1

Technology / How to change IP Address of my computer?

« on: 21/02/2011 14:39:11 »

If you're talking about your IP address as seen by the outside world, then you have very limited control over it.  If your computer is connected directly to your ISP via a simple modem, then the IP address of your computer will be whatever your ISP assigns when you connect.

Most residential packages use dynamic assignment by default, which means that each time your modem connects you will be assigned a free IP address from a pool, and that will remain your IP address until the connection is broken for whatever reason.   The address will almost certainly be different each time a connection is established, but will always be within one or more specific ranges of addresses which are used by your particular ISP.

You can request a static IP address, meaning that your IP address will always be the same, but it will still be whatever your ISP assigns to you from its own available pool of addresses (your ISP will also quite likely charge an additional fee for a static address).   You can't choose any address you like, because your ISP can assign to you only addresses over which it has control itself.

If your computer is connected behind a router with NAT, then all of the foregoing applies equally to the address which the router will get from your ISP which is how your computer will appear to the outside world. 

On your local area network, however, you can configure your router to use a specific IP address range, and assign static addresses to specific computers within that LAN.   The three main classes of reserved addresses are:

10.0.0.0/8 (i.e. 10.0.0.0 - 10.255.255.255)
172.16.0.0/12 (i.e. 172.16.0.0 - 172.31.255.255)
192.168.0.0/16 (i.e. 192.168.0.0 - 192.168.255.255)

You can configure your router to use a subnet within any of those ranges, and then make a static assignment of an address within the chosen range to each computer if you wish.   

It's possible to configure many routers to use any range you like for your LAN, but doing so will mean that those addresses in the outside world will then be inaccessible to you, because your router will treat them as LAN addresses and never send them outside your own side of the network.  For example, if you configured your LAN to be on 87.112.79.0 with a subnet mask of 255.255.255.0 you could then set your computer to a static address anywhere within that range (87.112.79.0 - 87.112.79.255).  But doing that would mean that the same range of addresses in the outside world would not be accessible.  That's why there are reserved LAN address ranges.

Of course, if it's an entirely private network with no connection whatsoever to the outside world, you're free to assign the addresses however you wish, because there will be no clashes.

2

Famous Scientists, Doctors and Inventors / who invented the lettering system on phones?

« on: 09/02/2011 18:48:56 »

Rather a late response, but here goes!

The idea of using letters on the dial goes back to a time when telephone numbers started to become longer than just a few digits, and it was believed that using a combination of letters and numbers made them easier to remember.  Initially, there was no standard for the use of letters, and there didn't need to be, because direct dialed calls weren't possible outside one's own local area.  Different local telephone companies could - and did - invent their own various unique schemes.  Often they would just put a single letter on each dial position, and numbers might be listed as A-1205, C-5984, etc.  Sometimes these letters were chosen to represent the names of different exchange districts.  To take a hypothetical example, the letters C, S, W, and R might appear on the digits 2 through 5 respectively, with numbers listed as Central 1246, South 2197, West 3118, Riverside 7412, and so on.  Users then dialed the letter plus the four digits.  In some cases, the full names appeared on the dial.  In the U.K., Brighton was one area which used such a scheme in early times, and the dials carried the names Brighton, Hove, Portslade, Preston, Rottingdean, and Southwick on the digits 2 through 7 respectively.

As dial service expanded, it became clear that a more universal system would be beneficial, based upon a standardized set of letters on the dial which could be used to represent exchange names.  In the U.S., a Mr. W. G. Blauvelt of AT&T is generally credited as being largely responsible for the adoption of the system, around the time of World War I, resulting in the now-familiar assignment of letters on North American dials:

2 - ABC
3 - DEF
4 - GHI
5 - JKL
6 - MNO
7 - PRS
8 - TUV
9 - WXY

No letters were assigned to 1 or 0, since the former was not used as the first digit of a telephone number due to the risk of false impulses, and the latter was reserved for reaching an operator.  Q and Z were omitted, although Z did make an appearance in the zero position of some dials in later years, but was dropped again at the end of the 1940's. 

