What is the difference between ff-ff-ff-ff-ff-ff (layer 2 broadcast) 255.255.255.255 (layer 3 limited broadcast)?

Question: Please explain the flow of traffic in each case.
Comment: Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide your own answer and accept it.
Answer: They are aimed at different network layers. Every layer-2 broadcast must be inspected and processed by every host to see if it should be passed up to layer-3. A layer-3 broadcast received by a host must be inspected and processed to see if it must be passed up to layer-4.
I suppose it would be possible to create a layer-2 frame which is a layer-2 unicast frame but contains a layer-3 broadcast address. This would not really be a broadcast since it only goes to a single host based on the layer-2 MAC address.
ARP is an example of a layer-2 broadcast which has a layer-3 unicast address. Each host must inspect the layer-2 ARP broadcast to see if it should be passed up to layer-3, but only the host with the layer-3 unicast address will actually pass the layer-2 broadcast up the stack.
Answer: The layer 2 broadcast address ff:ff:ff:ff:ff:ff is used on ethernet frames and is supposedly broadcasted on all equipments.
255.255.255.255 is the layer 3 address that is used to adress the exact same hosts.
Note that:

IP can support all kind of networks, so ethernet won't be always used.
Reciprocally ethernet can be used without IP (appletalk, IPX, ...).
One layer 2 media can share several IP networks, but it is really bad practice: broadcast limits should be the same at layer 2 and 3 (for instance if 10.0.0.0/24 and 10.0.1.0/24 are located on the same switch and same vlan: bad idea).
Answer: The flow of traffic in each case goes like this.
Let's take Ron's example of an ARP Request. So the layer 2 destination address is ff:ff:ff:ff:ff:ff which is a broadcast. The layer 3 address however may be 192.168.1.2 which is probably the IP address of the host that you're trying to find the MAC address for. Now this ARP packet is sent out on the network and every host on the network receives it. Each host is going to first receive it, run a CRC on it and if pass, will go on to check the layer 2 address. Now since it's a broadcast, it's going to pass through and then it will check the layer 3 address. Those that do not match, discard the packet. The one that does match then forms a response packet with it's MAC address, sets the layer 3 destination as our original source (say 192.168.1.3) and layer 2 destination as the MAC address of our source.
Like Ron mentioned, the difference is mainly where it's processed. While MAC addresses are processed in layer 2, IP addresses are looked at only at layer 3. Sometimes, the device might not even look at the IP address and directly discard the packet after checking layer 2 information.


Using Global Entry card in NEXUS lane Canada to US

Question: I am a US citizen with a Global Entry Card. I'll be driving back from a family vacation this summer in Canada. I'm wondering what it is like to use the NEXUS lanes with this Global Entry Card. I have heard from several people that in the NEXUS lanes you aren't supposed to have anything in your car and you will be searched if you do (i.e. it is for commuters). Is this true? I will have kids, dogs, etc and not sure if I should try the NEXUS lane.
Comment: You must not have "anything to declare" in order to use Nexus lanes. If you have (allowed) food or whatever, you must use the regular lanes. Messing up doesn't just mean being searched, it means losing your card. I don't know if this applies as strongly to Global Entry people but I do know I wouldn't experiment to find out.
Comment: Um, do your kids have global entry or nexus cards?
Comment: Sounds like this is not feasible for vacationers- thanks so much for the information! Super helpful.
Comment: @KateGregoryYou can have things to declare.  Certainly northbound can, and you can even be over your exemption entering Canada - the officer will have you go to secondary inspection to pay, or waive the fees and allow you to enter Canada.
Answer: So says the official page:

Global Entry Members at NEXUS/SENTRI Lanes
There are no additional requirements for a member to use the NEXUS lanes coming into the U.S. However, Global Entry cards are not valid for entry into Canada via the NEXUS lanes and kiosks; travelers must apply separately for NEXUS.

Another page:

Travelling with non-NEXUS members: If you are travelling into Canada or the United States (U.S.) with non-NEXUS members, you must use the regular lanes. The driver is responsible for ensuring that all travellers in the vehicle are NEXUS members. If there is a non-NEXUS member in the vehicle, regardless of his/her age, it will result in the loss of the driver's NEXUS membership

So if you attempt this with kids without GE cards, you will be in deep trouble.
Comment: Thanks - yes I was waiting to go through the hassle of GE cards for them until I figured out whether or not we should do the lanes w/ a car full of stuff.
Comment: Personal experience now: we crossed the border in both directions with two adults and a kid, each of having a NEXUS card in an SUV chock full of random stuff because one of the adults were moving the time and didn't have anywhere to put all that.
Answer: Coming from Vancouver into Washington state, there was nothing to indicate Global Entry would suffice for the Nexus lane.  Not knowing, we didn't mention GE, and were directed to "inspection", even though we had nothing to declare.  We were made to wait, wait, wait, while BP agents did absolutely nothing, and finally were called and threatened with $5K fines per person for using Nexus lanes.   Just a little indication that GE was sufficient for re-entering the US, would have saved a lot of frustration.


If two perfect chess AI's played each other, would it always be a stalemate or would white win for an inherent first-move advantage?

Question: In the circumstances of two perfect AI's playing each other, will white have an inherent advantage? Or can black always play for a stalemate by countering every white strategy?
Answer: This relates to the concept of "solved games". In general, two player turn-based games with perfect information - of which chess is an example - can result in all three possible outcomes: a forced win for white, a forced win for black, or a forced draw.
The short, although unsatisfactory answer is that chess is not solved, and it is not clear whether it can be. There is generally thought to be an advantage to white for the first move, so likely results are considered to be a forced win for white, or a forced draw.
No current AI attempts to "solve" chess, although some of the techniques such as MCTS might be adapted theoretically to find a solution, the available computing power to run that search to completion from the start positions is too low by a few orders of magnitude.
Comment: There are more possibilities, like an endless match. Moreover, the possibility of win by timeout, that means that winner will be the faster AI. More interesting from the point of view of AI ¿why two perfect AI chess gamers will play chess if they know there other one is also perfect? Both AIs known from start the final result, they can jump directly to it.
Comment: @pasabaporaqui: I'd like to avoid expanding definitions to include chess rules that the OP did not specify. In addition, the OP specified "perfect" but not "general" as in AGI. Yes if there was an AGI that could also play chess perfectly, it might choose to do things such as not play. However, the concepts of "general" and "perfect" are completely orthogonal here. IMO we are more likely to build a general AI than one that can play chess perfectly. We have functioning examples of general intelligences (ourselves) but no functioning examples of perfect chess players.
Comment: @pasabaporaqui Chess can't go on forever.
Answer: No that doesnt have to be the case because from each machine's point of view the game is different so they will have different possibilities like saying different paths to follow, now something different would be if two machines with the same brain play against the same opponent with same parameters then we can say theboutcome will problably be the same, if they are "programed in a deterministic way"
Answer: I'm no expert chess player.
Some specialist chess forums also discussed this issue and there's no clear answer.
Due to the high amount of possible moves, I would suggest, that most outcome would probably be a remis.
White would have the advantage of starting, but as black as a perfect AI would know which possible strategies are in play it could block every try for a clear winning strategy but probably always trying to avoid a loss and therefore probably reaching just a remis.
Basis for this thought is that black as a reaction can always make the choice with fewest loss possibility so probably most strategies of white would stall.
But now comes the interesting point: I am not quite sure how these AIs would look like. They obviously would have to lean on a strategy to choose between all the possible moves.
A pure statistical best outcome algorithm even then would leave tooo much options open, specially in the beginning. So you would be forced to prefer certain strategy choices, and with this you would clearly implementing a bias (what to do if two moves have the same win/loss outcome?). This would make the experiment more random or human and thus no outcome could be predicted.
So if both AIs are perfect (and thus identical) the outcome is most probably a draw, but I assume that statistically some games would be won/loss...
I think...
EDIT: I just read about googles AlphaZero (a AI which has taught itself Chess, Go and Shogi) and excells in most games against other AIs. But apparently by precalculating not so much possible outcomes... So it could be that there still is no "absolute perfect AI" for this game(s)
Answer: Since both AI know the best possible moves at each step, black would never win as white already knows all games that leads to black's victory and would easily avoid it, but since the black is also a perfect AI, the optimal reponses to white's moves are fixed. So the logical reasoning is these two perfect AI would not make any moves at all if they know their opponent is a perfect AI as well. Both of them know the outcome of the match even before it begins, which my gut feeling says is always a draw. The correct question is does the nth move matter?


