Wikipedia:Reference desk/Archives/Computing/2011 April 25

= April 25 =

How long is a bit?
Ok, so an ethernet cable transmits low voltage digital data along its length in bits. How long is the higer voltage (a 1 for example) segment in a ethernet cable? Maybe to make it simpler, imagine a cable in a single segment (forget standards) that runs for miles and miles. 69.180.160.77 (talk) 00:43, 25 April 2011 (UTC)


 * That depends in part on what speed you're running your ethernet at. In common use there's 10BASE-T, 100BASE-TX, and 1000BASE-T, running at 10 Mbit/s, 100 Mbit/s, and 1000 Mbit/s (1 Gbit/s), respectively. There's also 10 Gigabit Ethernet, 40 Gigabit Ethernet, and 100 Gigabit Ethernet. As you go faster, the bits come faster, so each would take up less space. The one you're probably using right now is 100BASE-TX, which the article says runs the raw signal at 25 MHz. That is, each bit of raw signal gets allotted 40 ns of time. If you're looking for length length rather than time length, the signals travel around 95% of the speed of light, which means that one bit traveling along a really long wire would occupy 11.4 m (37.4 feet). (Or more accurately, at any instant there would be 11.4 m between the start of one bit and the start of the following bit.) Note that not all of that time/space would actually be used in the detection of the bit. There's generally two ways of detecting a signal (and I'm not sure which one ethernet uses). You can either detect absolute levels at a regular intervals, in which case there would be a buffer space between the bits where the signal can switch, or you can detect transitions between the levels, in which case it's only a small fraction of time which is actually the bit itself, an the rest of the time you're just sitting around waiting for the electronics to stabilize. -- 174.31.219.218 (talk) 01:14, 25 April 2011 (UTC)


 * It's a bit more complicated than that -- 25MHz is the clock speed of the MII, and it carries 4 bits per clock.  The groups of 4 bits are run through 4B5B encoding, which expands them to 5 bits (and mixes them together).  The results of that is then MLT-3 encoded, which turns each of the 5 bits into a (possible) voltage level change.  Net result: each 11.4 m (37.4 feet) of signal contains up to 5 voltage transitions, which collectively encode 4 bits of actual data.  You can't really pick out the length of a particular bit on the wire, because they're mixed together in groups of 4.
 * Other ethernet versions can be even more complicated. For example, if I understand right, 1000BASE-T mixes the bits together in groups of 8 (clocked at 125MHz), which are then spread across 4 pairs of wires.  So each  2.3 m (7.5 feet) carries 8 bits.  Speaker to Lampposts (talk) 03:26, 25 April 2011 (UTC)

Is Firefox getting spacey?
Every so often Firefox (on Vista) gets another security upgrade because as usual anybody on the web can take over a windows system at any time. But it seems like after these upgrades Firefox keeps getting slower, so that now after the last one it is stopping in the middle of page loads, so I have to click the link again, stopping in the middle of scrolling down a page and waiting 5 seconds to continue. Using a laptop with 1 gig of memory only, but little else running. Is this a general problem of the program as upgraded or does it indicate a problem locally? Wnt (talk) 08:07, 25 April 2011 (UTC)


 * I have a similar setup (Firefox, Vista, and just 1 GB) and I have a similar problem occasionally, but I've always blamed my slow internet connection because the same happens in Opera, so I'll be interested in the replies to this. The stopping in the middle of the scroll down does sound like a resources problem on the laptop.  Do you have some bloatware anti-virus running?    D b f i r s   11:41, 25 April 2011 (UTC)

Please hook me up with donation brokers for old hardware.
i hope to donate an old PC system and cellphone to an impoverished country. Which brokers/organizations would help me make it so? Thanks. --70.179.169.115 (talk) 10:58, 25 April 2011 (UTC)


 * http://www.google.com/search?q=atlanta+georgia+donate+computer &brvbar; Reisio (talk) 11:58, 25 April 2011 (UTC)

Sadly, I'm from Manhattan, KS. I'll one day figure out if those Atlanta-area agencies will accept them and not have me pay the shipping. --70.179.169.115 (talk) 20:58, 25 April 2011 (UTC)


 * You get the idea: http://www.google.com/search?q=manhattan+kansas+donate+computer &brvbar; Reisio (talk) 21:09, 25 April 2011 (UTC)

Is there computer assistance in modern-day aircraft traffic control?
Lately in the news (in the US, at least) there's been a lot of coverage of recent events of air traffic controllers falling asleep on the job. Yes, I get that due to the high stakes involved, no fully automated system could be trusted to do that job, but I'd be very surprised to find that in this day and age with so many airplanes going into and out of airports, it's still a matter of a/some human(s) relying on their mental ability to handle schedule rearrangements. Do they at least have software products to suggest multiple possible reorganizations of influx and outflux of planes while taking into account each plane's constraints much more quickly than any human could, while leaving the final choice of which option (if any) to take up to the human? 20.137.18.50 (talk) 14:44, 25 April 2011 (UTC)