Several different numbering schemes were employed in those earlier times.  As well as a single-letter-plus-number scheme as already mentioned, a common format which soon became widely used was 2L-4N (i.e. numbers were two letters representing the exchange name, followed by four numbers), giving rise to numbers such as SUnset 4508 or DUpont 6200.   For many years, Los Angeles and Washington D.C. had numbers in this format.

Larger American cities which needed 7-digit numbering most often adopted a 2L-5N format, but a few initially used 3L-4N, changing to 2L-5N later.  New York and Chicago fall into the latter category, changing formats in 1930 and 1948 respectively. 

The 2L-5N format was to become the norm when numbers across the country were made up to seven digits for the uniform direct dial plan, and you'll see many references to numbers in this format in movies and TV shows of the appropriate era, not to mention Glenn Miller's famous recording of PEnnsylvania 6-5000, referring to the telephone number of the Hotel Pennsylvania in New York City.

Here in Britain, the G.P.O. adopted the same basic lettering scheme on the dial, with the single exception of moving the letter "O" onto the zero position, as already noted above.  London used a 3L-4N numbering system, giving rise to numbers such as HOLborn 1234, MAYfair 5678, and the famous WHItehall 1212 (the main number for Scotland Yard for many years).   A similar 3L-4N system was adopted in Birmingham, Edinburgh, Glasgow, Liverpool, and Manchester, but elsewhere numbers were entirely numeric (after those very early schemes such as Brighton's were eliminated).

Other countries often used similar schemes, but with appropriate changes to the letters on their dials to suit the local language.  In France, for example, Paris used the same 3L-4N numbering scheme as London, and the letters on dials were similar to those on British dials, but with Q added to the zero position (the city had an exchange named ROQuette).  In other countries, the letters assigned to the various digit positions were quite different.

In general, letters as part of the phone numbers vanished during the 1960's, London making the change to 7-digit all numeric numbers during the 1966 to 1968 period.  A few American cities maintained letters in numbers through into the 1970's, however.

There were two main reasons for abandoning letters.  First, it was becoming increasingly harder to come up with suitable exchange names to fit vacant codes.  Try making up any sort of meaningful exchange name (in English!) from the letters available for the codes 559 or 956, for example.  This is what had already prompted cities like New York to change from 3L-4N to 2L-5N, since needing only two letters instead of three made many more choices available.  Switching to all-figure numbering made all of those otherwise difficult-to-name codes available, which was important at a time when telephone use was expanding rapidly.

Second, international direct dialing was starting to become a reality, and the various incompatible schemes on different countries' dials made it impossible for somebody in one country to dial numbers involving letters in another country.  The letter Q was actually added to British dials ready for when direct dialing to Paris became available from some places in 1963.  That was relatively easy, since all the other letters were identical.  But for calls between, say, the U.K. and the U.S., there was the problem of the letter O being on a different digit.  The London number ACOrn 1234 was 220-1234 on a British dial, but anyone using an American dial would end up reaching CANonbury 1234 (226-1234) instead.   Obviously the problems with countries using vastly different dial letters were even more severe.

To address a few other points:

The first two digits of my current area code is 58, and that just happens to match to the letters LU, and the local phone area is Luton (I did not live here in the days when we had names to our phone exchanges, so I am assuming the fact that 58 is LU is not coincidental

It certainly isn't.  The original U.K. STD plan assigned the bulk of the codes on the basis of two letters plus a number, the letters representing the town name, or sometimes some form of the county name or some other geographical reference.  Hence, to take a few examples:

0AB4 = 0224 = Aberdeen
0BE4 = 0234 = Bedford
0BR2 = 0272 = Bristol
0NO3 = 0603 = Norwich
0PE6 = 0736 = Penzance
0SW2 = 0792 = Swansea

The exceptions to this general rule were London which was assigned just 01, and Birmingham, Edinburgh, Glasgow, Liverpool, and Manchester which were assigned 021 through 061 respectively (if you look at those codes, the first digit after the zero also happens to correspond with the name of the city, although these codes were always listed as numeric only).

As with local exchange names in London and the other large urban areas, the letters were dropped from the STD codes later in the 1960's, leaving them in the all-number form with which people are familiar today (allowing for the later insertion of "1" etc.).

