FCC regulations prohibit any faster speeds than like 53 kilobits.. why? something to do with the amount of power going through the phone line? why do they not allow faster than that for the POTS system, yet DSL, which is on a much higher frequency, is just fine?
Why did we stop at "56k" for dialup modems?
- Thread starter zsouthboy
- Start date
Mrpilot007
Senior member
- Jan 5, 2003
- 227
- 0
- 76
Actually the difference between dial up modems and DSL is the fact that DSL stands for Digital Signal Loop. The analog connection is limited to or maxed out on the phone lines. Some older lines even are limited due to the bandwidth that is available under analog. Digil Signal Loop allows a digital connection and at a higher frequency so it is still limited under the bandwidth available on that frequency but is much faster. DSL is not asynchronus so the download speeds are faster than the uploads. ADSL is asynchronus and allows the same up and down.
I know I just rambled on but the answer is maximum bandwidth. (please someone else enter in and put me in my place
)
I know I just rambled on but the answer is maximum bandwidth. (please someone else enter in and put me in my place
)What happens after 53k is that analog phone lines produce "cross-talk". This is where the power used in a phone line is so much that it actually interferes with communications with other lines (I'm simplifying...). I don't think new cordless phones do this much, but I know the old ones certainly did and sometimes you could do this with walky-talkies: you could pick up another conversation while you were talking - that's cross-talk. Of course, that's not due to the physical phone line but the radio transmission between the cordless phone and its base, but the idea is the same. You can imagine that it would suck really bad trying to transmit information - it would become pretty corrupted or screw with other communications, and FCC regulations forbid that for most devices (it may not cause interference, blah, blah).
DSL is different. First, though, I wanna clear up the asynchronous and synchronous thing. Synchronous DSL, which isn't common, is where the speeds are "synchronized" or the same; the upload and download speeds are the same. The DSL most common, especially to homes/residences is ADSL or asynchronous DSL. That's where the upload and download speeds are NOT the same, they are A-synchronous, or NOT synchronized. Now, about DSL: those copper wires can actually carry more frequencies, and so more bandwidth, than your voice calls. DSL takes advantage of this extra frequency left over; a lot of service will let you talk on the phone and use DSL at the same time. Why was there all this extra space anyway? Well, by using a narrow frequency range of what those copper wires are capable of, you have very low interference and can pack in a lot of copper wires in a smaller area, making stuff cheaper. The phone companies want it cheap and the FCC wants no interference... so you can see what happened.
However, your voice system uses about 0 to 4000 hz, but your copper wires are capable of up to a few million hz. Obviously, there's stuff there to exploit. It's limited in how clear the signal is, which is limited by your distance from a central telephone office. With a digital signal it's much safer to exploit this left over frequency for whatever you want, and not the interference problems like you do with an analog signal.
So... basically, you're trying to get faster than 53kbps in a very narrow frequency range, the space reserved for analog transmission. Without some AMAZING data compression techniques, there's just not much space to squeeze out of this small frequency range and use. We pretty much use it all right now.
DSL is different. First, though, I wanna clear up the asynchronous and synchronous thing. Synchronous DSL, which isn't common, is where the speeds are "synchronized" or the same; the upload and download speeds are the same. The DSL most common, especially to homes/residences is ADSL or asynchronous DSL. That's where the upload and download speeds are NOT the same, they are A-synchronous, or NOT synchronized. Now, about DSL: those copper wires can actually carry more frequencies, and so more bandwidth, than your voice calls. DSL takes advantage of this extra frequency left over; a lot of service will let you talk on the phone and use DSL at the same time. Why was there all this extra space anyway? Well, by using a narrow frequency range of what those copper wires are capable of, you have very low interference and can pack in a lot of copper wires in a smaller area, making stuff cheaper. The phone companies want it cheap and the FCC wants no interference... so you can see what happened.
However, your voice system uses about 0 to 4000 hz, but your copper wires are capable of up to a few million hz. Obviously, there's stuff there to exploit. It's limited in how clear the signal is, which is limited by your distance from a central telephone office. With a digital signal it's much safer to exploit this left over frequency for whatever you want, and not the interference problems like you do with an analog signal.
So... basically, you're trying to get faster than 53kbps in a very narrow frequency range, the space reserved for analog transmission. Without some AMAZING data compression techniques, there's just not much space to squeeze out of this small frequency range and use. We pretty much use it all right now.
