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The Guru's Corner

The Guru speaks on:

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bullet FLEX™ paging speeds

bullet Bandwidth

bullet FLEX™ Capcodes

O Enlightened One, please tell us the truth about the FLEX™ paging protocol.

Well grasshopper, if you want to know the truth about FLEX why do you not ask Motorola? They are the company that invented FLEX.

Master, I would never say that the great and honorable Motorola speaks with forked tongue, but they do speak with many different tongues. When I asked a Motorola engineer about FLEX I received a long and confusing technical explanation that I could not understand. When I asked a Motorola salesperson he told me not to worry and invited me to have lunch. A nice lady in marketing reassured me that Motorola knows what is best for the paging industry. I even asked a Motorola manager who said that his people would get back to me, but no one ever called. . .

Very well grasshopper, this humble teacher cannot aspire to match the wisdom of the all-knowing Motorola, but if you have some specific questions, I will try to help you.

Oh thank you Master! If you would be so kind to explain about all those different signaling speeds that FLEX offers. I have heard they work at 6,400, 4,800, 3,200, and 1,600 b.p.s.

Not really so grasshopper, FLEX transmitters send the paging data out at 1,600 b.p.s. (FLEX 3200 2-level-FSK and FLEX 6400 4-level-FSK both use a modulation rate of 3200 symbols per second: where each symbol contains 2 bits of information. Thanks to Gary Cromack).

Then what about those other speeds Master?

The other FLEX speeds refer to "throughput", and the 4,800 rate is no longer supported.

My apologies Maestro, but I am getting confused.

My son, it will be easier for you to understand if you think of a FLEX radio channel as a highway. Instead of trying to build a single-lane highway that would allow people to drive their cars at 200 mph it is more practical to build a four-lane highway that allows people to drive at 50 mph The same amount of traffic can be handled on either one and the four-lane highway is much safer.

Ah so! It is "as if" the radio channel was operating at a speed of 6,400 b.p.s.?

You are beginning to understand my young disciple. The combined throughput of four 1,600 b.p.s. "phases" is 6,400 b.p.s. (the actual transmission speed is only 3200 b.p.s.)

But Master, a radio paging channel is not the same as a highway, why do they not really transmit the data at 6,400 b.p.s.?

If my inquisitive student insists on knowing this too, some technical explanations are necessary. There are some things that even Motorola can not do:

  • current state-of-the-art technology does not support simulcasting at 6,400 b.p.s. Simulcasting techniques require one-eighth to one-forth of one bit in phase delay compensation. At 900 MHz a 6,400 b.p.s. radio transmission could not be adjusted with the exactitude to eliminate signal cancellation in the overlap zones. Equipment stability is not high enough, and synchronization methods are not yet accurate enough to accomplish this at 6,400 b.p.s.;
  • standard paging channel bandwidth allocations by the United States FCC and Government regulatory agencies in other countries are not wide enough for 6,400 b.p.s. modulation. Remember your basic radio theory lesson grasshopper. An un-modulated radio carrier is infinitely narrow - it does not occupy any spectrum. (Zero - zilch - nada.) As a carrier wave is modulated, it begins to widen and occupy more spectrum. As the modulation frequency increases so does the bandwidth of the transmitted radio signal.

Is that technical enough for you grasshopper?

I am very sorry my Mentor, but I am getting confused again. Everything I read on the Internet uses the word "bandwidth" interchangeably with "speed." Do they not mean the same thing?

Young man, remember Confucius say: "He who believe everything he read is very confused person." The classic use of the word "bandwidth" means "width of the band," the "width of the signal" or the "amount of spectrum occupied." This is not the same as the rate of transmission. While it is generally true that the faster the rate - the wider the spectrum requirement or "bandwidth," there are some good examples of exceptions. One of these being the sending of four FLEX phases in parallel through one radio channel, and the other being the ever-increasing MODEM speeds through plain old telephone lines. The voice bandwidth of a standard (unconditioned) telephone line has not changed over many years. A few years ago, 300 b.p.s. was the fastest a MODEM could do over a standard telephone line. Today, most MODEMs operate at 28,800 b.p.s. and many at higher speeds (30 to 50K b.p.s.). The voice bandwidth of the telephone line has not changed but the speed of the data transmitted over it has increased dramatically. This is accomplished with special modulation techniques that essentially send the serial data information in separate parallel paths to increase the overall speed or throughput.

Thank you O Guru, Great Teacher, I think I understand now. Could you honor us with a few words of wisdom about FLEX capcodes?

Ahhhh. . . grasshopper, capcodes! This is very long subject. Motorola has many books and papers about capcodes. I think I will just tell you just a few of the more important facts.

There are basically two kinds of FLEX capcodes, short and long. The short codes have seven digits and the long codes have nine digits. Seven-digit numbers allow for two million capcodes and nine-digit numbers allow for one billion capcodes. Most paging companies start out by using 7-digit capcodes because of better channel efficiency. This is due to the fact that one data code word is used to transmit a 7-digit capcode, and two data codewords are required to transmit a 9-digit capcode. Valid 7-digit codes run from 000,000,001 to 001,933,312. Valid 9-digit codes run from 002,101,249 to 999,999,999. So, once a paging company has approximately two million customers, it is necessary to start using long codes. I have heard that the great Motorola has even modified FLEX so that total system capacity can be five times more than this, but you will have to ask them about this. (Note: you will frequently see 7-digit capcodes written with two leading zeros. The actual FLEX encoders do not determine if the code is short or long by counting the number of digits. It is done by using the absolute value of the number. The leading zeros are ignored.)

But Honored Teacher, that seems like too many codes! I know there are a few paging companies big enough to need over two million codes but is this not far more than most companies will ever need?

No my young friend, anytime two paging systems are connected together over a network for subscriber roaming, it is not possible to reuse (or duplicate) the same capcode in either of the systems. This is necessary because if a subscriber to one system travels to the area of the other system his (or her) code must be unique in that area or two people would receive the same message on their pager.

Thank you O Most Exalted Guru, I am indeed beginning to understand. Would you please tell us about "Digital Voice Paging?"

Sorry my son, I am old and tired—maybe later—but for now, remember: there ain't no such thing as "Digital Voice Paging!"

(Note: this refers to the system that took in an analog voice message and converted it to digital within the telephone system and the paging terminal. It was then converted back to analog and transmitted over a radio channel in a compressed format. Once in the pager, the voice message was converted back to digital and then stored in RAM. Since it was transmitted as an analog signal, it shouldn't be called "digital voice paging." It exists as digital two places in the system but all radio transmission protocol names refer to the over-the-air, or radio portion of the transmission.)

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FLEX, ReFLEX, FLEXsuite, and InFLEXion, are trademarks or registered trademarks of Motorola, Inc.

Peace be unto you
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[Thanks to Jagadish Venkataraman for sending in this picture of a more suitable "Guru" than the "Wizard" shown above. — 02/06/03]


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