To be sure, but the relevant scale is the wavelength, which is about 5.5 cm at 5.5 GHz, and 33 cm at 900 MHz. There's nothing magic about those lengths -- a forest, or a building, is just as likely to have structures of either length, or any other length, or neither length. Certainly one can create frequency-selective surfaces that pass one band of frequencies and reject others. The statement to which I objected was that it was always true that 900 MHz signals travel farther than 5 GHz signals, due to some perceived difference in radiowave propagation. That is simply a falsehood -- a myth.
Please don't carry the discussion on any further. Instead, take a pair of parabolic dishes and run the test yourself -- through a forest, through a building, through the path of your choice -- then repeat the process with resonant dipole antennas.
I'd expect them to diffract around typical household corners differently...
They do, and you'll find that shorter wavelengths get through smaller openings that longer wavelengths cannot. It's a complicated field to model, and one can find cases where either is superior, but shorter wavelengths do have advantages in a scattering environment.
I design antennas and wireless links for a living, sir. Yes, I have done this. Since 1984.
Of course 900 MHz has better range, if one is using dipole antennas. The point is, the apparent difference in range is due to the antennas used, not some intrisic property of the propagation medium. (It's also likely that the 5 GHz transmitter has lower output power, and the 5 GHz receiver a higher noise figure, than the 900 MHz versions, making the 5 GHz range even less, but I'll ignore those factors for now.)
Give me parabolic dishes, and 5 GHz will go where 900 MHz will not. Give me resonant dipoles, and 900 MHz will go where 5 GHz will not. The behavior follows the antenna selection. It has nothing to do with "propagation," whether through trees, concrete block, or anything else.
Maybe numbers will help. Since the effective area goes as the wavelength squared, the amount of power captured from a resonant 5.5 GHz dipole, compared to that from a resonant 900 MHz dipole, from a given power flux density, is (900/5500)^2 = 0.027, or 2.7 percent. This means that, with identical efficiencies, matching loss, etc., the signal the 5.5 GHz receiver will get from its antenna is down 10*log10(0.027) = 15.7 dB from the signal the 900 MHz receiver gets. That's why your range at 5 GHz is less than at 900 MHz.
Now, let's move to parabolic dish antennas. Since the effective area of the dish antenna is the same for both bands, for a given power flux density the same signal level will be presented to each receiver at the antenna terminals. However, the power flux density generated by the transmit parabolic dish antennas will vary, but this time by the frequency squared, so the power flux density at the receivers on 5500 MHz will be 15.7 dB stronger than it will be at 900 MHz.
You can see where this is going. One has four possibilities:
1. Tx dipole, Rx dipole: 5.5 GHz signal 15.7 dB weaker than 900 MHz 2. Tx dipole, Rx dish: 5.5 GHz signal the same as 900 MHz 3. Tx dish, Rx dipole: 5.5 GHz signal the same as 900 MHz 4. Tx dish, Rx dish: 5.5 GHz signal 15.7 dB stronger than 900 MHz
You may ask how much range change is implied by a 15.7 dB change in signal strength. One simple range model is the use of a loss exponent; a typical value for the loss exponent might be 3 or 4. If we are optimistic, and take 3 as the value for the loss exponent, and assume a 15.7 dB change in loss, one can determine the ratio of the two distances as
10 * n * log10(d1) - 10 * n * log10(d2) = loss(dB)
30 * log10(d1/d2) = 15.7
d1/d2 = 10^0.524 = 3.34.
The band with the 15.7 dB strength advantage (the 900 MHz band, when dipoles are used, or the 5 GHz band, when dishes are used) has more than three times the range of the alternative. (If one assumes a loss exponent of 4, as might happen in an ugly indoor environment, the ratio works out to be about 2.47.)
when there are obstructions(i.e. real life) 900MHz reaches further than higher frequencies. It ain't no myth it's physics
No, it's a myth. If your 2.4 GHz radio had an antenna the size of your 900 MHz radio antenna, the performance would be the same. But because the 2.4 GHz dipole is smaller, the 2.4 GHz range is less. But it has to do with the antennas used, not any propagation phenomenon.
