They can't pass outside of the event horizon, and they will eventually be pulled back towards the black hole, but that doesn't mean that they can only travel radially inwards.
And while all of this is happening you, the observer, are also being affected by the gravity pull of the black hole.
It would seem that an observer that is being pulled into a black hole would not notice.
Consider that the earth is being pulled into a black hole. Even after we have passed the event horizon, look outwards (away from the black hole) and photons are being sucked in towards the black hole so you see other objects. Look inwards and you see the objects there, that have been pulled in before the earth was. Since the objects are all within the event horizon, some photons will be able to make it to us, observing from the earth, even though they'd never have made it completely out of the event horizon.
There has been a lot of research into passive and/or bistatic RADAR. Bistatic RADAR uses transmitter[s] physically seperate from the reciever[s]. Passive systems are similar, but use RF sources that are primarily intended for other uses, e.g. TV, radio.
What really amazes me is that the goof-offs exist in the commercial world. After all, for open-source you can just ignore them. But on commercial projects somebody is actually paying them so the goof-offs are displacing somebody who might actually be a useful contributer!
I once spent months getting a goof-off off of my team. After demonstrating that he had failed to actually do anything for 3 months, that he couldn't even turn the product on & make it work, he just got reassigned to another project. PHB's words: "I made sure I didn't hurt his feelings."
The number of developers that are actually contributing seems much like the commercial closed-source projects I've worked on. There's always a small team that really understands the code that does most of the work. The core tends to be about half of the team on small projects. Everybody else performs ancillary functions or just goofs off. On larger projects communication breaks down and the core is limited to a no more than a few subgroups of one to 7 developers each. The subgroups tend to work independently, only occasionally interacting with one another -- usually though some spokesman or leader.
So open source group dynamics are similar to closed source projects. Not really surprising, since both are staffed by people!
Larger more formalized projects, aerospace for example, improve on the above by making subgroups of subgroups. This layering of project & program management really increases the overhead. It seems to slow things down, but at the end you can put things together and have a hope of making it work. It's really a formalization & extension of the way we organize ourselves naturally.
There is a plan to send a mission to the outer planets and the Kupier belt. I'm sure a vist to the Kupier Belt's leading citizen will be added if at all possible.
Our old Toronado had a touch screen in the dash. It took a 3-finger exercise to enter diagnostics mode, the GM version of ctrl-alt-del, but then you:
had access to the engine diagnostics codes
could override/modify some things (e.g. shut down one injector, select Canadian shift regime)
had a summary screen that showed engine & transmission status
let you access the body computer (e.g. heat, A/C).
I had hoped that was the coming thing, but they charged too much for the touch screen. The dealership was forthcoming with info on the codes, or you could buy the service manual at a reasonable price.
Too bad. A great car, still looked good, my wife loved it, I could work on it, but we couldn't get parts to keep it on the road any more.
There are 2 major things stopping them from building this car today:
Power source: They need fuel cells & a fuel source that are comapact enough to fit into the "skateboard"
Light electric motors: Present designs are too heavy. Too much unsprung weight (i.e. in the wheels) adversely affects suspension dynamics.
Steps to migrate from today's common car to this design.
Use a more conventional drivetrain, with an engine & transmission above the skateboard but still allowing more modularity than existing cars.
Hybrid engine, with everything still mounted above the skateboard, including the electric motors.
Hybrid engine, but light electric motors now in the wheels.
Electric motors in the wheels, fuel cells in the skateboard, no auxiliary engine.
This looks like a good strategy. They've defined a goal, know the impediments that nust be surmounted, can build interim designs that lead to the goal. Making small steps will also get acceptance John Q. Publik -- who is wary of major change.
I'm not sure how many interim steps will actually reach the consumer, but I venture that this is more likely to result in a car acceptable to consumers that can be built affordably.
The Japanese hybrids are great ideas, but they're cars that many car buyers can't or won't buy, and the manufactureers are selling them at a loss. How will it help the environment if nobody will buy the car & nobody can afford to build it?
For ground based they generally use the 5400, though they might use some space qualified parts in circuitry requiring high reliability.
I seem to remember working with telephony equipment -- the central frequency reference at the site where multiple microwave & coax links came together used space-qualified parts. Even though there was a backup there'd be a glitch on hundreds of thousands of DS0s (8kBPS voice channels), so they wanted all the reliability possible.
