Aside from simply understanding your place in history, the underpinnings of mobile communications can be crucial to assuring your particular application, site or service works correctly. Decisions made long ago, whether technical or regulatory, influence how mobile telephony evolved, and are still felt today.
An example might help. My favorite is that SMS (text messaging) isn't data. It looks like data, because it's typed; email and IM are data, right? But SMS is in the "paging channel," or the part used for ringing the phone and sending caller ID data. What does that mean for pricing, availability, traffic management? Well, too much to go into here, but importantly different things than managing data services.
I have gone out of my way to take actual RF engineering classes. It's pretty arduous, and I no longer remember how to calculate Walsh codes by hand, for example. But as it turns out, almost no one does. I was in class with guys who had EE degrees, and had been working as radio techs for mobile operators for years, and still didn't know the history, or how parts of the system outside their domain work.
Hence, I feel pretty good about boiling several thousand pages of lecture slides and books into this short article. Just understanding the basics can matter a lot to your everyday work.
The Electromagnetic Spectrum
We'll start with junior high physics, to make sure everyone is on the same page. Everything from light to radio to x-rays (and much more) are all a part of the electromagnetic spectrum. These various parts of it are named, and discussed as separate elements based on the way the radiation interacts with physical matter. Visible light excites electrons at a frequency convenient to biochemical processes, so there are rods and cones in our eyes which detect it. Radio collectively oscillates materials, like all the electrons in an antenna at the same time.
Think of the entire electromagnetic spectrum in the same way as the spectrum of visible light; there is a clear area which is "red" and a clear area which is "orange" but also a space in between, parts of which could be considered "red-orange" or red, or orange. Though the individual components are discussed as though they are separate components, they are also part of a continuum and certain interactions overlap quite strongly.
Everything on the electromagnetic spectrum has a frequency, wavelength and power. Radio, especially, is very commonly discussed in these terms as they influence range and the speed and capacity of the information carried. Radio is generally considered to be the frequencies between 3 kHz to 300 GHz. That "collective oscillation" mentioned above means you can feed an RF frequency electrical signal into an antenna (of the right materials, size and shape) and it will send out waves through the air, radiating much like ripples in a pond. When another antenna (again, of the right configuration) gets struck by these waves, it vibrates in such as way that it generates electrical signals, and the receiving electronics do their job.
Frequencies are measured in Hertz, where 1 Hz is one "cycle" per second. Electrical power in the US is delivered at 60 Hz. Mobile phones are much higher frequency, measured in megahertz (MHz) or gigahertz (GHz).
RF DIAGRAM - REDRAW - FIND IT AND ASK????
In general, longer wavelengths travel further, and may go through and around objects. Very low frequencies have global ranges. High frequencies carry much more information, but cannot penetrate objects. Even without something in the way, Certain frequencies are unavailable due to physical phenomenon, such as cosmic rays and the sun. Frequencies are managed by national governments (in the U.S. by the FCC), generally through international agreements. The spectrum is quite full of traffic now, such that adding any service requires disabling another one. The switch to digital television, for example, freed up quite a bit of useful bandwidth, just now coming into service for some next generation mobile networks.
History
I always start presentations about mobile telephony by getting a baseline of the audience. I ask when mobile telephony was first instituted, and where. Occasionally someone will refer to the DynaTac, and correctly insist it all started in the US. Much to the derision of their youthful cohorts, who are sure it came about no earlier than 1985, in Japan or -- for the very clever, Finland.
No one guesses that it emerged directly from experience with miniaturization of radios in the second World War, and was first placed into service by the Bell system right after the war, in 1946.
These first systems were based in relatively small areas, covering a single city for example. A single radio antenna on a tall building downtown sent signals to a large radio in your car, and received signals from it. To dial a call, you had to get an operator to physically connect the call by dialing it on their switchboard, then plugging a patch cable between the mobile network, and the wireline phone network.