Does anyone know why the Americans use 911?

See http://www.thenakedscientists.com/forum/index.php?PHPSESSID=ea40a3b14881d3ebfb988be6417daaa5&topic=35127.0

It does not say why zero was already free, but my initial guess was that zero may have been the original number for calling the operator (a tradition that has continued for many PABX systems), where this number has now changed to 100.

Zero was indeed the number to reach the operator from the early days in dial service.  In Britain, the operator was moved to 100 in preparation for STD and the use of zero as the STD access prefix.

3

Technology / What do US railroad signals mean?

« on: 26/01/2011 08:42:10 »

Here you go, this might help to explain it a little better:

http://www.alkrug.vcn.com/rrfacts/signals/signals.htm

4

Technology / What do US railroad signals mean?

« on: 25/01/2011 21:28:57 »

Many U.S. railroads adopted a system known as speed signaling, rather than the route signaling which is the norm in Britain. 

Where there is a single signal head, as on long stretches of open track with no turnouts, the red, yellow & green indications mirror the equivalent British indications, except for the names, which are stop, approach, and clear respectively. 

Once diverging routes come into the picture, the norm is for the signal to indicate the speed at which the train may travel for the route selected by a combination of two or more lights.  The top head indicates the conditions for the full permissible speed of the line.  The second head, below, then indicates the condition for medium speed.  The definition for the latter can vary from one railroad to another, but typically limits speed to around 30 mph. 

The signal will show green for whichever speed route is set and clear.  So green over red indicates that it's clear to proceed at the maximum line speed.  Red over green indicates that the full-speed route is impassable, but that the medium-speed route is clear.  The indication is called medium clear. 

Yellow then comes into play as an approach warning (the "caution" indication of U.K. signals) for each speed route.  Yellow over red thus indicates approach for the full-speed route, just as for a yellow light alone.  Red over yellow is medium approach, meaning that the full-speed route is impassable, but that the train may proceed at medium speed, expecting the next signal to be at stop. 

It's possible for more than one light to be something other than red at a time.  Yellow over green is called approach medium, meaning that the full-speed route will be red at the next signal, but that the medium-speed route is clear for at least two blocks ahead.  In other words, the engineer may continue at full speed past this signal, but must slow to no greater than medium speed by the next signal.

A third signal head can be added at the bottom, which is used to indicate a route at which slow speed must not be exceeded.  Again, the actual speed can vary, but is typically 15 mph.   So green over red over red is clear; red over green over red is medium clear; red over red over green is slow clear.  In other words, the top light shows the condition for full speed, the middle light for medium speed, and the bottom light for slow speed.

Again, a yellow light with the others red indicates approach, medium approach, or slow approach, expecting all red at the next signal.  Yellow & green in combination indicates the speed at which an approach may be made to the next signal and the speed of the route which will be taken at that signal.  So yellow over green over red is approach medium (the red indicating that no slow route is available).  Red over yellow over green would be medium approach slow, meaning that the train may continue at medium speed past this signal, but must be down to slow speed for the next signal.

All lights red means stop, of course, since no route (full, medium, or slow speed) is clear. 

Those are the basics of speed signaling.  There are many extensions, such as lunar white to signal restricted speed, and flashing green or yellow for limited speed, which is higher than medium but below full speed. 

Some U.S. railroads did adopt route signaling, which operates in a similar way to U.K. signaling, the top light indicating the main line, the next light down the diverging route, and so on.  And some railroads adopted their own unique signals, like the position light signals of the old Pennsylvania R.R. or the position plus auxiliary lights of the B&O, which resemble no others.

5

Technology / 220Volts-110Volts

« on: 25/01/2011 21:02:10 »

Yes, a 3-wire single-phase 120/240V service has been the norm for residential supplies in the U.S. for many years.   The 120V loads (lighting, general-purpose receptacles etc.) are distributed as evenly as possible across the two sides of the supply.   The 240V loads (range, dryer, water heater, larger space heaters & air conditioners etc.) have the benefit of lower current draw at the higher voltage. 