Resonant dipole antennas are constant-gain antennas, meaning that their gain is constant with frequency, while their effective area varies inversely with frequency squared. There are also constant-aperture antennas, in which their effective area is constant with frequency, while their gain varies. A parabolic dish antenna is an example of the latter; its gain varies with the frequency squared. If you take two parabolic dish antennas, fit them with 900 MHz feeds, and then take the same dishes and fit them with 2.4 GHz feeds, you'll find that the 2.4 GHz antennas have (much) higher gain and the resulting system, much greater range than the 900 MHz configuration.
It's also possible to set up a link with a constant-gain antenna (e.g., a dipole) on one end and a constant-aperture antenna (e.g., a parabolic dish) on the other. In this case the two effects cancel out, and the user does not see a difference in range between the two frequencies.
You'll find, if you actually do this experiment, that it does work this way -- regardless of whether the path goes through a forest, a house, or both. It's physics, period.
900 MHz signals do NOT "propagate better." They propagate in free space just the same as 2.4 GHz signals and, in the presence of scattering (e.g., small openings in otherwise shielded areas) not as well as 2.4 GHz.
What people fail to realize is that these systems typically use some variant (often a physically shortened variant) of a dipole antenna, and the 900 MHz antenna is physically larger than the 2.4 GHz antenna. It therefore has a much larger effective area (the effective area being inversely proportional to the square of the frequency of operation), and so captures much more of the incoming energy. To the user, it looks like the propagation is better, but in reality the device just has a physically bigger antenna.
Look at it this way: If propagation got worse as the frequency went up, we'd never see light from the sun!
I was musing just the other day about a related calculating method that has fallen into disuse, the nomogram. Nomograms always impressed me as an especially clever way to perform specific mathematical tasks.
When I was young, and dirt was still sparkling and shiny new, nomograms were in every engineering textbook, handbook, and reference book. Their demise in engineering applications seems to have come with a whimper, not a bang, as no one seems to have noticed it.
The comment has been made before that the better analogy is to a tossed salad (or salad bowl), rather than a melting pot, as the ingredients tend to maintain their individuality, rather than becoming homogenized. A lot of it is bland lettuce, but one also has everything from olives to jalapeños to spice things up a bit. (Extra credit for identifying a good analogy for the salad dressing.)
In the early 1990s one could travel between Hong Kong and Kowloon by hovercraft. It was an interesting change of pace, but I could see how it couldn't compete against the Star Ferry.
[. ..] According to this prediction, the possible attack distance is approximately 16.78 cm using the same sound source that we used for the real-world attack with the maximum volume (113 dB). This attack distance range might not be sufficient for a malicious attacker. However, attackers can overcome this distance limitation by using a more powerful and directional source (e.g., a loudspeaker array) than the single speaker used in our experiments. For instance, SB-3F from Meyersound can generate sound of 120 dB at 100 m, and 450XL from LRAD and HyperShield from UltraElectronics can produce 140 dB at 1 m, which is equivalent to 108.5 dB at 37.58 m. Therefore, the possible attack distance is 37.58 m, if an attacker uses a sound source that can generate 140 dB of SPL at 1 m.
There may not be any need to panic and start fixating on what every little app is doing.
But then again, there might. How is one to know? That's the biggest problem I have with the mobile telecom computing model. I have no idea what the apps do, and no way, other than make it my life's work, to find out.
I hate having to trust the OS provider that everything is properly sandboxed, that none of the apps in their stores are malware, etc. What's going on, inside this box?
This was my thought, too. There is a hand-crankable version of the same thing in some hands-on museum in or around MIT that I visited some years ago. It's amusing to be able to crank the first gear in a chain as fast and as long as you like, with the final gear in the chain welded to the frame.
[T]he reason why older people don't use the technologies is because they suck, are intrusive, unreliable and fleeting.
This. I was born in the 1950s, when dirt was still relatively new, and this is exactly it. When one first sees new software and UI technologies as a young person, understanding them is an end in itself. After 50 years of watching them come and go, however, I am tired of the technology of the month, and instead find myself using something stable that will allow me to get my "real" work done -- that being the point of software, after all.
Learning to use a good software tool once is a useful and rewarding experience. Being condemned to a lifetime of re-learning to use the same tool every year just because some idiot changed the UI, not so much. (Sisyphus would understand.)