After reading my explianation, I realized it might suck. If you need a good explaination, head to:
http://www.howstuffworks.com/telephone.htm
It'll give a good explaination of how telephone and telephone lines work, and then head to:
http://www.howstuffworks.com/dsl.htm
for a nice description of how DSL works. That should tie in why more than 53k isn't possible over analog lines, unless there's some nifty data compression.
If you need a good explaination, head to:http://www.howstuffworks.com/telephone.htm
It'll give a good explaination of how telephone and telephone lines work, and then head to:
http://www.howstuffworks.com/dsl.htm
for a nice description of how DSL works. That should tie in why more than 53k isn't possible over analog lines, unless there's some nifty data compression.
The reason you can't get more than 56K out of the phone system is that > 95% of the phone lines are digitized to 8kHz, 7 or 8-bit sampling (usually 7), as soon as it reaches the first phoneco switch, then is changed back to analog coming out of the last switch on the other end. Fortunately the voltages represented by the digitized samples are spaced logarithmically rather than linearly so it sounds better than if you were to record something at only 8kHz 8-bit on the computer. 7-bit samples * 8kHz = 56kbps. The 56k modems actually rely on this digitization process and require that the ISP end has special modems that aren't on analog lines (stays digital at ISP end). Historically this conversion to a digital signal is so that many people's phone lines can be multiplexed together at the switch and sent to the next switch along 2 copper wires TOTAL for everyone's line rather than 2 for EACH line. Typically its 24 8kHz signals (either 56 or 64 kbps each depending on 7 or 8 bits/sample) multiplexed into one 1.5Mbps T1 connection, which is then multiplexed with other T1s into either a T3 or OC3 line. If your line was PURE analog, rather than getting digitized along the way, you could theoretically make a modem for it that connects at much higher speeds. A 45Mbps T3 is typically only 2 thin copper wires, just like your analog phone line.
BTW the limit *is* 56kbps, NOT 53kbps. K56 actually CAN connect at 56k if your line is immaculate. x2 and v.90 cannot, because as always, technology standardizes on the least common denominator (think VHS vs. Beta)
BTW the limit *is* 56kbps, NOT 53kbps. K56 actually CAN connect at 56k if your line is immaculate. x2 and v.90 cannot, because as always, technology standardizes on the least common denominator (think VHS vs. Beta)
Wow, so you can connect at 56k and not 53k. I thought that in a real world scenario the FCC wouldn't allow more than 53k. Otherwise why do all the ads and the boxes have that little disclosure? I'm a lil' confused.
Thank you. thats what i wanted to know
... okay, here's one for you smart people...
i can connect to my ISP(bellsouth.net) over at my grandmothers house at 115.2 kbps (the max for a com port btw).......... and, as strange as that seems, i actually get downloads of more than the 4-5 k a sec(which would be normal for when i sign on at 43333 at home
, i get almost 10 k a sec on the 115.2 ).... the number being dialed is over 60 miles away, so its not distance...i have connected to bellsouth.net at 115.2 in three different places, and not sporadically either..... explanations?
I would say you're getting faster than 56k speeds because of compression. Try downloading a compressed file, like an MP3, and I don't think you'd get those same speeds.
Originally posted by: Mrpilot007
Actually the difference between dial up modems and DSL is the fact that DSL stands for Digital Signal Loop. The analog connection is limited to or maxed out on the phone lines. Some older lines even are limited due to the bandwidth that is available under analog. Digil Signal Loop allows a digital connection and at a higher frequency so it is still limited under the bandwidth available on that frequency but is much faster. DSL is not asynchronus so the download speeds are faster than the uploads. ADSL is asynchronus and allows the same up and down.
I know I just rambled on but the answer is maximum bandwidth. (please someone else enter in and put me in my place
I sure that DSL is Digital Subscriber Line.
zsouthboy, you're being mislead. Your modem is reporting the serial line speed of 115200 bps rather than the phone line connect speed.
You don't get more than slightly over 5 KB/s of data across the phone line either. It's only that when the data is compressible, the sending end compresses BEFORE sending over the phone line, and your modem decompresses AFTER receiving and BEFORE sending it over the faster serial port line.
regards, Peter
PS: ADSL, SDSL are asymmetrical and symmetrical
You don't get more than slightly over 5 KB/s of data across the phone line either. It's only that when the data is compressible, the sending end compresses BEFORE sending over the phone line, and your modem decompresses AFTER receiving and BEFORE sending it over the faster serial port line.
regards, Peter
PS: ADSL, SDSL are asymmetrical and symmetrical
Crabbyman, et al,
Two things to head you in the right direction:
First: Data on your modem is not asymmetric. You get the full bandwidth in both directions.