A similar arrangement was employed in England in the early days, but it was soon decided to double up the voltage, which is how 240V (or thereabouts) came to be the standard here for all domestic appliances.   

Although more developed areas in Britain always employ 3-phase distribution, with regular houses just fed from one phase, in more rural areas we still have 1-phase 3-wire distribution networks which are similar in principle to the standard U.S. residential system, just operating at 240/480V instead of 120/240.    Most houses need only a 2-wire 240V connection, so about half the houses are connected to one side of the supply, the other half to the opposite side.  Some larger buildings, farms, and so on take a 3-wire supply.

6

Technology / 220Volts-110Volts

« on: 25/01/2011 13:04:58 »

I know this reply is three years late, but as no responses were made regarding 3-phase supplies in the U.K. and the U.S., I thought I'd comment.

Here in Britain, the standard system is a 4-wire wye (also called star) arrangement, which supplies 240V phase to neutral and 415V between phases.   It's used to distribute power around most neighborhoods of any size, even when only residential properties requiring only single-phase power are supplied.   

The 240V value became the standard by the early 1970's.  Prior to that there were minor variations in the declared nominal voltages from area to area, such as 220, 230, or 250V.   As noted previously, officially the U.K. now uses a nominal 230V, but that was pure political meddling with the permissible tolerances.   

The situation in the United States is rather more complex, as various systems have been employed at different times, and although some arrangements are now considered obsolete, there are still many of them in use.

Wye systems are generally installed as standard these days, with one arrangement being configured to provide 120V phase to neutral for regular portable appliances etc.  That results in a corresponding phase-to-phase voltage of 208V.  This configuration is sometimes written as 208Y/120V.   

Where larger amounts of power are required, a wye system at the higher voltages of 480Y/277V is employed.  The reasons for this are as mentioned earlier in the thread - Higher voltages result in lower currents for any given amount of power. 

Note that in all these systems the relationship between the two voltages is fixed mathematically due to the 120-degree difference between the three phases.  Phase-to-phase voltage is always equal to the phase-to-neutral voltage multiplied by the square-root of 3.

There are also 3-phase delta systems still in use around the U.S.  These are normally 240V or 480V (measured phase to phase, as there is no neutral).  Again, the higher voltage was provided where power demands were greater.   The old 480V delta system is the reason for the later adoption of the 480Y/277V arrangement, since the phase-to-phase voltages are the same.

Since 120V is required in most places in addition to any higher voltages, it's not at all unusual to have multiple systems within a commercial building, e.g. the power company supplies 480Y/277V then there's a separate transformer inside the building to provide 208Y/120V as well.

The old 240V delta system also gave rise to a 4-wire delta arrangement in order to get 120V from the same system at minimum cost.  A center tap on one winding is extended as a neutral, so that 120V can be obtained by connecting between one of two phases and neutral. 

When it comes to operations requiring vastly greater amounts of power, then in all countries it's common to supply three-phase power at much higher levels of thousands of volts, with transformers on site to step it down where required.

7

Technology / Did the 1983 film War Games feature the internet?

« on: 19/01/2011 17:28:01 »

I guess he was using some from telnet protocol - as all of his remote access was in a text / command line type environment.

Telnet was designed to work over a packet switched network.  The type of dial-up access depicted in the movie involves nothing more complicated than direct data connection to the distant computer into which he was dialing - Effectively just like typing commands on a terminal connected to a computer right next to you, but with the telephone line and modems extending the distance between them.

He might have gained access to the Internet via one of the systems into which he dialed, although there's nothing to suggest that in the movie.  Many commercial systems at the time did not have such access, the dial-up connections via telephone lines being the only link with the outside world.

The other tech you missed mentioning was the acoustic coupler, which he puts the telephone handset into, even though the first true modems were becoming avaliable... 
For the young uns out there, this is a device that physically connects the telephone itself to the computer and effectively predates modems

It was still a modem (MODulator-DEModulator); it just used acoustic coupling to and from the telephone handset rather than being connected directly the line. 

At the time of Wargmaes film, 300 baud (i.e. 300 bits per second 38 bytes per second)was the standard rate for data transfer... 

