampled at a known time interval. The amplitude of the signal at each sample is rounded off to a power of 2 (2,4,8,16 etc) a process is called quantization. This is represented by binary digits three, four, five, or six binary digits (aka bits) respectively. So the output is now just a string of binary numbers. See the chart below:

Decimal	Binary number
0	0000
1	0001
2	0010
3	0011
4	0100
5	0101
6	0110

In 1926 Paul M. Rainey at Western Electric patented a facsimile machine which transmitted its signal via PCM. It was using a 5-bit encoded PCM. It was never sold commercially but the idea was taking hold. This was it's first use transmitting an image. Though telegraph and fax are both both used to transmit text, the fax sends an image with text on it. The telegraph sends dots and dashes directly correlating to alphanumeric characters. A fax can send non text data. You can fax a xerox image of your middle finger which I have done on occasion. Alec Reeves invented PCM on his own in 1937 unaware of any prior work. He was a British expat working at the International Telephone and Telegraph in France. He patented the concept in 1938 but no working model. His U.S. patent # 2,272,070 was granted in 1943. More here. He lived long enough to see it used in WWII by the allies, though it was so top secret he probably didn't know until after the war. SIGSALY went into service in 1943 and was in military-only use until 1946.

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.