 * One note that's a little of a tangent &mdash; pilots I have talked with say that the recent news stories about sleeping air traffic controllers are overhyped, and it's not a very big deal. Pilots land at unattended airports all the time, and they all are trained on how to talk to each other on their radios to determine position and decide who gets to land first.  At a nighttime airport without a lot of air traffic, the lack of a controller isn't a significant safety issue.  Comet Tuttle (talk) 17:15, 25 April 2011 (UTC)


 * As I understand it, they were not allowed to land without tower clearance, and this was the problem. So they either need to allow pilots to land on their own when the air traffic controller dozes off, or they need to make sure they never doze off (how about requiring those angle sensitive ear alarms, as well as avoiding insanely long shifts ?). StuRat (talk) 18:15, 25 April 2011 (UTC)


 * One problem with automating systems is that people soon begin to over-rely on them. Thus, while the air-traffic controller SHOULD notice that, due to a glitch in the program, it has recommended smashing all the planes together into a big fireball, it's not certain that they WOULD notice this, rather than just doing what the program suggests. StuRat (talk) 18:18, 25 April 2011 (UTC)

There is significant computer assistance, at many levels, in modern air traffic control. I ran down a brief overview of the modern ASR systems in response to a question in February 2011. Digital ASR means that a complicated computerized system is responsible for coordinating every aspect of the system, from the electrical properties of the RADAR signal all the way to the "animated blip" on a "Google Maps" style software overlay map that the controller/operator uses. The rise of GPS and the integration of ground-control RADARs, approach RADARs, GPS, and regional air traffic surveillance, have changed the technology significantly; it's fair to say that the policies and procedures need to do some catching up to make sure there are no safety gaps. Probably, the recent media hype has been overblown, and has not affected commercial/civil aviation safety in an adverse way. Nimur (talk) 20:48, 25 April 2011 (UTC)

Gigabit Ethernet Jumbo Frames — performance and switches
How can I determine if my Ethernet switch can handle Jumbo Frames? Generally speaking, what kind of performance gains there with Jumbo Frames with large streams of data (like uncompressed video)? --24.249.59.89 (talk) 14:56, 25 April 2011 (UTC)
 * You could look at the specifications or user manual to see if it make a statement on the matter, it may be listed as the Maximum transmission unit size. Jumbo frames can only be found on gigabit Ethernet and should be able to support 9000 bytes. All devices connected to the switch network have to support jumbo frames and have the option turned on if you want to use it.  I will leave the performance gain for someone else.  For simple networks with one switch there may be little benefit, and the main advantages will come if there are many switches or long latencies involved, or if you go to speeds of 10 gigabits per second or higher. Graeme Bartlett (talk) 11:42, 28 April 2011 (UTC)

Server?
"The server did not recognize your user name or password for account" xxxxxx .” Make sure you enter them correctly."  My iCal programme comes up with this message before giving me the programme, as I am a non-techie can someone please advise what server it is writing about since so far as I can see all my passwords are in order.


 * Is this a program which runs over the Internet ? If so, the server is the computer at the other end of the Internet where the program actually runs, then it returns the results to your computer.  As for not recognizing the password, the most common problem is that you have the case wrong for some or all of the password.  Since most passwords are case-sensitive, this will give an error.  (Unfortunately, it's the same error as if you typed in something random.) So, check whether the CAPS LOCK light on the keyboard is on, then be very careful to enter the password exactly as you did before, including uppercase versus lowercase.  StuRat (talk) 18:08, 25 April 2011 (UTC)


 * iCal is groupware - it synchronizes your calendar with other peoples' calendars in your network (business, school, etc). To do this, it needs to talk to a server - kind of like an email program, only for calendars.  If you did not intend your iCal to be set up as a networked calendar, you can disable the hosted calendar by removing that account from "Accounts" in your iCal preferences.  Nimur (talk) 23:49, 25 April 2011 (UTC)

Thank you both, I have changed the password and will see what happens. By the way would Nimur care to move in please, we have a spare bedroom, he always answers my problems so promptly.--85.211.238.236 (talk) 16:53, 26 April 2011 (UTC)

No, changing the password did not work. I used the password in my Keychain, so I don't know what it wants or where to find it.--85.211.210.107 (talk) 08:59, 27 April 2011 (UTC)


 * Check the account preferences to determine where this networked-calendar is hosted. You will need to use the account credentials that are appropriate for the account on that calendar server, which can be different from the login you use on your local machine.  If you don't know what account that is, you should contact the IT department who set up your machine or calendar system.  Nimur (talk) 17:12, 28 April 2011 (UTC)