If they wanted to buy the rights and the documentation to the obsoleted parts I guess they could. It'd get some of our representatives in DC bent out of shape though. Generally they want to keep this sort of work in the private sector -- free markets, competition, and all that. It lets the senators & representatives claim that they kept jobs for their constituents.
The 54 series parts were like the 74 series, but in a hermitically sealed case, 100% tested over a wider temperature range, and burned in to remove infant failures. For this application they used space qualified components. The same as 54 series parts, more stringent tests, and now the chips are also evaluated for radiation resistance. Any change in the design or production process and the 54 & space qualified chips must be requalified. What can happen is that a chip is produced to be fuctionally the same, but using smaller geometries, and now is more suseptiple to ESD and radiation.
CMOS chips, because of their high impedances, are notorious for ESD and rad sensitivity so they won't do.
With the reduction in military, aerospace, and space spending many manufacturers have dropped the 54 series and space qualified components. They haven't made any attempts to add replacements in their product lines.
When a part is dropped, the manufacturer usually informs the industry of their intent. You're given a date & price for a final order. the theory is that you can buy a lifetime supply of these parts. Industry isn't likely to but any more than they need to complete existing contracts plus a few spares, there's no guarenty that you'll get any more contracts to build items requiring these parts so these purchases will cut into your profits. Government procurment may buy additional components, but lack funding to really buy large quantities.
An opportunity is presented, and they will be taken advantage of. A distributer might buy some additional parts -- since the distribributer has several customers buying a particular part from him, his risk of being stuck with an unseable component is small.
After the final production run, the chip manufactorers will sell the documentation, tooling, and rights to make a chip. There are small manufacturers who buy these, all well as the out of date machinery to produce these parts. They can then make small production runs, sometimes under a hundred components, for a price. In addition, they might buy untested dice or wafers from the last production run. The untested & unpackaged componets are very cheap, so it's more affordable & less risky to buy and store these than the completed components.
So it is possible to still get the parts needed? -- at a price!
As the AC posting at 0 (likely -1 by now) pointed out in such a anonomously & cowardly manner: what I meant to say was that that two legislators took the money and voted against the banks' interest.
So who is writing code for an OS mirror program?
What a great idea for a browser plugin, a plugin that re-mirrors the text on the user's display so it can be read.
The end of the article mentions two legislators that took the banks' money but then voted against the bill.
I'm not sure if we should applaud these folks for voting their mind or treat them as a pariah for taking the cash then not delivering the goods.
In one of Heinlein's books a recurring character defines an honest politician as one who "once bought stays bought".
Dot-commers were always a diverse group, from the founders down to the mailroom these companies tended to attract folks with a wide range of interests & capabilities. That's one of the things that I found fun about working for a startup & one of the things
I found scary about working for a startup.
Is it any surprise that these same folks have movd on to varied new vocations & avocations?
You don't own it until you "improve" it. That is have permanent residents living there without significant outside assistance. This rule of international law has applied to everything from continents (e.g. Australia) to homesteads (e.g. some company wanted land in the US west, or rather the oil under it, and sued for the government to take it away from the homesteader since he hadn't built a good enough cabin and hadn't cleared land for a garden -- they only lost because of a statute of limitations issue).
The next step is bulldozing everything in sight -- so when you look up at night and notice that all the peaks have been flattened and all the craters have been filled in, then you'll know that somebody really owns it!
You make a good point, and certainly if it makes economic sense for me I'd get the fiber. However:
1. I work for an R&D organization so it isn't just a 100 people browsing the web & emailing, it's mostly engineers moving schematics, board designs, code, & binaries between us & contractors, customers, & other company sites.
2. I downloaded the RedHat 7.3 ISOs on my cable modem at home (Comcast) in about 5 hours. That's after they started capping rates.
Considering the comments you've made, I'll certainly investigate & compare my options carefully before deciding the route to go.
The orginal post suggestted that fiber was overkill, and I'd have to agree. A fully switched 100MBPS network with 802.1a WAP in the house and 1 DSL or even cable modem pipe up to the house would do fine. Right now I work in a building setup like this, except with a T1 to the home office, and it supports >100 people.
Though your certainly right. I might pay to get fiber into the house, depending on cost, but take it from there myself.