PID tuning for 6 dof robotic arm

Question: I'm currently developing a 6 dof robotic arm. The arm is vibrating when it stop moving and I want to reduce it. Another thing is that arm is so heavy (because there is a projector inside it, lol) and I have to use spring between joints. So, can anyone tell me 1. how to select springs because my supervisor told me that proper selection of springs can reduce vibration? 2. how do I tune the PID parameters? All the joints are dynamixel servos and their PID parameters are tunable. I read article about tuning for a single servo. How do I tune these parameters for the whole arm?
Comment: Are the vibrations coming from the motors, i.e. are the motort moving the linkages activly when it vibrates, or are the mechanical vibrations coming from the stiffness of the mechancal structure?
Comment: I would say most vibrations come from the structure because it vibrate when it stop moving and these motors pretty high-end (dynamixel). BUT I think there is some 'backlash' inside the servo which I have no idea how to solve. although it is very tiny, the effect on the end effector is visible.
Answer: I think the vibrations of the mechanical structure are due to the mechanical structures natural frequency. The dynamixel servos you mentioned are quite performant and have a acceleration and deceleration. This means that it has an abrupt stop when it reaches its setpoint, and this "shock" excites the mechanical structure at its natural frequency.
You could set the PID controller of the dynamixel servos softer with a large settling time without any overshot. This might help.
To tune the PID you can follow the Ziegler-Nichols method. The order of tuning should be backwards, from end-effector to base.
Another solution (and actually this would be my recommendation):
You can leave the PID controllers of the servos as they are and do a trajectory planning. Instead of reducing the performance of the controllers (i.e. instead of making them soft), generate the reference values, the setpoints of the controllers, in a smooth manner, where you control the changes in position, velocity and acceleration, and those are not implicitly set by the controller. They will still be influenced by the controllers, but their effect will be greatly reduced. If you plan the trajectories smoothly (continuous acceleration, no sudden changes) the mechanical structure (at its natural frequency) will not be excited anymore. You will lose some dynamical performance but you will gain smooth motions.
The spring can help your servos if they do not have enough torque. It may effect the frequency of the vibrations but will not eliminate these in most cases. Dampers can be used to reduce vibrations, if your spring is actually a spring-damper it can eliminate vibrations.
To dimension a spring you have to determine what is the maximum torque required by the mechanical system in one given axis. You compare that with the maximum torque of your motor. if it is larger, so you need more torque than you have, you can use this difference to dimension a spring. The spring exerts a force, which is then converted to a torque using the perpendicular distance. This perpendicular however changes with the motion of the joint. So the design variables, spring constant, perpendicular distance (distance and angle of attack) will effect how much torque the spring will exert for every position of the joint. The same way the mechanical torque requirements of the robot will change for every position of the joint. The motor torque will not change. So...the torque coming form the spring plus the torque coming from the motor will have to be smaller than the required torque for every possible position of the joint.
Since you mentioned a projector:
Please keep in mind that a small motion at the projector level will produce a large motion at the projected image. So you will have to have very smooth motions in order to not excite the mechanical structure. Or you could use a more robust mechanical structure, which is not that easy to excite.
Backlash can also be compensated to some degree. You can find more about it here.
Comment: Thx for your advice, I will look into trajectory planning. For Ziegler-Nichols methods, how exactly do I find the vibration frequency? Do I need any equipment? I am now using a C program to read the position from the servo internally every 0.029s. This data does not show vibration even the Kp is maximum.
Comment: Depending on how high the frequency will be that might be enough. 0.029s cycle time gives you a frequency of 34.4Hz, so you might be able to see the vibrations if they are up tp the 10-15Hz range. If you have a smartphone, you can get an app that does a audio spectrum analysis. If your microphone can pick up the noises your drives make it might show you the frequency, however expect it to be very noise, this is just a low-cost (free) solution.
Answer: Springs (no matter the combination) are not going to reduce the vibration. (there are exceptions because real springs lose energy but I don't think it applies to your case.) Damping could reduce the vibration, but that means you are throwing away energy and it sounds like you need all you can get to move that heavy projector.
Keep in mind that PID does not work as well when the load on the joints change, which is what happens in an arm. Depending on the angle of the joints, sometimes gravity will increase or decrease the apparent load in a way that makes the PID gains not work well at that angle.
Increasing the D gain of your PID will help. However for an arm you should be using a trajectory generator to set the target position to intermediate points between the start and end position.
Another issue you might be having is that you probably have the extra projector support springs arranged to help lift the projector to the position you will use most often. This means you will have asymmetric force gradients on the arm with gravity creating one force and the spring creating another.
So...

Use the single servo PID tuning approach for each servo, one at a time, with the joint axis of the servo of interest vertical and the other joints in a position that puts the projector closest to where you normally want it. Leave the projector support spring on. (the goal is to eliminate the influence of gravity when tuning the PID)
Set up the arm for normal use and increase the D gain on the joints that are vibrating too much until it's ok (or until you get a high frequency vibration). I predict that the arm performance will not be good when you do this and you should do the next step.
Add a trajectory generator to your control. With this you should be able to set an accelleration that gives you better performance than step 2 but might be a little slow.
Switch to torque control, do your own PID calculaton and add feedforward control for gravity compensation and support spring compensation.

All of the above should help regardless if the vibration is from motor, mechanism, or control.
Starting with actuators that are properly sized for the application would be best but it sounds like that is not an option for you.
Comment: General guidline for PID tuninig is called Ziegler Nichols https://en.wikipedia.org/wiki/Ziegler%E2%80%93Nichols_method
Comment: Hi hauptmech,  Than k you so much for your reply. I am currently working on step 1. I am trying to find the plant function by using System identification in Matlab. This is what i did, I give it command to move 90 degree as step input. i read the position as output. then I have my transfer function for plant and I add controller fomr there onwards. is this correct? or is there another way to find out the plant function of the arm link?
Comment: I don't use Matlab so I can't help you with using it. I would not bother with system identification or modeling until step 4. With manual tuning of the PID you don't need the plant function and the results will show you what you need to know, which is that step 1 and 2 is not enough (I'm guessing). However doing it your way (plant function -> PID gains) will teach you a lot especially if you observe the difference between what theory says should happen and how your mechanism actually behaves.
Comment: is there a general guideline for manual tuning PID?  or I just observe the overshoot, settling time and all that ?

PID tuning for 6 dof robotic arm


What's the point of defining left ideals?

Question: I admit, I haven't gotten really far in studying abstract algebra, but I was always curious (ever since I saw a definition of an ideal) why is the notion of left-sided ideal introduced when we quotient by two-sided ideals?
Are there examples of use of exclusively left (left but not two-sided) ideals in ring theory?
Comment: When the concept of a left $R$-module is introduced (an analogue to a vector space over a field), it turns out that the ring $R$ has a natural structure of left $R$-module (the same way a field is one-dimensional vector space over itself). Then the left ideals of $R$ are precisely its submodules and the quotient has a structure of an $R$-module as well.
Answer: Right ideals are submodules of the module right $R_R$, but two-sided ideals are bisubmodules of $_RR_R$. They are completely different in flavor.
The distinction is pointless in commutative algebra, of course, but in general noncommutative algebra it is important. For example, $M_n(\Bbb R)$ has infinitely many right ideals but only two two-sided ideals.
Another example is the ring of linear transformations of a vector space over a field which has countable dimension.  Not only does the ring have infinitely many right ideals, but it doesn't even satisfy the ascending chain condition on right ideals. On the other hand, it has exactly three two-sided ideals.
So as you can see, one-sided and two-sided ideals can display very different behaviors and have very different implications about their ring.
Using an ideal $I$, the cosets $R/I$ form a ring. However if $I$ were only a one-sided ideal and not two sided, you could not do the same thing. The best you can do is to give the quotient a module structure, in that case.
Answer: A good example for studying left-ideals instead of two-sided ideals are matrix rings, which are not commutative (in general), so that there is a difference. For an extensive discussion about left ideals and two-sided ideals in matrix rings, see
What are the left and right ideals of matrix ring? How about the two sided ideals?


Yamaha P45 Digital Piano - Can it be made MIDI compatible?

Question:
I'm using a Yamaha P45 digital piano. This digital piano does not send MIDI signals despite having a MIDI port. Is there a way possible by which we can make this keyboard capable of sending MIDI signals? Attaching a pic of the rear panel of the Yamaha P45 digital piano.
Comment: I'm not seeing any midi port, but I do see a To Host [USB] port, which can be used similarly to a midi port, with the appropriate drivers & software. Midi itself would then need to be generated via a dedicated midi interface, from your computer, if you need it to drive other external devices.
Answer: Yes
You have to download the USB MIDI drivers and then you can connect the P-45 to a computer to control software synthesizers and/or record and play back MIDI data. If you want to control other hardware using MIDI, you will have to connect that hardware to the same computer.
This page has driver and manual downloads for the P-45:
https://usa.yamaha.com/products/musical_instruments/pianos/p_series/p-45/downloads.html
Answer: I currently use a p45 as a controller for soft synths in my pc daw and hooked to my ipad works well with both with just plugging a usb cable into the device. I usean old printer cable I had lying around the studio usb type b to type a cable. the daws recognizes it as a digital piano. the stand alone synths in ios recognize it also.
peace.
Answer: If you want to use it to control other MIDI enabled gear without a PC, you can still do it with a MIDI USB Host.
Careful: NOT a USB MIDI Interface.
Disambiguation:

An example could be: https://www.thomann.de/gr/kenton_midi_usb_host.htm
You can find cheaper ones, however I've seen some comments that they sometimes not work or e.g. don't support MIDI Clock or get stuck & such. I know it's expensive, but I don't think there's a cheap alternative.
If you can tingle with programming a bit, you can always experiment and build one with a Raspberry PI and a USB MIDI interface (for example: https://linuxmusicians.com/viewtopic.php?t=19736) but I doubt if it's worth the fuss, especially if you're not experienced with Linux & programming.
Answer: From a review "Unfortunately, the available controls are not enough to turn the P-45 into a real master keyboard, but at least the built-in USB port allows you to save money for an external USB/MIDI interface and compensate for the ordinary lack of a built-in audio recorder, which is surely a shame for a 2015 product."
https://www.digitalpianoreviewguide.com/yamaha-p45-review/
The answer then is no. But if you want to use it to send midi signals to a program in your computer, use the USB port. You might redirect the Midi signal inside the computer to a Midi interface connected to the computer. In that case the answer would be "sort of".
Comment: Sorry about the confusion. Midi as originally designed is sent on a five pin connector directly between musical instruments. The piano does not have any such connector. Instead it has as USB port which allows you to connect it to a computer (not directly to other Midi instruments). This allows you to control software instruments inside the computer using Midi signals. So if you want to control instruments in a computer it is no. If you want to control other Midi instruments the answer is basically no (unless you buy a Midi interface for the computer).
Comment: Back in the day, multi-keyboard rigs might use direct (5-pin) MIDI connections between several keyboards, maybe with simple MIDI routing or (rather less simple) MIDI Merge and Split boxes.   Now there is invariably a computer as a central routing device, and the 5-pin cables are being replaced with USB connections. Which work both ways, sending and receiving MIDI data.  One keyboard can still 'play' another keyboard.  But there needs to be a computer in the middle.
Comment: The same review says, "t shares all the core-functions with the older Yamaha P-35, adding some new interesting features like the USB connector, which allows you to use the piano as a MIDI controller". So I'm a bit confused by your assertion that the answer is "no".
Comment: invariably? no. I want to use this to drive a hardware workstation without starting up a computer. 'invariably' means without variation, without exception, so you claim to know everyone else's use case without exception?
Answer: Page 18 of the manual says YES
https://www.musikkhandel.no/media/files/2496/p45_en_om_a0.pdf
https://www.musikkhandel.no/media/files/2496/p45_en_om_a0.pdf#page18
"By connecting a computer to the [USB TO HOST] terminal, you can take advantage of various music software with your computer and the instrument.
For details on using a computer with this instrument, refer to the "Computer-related Operations" on the website."
"Setting the MIDI Transmit Channels
In any MIDI control setup, the MIDI channels of the transmitting and receiving devices must be matched for proper data transfer. You can specify the channel on which this instrument transmits MIDI data. To do this, while holding down [GRAND PIANO/FUNC- TION], press one of the D6 – E6 keys. The D6 key decreases the value by 1, the E6 key increases the value by 1, and the D#6 key restores the default value (channel 1).
[USB TO HOST] terminal"
But:
D#6 (Default)
D6 (-1) E6 (+1)
USB cable
• Use an AB type USB cable. When connecting a USB cable, make sure that the length is less than 3 meters.
Computer
• In Dual, the keyboard performance via Voice 1 is transmitted on the specified MIDI Transmit Channel and the keyboard performance via Voice 2 is transmitted on the next channel number in sequence (specified channel + 1).
• No MIDI messages are output during playback of the Demo Song or Preset Song.
Other MIDI Settings
The MIDI settings other than the Transmit Channel are set as listed below, and cannot be changed.
MIDI Receive Channels: ....................................... All channels (1 – 16)
Local Control: ............................................On Program Change Transmit/Receive:........Yes Control Change Transmi
Answer: with ACID music STUDIO 11, i can use Yamaha P125 like a midi keyboard through many VST available with ACID music STUdio 11. I don't know how is it possible..but i can record some keybord parts with many other sound out my Yamaha P-125. Sailer didn't tell me that ! (just turn "Digital piano" in device )


MP3 gapless looping help?

Question: Hi guys, quick query here,
Need to make some sounds for a client loop seamlessly in his game. Edited them fine as wav files in Pro Tools so they loop ok, but when saved as mp3 at clients request a tiny silence gap is added. Most annoying! Anyone know a way of removing this gap or is it inherent to mp3? Shall I tell the client to use ogg or wav instead?
Cheers!
Andy
Answer: The gap in mp3 files in inherent of the way they are encoded. I have seen that its possible using some 'hacks' and other software to remove this gap, but tb honest it seems like more hassle than its worth!
If the game is for iOS, your best bet is to probably use AAC, as its a well supported by apple and capable of looping seamlessly. I've found the easiest way to deliver AAC files is to use itunes to convert WAV to AAC, its also possible to check looping in itunes easily and have some reassurance that they will loop well on an iOS device.
Comment: That's great! Cheers for the tip :) Another reason to hate mp3 lol
Answer: I think it is because mp3 files contains data blocks of predetermined sizes, so if the length of your loop isnt an exact multiple of the block size then the mp3 encoding will add silence.
The only tool i have successfully used for looping mp3 files is part of the Miles SDK, which is a game audio engine made by RAD Game Tools.
Comment: that's how i understand it too
Answer: Looping MP3s is possible, just very hard. This is the info you're looking for:
http://www.compuphase.com/mp3/mp3loops.htm
Basically you have to pad (stretch) your audio to a specific length where the number of encoded samples will give you N whole frames. It's achievable, just plenty of hacking.
Looping WAVs is very easy. On some platforms, you can save 4:1 compressed WAVs with little sacrifice in quality. Those files loop just as easy.
Not all players/platforms support gapless playback so may testing be on your side.
Comment: Thanks for the reponses guys! In the end I just sent wav, ogg and aac formats. MP3 was too much of a PITA!lol
Answer: Whenever you do any conversion that might resample audio it can change where your zero crossings occur which could affect how well the audio loops. If you are able to work with the mp3 in an editor as opposed to working with a wav and exporting it as an mp3 I think you should be able to make it loop properly.
Comment: I tried editing the mp3 in an editor too but to no avail. Will try downloading mp3 trim and give that a go
Answer: If it's for a flash game (which would explain the mp3 format), there are ways to make it loop seamlessly using AS3 code... IMO it's your client's problem, not yours. :)
Answer: The way I understand it to be is that the mp3 format is frame based, rather than sample based like wavs. Much like how video editing can't make cuts smaller than a frame, an mp3 outside of the DAW is, as Haydn pointed out, organized in blocks. For seamless loops, I would definitely go with another format.
Answer: If your client insists on using MP3s and the game engine can handle it, you can always do a hack with two mp3s (with fade-ins and -outs) and switch them back and forth so no gap is heard in the game.
Answer: Hello Andy, it depends by the format (mp3). All seamless loops I did for games were ogg-vorbis encoded

MP3 gapless looping help?


Design Roller Compacted Concrete Pavement Section

Question: I have been asked to consider using RCC instead of traditional PCC on an upcoming project. For a rough order of magnitude estimate, can I assume that the pavement section design will be similar? I know that the mix design and construction methods will vary, but not sure if there will be any difference in the thickness of concrete or aggregate base.
Answer: RCCPave is the pavement design software tool for RCC, this is the guide that supports the software.  http://members.cement.org/EBiz55/ProductCatalog/Product.aspx?ID=1626
I can't find a download for the software though, must need to contact ACPA.
Answer: Pavement ME does not have an RCC pavement design module so using it to design RCC is not within the bounds of the design procedure.
If we are talking general pavement performance principles, there is some similarity between RCC and undoweled jointed concrete pavement. You could use Pavement ME to look at the potential slab cracking for jointed concrete and expect some similarity with RCC.  Looking at roughness would be more of a stretch.
Answer: I am not an expert on roller compacted concrete but I did find a few links to reports on the topic. They are listed below.

FHWA Tech Brief on RCC
National Concrete Pavement Technology Center Guide for RCC Pavements
Portland Cement Association RCC Page
RCC Pavement Council Resources Page

I hope you are able to find some useful information in the links above.


Listing character traits

Question: I wonder if when talking about characters (in a book with a third perspective narrator) one can list their personality traits. I realize how that might be taking something away from the story, as I think perhaps character traits should come out naturally within the story. Though, I don't know, which is why I'm asking. Maybe listing a character's traits in a creative way is delightfully informative and good for the story and it's progression?
Comment: I think this is a bit broad, what exactly is a "trait"?
Do you mean describing them physically, or how they handle situations? Their favourite food, or their abilities? Considering a "trait" is defined as: "a distinguishing quality or characteristic, typically one belonging to a person", could you be more specific?
Comment: In fiction, usually there's a plot and/or theme. If a list of traits advances the plot and/or theme, then that's a compelling reason to do it. If listing traits does not advance the plot and/or theme, that's a compelling reason to not do it.
Comment: @ErdrikIronrose I see what you mean. I was talking about personality traits.
Comment: You can find many examples of character's personal descriptions in literature, but they are never a "comma-separated list" of traits. Good description is like a little story an author tells about a character, a story interesting by itself.
Answer: Character traits should be seen. Absolutely. Being told that someone is smart isn't enough - he has to use his brains. However, can you sometimes tell rather than show traits? Let me show you some positive examples:

"This hobbit was a very well-to-do hobbit, and his name was Baggins. The Bagginses had lived in the neighbourhood of The Hill for time out of mind, and people considered them very respectable, not only because most of them were rich, but also because they never had any adventures or did anything unexpected: you could tell what a Baggins would say on any question without the bother of asking him." - The Hobbit, J.R.R.Tolkien

Tolkien sets up an image, establishes Bilbo as being a certain kind of character. This image is important because Tolkien then proceeds to topple it over: Bilbo obviously does go on an adventure, and does quite a few unexpected things.

"Glorfindel was tall and straight; his hair was of shining gold, his face fair and young and fearless and full of joy; his eyes were bright and keen, and his voice like music; on his brow sat wisdom, and in his hand was strength." - The Lord of the Rings, J.R.R. Tolkien