It depends upon the application.  300 bps was very common in hobbyist use at the time, but 1200 bps was also in widespread use commercially, and had been so for quite a few years (at a cost).   But going the other way, there were still a large number of teletypes in use, such as the ubiquitous ASR33, which operated at a slower 110 bps, a limitation of the mechanisms employed.

In the most widespread use of 300-bps modems which used asynchronous communication, the maximum character rate was actually limited to 30 characters per second, since each character needed to be preceded by a start bit and followed by a stop bit, i.e. 10 bits per character.   Many teletypes running at 110 bps actually printed at 10 characters per second, since they need 2 stop bits for mechanical reasons (i.e. 11 bits per character).

And although the standard 300-bps modems were also 300 baud, it's worth pointing out that baud and bits per second are not the same thing, despite common misuse which suggests otherwise.

They are still avaliable today and can be used to connect a computer to the internet anywhere in the world with a phone line and can now acheive speeds of up to 28kB / s     

And higher.  56 kbps modems have been available for some time, although telephone line limitations generally limit the actual achievable speed to somewhat less.

8

Technology / Why 999?

« on: 19/01/2011 17:09:18 »

As far as Geezer's suggestion...  I don't think it worked for international calls from Italy.

I'm not familiar with the arrangements used in Italy, but as far as the old British A/B coinphones were concerned, pulsing out the number on the hookswitch to get a free call worked only for local calls, since all long-distance calls (and international, if you had a pocket big enough to hold the coins!) were placed via the operator who would then listen to the bell and gong signals as you dropped coins into the phone, just as with the old three-slot coinphones in America.    Although you could dial beyond the local area with this method by using the local routing codes as described, if you knew what to dial and there were routes which weren't barred in your particular area, as described above. 

Because of the way the coin mechanism worked, as described previously, it was actually possible to call TIM (the speaking clock) or any of the other prerecorded services such as weather for free from a coinphone by actually using the dial, so long as you had the coins to make the initial deposit.   You could hear the distant party answer before pressing button "A" to drop your coins into the box, but could not speak to him - Something which clearly wasn't needed when just listening to a recorded announcement!   So you just deposited the local call fee, dialed the number as normal, listened to the announcement, and then pressed button "B" to get your money back when finished.  So coinphone users could call those services for free while regular residential customers were charged for them.

All of these "tricks" became impossible with the postpayment phones which were introduced for STD and which could be used to dial long-distance as well as local.

9

Technology / Why 999?

« on: 16/01/2011 12:27:15 »

You're welcome.  And to pick up on a few other comments:

All true, but I think 911 came into use in the US prior to new fangled push button phones.

DTMF (TouchTone) dialing started to appear in the U.S. in the early/mid 1960's, but only in a few places, so certainly wasn't in widespread use when the decision to adopt 911 was made.

I assume that 911 had never been assigned an area code prior to its adoption as an emergency number, thus quicker routing.

None of the n11 numbers have ever been used as area codes, as they were already in use in many places as service codes (information, long distance etc.) when the area-code plan was drawn up in the late 1940's. 

The original plan was that the middle digit of an area code would be 0 or 1 (which continued to be true until the 1990's).  The reason for that was to allow equipment to distinguish between area codes and a number within one's own area from the second digit dialed.  Seven-digit numbers within each area code with the first two dialed as letters was to be the norm (as in, for example, the famous PEnnsylvania 6-5000 number in New York).    As there are no letters on the 0 or 1 digits, that meant that if the second digit dialed was 0 or 1, then the caller must be dialing an area code and the equipment knew to wait for ten digits in total.   

Because the n11 numbers were already in use as service codes, however, they were excluded from being assigned as area codes, and the equipment "knew" that if both second and third digits were a 1, then it wasn't an area code after all.

I've been told (possibly by Dave's dad) that it used to be possible to make "trunk" calls at local rates (which covered the local exchange plus nearest neighbours) by dialling through to the neighbouring exchange, then (because you appeared to be a local call) the next one over, then the next one, and so forth, thereby bypassing the transition to (much more expensive) national calls.