... and personally, I'd buy a house without the wiring and do it myself. Most of the "several thousand dollars" to rewire the place after it's built is labor costs. This sounds like an easy way to increase a house's value with via sweat equity, and there was a lot less sweat involved with running CAT5 than with the kitchen cabinets & tial in my present house!
From grad school experience & working with people who came from Academia I must say that such violations are far too common. Most profs know that they're not worth enough for anybody to sue, so they'll take the easy way. Several former profs that I work[ed] with are fond of saying "It's easier to apologize than to get permission."
Am I the only one who has seen profs dump hundreds of pages of the web to create student notes -- copied & bound & sold at the student book store as a "required text"?
Let me see if I can expound a bit. There are actually two different digital hierarchies in this discussion.
The basic unit in North American TDM is the DS0, 8 bits of data at a constant 8 kHz rate for 64 kbps unless the carrier is using robbed bit signalling where 1 bit is used for in band signalling information and the remaining 7 are used for data at a 56 kbps rate. An entire DS1 can be used to carry non-voice data. In this case some control & signalling overhead is not needed and can be given to the user. Also there is fractional T1, where you rent 2 to 24 DS0s within a DS1 for data. Thus you buy nx64 bps of bandwidth (or nx56 if your provider is using robbed bit signalling).
Take 24 DS0s along with some additional bits for signalling, synchronization and maintenance and you get a DS1. The DS1 describes a particular arrangement of bits, it really comes in many variations. if this DS1 data is transmitted over 2 twisted pairs of wire with proscribed impedance, drive, and levels then this electrical representation of the DS1 data is called T1.
This was still too little bandwidth, so 28 DS0s were aggregated into a DS3, which is T3 when it's pumped over a pair of wires. There were also other attempts at aggregating data: DS2 is 4 DS1s and had some limited use as a means of transmitting digital video and DS4 contains 6 DS3s. Each of these conglomerations of data were independently conceived, so are quit different. Only the DS3 achieved widespread use and it is being superceded by the STS1.
Clumping more and more DS0s into a single glob meant that at each node you had to track each and every DS1. So the DS3's get torn apart into DS1s that then get grouped together in different compinations depending on their destination. New DS3s are formed to be sent out on T3s to various destinations. Of course, DS1s for subscribers local to this node get torn apart into DS0s and sent out (converted to analog as needed). Incoming DS0s get aggregated into DS1s, that get clumped into DS3s, that get sent out as T3s. Oh yea, smaller towns might just get T1s. So you have a mix of everything, and everything is different -- very confusing.
In an effort to make something better, the STSs & OCs, the SONET hierarchy, came about. The STS1 is a facility (an electrical method of transporting data, as a T1 or a T3 are). The STS is designed to contain tributaries of various size. there are Virtual Tributaries (VT) within the STS1 that are sized to contain various sizes of tributary within them. VTs have header information that helps the STS maintain time synchronization of the data contained within the VT. Furthermore you can aggregate VTs within VTGroups. A common way of carrying 28 DS1s within an STS1 is to put 7 VTGroups into the STS1, 4 VT1.5s into each VTGroup, and each VT1.5 can contain a DS1. If you transmit the bits of an STS1 optically then this is called an OC1. Confusing? The advantage is that as you grow the STS/OC larger you have a common interface to lower levels, the VTs act as wrappers around other conglomerations of data, and smaller STSs can be contained in larger STSs in a well defined manner. As the STS/OC hierarchy grows there's no need to invent new ways of packing the resultent bits. At the lower end, phone companies could just pipe their DS1s & DS3s into the STS1s. This allowed them to gradually add a mix of TDM & SONET, slowly growing into SONET with minimal disruption to service and gradual training of their workforce as they made the transition.
Now an STS3 is three times the size of an STS1. Three STS1s are contained within the STS3. Alternatively, if you're using the DS3 to just move a large amount of generic digital data, you can use an STS3c, c stands for concatenated. This eliminates some header information, and the huge field of data is left open for whatever you wish. A veru common use is ATM, which is a packet based data transfer system. unlike ip, it has a numer of different services available (e.g. CBR - constant bit rate, VBR - Variable Bit Rate) as well as Quality of Servis feature.
This is extended upwards, although for STSx & OCx there is no equipment available for most x. As you say, there is oc1 & oc3, but no oc2, then it skips to oc12. One reason is the desire to use common transmission equipment to carry both North American TDM, as well as European TDM which is based on 64 kbps E0s, of which 31 are grouped into an E1 (though 1 is usually used for signalling leaving 30 for voice)... The European equivalent of SONET is SDH. Oh yes, Japan is slightly different. The have a slightly modified version of North Amaerican TDM, and their T1 is often called a J1 to differentiate it.