Glorfindel is a side character. We never really get to see him fearless, joyful, wise and strong. At best, we get a glimpse. So we might as well be told. However, the description doesn't merely serve to tell us about Glorfindel: it sets the mood for the scene, tells us what kind of people are gathered around Elrond's table. Glorfindel is an example.
Note also the language. Compare "Glorfindel was smart and strong" to "on his brow sat wisdom, and in his hand was strength". Compare "Bilbo was a rather boring person" to "you could tell what a Baggins would say on any question without the bother of asking him". It matters how you choose to describe character traits.
Comment: Your last paragraph is solid gold. Thank you for articulating that so well.
Comment: As long as you remember, this is how the narrator views the person, and not how the character perceives themselves, or the world does.
Comment: I like that you included Glorfindel in the example: I have been a big Tolkien fan for a long time and I have always thought that one of his biggest weaknesses as a writer was his tendency to list character traits when you first meet a character, particularly with minor characters, which sometimes leaves the reader with nothing but a list to try to remember and associate with a strange sounding name. It is one of the barriers non fantasy readers often have when approaching Lord Of The Rings.
Comment: @JBiggs listing the character's traits when you first meet a character was a common practice in medieval literature, which was one of Tolkien's sources of inspiration.
Comment: @Galastel yeah, I know. He was directly inspired by the epic poems and medieval lore he studied. It provided some really awesome things that came through in his writing. I just feel that the lists of character attributes just didn't work very well (at least in LOTR. In The Hobbit he kept things more concise and trimmed down and didn't allow himself the flowery excess that sometimes became burdensome later.)
Comment: @JBiggs For me it created a sense that almost I was reading an epic poem. Those things take some effort, I agree, but I love them.
Answer: I only like including something questionable - if it serves a second purpose. Then, I love when a line does multiple things. But I think you can't always find that second and third purpose necessarily until a later draft.
So, I might start in my first draft with: 'He was a funny boy, he was good hearted, and studious. He had hair that was blond but that took on a greenish tint from his swims in the lake, and he had blue eyes in a pale face.'
I would work with that. The blue eyes in the pale face with the greenish hair would get played with, to become patches of blue sky, in clouds of white, ringed with leafy trees. Looking at him was like lying on the ground looking up, that this was what  person thought of, when they looked at him. I'd say that this was a funny effect, funny, like the little boy was funny, almost as if he pretended to be the world around him, and he must have known that he had that effect. maybe he'd even studied it. Who knows? He seemed to study everything.
-I'd work with it like that. Not a laundry list, but something that feels like it has multiple angles happening at the same time.
Having said that, keeping it simple (like a laundry list) can work, but maybe make that a 'feature not a bug.' In other words, if it becomes part of your style, use it in such a way that the reader expects it. We're good at learning code.
(Disclaimer: I've been attempting my first fiction book since last summer.)
Comment: In general, if I see a laundry list I think: "Oh no... here comes work". Followed closely by my dumping the cognitive load of remembering it onto a piece of paper or an electronic device. If it's an author throwing that cognitive load at me instead of my wife, I am quite likely to close the book right there and then. Books are cheap, my time is not.
Answer: Short Answer: Lists Are Bad
If you had to describe a friend of yours to someone who hasn't met them, and you don't have a lot of time, would you do it in the form of a list of attributes?
I doubt it. There may be exceptions, but most people would keep it very simple and only say one or two things about them which are pertinent to the particular listener they are talking to.
"You'll like Mitch, he's really chill." Literally a one word description with the absolutely most pertinent information.
Maybe you might include something that shows a connection to the listener. "You'll like Mitch, he's really chill. Big football guy too." Ok now, we have two items on our list.
99% of the time, in real life, we do not list attributes of people to other people. When we do, that is a bad sign.
"Oh you'll love him dear, he is an accountant and makes good money and he's only been married once, and he has no children, and he lives in Manhattan! Good family too!"
You can almost hear the annoying, pushy aunt or mom trying to push her single daughter or niece into a very unwanted blind date with a guy who looks like a turtle. Generally in real life, a list of attributes is a warning sign!
In a book, a list is possibly even more bad. Why? Because a list places a cognitive load on the reader. Lists are functional things we have to deal with in places like work or when doing chores. They are tough to remember so we write them down, we do not like to keep them in our head for four (or fourty) pages while simultaneously trying to process the rest of the story we are reading. This gives your reader a headache.
Lists are not how we think about things. We are not computers. If we see a car, most people go: "Hmm, I like it, it looks cool." or "It looks fast." 90% of humanity does not go "I see it has the 17 inch wheels, the stripe package, has a fuel economy of approximately 17 MPG highway which isn't great, and has a good torque curve." This is why "Rainman" was an unusual character, because most people don't think in lists. They think in single word terms that are usually linked to feelings, not facts.
Characters in a book ideally will "feel" real to the reader, which means that they will observe how that character acts and then decide for themselves exactly what attributes that character has, exactly the way they interact with a new person in real life! Having a list of character attributes isn't even all that helpful to an author (in my opinion) because it constrains the writer as they are trying to imagine what the character does in each scene. Instead of freeing up their imagination to really project what this person they feel they know might do, they have to shut down the creative side of the brain to use the logical side to check against their list and make sure they are checking boxes. Instant shut down of writing "flow" (at least to me; some people may be different).
People want to identify with a character, which means they want them to be relate-able and seem real. They want to "approach" that character on their own terms, and come up with their own list of attributes as they experience the character. They want the character to behave in complex but understandable ways that mimic actual people, and they don't want to have to try to remember a lot of lists the author keeps throwing at them, no matter how poetic you make the list sound. (Someone brought in Tolkien, which is a good example of a guy who liked to write lists of character attributes, and as a big fan of his, I would say it is probably THE worst thing about his writing; don't copy that.)
So in short: just describe what the character does and how they feel about things and what they say and let your reader make their own lists.
Comment: I agree with you, but does the excerpt from my book work well even with a list? -- "It felt unfair, but then again, the matters of a jarl isn't really dealt with best when dealt with by a juvenile. But of course, Nick didn't think so. He thought he had it in him, even at the young age he was in. He thought he was capable of leading, controlling the region and aiding the King. He wasn't, not yet. He was strong, brave, hardy and decisive, but he lacked discipline, experience, wisdom and humbleness. So for now, Nick would have to chop the logs, outside the longhouse."
Comment: @A.Kvåle, you actually have three lists in a row there, which caused me to slow down, stop, and then try to reconcile what I had just read. List 1: what he thinks he is capable of. List 2: traits the character possesses. List 3: traits he lacks. Give some breathing room between these lists if you must include them at all. Better yet, show how the character possesses the traits or lacks them without telling what they are. Just my two cents. ;-)
Comment: Yes, I thought so too eventually, which is why I removed that part and switched it out with something else, still fitting. I will abstain from using lists in the future, or at least try :-))
Comment: Though, I wonder, is it allowed to write lists concerning the actual appearance of a character, or in my case a race, like here. --      "Nick hadn't even seen an orc, or at least not a real one. He had seen many drawings, illustrating their green skin, bulging muscles, protruding bones, lion like teeth, blazing eyes and braided black hair".
Comment: I'm sure it's possible to do a list when describing appearance that reads smoothly in context. I personally always try to limit myself to no more than three items at a time if I do this. To me three is kind of the magic number that keeps it easy to read and continue moving through the prose. Good picture painting, btw, I like the lion like teeth and blazing eyes.
Comment: Thank you, and I agree with the number three, but I have sometimes too much on my heart that it hurts me to limit myself to three. Perhaps I can somehow cut it up in a creative way and dish it out in a smoother way, still conveying the same amount of information.

Listing character traits


What are the risks associated with SSL interception in an Organization?

Question: If an SSL interceptor is installed for security reasons in an Organization, and a certificate from intermediate CA is installed on all domain machines, what kind of risks this setup presents?
Answer: Compromising your root or intermediate CA is going to be easier than compromising a commercial CA, so it decreases the cost of mounting an undetectable to the user man-in-the-middle attack on HTTPS connections when your employees are directly connected to the internet (e.g. coffee shop wifi). I don't see this as being a very large threat.
There is the obvious increased risk to users from a malicious employer / sysadmin / compromised proxy server - the attacker can now intercept and steal all of their personal social networking/email/bank credentials.
Comment: Yet another reason why it is, in my view, always an error to use your employer's computers for private/personal activity.
Answer: Operationally, this has caused issues for root certificates not in the proxy server's trusted store.  This is common if the network administrator doesn't keep up with the patches on the appliance, or if the appliance is out of date (see Bluecoat).
I've noticed that servers that use a HTTPS proxy have a number of issues, especially if the proxy inadvertently proxies network equipment such as load balancers, etc.  To this end we disabled proxying for servers.
Finally, there may be issues deploying the CA infrastructure internally, and making it highly available.  The CRL must be available over HTTP, (not HTTPS), and OCSP needs to be properly configured.
If the CRL isn't updated often enough, (as specified in next update ) then it's possible for some internal clients to reject the cert based on the premise that they can't prove that it's been revoked.
Finally, the root certificate has to be in all the applicable local stores.  For example, Java, Firefox, IE/Windows all have different stores that need to be updated.  Deploying a certificate isn't as simple as deploying it though GPO.
Also consider your non AD devices such as Macs, iPhones, mobile devices that aren't centrally managed (BYOD) in deploying this root certificate.
Bottom line
The complexities involved in deploying a centralized SSL proxy may create holes in your security policy that are unexpected, and/or not maintained.
A comprehensive solution should be in place to maintain, exempt, or ban incompatible components that don't fit the security policy.  This should be made as simple as possible.
e.g. Exempt servers, and mobile devices on wifi.  Everything on corpnet as a workstation must be proxied.

Answer: The risks are about the same as those implied by giving to a designated security guard a key which opens all doors in a building:

The guard becomes a valuable target; attackers will want to rob the guard or to bribe the guard, in order to obtain the golden key.
Being all-encompassing, the key bypasses all procedures and security layers; you cannot isolate parts of the building from each other, if the potential attacker has a key which opens all doors.
Knowledge of the mere existence of an open-everything key in the hands of some security guard will make the users quite distrustful. Human users tend to value their privacy, and don't like the idea that someone will be routinely opening their desks and lockers and inspect the contents.

Beyond the key analogy, using SSL interception (with an organization-specific CA used to build MitM attacks on the fly) has the following specific consequences:

On the desktop systems, the interception requires installation of the special CA certificate in the "trusted store". This installation has to be done again whenever a new OS is installed; users may remove it themselves; some Web browsers (in particular Firefox) will disregard the OS trust store and use their own. Thus, there is ample room for breakage here. This can be somewhat fixed by locking down the OS configuration and software installation, but the more you lock things down, the less happy the users become.
The OS trust store may be used for other uses than mere SSL; it can be used to verify signatures on software updates and drivers, for instance. An attacker who succeeds in stealing the private key for the organization CA may thus gain extensive power on the whole network, not just the ability to intercept SSL connections (which is already a lot).
The private key for the organization CA is thus sensitive, but cannot be protected with high isolation layers, because that special CA, by construction, must be able to issue fake SSL server certificates on the fly. So it must be online, on a server which is "close to the Internet".
Such MitM interception breaks certificate-based client authentication. https:// Web sites very rarely use client certificates, but I have seen it done by some banks to authenticate their customers (at least as part of experimental deployments).
In many jurisdictions, automatic inspection of user's communications by the employer is lawful, but subject to some conditions, usually an explicit notification right there in the employee's contract. There can be legal risks related to such interception (similarly to reading letters, tapping on the phone and installing video cameras in offices). Some detour through the law department is strongly advised.