Second: So what about the extra bandwidth you are missing? The articles that figgypower mentioned give a good overview about how your analog voice is taken off of your phone line and converted into digital data, 64kbps worth. The one thing it doesn't explain is why do you only get 56kbps, and sometimes less (although it alludes to the latter part). The 56k is due to the fact that your phone, and 23 of your neighbors' are all gathered up, put on a T1 and sent down to the telephone company's Central Office (CO). Now, although a T1 does allocate 8 bits of data 8000 times a second for your phone, it does not allocate any extra bits for telling the CO whether your phone is On Hook, Off Hook, Ringing, etc. Therefore, the fancy people at Bell Labs decided to use one of your 8 bits every so often to tell the CO what state your phone is in. As a result, modem designers can not count on having all 8 bits of data all of the time. So modem designers only use 7 bits (how they do that is a whole other story) worth of data. Therefore, doing the math, you get 7 bits 8000 times a second, or 56 kilobits per second.
Now, last but not least, about that last 3 kpbs. The 3kbps loss is due to what is explained under Telephone Bandwidth section in the article. That 3400 hertz number is responsible for that. The filters used on analog lines further reduce what modem designers can pump down a line. But, lucky for us, different installations (equipment and distance) of lines running to your houses can result in a variance in the effect of the filters. So part of what happens durring the noisy periods of your modem handshake is figuring out how bad your POTS line is filtered (and attenuated). So once the modem figures that out it configures itself for the maximum amount of reliable speed.
I hope this helps
Jason
Two things to head you in the right direction:
First: Data on your modem is not asymmetric. You get the full bandwidth in both directions.
Second: So what about the extra bandwidth you are missing? The articles that figgypower mentioned give a good overview about how your analog voice is taken off of your phone line and converted into digital data, 64kbps worth. The one thing it doesn't explain is why do you only get 56kbps, and sometimes less (although it alludes to the latter part). The 56k is due to the fact that your phone, and 23 of your neighbors' are all gathered up, put on a T1 and sent down to the telephone company's Central Office (CO). Now, although a T1 does allocate 8 bits of data 8000 times a second for your phone, it does not allocate any extra bits for telling the CO whether your phone is On Hook, Off Hook, Ringing, etc. Therefore, the fancy people at Bell Labs decided to use one of your 8 bits every so often to tell the CO what state your phone is in. As a result, modem designers can not count on having all 8 bits of data all of the time. So modem designers only use 7 bits (how they do that is a whole other story) worth of data. Therefore, doing the math, you get 7 bits 8000 times a second, or 56 kilobits per second.
Now, last but not least, about that last 3 kpbs. The 3kbps loss is due to what is explained under Telephone Bandwidth section in the article. That 3400 hertz number is responsible for that. The filters used on analog lines further reduce what modem designers can pump down a line. But, lucky for us, different installations (equipment and distance) of lines running to your houses can result in a variance in the effect of the filters. So part of what happens durring the noisy periods of your modem handshake is figuring out how bad your POTS line is filtered (and attenuated). So once the modem figures that out it configures itself for the maximum amount of reliable speed.
I hope this helps
Jason
Crabby,
You say you are getting "bursts" up to 25kbps. Do you mean 25kBps? As in Bytes per second?
If so, that would be due to compression. The v.92 standard your modem uses has a compression component. If you are downloading a lot of text or other easily compressible data, then your rates will be higher. As for the songs, there must be some parts of mp3s that can be compress by the v.44 compression algorithm used within v.92.
J
You say you are getting "bursts" up to 25kbps. Do you mean 25kBps? As in Bytes per second?
If so, that would be due to compression. The v.92 standard your modem uses has a compression component. If you are downloading a lot of text or other easily compressible data, then your rates will be higher. As for the songs, there must be some parts of mp3s that can be compress by the v.44 compression algorithm used within v.92.
J
jasonpetras, great post. Very informative. I didn't realize the bit about only 7 bits rather than 8 bits. Now quite a few things make more sense to me.
Well, not exactly.