Yes, in the old British network that was possible in some places.  Long before STD (Subscriber Trunk Dialling) came along, there were local dialing codes to permit calling nearby exchanges without the assistance of an operator.  Typically you dialed 9 (as noted above) to reach the parent exchange in town, and subscribers there dialed codes such as 81, 82, 83, etc. to reach the outlying places.  Calling from one rural exchange to another which was parented on the same town exchange often involved dialing 981, 982 etc., the stages of the call progress there being fairly obvious.  There were numerous other variations to cater for local needs, and subscribers were issued with booklets containing all the local dialing codes for places they could dial direct (officially).   Steps were usually taken to prevent calling beyond the official local area, but it wasn't always easy and sometimes it could still be done with the right sequence of codes.    The local dialing codes finally vanished from the system during the 1980's.

Not that I ever tried it myself you understand, but I seem to remember it was possible to dial numbers in the UK from a call box by tapping the handset cradle to simulate the pulses produced by the rotary dialer. Consequently, it was possible to avoid the tiresome inconvenience of actually inserting money into the device.

Yes, with the old prepayment boxes (the ones which had the "A" and "B" buttons on them) that was possible.   As noted above, the dial didn't work (except for 9 and 0) until the local call fee had been deposited.  In normal operation, making that deposit also shorted out the transmitter in the handset so that you couldn't talk to your distant party until you pressed button "A" to drop the coins into the box and restore the contacts (pressing button "B" also restored everything, but returned the coins and disconnected the line for a few seconds to release the connection). 

So if you didn't make any initial deposit, the dial wouldn't work, but the transmitter was still active.  Pulsing out the number on the hookswitch would thus give a free call.

That ceased to be possible with the postpayment coinphones which gradually came into use during the 1960's, as all the coin control was done by relay sets at the exchange and both the dial and the transmitter were active all the time. 

10

Technology / Why 999?

« on: 15/01/2011 18:26:45 »

Hi all,

There were multiple reasons which led to the adoption of 999 by the British G.P.O. in the 1930's.  One of the major considerations was to use a number which involved minimum disruption to the existing numbering schemes in use. 

London used a step-by-step director system in which the first three digits of the seven-digit number (dialed as letters at that time) determined the exchange the call was to be routed to.  Except for the special case of dialing 0 for operator, all calls thus required a minimum of three digits before the equipment could decide how to connect the call, so a three-digit emergency number was needed.  As the digit 9 corresponds to the letters WXY, from which no useful names could be made up, the code 999 was vacant.  The same was true for the other major cities which used the director system, such as Glasgow (which was to get 999 service soon after London).

Elsewhere in the country, no subscriber numbers started with 9, because the 9 level on first selectors was used to access various services (directory enquiries, telegrams, etc.).   Moreover, from many rural and village exchanges, a first digit of 9 already connected to the parent exchange to allow subscribers to dial directly to that larger place.   As emergency calls needed to go to such places where the operators were located, the initial 9 of 999 already routed the call to the parent exchange without the need for any modifications at those smaller exchanges. 

The other very important factor which affected the choice was to allow for payphone users to call without needing to deposit any coins.   The prepayment phones which were standard at that time worked by preventing the dial from operating until the correct minimum fee for a local call had been deposited (2d. in the 1930's).  However, extra contacts on the dial opened when it was rotated right round to the zero position in order to permit callers to dial the operator without coins.  It was relatively easy to modify the dials to allow for both 9 and 0 to be dialed, thus allowing coin-free calls to 999 as well. 

As far as the adoption of 911 in the U.S.A. is concerned, this happened much later (the first place to get 911 service was Haleyville, Alabama in 1968).   Codes such as 211, 411, 611, and so on were already well established, having been in use for several decades by that time in many parts of the country.  911 was part of this reserved service-code sequence so fitted the need well.  Another consideration at the time for using 911 rather than some other spare n11 code was to make it easier to implement in areas using step-by-step switching equipment, since the 9 level was more likely to be unused for subscriber numbers.   More complications arose with 411, 611 etc. in step-by-step systems, which is why many of these had traditionally used codes such as 113 for information and 114 for repair service instead of 411 and 611.