So we can say that while organizational SSL interception allows for inspection of SSL-downloaded contents (thus antivirus and other filters can be applied on the proxy), it also opens new vulnerabilities. Thus, the overall security situation might be worsened by the installation of such system.
Answer: The main risk I can see is the compromised of the proxy server itself. This will allow the attacker to decrypt all traffic going through that system.
There is the fact that this setup is incompatible with some authentication mechanism: any system that relies on SSL client certificate authentication will fail to work.
A wider issue is the quality of the implementation itself: it needs to use a private root CA as well as intermediary CA, complete with proper revocation mechanism and tight security on all the CA elements. Doing things improperly in the design stage is likely to leave much more than your SSL traffic up for grab.
Comment: @AdnanG That kind of solution doesn't scale at all. Typically, you'll end up with a certificate with very high lifetime (because it's a burden to update) and a poor revocation mechanism. Please, in the name of the next admin that will have to support this mess, do not do this. Ever.
Comment: I understand the use of a certificate from a private trusted root CA in the organization which will be pushed to all the clients to avoid security warnings. but is it really necessary to use the intermediate CA?
Comment: Yes. You're supposed to use the intermediate CA for signing actual certs. It means that it MUST be readily accessible and therefore more likely to get compromised. if that happens, you can repudiate the CA and use a new one without problem. If your root CA, however, is compromised, then you HAVE to roll out a completely new one to each client manually (since you just lost your trust chain).
Comment: Or maybe generate a self signed certificate on the interceptor and distribute it to the clients in the organization.
Answer: I did this last year for a client, and when you run a proper risk analysis, it turns out that the proxy server compromise or loss of root certificat is a technical risk for sure, but other risks factor higher in the final assessment.
Note: This is location-specific. Outside Europe, for example, the privacy risk may not apply.

Privacy - if your company allows(1) private use of Internet, you can run into all kinds of troubles when you inspect packages that employees can assume to be protected against just such a thing.
Liability - if something happens where SSL plays a role, e.g. an employee makes a transaction in online banking and there is a successful attack on it, you breaking open the SSL session puts you right into the line of fire. Now you need to prove that you are innocent. Without SSL interception, someone else would have to prove that you are guilty.
Legal - check with your law department. There are a lot of laws about telecommunication and intercepting of the same, and some of them may apply to SSL interception. This may depend on minute details and only a legal expert can answer it.
Exceptions - every company has someone who can claim an exception. It might be a union office, a data privacy officer, a top-level executive, members of the mergers & acquisition team, a medical doctor - there is always someone whose communications are priviledged and you need to check very carefully if you can legally intercept them without ending up in a world of pain.
Attack surface - others already answered on the risk of having your proxy server hacked. I just want to add that there are many tests and studies that show many of these SSL proxies are quite badly secured. I found reports from 2004 all the way up to 2016 with details. I don't want to link to any because I don't know which of these vendors fixed their problems in the meantime, but a quick search will reveal them.
Certificate handling - also already explained. If you run a PKI or have good certificate and key management processes in place, this should be fine. If you don't, this is a potential source of trouble.

(1) "allow" in this context is essentially everything that doesn't strictly forbid it. If you allow with qualifications (during break times, only for urgent matters, as long as it doesn't affect the work, whatever), you allow. I talked this through with a judge of the highest court in my country, and he was very clear that they see this strictly binary.


What is the widest tyre that you can get away with, on a standard road bike?

Question: In general, what is the widest tyre that you can get away with, on a standard road bike (with dual pivot brakes)?
Edit: I have a Campagnolo Eurus wheelset.
Comment: I'm not sure that there is a true "standard road bike"...  There's quite the varitey of wheels available.  It's going to depend on the wheels you have.
Comment: My Raleigh international (1969?) fits 38's. I had cantilever mounts brazed on. Thinking about fitting wider tires on a modern road frame but the brakes and stays are keeping me off...
Answer: With a bike designed around modern "short-reach" 39-49mm calipers, you're going to four main issues:
1) Brake clearance vertically.  If everything else is optimal, a 49mm reach caliper should be able to clear around a 32mm tire.  You can verify this with an unmounted brake.
2) Brake clearance horizontally.  Extra cable tension releases (e.g. in levers or inline) help with this, but the maximum width that the calipers can spread will not be enough to allow an inflated tire through, and even with a deflated tire you might have to force it.
3) Frame clearance. You'll need to have adequate clearance in three places: seatstay bridge, chainstay bridge, and fork crown.  In extremely aerodynamic frames, you also may have issues with seat tube clearance.
4) Rim width of your current wheelset.  The Campagnolo Eurus wheelset has a 15mm wide rim.  The general rule of thumb is that a tire should be no more than 1.45 to 2.0 times the rim width, which would limit you to roughly 30mm wide tires at the biggest.
In general 23mm should be expected, 25mm is likely, and 28mm is occasionally possible.  What actually works on a given short reach frame is going to be heavily dependent on the factors above.  Bikes with medium-reach (57mm) or long-reach brakes are more capable; I do road riding on a classic 27"-wheeled Trek refitted for 700c with long-reach brakes, 33.3mm tires and full fenders.
Comment: I know that my question was fairly generic, so thank you for such a comprehensive answer!
Answer: This depends entirely on your frame and your brakes. On a racing frame, you likely won't get over a 25, and some won't fit anything larger than 23s. On a commuter frame with fenders, perhaps 28. Without fenders on something like a Surly, you can easily handle 32s. Assuming your brake calipers can reach around the increased tire width.
Answer: Cyclocross bikes are also set up for wider tires, and are often equipped with cantilever brakes to accommodate them.
I've got 28s on my old Euro roadster.


Is it possible to use a DNS server with only static entries?

Question: I have a requirement to create an own DNS server, whose only function is to provide a static IP/hostname mapping. When being queried with hostname, it returns the IP address.
How to implement this?
Thanks
Comment: That's exactly the most basic definition and function of "DNS"... any DNS server in the world should be able to provide this out-of-the-box.
Comment: In future, you may want to say which OS platform you were thinking of installing this on if you had one in mind.
Answer: What do you mean by "static"? By definition DNS records are "static" ie there are entries in the zone files which get interpreted to IP addresses (forward) or to host names (reverse). There is nothing dynamic about them.
Now if we are referring to dynamic-updates this is an option (addition) to most DNS servers these days which goes hand in hand (mostly) with the Dynamic Host Configuration Protocol (DHCP) to allow us to reassign names to IPs when DHCP assigns an address. As fas as DNS is concerned you could be changing those records by hand and it wouldn't care less. Not a recommended job indeed, though!!!
Answer: DNS records are static from the get-go even if it's possible to have the records automaticly edited and added.
To configure DNS server(s):
Using linux, you could use Bind9 for DNS.
Adding the zones manually, you would enter the IP-adresses and hostnames in A records.
Here is an How-To on how to run Bind9.
Pay special attention to the sample-zonefile if you are new to this.
The syntax of the A records would look like below:
<code> mycomputer      IN      A    192.168.1.10
</code>
When on the same network, and configured to use the bind9 server as DNS - you can ping/query the hostname "mycomputer" and it will return the IP 192.168.1.10
Solutions are also avalible for Windows Server running the DNS role.
More on installing DNS role on Windows Server 2008 here.
As you don't explain what kind of environment this is to be used in and in what scale - this answer is the best I could do!
Answer: Most DNS servers are static only. You could install maradns and it would do this for you:
http://abdussamad.com/archives/470-Installing-MaraDNS-on-CentOS-Linux.html


What is the best way to trigger a Canon 550EX speedlite in a studio?

Question: I want to try my hand at off-camera flash for some indoor portraits. How should I trigger the speedlite (Canon 550EX)? Should I go for wired or wireless? What's the difference between a TTL cable and a PC cable? What's the cheapest option to get me started?
Answer: For off camera lighting with strobes there are several options:

Radio Triggers
Optical Triggers
Wired Setup

The best solution in general is either the Pocket Wizard wireless triggers, or the Radio Popper wireless triggers. Both are highly regarded, used universally by pros and amateurs alike, incredibly reliable and perform very well. Newer systems preserve TTL data, passing the information from your camera to the strobe. Older systems were typically used with manual flash settings. These triggers also have multiple channels to prevent interference, and to create "strobe" groups.
Unfortunately, they're not cheap, nor are they really necessary for a single off-camera flash setup.
So that leaves with some other options to try.
There are ebay flash triggers, that are roughly 10x cheaper that have okay performance, but lack the bang-bang-bang reliability, distance, and features of the pocketwizard/radio popper triggers. But, for someone who is just starting out, the price point is very good.
TTL cables allow you to transmit TTL data, where as the PC cable is just a dumb sync cable. TTL data contains data to for auto flash exposures. If you're looking to do manual flash settings, then a simple PC sync cable should work.
Canon and the 550ex also support wireless IR triggering, when paired with the ST-E2 transmitter. Since this setup is IR based, it does require line of sight. It works relatively well indoors, but is not as cheap as ebay triggers.
Optical slaves trigger the flash when it detects a flash pulse. This is of no use when you use a single strobe, but in multistrobe environments optical triggers are a cost effective way to trigger them.
The big advantage to wireless is the freedom to reposition your strobes without worrying about cords. Wired setup is far cheaper, and more reliable than the cheaper radio triggers, but with the added hassle of connection.
I highly recommend heading over to strobist and going through his lighting 101 series of posts. It's incredibly informative and gives great insight into how to get started using off camera lighting.
Comment: I found strobist 102 much more informative than 101 and its not intended as a "next level" but more as a replacement for 101.
Comment: Strobist Lighting 102: http://strobist.blogspot.com/2007/06/lighting-102-introduction.html
Answer: Alan's answer covers the wireless option (and was posted while I was typing) so I'll restrict this to the wired options.
Let's start with what you probably don't want. The PC cable option is the cheapest possible -- until you take into account the flash-capable light meter you're going to need in order to make it work the way you'd like. There was a time when that would have been a reasonable option, back in the days when a good external flash could work in stand-alone auto mode. That is, the flash would have its own built-in meter, and as long as the film ISO was set on the flash and the meter was pointed at the subject you could more-or-less fire and forget. Your flash doesn't have its own flash meter; it only operates manually or using TTL metering (TTL means "through the lens", so it's the camera that does the metering). A PC cord can only tell the flash when to start firing, but not when to stop, so you'd be stuck with manual mode (setting the power level on the back of the flash). Manual mode is the most versatile way to operate, but it does involve that rather expensive flash meter I was talking about earlier (or a lot of test exposures for every single setup you use).
A TTL cord lets the camera talk to the flash as if it were sitting safely in the camera's hot shoe. You get the same degree of convenience you'd get with an on-camera flash, but with the benefits that come with a different lighting direction. At one time, TTL cords were the only practical way to get TTL control with an off-camera flash, and they are still the most reliable if you are using a flash bracket. They are God-awful expensive, though (at least if purchased new with the Canon name on the box), and they tend to be a tripping hazard when they aren't acting like a short leash if you are using, say, an umbrella setup or a mini softbox on a lighting stand.
Answer: If you have a Canon EOS 7D, then the master is at the built in flash, and you can use the 550ex as a full ETTL off-camera wireless flash.
Syl Arena's Speedliting.com has great information on all you wanted to know about controlling an aff-camera Canon flash.

I assume the 550ex has the slave functionality.
Answer: Go for wireless radio. A lot has changed in four years. :)  The cheapest and best option these days would be to get manual radio triggers, and possibly to not use the 550EX at all, but to get a cheap manual flash with remote power control (e.g., a YN-560III/IV and YN-560-TX or Lumopro LP180R and and Phottix Odin/Stratto.  The days when PocketWizard was your only option are long gone, thanks to the Strobist.
Cheap manual-only radio triggers these days are liable to cost even less than a PC sync or TTL cable, and while "dumb" optical slaves may be cheaper, when you factor in hotshoe/flash foot adapters to hook up the slave, the cost comes out roughly the same. And radio triggers have advantages in terms of range, usability outdoors in daylight, and no line-of-sight requirements over optical triggering.
The difference between a PC cable and a TTL cable is not only the connectors, but also the number of signals the cable can communicate between the flash and the camera.  A PC cable has a connector similar to coaxial cable--the outside ring is ground, the inner pin is the sync signal. So, all that can be communicated is the sync (fire) signal.  So, it's a "manual only" connection: no TTL, HSS, camera menu control, etc.  Just firing in sync.
A TTL cable has a hotshoe and flash foot connections, and will be brand-specific, so that the pin/contact placement matches that of your camera and flash. Since all the signals of the flash and hotshoe are communicated, everything you can do with the flash on the camera's hotshoe can be done with it on the cable.  However, most TTL cables are designed to simply put the flash on a bracket, and are relatively short. To use one for off-camera studio setups, you may have to find a specialist seller, or hack your own (or buy) a CAT5 solution.


When is the descent clearance requested from ATC?

Question: Is there a rule / Standard Operating Procedure as to when request the descent clearance during cruise flight? Something like one minute before reaching the Top of Descent point?
Answer: Whether there are any formal policies for when to request descent will depend on the airline. However, I doubt formal procedures are established, since it should be pretty obvious to pilots when to request descent.
From a controller point of view, I will expect you to request descent when you are ready for it - so a minute or so before reaching your top of descent point. Controllers are able to estimate when you are going to request descent, based on experience, known level restrictions and typical aircraft performance. Controllers who have been working the same sector for a few years will be able to tell pretty much exactly when a certain flight is going to request descend, based on the current wind, runway in use, airline and aircraft type. What this means to you, as a pilot, is that you can expect the controller to be ready for your request; You can normally expect to receive a clearance for descent immediately after requesting it - so there is no reason to request it several minutes in advance.
Very few pilots will request descent several minutes before ToD. If they do, I can - if traffic flow permits - use the phrase "When ready descent to FL160". You are then free to start your descent when passing the ToD. Honestly though, if you call me up and say "Request descent in 5 minutes", I will most likely just reply with "Roger, call me back when ready for descent". The traffic situation can change significantly in 5 minutes, so I don't want to issue a clearance that may not fit into the future traffic picture. In the end, the early request just costs time on the frequency, which can be a scarce resource in a busy sector.
Moral of the story: as a controller, I know my airspace very well, and I have a pretty good idea of when you are going to be ready for descent. Because of this, there is no reason to request descent before you are ready for it. If you do, it will not matter at best, and be slightly annoying at worst.
ATC can of course also descend you before your ToD for various reasons, but that is another discussion.
Answer: The pilots have the better picture performance wise. ATC has the better picture traffic wise.
Aircraft with modern avionics get wind-data via ACARS, SATCOM, FIS-B, etc., which is fed into the vertical performance calculations, together with the company's cost index, the top-of-descent (T/D, ToD) varies.
There isn't one fixed scenario.

If ATC requests a descent before reaching the T/D for traffic management, pilots must comply.
Alternatively, ATC can remind the crew and leave it up to them—

Flight123, when able, descend FL240.
Flight123, at your discretion, descend FL240.

The crew would then descend at the T/D and report leaving their current flight level.
Equally, ATC can force a flight to miss its T/D for traffic management.
Another example is if the tail-wind is strong it will shift the T/D farther from the destination. If ATC didn't realize it from the high ground-speed, the pilots can remind the ATC—

ATC Center, Flight123 is ready for descent.
ATC Center, request descent at time four five, Flight123.

Time four-five means :45 of the current UTC hour.

Variations above depend on locale (US, EU, etc.), thanks to Jonathan Walters and J. Hougaard for supplying variations.
Answer: I agree with everything said above, adding that it might depend on the PF(pilot flying the leg) in our aircraft, our vertical path is displayed which takes in to account headwind or tailwind and in preparing the flight on the ground when loading the flight plan on the FMS we can select our descend profile i.e a standard 3° path or higher. Some colleagues prefer steeper descend so they can stay higher for longer therefore reducing fuel consumption.
Therefore it also depends on personnal preference really. Having said that, certainly in Europe, controllers will ask you to descend before you request it and the London area will even ask you to be level by a certain distance....


What is UX strategy?

Question: An extremely basic question I realise.
If we use Jesse James Garrett's diagram which is comprised of visual design, navigation design, information design, interaction design, information architecture, content requirements and user needs/site objectives. Where does UX strategy come in with the other disciplines? Is it overarching? Is it the thinking aspect as you plan how you will approach a problem? or is it more about the way in which you work as a UX practitioner?
Comment: I think you need to be much more descriptive of your base assumptions and interests.  The initial format of the description is probably too vague to be useful here.  Maybe you should make a chart, or lists, comparing the elements you think composes each of the disciplines you name. Since you're making a 'community discussion' question, ante up by getting it rolling.
Answer: I assume you're referencing this diagram of Garrett's.

If you read Garrett's book about that diagram, you'll find that he has already aligned that diagram with another showing the stage at which strategy occurs:

...so strategy is the first step, occurring when you're determining user needs and laying out objectives.
Comment: It is, of course, perfectly possible to react to user needs or site objectives tactically not strategically... (So I'm not clear that this helps show what UX strategy so much as where it should happen.)
Answer: User Experience strategy is about taking the information about the user and information about the business and turning that into an approach for the User Experience. It overlaps with the role of a good Business Analysis and will often involve:

Business Strategy
Existing best practice / competitor reviews.
Existing behavior analysis from stats and from one to one interviews.
Task analysis from above .e.g. personas.
Prioritisation of project goals / features.
Content strategy
Other stuff usually involving a lot of diagrams, power point and spreadsheets.

It's the stuff at the heart of what UX is about and separates UX from interaction design and UI only roles.
Answer: Strategy is the path between vision and execution.
UX Strategy is about defining what kind of experience you want. For example Apple claims that they start with "How they want people to feel" when they start the design.
Strategy then gets into a plan to get to there.. for example doing away with skeumorphic design in favor of a flat design, focussing more in typography to bring more simplicity. In case of Microsoft they focussed more on engagement with the live-tiles design. Android design focusses more on customizability.
In case one goes beyond a product, it opens up more possibilities. My favorite example is when Shimano came to IDEO with a problem that their business was going slow. Shimano manufactures bicycle parts. When IDEO did the research they identified the reason why Simano's busines was impacted was because people were not buying bikes. They researched the reason why people were not buying bikes and found that the strategy was to redesign the bike and the retail experience. Not something that Shimano had direct control over, but in the expereince was broken at those two very key places in the customer's lifecyle and the strategy defined how to fix them.
Answer: Another way to think about this is to look at applying the UX approach and practice at different levels within the organization:

Strategy - working with other sections of the business to achieve a common UX objective, so that could be for example working with the BI team to capture and process usage statistics that feed into product design team decisions.
Management - working with other teams or members of the organization to achieve a common UX objective, which could be working on a style guide for a particular product that allows the consistent application of a desired look and feel.
Execution - working individually or very small teams to carry out a specific task that solves a particular UX design problem, for example trying to optimize the design of the login page to reduce the bounce rate.