I sometimes ask these UI wizards what they think would happen if I moved the keys on their keyboards around with every software release, in response to the latest theories on typing speed and accuracy, and perhaps added and/or subtracted a few just, well, just because I thought it would be a good idea. If one is, say, ten years old and just learning to touch-type, perhaps the new keyboard layout indeed would be better. However, the installed base of zillions of users that are used to, and expected to see, the old keyboard arrangement would be totally hosed, and would need to retrain themselves just to get back to the productivity levels they had before I "helped" them. Do you really, really want to do this kind of thing to your customers? Repetitively?
I am always amused to find that the same programmers that gleefully shuffle, delete, and obfuscate menus and other UI features for their users strenuously object if you even suggest taking away their precious Dvorak, XP, or simple QWERTY keyboards -- let alone, say, reverse the order of the bottom row of keys, transpose the T and H keys, and move the "@" symbol so that it is now CTRL - ALT - ].
. . . so a company would outsource the design and manufacture of the one feature their customers cared most about? I don't think you'll find many examples of that in history -- at least, examples in which the parent company survived very long. You're more likely to find examples in which the outsourcing company found itself in competition with its vendor, and then went out of business when the vendor kept feature improvements for its own designs.
You really believe that Apple or Google or Tesla would be content making just the software for cars, when the rest of the car is a commodity? (Check their cash reserves before you reply.) Or that GM or Toyota or Volkswagen could defend itself from them, once they made their move into a commoditized car industry serving customers who only cared about the car's OS?
I don't doubt that the industry has such contingency plans, but I wonder just how effective they will be. Business history is rife with cases of large companies that failed to move as rapidly as their industries, and disappeared as a result. See Clayton Christensen's The Innovator's Dilemma for a bookload of examples.
The problem is that the companies are built on a certain premise of customer wants and needs and, due to their installed asset base, organizational structure, and culture, can't react fast enough to supply products emphasizing the new customer wants and needs. By the time the company is willing to invest the capital needed to meet the needs of the new customers, they are so far behind their smaller, innovative competitors that they cannot compete. Whether they buy one of their upstart competitors or try to compete with their own new division, the corporate mass and culture almost inevitably dooms the venture.
Suppose the automotive market did change, to one in which customers didn't care about fuel mileage, or number of seats, or whatever it is they do now, and instead cared only about what OS the car was running. How many decades do you think it would take to remove all the car- and engine-geeks from the company and replace them with digital-geeks?
Kodak was aware of the digital photo revolution (it invented the digital camera, and its Board of Directors hired George Fisher from Motorola as CEO back in 1993), but a fat lot of good it did them. They had too many chemists, dye specialists, and high-performance camera designers, and an organization and profit structure built around discrete cameras and physical camera film. It was never clear how Kodak could maintain its market dominance in the digital camera world -- and it didn't.
Sitting on the floor is still easy, but getting up involves a lot of aching bones/muscles.
I think a good working definition of "the threshold of getting old" is the age when overexertion causes more pain in the joints than in the muscles.
As a young person, running an unusually long distance or lifting a weight an unusually large number of times causes sore muscles. As a not-young person, running an unusually long distance or lifting a weight an unusually large number of times causes sore joints -- and, unfortunately, it takes a lot longer to recover from sore joints.
I always thought the most practical combination of aircraft and submarine was the FA 330, a rotary-wing kite used by Nazi submariners to get their lookout higher to see farther. It was tethered and unpowered, but it was quick to set up, simple to use, and provided a great benefit to the sub in the last few days before radar.
. . . training which can curve some of their impulsive tendencies... however at the same time insure if they need to use force it is more affective.
should actually be written,
. . . training which can curb some of their impulsive tendencies... however at the same time insure if they need to use force it is more effective.
This uses curb with the definition of, "to check or restrain," and effective with the definition of, "producing a desired result" (as opposed to affective, which may be defined as, "influenced by, or resulting from, the emotions").
These types of errors (using similar-sounding words instead of the correct words) are called "malapropisms." The speaker knows which of the two words is correct, but somehow when speaking (or writing) the brain pulls the wrong word out of memory. It's an interesting neuropsychological phenomenon.
It's my understanding that the flight deck by international regulation is a "no alone" zone, meaning that when the pilot left, a flight attendant should have entered the flight deck so that the copilot was not alone. This rule is why it made sense to have a "Locked" position on the door.
The real question, to me, is, why was the flight attendant not on the flight deck while the pilot was away?