It does make sense in a historical perspective. Different carriers had different needs (rural carriers with few customers space wide apart vs. urban carriers with many customers close together). Various solutions came about, and then there was an attempt to fit all of these together into one method of transmission, since phone companies often use their equipment for decades. Nothing gets thrown away so new standards must be able to work with decades old standards.
Slashdot Mirror
I probably should have talked about a reference frame. However, just how do the enormous events within a black hole affect the local physics?
And while all of this is happening you, the observer, are also being affected by the gravity pull of the black hole.
Consider that the earth is being pulled into a black hole. Even after we have passed the event horizon, look outwards (away from the black hole) and photons are being sucked in towards the black hole so you see other objects. Look inwards and you see the objects there, that have been pulled in before the earth was. Since the objects are all within the event horizon, some photons will be able to make it to us, observing from the earth, even though they'd never have made it completely out of the event horizon.
Here are some links I found: DARPA research, Canadian project (they're pretty tight -lipped about this), and German work is ongoing too.
It seems to have been used in astonomy for counting meteors & observing auroras.
I once spent months getting a goof-off off of my team. After demonstrating that he had failed to actually do anything for 3 months, that he couldn't even turn the product on & make it work, he just got reassigned to another project. PHB's words: "I made sure I didn't hurt his feelings."
So open source group dynamics are similar to closed source projects. Not really surprising, since both are staffed by people!
Larger more formalized projects, aerospace for example, improve on the above by making subgroups of subgroups. This layering of project & program management really increases the overhead. It seems to slow things down, but at the end you can put things together and have a hope of making it work. It's really a formalization & extension of the way we organize ourselves naturally.
There is a plan to send a mission to the outer planets and the Kupier belt. I'm sure a vist to the Kupier Belt's leading citizen will be added if at all possible.
- had access to the engine diagnostics codes
- could override/modify some things (e.g. shut down one injector, select Canadian shift regime)
- had a summary screen that showed engine & transmission status
- let you access the body computer (e.g. heat, A/C).
I had hoped that was the coming thing, but they charged too much for the touch screen. The dealership was forthcoming with info on the codes, or you could buy the service manual at a reasonable price.Too bad. A great car, still looked good, my wife loved it, I could work on it, but we couldn't get parts to keep it on the road any more.
The history seems to be of losing. Not to say that we're winning the war on drugs either.
Give the poor slob some free hits, get him hooked before he knows how bad the dope is, then start charging big bucks.
There are 2 major things stopping them from building this car today:
Steps to migrate from today's common car to this design.
This looks like a good strategy. They've defined a goal, know the impediments that nust be surmounted, can build interim designs that lead to the goal. Making small steps will also get acceptance John Q. Publik -- who is wary of major change.
I'm not sure how many interim steps will actually reach the consumer, but I venture that this is more likely to result in a car acceptable to consumers that can be built affordably.
The Japanese hybrids are great ideas, but they're cars that many car buyers can't or won't buy, and the manufactureers are selling them at a loss. How will it help the environment if nobody will buy the car & nobody can afford to build it?
For ground based they generally use the 5400, though they might use some space qualified parts in circuitry requiring high reliability.
I seem to remember working with telephony equipment -- the central frequency reference at the site where multiple microwave & coax links came together used space-qualified parts. Even though there was a backup there'd be a glitch on hundreds of thousands of DS0s (8kBPS voice channels), so they wanted all the reliability possible.
If they wanted to buy the rights and the documentation to the obsoleted parts I guess they could. It'd get some of our representatives in DC bent out of shape though. Generally they want to keep this sort of work in the private sector -- free markets, competition, and all that. It lets the senators & representatives claim that they kept jobs for their constituents.
The 54 series parts were like the 74 series, but in a hermitically sealed case, 100% tested over a wider temperature range, and burned in to remove infant failures. For this application they used space qualified components. The same as 54 series parts, more stringent tests, and now the chips are also evaluated for radiation resistance. Any change in the design or production process and the 54 & space qualified chips must be requalified. What can happen is that a chip is produced to be fuctionally the same, but using smaller geometries, and now is more suseptiple to ESD and radiation.