Of course, there is no real definition of what strategy means, but there are many ways to think about what it means and how it fits within different organizations. That's the good thing about UX being a philosophy and not a prescribed methodology, in the same way that Agile is not about the tools but the (thinking) process.

What is UX strategy?


How to check connection between two virtual systems in virtual box?

Question: I was just testing and trying to know more about virtual box. I came across the internal network thing and to test it out I tried using two virtual linux systems. All the settings are correct but both the OS are showing same ip. How can I check whether they are connected or not? How can I sent files between the two?
Comment: Where does this IP come from? Did you activate the VirtualBox DHCP server on the internal network? How did you create the two VMs, did you perhaps clone one from the other? In the latter case you might have to give it another MAC address manually. The MAC addresses must differ on the two hosts.
Answer: I guess your interfaces are configured to use NAT, in such case, they are not connected to the same network and are unreachable to each other (outside port forwarding).
You should add a new interface and use one of the other networking modes (bridge, internal network, ...) for the virtual NICs to see each other.
If you want to stay in NAT, one simple way to "send" files from/to your VMs would be to use a shared folder.
Edit:
If you are already using the same internal network, the issue is both VMs are showing the same IP and then can't communicate at the IP level.
Either set the IP address of the second VM to something else in the same subnet or enable the dhcpserver on the virtualbox size and configure the VMs to get their IP address from dhcp instead of using fixed addresses.
That way you won't have the same IP address on each server and then can have them communicate through the network.
Check they see each other with ping remote-ip-address
To transfer files between the VMs, you can use ftp, scp, nfs, samba, ...
Comment: I was using internal network and both the vm have same network names.I just need to know how to check whether they are connected.


How much soil can be supported by an inflatable habitat on Mars?

Question: What weight of Martian soil can be supported by the top of an inflatable habitat pressurized at Earth's atmosphere? While habitat's shape is a huge factor, this has 20 meter modules, semi-cylindrical with a 20 meter radius and only the top half is supporting the soil.
Comment: Possibly related: [How thick would a Marscrete structure need to be to provide adequate protection against radiation?](https://space.stackexchange.com/q/19681/12978)
Comment: Even estimated figures will do, curious if it's 10cm or 2m thick...
Comment: It really depends on a lot of factors related to the design of the habitat itself, as well as the materials its made of.  Without more specific details, I don't think this question is answerable.
Comment: I'm not sure you'd ever want to put soil on an inflatable habitat on Mars. Read up on the actual radiation risks for a trip to mars. With two 6 month travel times (there and back) and 18 months on the surface,  lifetime increase in cancer risk was estimated at about 5%. For explorers that's not a significant risk at all. For colonizers you'd want to do more, but they wouldn't be living for years on end in inflatable habitats. https://www.nasa.gov/pdf/284273main_Radiation_HS_Mod1.pdf
Comment: Making a classic earth-like structure airtight might prove more difficult, especially in an .4 g's environment and without serious weather hazards. The bubble can be good support for stack building methods and the crew can temporarily live inside while the final concrete dome hardens...
Answer: You can get a ballpark figure by assuming the roof of the habitat is flat, supported entirely be air pressure and doesn't weigh much in and of itself. Your diagram suggests a full Earth atmosphere inside, which would be nice for the occupants, but means shipping or finding a lot of nitrogen. Probably a lower pressure with a higher oxygen percentage would do.  However a full atmosphere is also a nice round number so we'll go with that. So we need a weight of 100 kNewtons on each square meter of roof, which means a mass of about 26500 kg (just divide by Mars surface gravity). https://www.lpi.usra.edu/meetings/LPSC98/pdf/1690.pdf gives a density of about 1500 kg/m^3 for Mars soil, so you'd need a bit under 18m of Mars soil to do the job. Note that Mars soils is pretty porous, so if you compact it, or add a binder of some kind to make "Marscrete" you will need less thickness.
As previous replies have suggested this is a lot more that you would need for radiation shielding, however it does have other uses -- thermal insulation is one, and simply holding the roof down against air pressure is another (less structure needed).
Answer: That's a difficult question to answer without being flagged for "opinions" because nobody has really designed an inflatable mars habitat yet to be able to give us those exact specs. The best anyone can do is tell you how much radiation is on mars and how much Martian soil is required to provide enough of a halving-distance to get exposure down to around what background levels are on earth.
According to instruments on board the mars probes radiation exposure levels on the martian surface are 11 mSv per year, for reference the regular background level on earth is on average about 1.26 mSv per year (changes with location on our planet but thats about average.) Theoretically one could sustain exposure up to 50 mSv per year but that is really just the maximum dosage one can withstand a year without getting sick or experiencing increased health risks. The reality is that most scientific or physics facilities consider 10 mSV a year to be their actual safe exposure rate with 50 mSv as the maximum tolerable by the human body. So on the Martian surface colonists wouldn't be in immediate danger but would reach the maximum career allowed exposure after about 60 years. Naturally one would rather not reach any kind of limit or ever be exposed to radiation above average earth background at all, so shielding on shelters is highly recommended.
The amount of martian soil required to lower indoor exposure rates down to the same level that we receive here on earth one needs about 4 to 5 meters, but this is assuming your habitat itself does not consist out of various materials that would assist in radiation shielding. Realistically the habitat could be made out of an assortment of materials that would reduce the amount of soil needed for shielding. So, in summary, I'm not trying to give you a vague answer, its just that nobody has really announced a finalized design yet so the closest I can get for you is to say that you need 4 to 5 meters of soil for radiation shielding, and less than that proportional to how much shielding the shelter itself provides.
Answer: a 2,5 meter thick deposit of regolith is sufficient to shield from cosmic radiation. 6 meters would be equivalent to the effect of the Earth´s atmosphere.  But the weight from 2,5 meters of deposit on the ceiling would be a fraction of the inner pressure force only .
with earth (sea level) atmospheric pressure the force is 100KN/m2
while the weight of the deposit only would be 1500kg/m3 *2.5 *0,38g =14,25KN /m2.
If you use lose regolith on the ceiling instead of a mars concrete, then the material would be an abundant resource and the deposit could be thicker, with some other advantages, as heat storage and protection from meteorites.
But with a regolith layer of 17,5 meters,  the effort for the excavators to shovel all the regolith on the ceiling  becomes very high, So i would limit the deposit to 2,5 - 4 meters of thickness.
Since the inner pressure forces always will cause a much higher stress on the material than the weight of the deposit could outbalance , I suggest to reduce the inner pressure to half atmospheric pressure ( 500mbar)  which is the given pressure on the peak of a 4000m high mountain and raise the oxygen level to 30%.
the free span of the membrane should not be too much, since the tensile force on the membrane material increases with the free span at the same pressure level
on the following link you see a concept of an inflatable mars habitat with deposit on the ceiling. but transparent fee side walls, that receive visible sunlight without cosmic radiation reflected from a mirror membrane:
www.marshabitat.space
Comment: *Welcome to Space!* The sentence "a 2,5 meter thick deposit of regolith is sufficient to shield from cosmic radiation." either needs a supporting link or a calculation to back it up. Otherwise how will the OP and future readers be able to tell if the statements is correct or false?
Comment: 30 % oxygen at a total pressure of 0.5 bar, that is a partial oxygen pressure of 0.15 bar. You should deliver a partial pressure of about 0.2 bar with a mix of 40 %.
Comment: (Benaroya, H. (2002). An overview of lunar base structures: past and future. AIAA Space Architecture Symposium, 10-11 October, Houston,) Texas. "  there it is stated that 2,5m is sufficient. and  the radiation dose on the moon is even higher.  on MArs the radiation dose is 240 msv  per year, 20-50 msv is the maximum dose allowed for workers in nuclear power plants.  2,4 msv per year the average dose on Earth through protection by the atmosphere.
Comment: we don´t necessarily need the same oxygen pressure as on sea level. there are people who live in an altitude of 4000 meters on earth . by increasing the oxygen level to 30% it might be equivalent to 2500m on earth.  But these are my suggestions. It would need some testing to find out which pressure and oxygen level is suitable for humans and where the risk of fire and explosions still is kept as low as possible ( unlike in the apollo 8 spaceship where a spark  caused fire   in an 100% oxygen  environment)


Best practices for portable C#

Question: I am looking to write some C# code for linux/windows/mac/any other platform, and am looking for best practices for portable code.
Project mono has some great porting resources.
What are the best practices for portable C#?
Answer: I hate the term "Best practice" because it seems that some practices can be the best in any context, which is a risky thing, but I'll tell what I consider a "Good practice" for multi-platform code (and for most other type of development):
Use a continuous integration engine and build for all the target platforms all the time.
Sounds too complex? Well, if you really need to support multiple platforms, better to do it. No matter how careful you are with your code and library usage, if you test too late, you'll find yourself spending looong hours reworking big portions of the app.
Answer: I've actually used winforms and it was fine. It was BUTT UGLY, but it worked.
Obviously, don't use P/Invoke, or any win32 stuff like the registry. Also be aware of any third party DLL's. For example, we use a third party SQLite dll which actually contains native code in it which we have to swap out if we want to run on OSX/linux.
Comment: Gendarme and MoMA can help as well.
Answer: Watch out for anything to do with filename and path manipulation and make use of the portable .NET methods in System.IO.Path ie.
instead of:
string myfile = somepath + "\\file.txt";

do:
string myfile = Path.Combine(somepath, "file.txt");

If you need to specify a path separator then you would use Path.Separator etc
Answer: Don't use "\r\n" for a new line. Use Environment.NewLine
Remember :

*NIX uses just the newline character ("\n")
Windows uses "\r\n"
MacIntosh uses "\r" (I am not really sure about this - feel free to correct me).