Slashdot Mirror
To be sure, but the relevant scale is the wavelength, which is about 5.5 cm at 5.5 GHz, and 33 cm at 900 MHz. There's nothing magic about those lengths -- a forest, or a building, is just as likely to have structures of either length, or any other length, or neither length. Certainly one can create frequency-selective surfaces that pass one band of frequencies and reject others. The statement to which I objected was that it was always true that 900 MHz signals travel farther than 5 GHz signals, due to some perceived difference in radiowave propagation. That is simply a falsehood -- a myth.
Please don't carry the discussion on any further. Instead, take a pair of parabolic dishes and run the test yourself -- through a forest, through a building, through the path of your choice -- then repeat the process with resonant dipole antennas.
They do, and you'll find that shorter wavelengths get through smaller openings that longer wavelengths cannot. It's a complicated field to model, and one can find cases where either is superior, but shorter wavelengths do have advantages in a scattering environment.
I design antennas and wireless links for a living, sir. Yes, I have done this. Since 1984.
Of course 900 MHz has better range, if one is using dipole antennas. The point is, the apparent difference in range is due to the antennas used, not some intrisic property of the propagation medium. (It's also likely that the 5 GHz transmitter has lower output power, and the 5 GHz receiver a higher noise figure, than the 900 MHz versions, making the 5 GHz range even less, but I'll ignore those factors for now.)
Give me parabolic dishes, and 5 GHz will go where 900 MHz will not. Give me resonant dipoles, and 900 MHz will go where 5 GHz will not. The behavior follows the antenna selection. It has nothing to do with "propagation," whether through trees, concrete block, or anything else.
Maybe numbers will help. Since the effective area goes as the wavelength squared, the amount of power captured from a resonant 5.5 GHz dipole, compared to that from a resonant 900 MHz dipole, from a given power flux density, is (900/5500)^2 = 0.027, or 2.7 percent. This means that, with identical efficiencies, matching loss, etc., the signal the 5.5 GHz receiver will get from its antenna is down 10*log10(0.027) = 15.7 dB from the signal the 900 MHz receiver gets. That's why your range at 5 GHz is less than at 900 MHz.
Now, let's move to parabolic dish antennas. Since the effective area of the dish antenna is the same for both bands, for a given power flux density the same signal level will be presented to each receiver at the antenna terminals. However, the power flux density generated by the transmit parabolic dish antennas will vary, but this time by the frequency squared, so the power flux density at the receivers on 5500 MHz will be 15.7 dB stronger than it will be at 900 MHz.
You can see where this is going. One has four possibilities:
1. Tx dipole, Rx dipole: 5.5 GHz signal 15.7 dB weaker than 900 MHz
2. Tx dipole, Rx dish: 5.5 GHz signal the same as 900 MHz
3. Tx dish, Rx dipole: 5.5 GHz signal the same as 900 MHz
4. Tx dish, Rx dish: 5.5 GHz signal 15.7 dB stronger than 900 MHz
You may ask how much range change is implied by a 15.7 dB change in signal strength. One simple range model is the use of a loss exponent; a typical value for the loss exponent might be 3 or 4. If we are optimistic, and take 3 as the value for the loss exponent, and assume a 15.7 dB change in loss, one can determine the ratio of the two distances as
10 * n * log10(d1) - 10 * n * log10(d2) = loss(dB)
30 * log10(d1/d2) = 15.7
d1/d2 = 10^0.524 = 3.34.
The band with the 15.7 dB strength advantage (the 900 MHz band, when dipoles are used, or the 5 GHz band, when dishes are used) has more than three times the range of the alternative. (If one assumes a loss exponent of 4, as might happen in an ugly indoor environment, the ratio works out to be about 2.47.)
No, it's a myth. If your 2.4 GHz radio had an antenna the size of your 900 MHz radio antenna, the performance would be the same. But because the 2.4 GHz dipole is smaller, the 2.4 GHz range is less. But it has to do with the antennas used, not any propagation phenomenon.
Resonant dipole antennas are constant-gain antennas, meaning that their gain is constant with frequency, while their effective area varies inversely with frequency squared. There are also constant-aperture antennas, in which their effective area is constant with frequency, while their gain varies. A parabolic dish antenna is an example of the latter; its gain varies with the frequency squared. If you take two parabolic dish antennas, fit them with 900 MHz feeds, and then take the same dishes and fit them with 2.4 GHz feeds, you'll find that the 2.4 GHz antennas have (much) higher gain and the resulting system, much greater range than the 900 MHz configuration.