These Mobile Telephone Systems were replaced starting in 1963 with an Improved version, which added antennas for larger areas such as the St. Louis region above, and had some capacity for handoff between them, allowing larger range, and lower power. Devices became smaller (though still installed, they were briefcase, not whole-trunk sized). The customer could even direct-dial calls straight from his car.
These were mostly replaced by 1995, but at least one IMTS network was operating in Canada until 2002. The long range of the individual towers (up to 25 miles) was an advantage in the specific locale.
But these were still not cellular devices.
Legal & Regulatory
Much of the behavior of these systems, and even how they work today, is tied up in regulatory requirements. The FCC allocates frequencies in the US; other countries have their own bodies, but radio waves do not respect international borders, so they generally coordinate through an organization called the ITU.
The delay moving to IMTS, for example, was due entirely to the allocation of bandwidth to the mobile operators. The FCC denied anyone wanted or needed mobile phones, so sat on the requests for over ten years.
Despite the name, mobile phones are not really phones. The wireline phone network is more precisely called the PSTN, or Public Switched Telephone Network. It is a single system that connects to (more or less) the entire globe. It also must generally provide a certain level of service, offering highly subsidized service, for example. As usual, specifics vary by country.
Related to this is Quality of Service. This is not quality in the sense of resolution or clarity, but degree of service. For example, If the PSTN is totally jammed up -- perhaps due to some disaster -- when the fire department picks up their phone, someone else (you or me) is forcibly dropped to provide room. Gradations between this public safety over-ride and general access, where you may pay for better access for your business, also exist.
Traditionally, there is no such equivalent practice in any private network, such as satellite or mobile telephony. In some countries, the devices must dial emergency services, and there is a small trend, starting in Scandinavia, to make mobile service and complete coverage a right of every citizen.
Additional regulations are equally or more crucial to understanding how and why services work in a particular manner, or what you must do to comply with them. In the US, location must be made available to emergency operators by offering GPS telemetry (this under the e911 mandates), but cannot be used for any other purpose for individuals under the age of 13 without quite specific consent rules. How do you enforce this, without overly burdensome rules?
Likewise, there are restrictions on how your personal data can be used to market to you, and how it can be sold. In the US these are all consolidated under a concept of CPNI (WHAT?????????) which has more recently been used to assure the security of your data as well.
CHECK ALL THE ABOVE. 13 Y.O. may be CPNI INSTEAD.
This all means that, aside from technical and moral limits, there are significant and generally well-enforced restrictions to what can be done with personal information, and is much the reason that it is difficult to get customer information from the mobile operators. Understand local technology, markets, laws and regulations.
Early Cellular
While it seems no one could have foreseen the growth in mobile we are continuing to experience, there was a great deal of pent-up demand for more, more portable, and more flexible mobile telephony. After numerous delays, mostly due to regulatory approval, a new service was launched starting in 1983. The day this service launched, there were some 10,000 people on waiting lists for an IMTS device in New York City alone.
The Advanced Mobile Phone System, what we all call the "old analog system" today -- again, first launched in the U.S. -- was a massive success, and was used in one guise or another throughout the world.
http://en.wikipedia.org/wiki/Advanced_Mobile_Phone_System Use HISTORY of this section http://en.wikipedia.org/wiki/Digital_AMPS This was the first true cellular mobile network because it shared two key characteristics, both designed for efficiency at multiple levels
- Handoff between base stations
- Frequency re-use
These allow much more traffic in the network, MULTIPLEXING… fdma does still keep you on a channel, but it's much narrower and is not fixed for the duration of the call; when you switch to another tower, the handoff may change the channel to suit the available space…
… NEED TO LOOK UP HOW FDMA IS A MULTIPLEXING SCHEME. FEELS LIKE JUST SWITCHING TO ME.
Aside from some cleverness with handoffs, and being over radio instead of wires, even AMPS was very similar to the wireline phone network, with a complete, dedicated voice circuit at any one moment. Replacing the analog voice carrier with a digital signal allows even more capacity, as well as reductions in power. This is what D-AMPS (D for digital) sought to do in an effort to extend their network life in the face of next generation networks.