CMOS chips, because of their high impedances, are notorious for ESD and rad sensitivity so they won't do.
With the reduction in military, aerospace, and space spending many manufacturers have dropped the 54 series and space qualified components. They haven't made any attempts to add replacements in their product lines.
When a part is dropped, the manufacturer usually informs the industry of their intent. You're given a date & price for a final order. the theory is that you can buy a lifetime supply of these parts. Industry isn't likely to but any more than they need to complete existing contracts plus a few spares, there's no guarenty that you'll get any more contracts to build items requiring these parts so these purchases will cut into your profits. Government procurment may buy additional components, but lack funding to really buy large quantities.
An opportunity is presented, and they will be taken advantage of. A distributer might buy some additional parts -- since the distribributer has several customers buying a particular part from him, his risk of being stuck with an unseable component is small.
After the final production run, the chip manufactorers will sell the documentation, tooling, and rights to make a chip. There are small manufacturers who buy these, all well as the out of date machinery to produce these parts. They can then make small production runs, sometimes under a hundred components, for a price. In addition, they might buy untested dice or wafers from the last production run. The untested & unpackaged componets are very cheap, so it's more affordable & less risky to buy and store these than the completed components.
So it is possible to still get the parts needed? -- at a price!
Cool -- I knew that there were folks out there that could hack this quickly. Being an embedded sort of guy this is outside of my skill set.
As the AC posting at 0 (likely -1 by now) pointed out in such a anonomously & cowardly manner: what I meant to say was that that two legislators took the money and voted against the banks' interest.
So who is writing code for an OS mirror program? What a great idea for a browser plugin, a plugin that re-mirrors the text on the user's display so it can be read.
The end of the article mentions two legislators that took the banks' money but then voted against the bill. I'm not sure if we should applaud these folks for voting their mind or treat them as a pariah for taking the cash then not delivering the goods.
In one of Heinlein's books a recurring character defines an honest politician as one who "once bought stays bought".
Dot-commers were always a diverse group, from the founders down to the mailroom these companies tended to attract folks with a wide range of interests & capabilities. That's one of the things that I found fun about working for a startup & one of the things I found scary about working for a startup.
Is it any surprise that these same folks have movd on to varied new vocations & avocations?
You don't own it until you "improve" it. That is have permanent residents living there without significant outside assistance. This rule of international law has applied to everything from continents (e.g. Australia) to homesteads (e.g. some company wanted land in the US west, or rather the oil under it, and sued for the government to take it away from the homesteader since he hadn't built a good enough cabin and hadn't cleared land for a garden -- they only lost because of a statute of limitations issue).
The next step is bulldozing everything in sight -- so when you look up at night and notice that all the peaks have been flattened and all the craters have been filled in, then you'll know that somebody really owns it!
You make a good point, and certainly if it makes economic sense for me I'd get the fiber. However:
1. I work for an R&D organization so it isn't just a 100 people browsing the web & emailing, it's mostly engineers moving schematics, board designs, code, & binaries between us & contractors, customers, & other company sites.
2. I downloaded the RedHat 7.3 ISOs on my cable modem at home (Comcast) in about 5 hours. That's after they started capping rates.
Considering the comments you've made, I'll certainly investigate & compare my options carefully before deciding the route to go.
The orginal post suggestted that fiber was overkill, and I'd have to agree. A fully switched 100MBPS network with 802.1a WAP in the house and 1 DSL or even cable modem pipe up to the house would do fine. Right now I work in a building setup like this, except with a T1 to the home office, and it supports >100 people.
Though your certainly right. I might pay to get fiber into the house, depending on cost, but take it from there myself.
... and personally, I'd buy a house without the wiring and do it myself. Most of the "several thousand dollars" to rewire the place after it's built is labor costs. This sounds like an easy way to increase a house's value with via sweat equity, and there was a lot less sweat involved with running CAT5 than with the kitchen cabinets & tial in my present house!
From grad school experience & working with people who came from Academia I must say that such violations are far too common. Most profs know that they're not worth enough for anybody to sue, so they'll take the easy way. Several former profs that I work[ed] with are fond of saying "It's easier to apologize than to get permission."
Am I the only one who has seen profs dump hundreds of pages of the web to create student notes -- copied & bound & sold at the student book store as a "required text"?