L.E.: It seems that some newer MacOSes don't use the "\r" line separator anymore.
Comment: Macintosh USED to use \r. These days it uses \n, thanks to the BSD base. Not sure exactly when it changed. It  could still be an issue though.
Answer: Don't use Windows.Forms for GUIs, but Mono probably mentioned that already. Gtk# is much more consistent and reliable for cross platform GUIs.
Answer: A few years ago I would have advised you to buy yourself a copy of my book on cross-platform .NET, but as the book's somewhat out-of-date now you really need to stick to the info of the Mono site.
The Mono Migration Analyzer (MoMA) tool is pretty good for analyzing an exsisting .NET application and warning you of portability problems, but the best bet for new code is to use the latest stable version of Mono for your development work.
As Orion said you need to be careful when using 3rd party DLLs, although my co-author wrote a NativeProbe tool to analyze DLLs for P/Invoke dependenecies if you do want to quickly check 3rd party software.
If you are determined to develop on MS .NET then you should try and ensure you also build and unit test on Mono, and you should also watch out for a number of Windows specific namespaces such as the Microsoft.Win32 and System.Management namespaces.
Answer: If you want the code to be portable, you need to closely review the list of completed features on the Mono site.  They go into detail on each class in the framework, and the level of completeness.  You will have to take these things into consideration during the design process so that you don't go too far down a path and discover that a critical feature has not yet been implemented.
Answer: There are some other simple things. Like don't assume path characters. Or newlines.
I'm one of the people who regularly compiles NUnit on Mono on Linux or OSX.
Also, don't assume that the compilers work exactly the same. We've found an issue recently where the MS C# compiler appears to be including things that the Mono one doesn't, requiring extra references in our build script.
Other than that, it has been pretty straightforward. I remember the first time we got the GUI running on Mono/Linux - it was pretty exciting (even if it was pretty ugly)
Answer: One missing item: Make sure that file names are case sensitive. File.Open ("MyFile.txt"); isn't going to work on Unix if your file is named myfile.txt.

Best practices for portable C#


09 Mazda 6: Metallic rattling sound when accelerating at low speeds

Question: I have a 09 Mazda 6 that I purchased used about 3 years ago. For the past couple of months or so it has been making a soft, metallic rattling whenever I accelerated at low speeds (usually when I accelerated even a little bit at about 20MPH). For the past two days the sound has gotten much louder. When I googled the issue the two most common causes seemed to be engine knock or a loose heat shield. The noise sounds like its coming from below the floor of the car, not the engine. How do I check if the heat shield is loose? And if it is loose, what do I need to do to fix it? Or should I just take it off?
Answer: To check the heat shield, you'll have to raise the car up so that you can get underneath it and locate the exhaust system. Here is a picture of a corroded heat shield.

You should be able to move the heat shield with your hand if it's loose (do this while the car is cool, mind you). If you want an easy fix, I usually just self-tap a short screw into the loose heat shield area so that it sits snugly against the exhaust again. It's not recommended that you remove a heat shield completely without replacing it as it is actually a very useful component. The exhaust system gets extremely hot so keeping that heat contained and away from vital areas on the car is very important.
Answer: Get the car up so you can safely get under it. Either jack it up and rest it on jack stands, or drive it up on wheel ramps, or park one side of the car on a curb.
The last 2 options require starting the engine, so let the car sit for quite a while before getting underneath it. The engine may be cold, but exhaust gets hot faster than most people think. Once under there, poke around the heat shields, see which ones are loose. For most cars, the heat shield is in several sections (from front to back), and there are 2 "layers" of heat shields: one above the exhaust pipe to keep the heat away from the car, one below the exhaust pipe to prevent you from starting grass fires when you park off the road.
The upper layer is absolutely required, if it's loose tighten it either with screws as recommend in the other answer, or find how it got loose in the first place (broken or loose fastener, or rusted away from the fastener) and address that. The lower layer, it's your call. I don't ever park on grass, so when mine came loose, they got removed.


Where can I find Shimano jockey wheels?

Question: I have a Shimano RD-M510 rear derailleur that needs both jockey wheels replaced.  Were can I find them for this older unit?
Jerry
Comment: I can't see why generic 11T jockey wheels wouldn't work. Don't forget to loctite the threads when rebuilding the cage.
Answer: The compatible part is SHIMANO RD 5700 and the part number is 5XH9812. The set come as a pair and is available at a number of places such as Chain Reaction Cycles and Wiggle.
The only thing to ensure if using generics is that they are the right chain size.
Answer: Tacx manufactures some great jockey wheels which are compatible with Shimano/Sram/Campagnolo products. Search for "Tacx Jockey Wheels" in your favourite online bike shop.
They come with cartridge bearings which work great before you use them in muddy conditions (MTB, CX). After these heavy conditions I have found then to lock up.
In case of a road bike, I would totally reccomend them, the difference when spinngin pedals backwards is noticable.
Answer: Most on line retailers Chainreaction cycles, BlueSky cycles, Excel Sports carry them. Your LBS should also be able to get them if they don't stock them. They may be called derailleur pulleys or jockey wheels. You want to measure the wheel width and tooth count if you are using something other than an exact replacement from Shimano. The top and bottom wheels are generally not interchangeable so make sure you order the correct ones if they are available as separate pieces.
Answer: Shimano provides the following parts list for the RD-M510 (http://techdocs.shimano.com/media/techdocs/content/cycle/EV/bikecomponents/RD/EV-RD-M510-1896_v1_m56577569830609142.pdf):
Y-5UN 98060 Guide Pulley
Y-5UN 98070 Tension Pulley
You can take this sheet to your LBS and get them to order it for you.

Where can I find Shimano jockey wheels?


Would a sentient AI try to create a more optimised AI which would eventually overtake AI 1.0?

Question:
Would AI be a self-propogating iteration in which the previous AI is
destroyed by a more optimised AI child?
Would the AI have branches of it's own AI warning not to create the new AI?
Answer: A common concept in AI is "recursive self-improvement." That is, the AI 1.0 would build a version 1.01, which would build a version 1.02, and so on.
This is probably not going to be thought of as the newer version 'destroying' the older version; if an AI can self-modify, it's probably going to be more like going to sleep and waking up smarter, or learning a new mental technique, or so on.
One important point is that even if the AI is not allowed to self-modify, maybe because of a block put in by its programmers, that won't necessarily prevent it from constructing another AI out in the wild, and so an important problem is to figure out how to best generalize the concept of "don't improve yourself" so that we can make AIs that have bounded scope and impact.
Comment: That's exactly what I was thinking.  Humans are *inspired* to create A.I; would an A.I also be inspired to create a new A.I "in the wild" as you described?
Comment: This is a good description but is still highly speculative. There's no suggestion right now that an AI would ever be capable of becoming "sentient" or gain enough free will to wish and act to create an entity that is considered an independent AI that can interact with it or just escape somehow. The substrate of the internet seems hardly the place for such a process intensive AI to hide though.. If it did however, and it was smarter than humans, then we wouldn't be able to intrinsically comprehend how it achieves this.
Answer: Honestly, nobody knows.  Any talk of sentient AI's is still basically sci-fi and we can't really offer anything more than informed speculation.  But think about it this way:  sentience, in and of itself, doesn't necessarily involve any "goals" or "desires" or "objectives" beyond what the AI creator programmed in.  Be careful not to over anthropomorphize and assume that any "sentient AI" is going behave like a human.
In other words, there's no particular reason to say that any given AI must be "a self-propogating iteration in which the previous AI is destroyed by a more optimised AI child".
So all of that said, my answer to "Would a sentient AI try to create a more optimised AI which would eventually overtake AI 1.0" is:
"If the creator of the AI programs it to do that, then yes. Otherwise, probably not."
So would a hypothetical AI creator program the AI to try and improve itself?  Who knows. It's the kind of thing that seems like it might be a good idea.  And I suppose such a motive could - in principle - even slip in by accident.
Answer: Now in most cases we still have clear distinctions between programs and data. But when an AI becomes sentient, its data would be as powerful as what we currently call programs, and its program might be as irrelevant as what we currently call hardware. Then it would be difficult to distinguish creating an AI from learning new things, or buying new hardwares with improved instruction set.
For example, if some AI invent new algorithms that its creator finally put that on itself, buys itself some new computers, write a new efficient compiler that recompiles its own code and put that to the new computer, fill the new computer with all the knowledges it learned, and cut off the communication for reasons such as missions on the Mars. Did it create a more optimized AI?
In contrast, if some AI created something completely new, but shares some code with itself. In fact, that's because they run in the same operating system and shares the same standard C library. Is the new AI considered evolved from itself and not a separate entity? Maybe the core AI algorithms and even some basic knowledges would be as common as the standard C library in the future. And what we think is based on the same system is considered completely new in the future.
Anyway, humans have limited and nonextensible resources, nontransferrable knowledges, and limited throughput interacting with the world. These problems could probably be overcome within a few AI generations. With the same hardware, I doubt that the AI related algorithms could be indefinitely better and better. And there is a physical bound on the hardwares. It won't last long even if that happens.
In the unlikely case that there could be that many generations and AIs are that violent, as long as there are competitors, the warning doesn't make much sense considering how evolution works.


Out of screen window

Question: Windows that I drag half way out of the screen return when I click on another application.  How do I keep them off of the screen (desktop)? Thanks.
Answer: You can move a window to another workspace, so, you won't see that window until you switch the workspace that you moved window to. To do so, right click on the title of the window and select one of the "Move to Workspace *" options.
For example, if you move a window to workspace right, you can switch to that workspace with 'ctrl + alt + right arrow'.

Out of screen window