It's also possible to set up a link with a constant-gain antenna (e.g., a dipole) on one end and a constant-aperture antenna (e.g., a parabolic dish) on the other. In this case the two effects cancel out, and the user does not see a difference in range between the two frequencies.
You'll find, if you actually do this experiment, that it does work this way -- regardless of whether the path goes through a forest, a house, or both. It's physics, period.
I am so tired of this myth.
900 MHz signals do NOT "propagate better." They propagate in free space just the same as 2.4 GHz signals and, in the presence of scattering (e.g., small openings in otherwise shielded areas) not as well as 2.4 GHz.
What people fail to realize is that these systems typically use some variant (often a physically shortened variant) of a dipole antenna, and the 900 MHz antenna is physically larger than the 2.4 GHz antenna. It therefore has a much larger effective area (the effective area being inversely proportional to the square of the frequency of operation), and so captures much more of the incoming energy. To the user, it looks like the propagation is better, but in reality the device just has a physically bigger antenna.
Look at it this way: If propagation got worse as the frequency went up, we'd never see light from the sun!
I was musing just the other day about a related calculating method that has fallen into disuse, the nomogram. Nomograms always impressed me as an especially clever way to perform specific mathematical tasks.
When I was young, and dirt was still sparkling and shiny new, nomograms were in every engineering textbook, handbook, and reference book. Their demise in engineering applications seems to have come with a whimper, not a bang, as no one seems to have noticed it.
The comment has been made before that the better analogy is to a tossed salad (or salad bowl), rather than a melting pot, as the ingredients tend to maintain their individuality, rather than becoming homogenized. A lot of it is bland lettuce, but one also has everything from olives to jalapeños to spice things up a bit. (Extra credit for identifying a good analogy for the salad dressing.)
In the early 1990s one could travel between Hong Kong and Kowloon by hovercraft. It was an interesting change of pace, but I could see how it couldn't compete against the Star Ferry.
From TFA:
[. . .] According to this prediction, the possible attack distance is approximately 16.78 cm using the same sound source that we used for the real-world attack with the maximum volume (113 dB). This attack distance range might not be sufficient for a malicious attacker. However, attackers can overcome this distance limitation by using a more powerful and directional source (e.g., a loudspeaker array) than the single speaker used in our experiments. For instance, SB-3F from Meyersound can generate sound of 120 dB at 100 m, and 450XL from LRAD and HyperShield from UltraElectronics can produce 140 dB at 1 m, which is equivalent to 108.5 dB at 37.58 m. Therefore, the possible attack distance is 37.58 m, if an attacker uses a sound source that can generate 140 dB of SPL at 1 m.
you initiate the feather to soon
. . . or even too soon.
There may not be any need to panic and start fixating on what every little app is doing.
But then again, there might. How is one to know? That's the biggest problem I have with the mobile telecom computing model. I have no idea what the apps do, and no way, other than make it my life's work, to find out.
I hate having to trust the OS provider that everything is properly sandboxed, that none of the apps in their stores are malware, etc. What's going on, inside this box?
This was my thought, too. There is a hand-crankable version of the same thing in some hands-on museum in or around MIT that I visited some years ago. It's amusing to be able to crank the first gear in a chain as fast and as long as you like, with the final gear in the chain welded to the frame.
[T]he reason why older people don't use the technologies is because they suck, are intrusive, unreliable and fleeting.
This. I was born in the 1950s, when dirt was still relatively new, and this is exactly it. When one first sees new software and UI technologies as a young person, understanding them is an end in itself. After 50 years of watching them come and go, however, I am tired of the technology of the month, and instead find myself using something stable that will allow me to get my "real" work done -- that being the point of software, after all.
Learning to use a good software tool once is a useful and rewarding experience. Being condemned to a lifetime of re-learning to use the same tool every year just because some idiot changed the UI, not so much. (Sisyphus would understand.)
I sometimes ask these UI wizards what they think would happen if I moved the keys on their keyboards around with every software release, in response to the latest theories on typing speed and accuracy, and perhaps added and/or subtracted a few just, well, just because I thought it would be a good idea. If one is, say, ten years old and just learning to touch-type, perhaps the new keyboard layout indeed would be better. However, the installed base of zillions of users that are used to, and expected to see, the old keyboard arrangement would be totally hosed, and would need to retrain themselves just to get back to the productivity levels they had before I "helped" them. Do you really, really want to do this kind of thing to your customers? Repetitively?