D-AMPS encoded the voice (by adding a "vocoder," which turns it into data), then chunks the data stream into "frames," which are then sent in intervals, with a specific time slot for each user on the network in that cell.
This time based system is called Time Division Multiple Access, or TDMA. Digital signals allow compression of the voice signal. Combined with very rapid time slice switching, the same frequency can be used for several users at once.
Cells and Backhaul
There are two fundamental components of a cellular network. The first is the most common, the handset (or aircard, or tablet, or eReader, or home modem, etc.). Operators also call this a terminal, or customer terminal, or use the old CPE terminology, for Customer Premises Equipment.
The other side of the equation is the "cell site," which is actually a small complex of items. Although we all use the term as a shorthand, the tower is actually just the mast to which antennas are mounted. Wires run down this to a shed which houses transmitters, and often backup power such as generators. GPS antenna and other equipment is also included on the tower or in the shelter. The whole arrangement is more technically called a BTS, or Base Transceiver Station.
The antennas themselves are arranged around the antenna, in a flat ring of three (or sometimes four), grouped with two or three antennas pointing each direction. The multiple antennas work together for power management and position finding, using math not worth getting into.
Whenever you see more than one stack of antennas on a tower, this is just because they lease to their competitors. This is a very profitable business alone, so competing operators rarely restrict each other from mounting antennas to their tower, at least for the right money.
DIAGRAM OVER TOWER PHOTO - ADD IN CELL/SECTOR COVERAGE AND OVERLAP???
Each group of antennas pointing a specific direction constitutes a "sector." When on a call, or using data services, and moving through the network, your handset stays connected by negotiating with the BTS to "hand off" between cells, and between sectors. When you run out of available coverage in your home network, you can even be handed off to a roaming network.
There are differences in the various types of handoff, but they are fairly unimportant for our purposes. Networks are generally voice-optimized still, so spend a lot of time making sure calls are not interrupted during handoff. Data services often are interrupted, and this can have implications to network based services. Websites in particular can become confused, and suddenly reset when they find you coming from a new IP range.
Handoff does require continuous coverage, so cells and sectors overlap. Your handset is, as you now might expect, communicating not just with the tower it's getting voice and data from, but with all adjacent towers. They all work together to decide when to hand off to assure maximum efficiency, and prevent drops in coverage.
As discussed above, the mobile network must attach to the PSTN at some point. The first step is to connect the BTS to all other nearby BTS in that operator's network. Then, they connect to a central office, and at various points interchange with the PSTN. The whole scheme of connecting the towers to each other and to the central office is called "backhaul." This can severely limit the speed or capacity of the network, especially when not considered in the design of new radio technologies.
Last year I was on vacation in Door County, Iowa. It is a relatively remote peninsula, but we were able to get mostly decent mobile coverage (high signal strength, low noise). However, the data speeds were awful and it even sometimes took forever to connect a call. It turned out that the backhaul was all done by a microwave link, one tower to the next in a line down the peninsula. It was not able to handle the traffic load of tourist season, even though the individual towers easily could.
Backhaul is generally microwave, via yet another antenna on the tower, or cables in the ground. Some other radio technologies are used, and some have backups so provide both. Central offices (which may be co-located with a BTS) sometimes have satellite links as a last-resort backup even, to offer service in the event the PSTN fails.
The 2G Networks
Quite soon after AMPS takes over the world, it becomes clear that mobile telephony is a big deal, and eventually the current networks simply won't be able to handle the load. Several schemes were put forward, and these all new digital networks took over the mobile landscape by the late 1990s.
Since these were considered an upgrade to AMPS (et. al.), or the second generation, they came to be called 2G networks. In the US, the FCC codified the system under the moniker of Personal Communications Service, or PCS, which you could see appended to the name of several operators.
It also became clear that mobile telephony was a huge industry, and would only grow. This ended up changing the industry landscape, as it suddenly became very expensive to be a mobile operator. The reason is not strictly one of scale or technology, but of competition. As discussed already, national governments control the bandwidth in their borders. When a new type of network launches, it generally appears on an all new band, so it can co-exist with the older network and we can all take most of a decade to switch over.