Let me see if I can expound a bit. There are actually two different digital hierarchies in this discussion.
The basic unit in North American TDM is the DS0, 8 bits of data at a constant 8 kHz rate for 64 kbps unless the carrier is using robbed bit signalling where 1 bit is used for in band signalling information and the remaining 7 are used for data at a 56 kbps rate. An entire DS1 can be used to carry non-voice data. In this case some control & signalling overhead is not needed and can be given to the user. Also there is fractional T1, where you rent 2 to 24 DS0s within a DS1 for data. Thus you buy nx64 bps of bandwidth (or nx56 if your provider is using robbed bit signalling).
Take 24 DS0s along with some additional bits for signalling, synchronization and maintenance and you get a DS1. The DS1 describes a particular arrangement of bits, it really comes in many variations. if this DS1 data is transmitted over 2 twisted pairs of wire with proscribed impedance, drive, and levels then this electrical representation of the DS1 data is called T1.
This was still too little bandwidth, so 28 DS0s were aggregated into a DS3, which is T3 when it's pumped over a pair of wires. There were also other attempts at aggregating data: DS2 is 4 DS1s and had some limited use as a means of transmitting digital video and DS4 contains 6 DS3s. Each of these conglomerations of data were independently conceived, so are quit different. Only the DS3 achieved widespread use and it is being superceded by the STS1.
Clumping more and more DS0s into a single glob meant that at each node you had to track each and every DS1. So the DS3's get torn apart into DS1s that then get grouped together in different compinations depending on their destination. New DS3s are formed to be sent out on T3s to various destinations. Of course, DS1s for subscribers local to this node get torn apart into DS0s and sent out (converted to analog as needed). Incoming DS0s get aggregated into DS1s, that get clumped into DS3s, that get sent out as T3s. Oh yea, smaller towns might just get T1s. So you have a mix of everything, and everything is different -- very confusing.
In an effort to make something better, the STSs & OCs, the SONET hierarchy, came about. The STS1 is a facility (an electrical method of transporting data, as a T1 or a T3 are). The STS is designed to contain tributaries of various size. there are Virtual Tributaries (VT) within the STS1 that are sized to contain various sizes of tributary within them. VTs have header information that helps the STS maintain time synchronization of the data contained within the VT. Furthermore you can aggregate VTs within VTGroups. A common way of carrying 28 DS1s within an STS1 is to put 7 VTGroups into the STS1, 4 VT1.5s into each VTGroup, and each VT1.5 can contain a DS1. If you transmit the bits of an STS1 optically then this is called an OC1. Confusing? The advantage is that as you grow the STS/OC larger you have a common interface to lower levels, the VTs act as wrappers around other conglomerations of data, and smaller STSs can be contained in larger STSs in a well defined manner. As the STS/OC hierarchy grows there's no need to invent new ways of packing the resultent bits. At the lower end, phone companies could just pipe their DS1s & DS3s into the STS1s. This allowed them to gradually add a mix of TDM & SONET, slowly growing into SONET with minimal disruption to service and gradual training of their workforce as they made the transition.
Now an STS3 is three times the size of an STS1. Three STS1s are contained within the STS3. Alternatively, if you're using the DS3 to just move a large amount of generic digital data, you can use an STS3c, c stands for concatenated. This eliminates some header information, and the huge field of data is left open for whatever you wish. A veru common use is ATM, which is a packet based data transfer system. unlike ip, it has a numer of different services available (e.g. CBR - constant bit rate, VBR - Variable Bit Rate) as well as Quality of Servis feature.
This is extended upwards, although for STSx & OCx there is no equipment available for most x. As you say, there is oc1 & oc3, but no oc2, then it skips to oc12. One reason is the desire to use common transmission equipment to carry both North American TDM, as well as European TDM which is based on 64 kbps E0s, of which 31 are grouped into an E1 (though 1 is usually used for signalling leaving 30 for voice) ... The European equivalent of SONET is SDH. Oh yes, Japan is slightly different. The have a slightly modified version of North Amaerican TDM, and their T1 is often called a J1 to differentiate it.
It does make sense in a historical perspective. Different carriers had different needs (rural carriers with few customers space wide apart vs. urban carriers with many customers close together). Various solutions came about, and then there was an attempt to fit all of these together into one method of transmission, since phone companies often use their equipment for decades. Nothing gets thrown away so new standards must be able to work with decades old standards.