I am always amused to find that the same programmers that gleefully shuffle, delete, and obfuscate menus and other UI features for their users strenuously object if you even suggest taking away their precious Dvorak, XP, or simple QWERTY keyboards -- let alone, say, reverse the order of the bottom row of keys, transpose the T and H keys, and move the "@" symbol so that it is now CTRL - ALT - ].
A thunderstorm must torture these people terribly.
How does that first sentence read again? I think someone left out a verb.
I disagree. Let's wait a few years and see who was right.
. . . so a company would outsource the design and manufacture of the one feature their customers cared most about? I don't think you'll find many examples of that in history -- at least, examples in which the parent company survived very long. You're more likely to find examples in which the outsourcing company found itself in competition with its vendor, and then went out of business when the vendor kept feature improvements for its own designs.
You really believe that Apple or Google or Tesla would be content making just the software for cars, when the rest of the car is a commodity? (Check their cash reserves before you reply.) Or that GM or Toyota or Volkswagen could defend itself from them, once they made their move into a commoditized car industry serving customers who only cared about the car's OS?
In any event, I think we can agree that old age doesn't begin at 25.
Indeed. Talk to me in 15 years (that is, if I'm still around).
I don't doubt that the industry has such contingency plans, but I wonder just how effective they will be. Business history is rife with cases of large companies that failed to move as rapidly as their industries, and disappeared as a result. See Clayton Christensen's The Innovator's Dilemma for a bookload of examples.
The problem is that the companies are built on a certain premise of customer wants and needs and, due to their installed asset base, organizational structure, and culture, can't react fast enough to supply products emphasizing the new customer wants and needs. By the time the company is willing to invest the capital needed to meet the needs of the new customers, they are so far behind their smaller, innovative competitors that they cannot compete. Whether they buy one of their upstart competitors or try to compete with their own new division, the corporate mass and culture almost inevitably dooms the venture.
Suppose the automotive market did change, to one in which customers didn't care about fuel mileage, or number of seats, or whatever it is they do now, and instead cared only about what OS the car was running. How many decades do you think it would take to remove all the car- and engine-geeks from the company and replace them with digital-geeks?
Kodak was aware of the digital photo revolution (it invented the digital camera, and its Board of Directors hired George Fisher from Motorola as CEO back in 1993), but a fat lot of good it did them. They had too many chemists, dye specialists, and high-performance camera designers, and an organization and profit structure built around discrete cameras and physical camera film. It was never clear how Kodak could maintain its market dominance in the digital camera world -- and it didn't.
Sitting on the floor is still easy, but getting up involves a lot of aching bones/muscles.
I think a good working definition of "the threshold of getting old" is the age when overexertion causes more pain in the joints than in the muscles.
As a young person, running an unusually long distance or lifting a weight an unusually large number of times causes sore muscles. As a not-young person, running an unusually long distance or lifting a weight an unusually large number of times causes sore joints -- and, unfortunately, it takes a lot longer to recover from sore joints.
I always thought the most practical combination of aircraft and submarine was the FA 330, a rotary-wing kite used by Nazi submariners to get their lookout higher to see farther. It was tethered and unpowered, but it was quick to set up, simple to use, and provided a great benefit to the sub in the last few days before radar.
He's going to get a lot of notes.
. . . training which can curve some of their impulsive tendencies... however at the same time insure if they need to use force it is more affective.
should actually be written,
. . . training which can curb some of their impulsive tendencies... however at the same time insure if they need to use force it is more effective.
This uses curb with the definition of, "to check or restrain," and effective with the definition of, "producing a desired result" (as opposed to affective, which may be defined as, "influenced by, or resulting from, the emotions").
These types of errors (using similar-sounding words instead of the correct words) are called "malapropisms." The speaker knows which of the two words is correct, but somehow when speaking (or writing) the brain pulls the wrong word out of memory. It's an interesting neuropsychological phenomenon.
Manure?
It's my understanding that the flight deck by international regulation is a "no alone" zone, meaning that when the pilot left, a flight attendant should have entered the flight deck so that the copilot was not alone. This rule is why it made sense to have a "Locked" position on the door.
The real question, to me, is, why was the flight attendant not on the flight deck while the pilot was away?