To decide how to allocate this bandwidth to the various operators, there are auctions. Other portions are involved (you have to prove there's a valid business plan, a market for the service and agree to actually offer service) but basically, the one with the most money wins. Many of the small or local operators simply could not compete in the face of big telecoms, and consortia of smaller ones. In some countries, of course, the nationalized telecom simply assigned the rights to themselves.
2G networks led to much more reliable, wide-reaching, and eventually data-driven mobile use. However, despite being able to be grouped as a class, they are not a single service like AMPS became. Instead, two competing network technologies emerged, and are still dominant in their follow-on versions.
GSM
hill the US (and some other locales) implemented AMPS with fairly good consistency, Europe suffered several fragmentation, both by carrier and across international borders. By the 1980s, it became clear that a common standard would be needed, and it should be built into a next generation system.
TOO DETAILED: Though developed totally separately, and with much more capacity, the principles described in D-AMPS are employed in GSM. For each of 124 channels, there are 26 "frames" which can each carry voice or data, paging (or SMS), or network control signals. These frames are differentiated by frequency and time, though GSM is generally boiled down to being considered a TDMA system.
Advantages: Interference-resistant (guard freq), portablity (SIM)
DRAW SIMS - ELSE PULL EXISTING DIAGRAM
Issues: Hard capacity (unused capacity is wasted, not used to enhance other's throughput), multipath several kinds of SIMs. Matters as not all are portable. Stabilized for a few years on Mini-UICC, but then SMALLER? What is that??? NEED TO ADD TO DRAWING.
GPRS: Asymmetric, packet-switched method of using GSM channels
Adds Packet Data Channel (PDCH) to the architecture. May use any physical channel, or several of them as capacity allows.
functionally owned by qualcomm PULL EXISTING DIAGRAM
*** SUMMARY OF DATA AT THE END. IT'S BASICALLY NOW STILL TWEAKS TO A VOICE-CENTRIC NETWORK ***
Future Networks
INSTEAD, MAYBE WRAP UP WITH A GRAF OR TWO ABOUT DATA NETWORKS OF THE FUTURE. HOW WIMAX AND LTE ARE THE DIRECTION, AND INSTEAD OF ALSO-DATA WE'LL MAYBE END UP AS ONLY-DATA, WITH VOIP WHEN VOICE IS NEEDED.
LOTS OF OTHER FUTURE STUFF IS POSSIBLE. SEEMS UNLIKELY NOW THAT AD HOC WIFI WILL BECOME A DOMINANT NETWORK, THOUGH IT WILL CONTINUE TO BE USEFUL. THERE CONTINUE TO BE PERIODIC PUSHES TO SATELLITES, BUT THERE ARE PHYSICS REASONS THIS WILL NEVER QUITE WORK, THOUGH IT CAN THEORETICALLY PROVIDE GAP COVERAGE IN REMOTE AREAS, ETC.
BUT MOST OF ALL... If there is anything I have learned from my time in the industry it is not to predict anything. No specific network technology is clearly going to win out. Whatever does emerge will probably be used in a way somewhat different than it was conceived, and will be replaced by something else then.
END END END END END
One clear trend is to data-only operation. This is far past simply encoding voice traffic. The expectation of more and more pure data services, the possibility of using any traffic channel for any type of traffic, and other efficiencies mean that VoIP and data networks will rule in the future. This requires all new networks, not just updates, partly so things like data handoff happen seamlessly. Without this, voice service (still by orders of magnitude the largest communications method on mobile networks) will suffer a severe degradation, and operators will not be able to sell their customers on the changeover.
This theoretically means that any data connection could be used as well, such as ad hoc WiFi networks. Anyone who uses serices such as Skype can tell you there is no overwhelming technical impediment to this today. However, the whole model of operating a mobile network makes this unlikely in the forseable future; operators want to retain control, for revenue, trust and customer support purposes.