The FCC limits the power that can be put on the telephone line. That amount of power, used with the current modem standards, limits data throughput to ~53kbps.
The FCC doesn't regulate transmission speed, it regulates the power on the line. That has an effect on maximum throughput that can be achieved using current modem signalling schemes.
pm, I am glad I can help!
It seems to me that the power limits that everyone is talking about do not directly correspond to a bit-rate. The power limits refer to the actual strengths of the signals traveling in both directions on the copper wire pair runing from your house to the telcom equipment in the street or CO. In the direction from CO to your house, the limit is +0dBmW or 1mW. In the opposite direction, it is +3dBmW, or 2mW.
So how does this effect bit rate? : The same way as it effects how fast your DSL connection can be. Hopefully most out there are not as far away from the CO as I am. Due to my distance, the signal reaching my house is very small. So SBC must talk slower (only 384kbps, sucks!) to me than they can to someone who is close to the CO.
Jason
It seems to me that the power limits that everyone is talking about do not directly correspond to a bit-rate. The power limits refer to the actual strengths of the signals traveling in both directions on the copper wire pair runing from your house to the telcom equipment in the street or CO. In the direction from CO to your house, the limit is +0dBmW or 1mW. In the opposite direction, it is +3dBmW, or 2mW.
So how does this effect bit rate? : The same way as it effects how fast your DSL connection can be. Hopefully most out there are not as far away from the CO as I am. Due to my distance, the signal reaching my house is very small. So SBC must talk slower (only 384kbps, sucks!) to me than they can to someone who is close to the CO.
Jason
most people consider the 53kbps limit to be an archaic limitation set by the FCC for phone lines that have been severely upgraded in the past few decades. of course there are still some places where higher speeds will cause problems, that is, rural areas (boon docks, middle of no where, etc). why they set 53kbps specifically, i have no idea. but that limit exists to prevent problems that few ppl are aware of, including myself. of course, it is my opinion (based on no facts at all) that the limit is out dated given current telecommuncations routines and hardware.
well phone lines are still muxed at somepoint to a 64 kbs channel with 8 kbs of signalling. Isn't that the reason? T1/T3s are still the dominate digital interface. Sure the backgone network is ATM but on the edge of the ATM core is still a channel mux, working with 64k channels. All phone switches revolve around the 64k channel.
Also isnt the upload limited to 33.6 kbs? only download can achieve 53k
Also isnt the upload limited to 33.6 kbs? only download can achieve 53k
The fundamental limit for modem link speeds is the bandwidth of the PSTN channel - for many years this has been set at 56 kbps. (8ksps at 7bit precision). Within the network, this is actually treated as 8bits, with the MSB set to 0.
The asymmetry in up-link/down-link speeds comes from the asymetrical equipment required for such high-speed links. The local loop of your phone line is analogue, and is connected to an ADC/DAC at the telco's linecard. The resulting 64kbps bitstream is them multiplexed and switched into the rest of the network.
The ISP uses ISDN equipment which receives the 64kbps digital bitstream directly, without conversion.
Due to noise in the analogue-digital conversion, the uplink cannot achieve 56 kbps (33.6 is the highest found to work reliably). On the reverse link, ananlogue-digital conversion is done by high-speed/high-precision converters in the modem which do not degrade the signal to such an extent.
The asymmetry in up-link/down-link speeds comes from the asymetrical equipment required for such high-speed links. The local loop of your phone line is analogue, and is connected to an ADC/DAC at the telco's linecard. The resulting 64kbps bitstream is them multiplexed and switched into the rest of the network.
The ISP uses ISDN equipment which receives the 64kbps digital bitstream directly, without conversion.
Due to noise in the analogue-digital conversion, the uplink cannot achieve 56 kbps (33.6 is the highest found to work reliably). On the reverse link, ananlogue-digital conversion is done by high-speed/high-precision converters in the modem which do not degrade the signal to such an extent.
TRENDING THREADS
-
Discussion Zen 5 Speculation (EPYC Turin and Strix Point/Granite Ridge - Ryzen 9000)- Started by DisEnchantment
- Replies: 24K
-
Discussion Intel Meteor, Arrow, Lunar & Panther Lakes Discussion Threads
- Started by Tigerick
- Replies: 19K
-
Discussion Intel current and future Lakes & Rapids thread- Started by TheF34RChannel
- Replies: 23K
-
-
Facebook
Twitter