At low amplitudes an increment of 1 in the PCM byte value represents a much smaller increment delta in analog signal amplitude than would be the case if the amplitude being sampled were much higher. Thus for low amplitudes it's difficult to distinguish between adjacent byte values. To make it easier to do this for 56k modems certain PCM codes representing very low amplitudes are not used.
This gives a larger delta between possible amplitudes and makes correct detection of them by your modem easier. Thus half of the amplitude levels are not used in the downstream direction by V. This is tantamount to each symbol valid amplitude level representing 7 bits instead of 8.
Of course each amplitude symbol is actually generated by 8-bits but only bytes of the possible bytes are actually used by the ISP sender. There is a code table mapping these 8-bit bytes to the 7-bit bytes. It's not just a simple mapping like ignoring the last bit. Thus to send 7 normal data bytes 8-bits will take 8 of the above mentioned bytes.
But it's a little more complicated that this. If the line conditions are not nearly perfect or if the direction is upstream V. Also due to US government rules prohibiting high power levels on phone lines, certain high amplitudes levels can't be used resulting in only about Note that the digital part of the telephone network is bi-directional.
Two such circuits are used for a phone call, one in each direction. For V. For this V. But due to sophisticated cancellation methods it's able to convey data simultaneously in both directions as explained in the next subsection. It's claimed that with V.
Modern modems are able to both send and receive signals simultaneously. One could call this "bidirectional" or "full duplex". This was once done by using one frequency for sending and another for receiving.
Today, the same frequency is used for both sending and receiving. How this works is not easy to comprehend. Most of the telephone system "main lines" are digital with two channels in use when you make a telephone call. What you say goes over one digital channel and what the other person says goes over the other reverse digital channel. Unfortunately, the part of the telephone system which goes to homes and many offices is not digital but only a single analog channel.
If both modems were directly connected to the digital part of the phone system then bidirectional communication sending and receiving at the same time would be no problem because two channels would be available. But the end portion of the signal path goes over just one circuit. How can there be two-way communication on it simultaneously? It works something like this. Suppose your modem is receiving a signal from the other modem and is not transmitting.
Then there's no problem. But if your modem were to start transmitting with the other received signal still flowing into your modem it would drown out the received signal.
If the transmitted signal was a "solid" voltage wave applied to the end of the line then there is no way any received signal could be present at that point.
But the transmitter has "internal impedance" and the transmitted signal applied to the end of the line is not solid or strong enough to completely eliminate the received signal coming from the other end.
Thus while the voltage at the end of the line is mostly the stronger transmitted signal a small part of it is the desired received signal. All that is needed is to filter out this stronger transmitted signal and then what remains will be the signal from the other end which we want. To do this, one only needs to get the pure transmitted signal directly from the transmitter before it's applied to the line amplify it a determined amount, and then subtract it from the total signal present at the end of the line.
Doing this in the receiver circuits leaves a signal which mostly came from the other end of the line. An analog signal traveling down a line in one direction may encounter changes in the line that will cause part of the signal to echo back in the opposite direction. Since the same circuit is used for bi-directional flow of data, such echos will result in garbled reception.
One way to ameliorate this problem is to send training signals once in a while to determine the echo characteristic of the line. Like CAP, the system checks the line quality at start-up to determine the capacity of each of the subchannels. To minimise the overlap in the subchannels, which leads to interference, other techniques such as the Discrete Wavelet Multi-Tone DWMT are used to increase the isolation between adjacent channels. The following diagram illustrates this.
Based on the above assumptions, the theoretical maximum upstream and downstream rates can be calculated as [9] :. Typically, lower frequency sub-channels are utilised more than higher frequency sub-channels because signals are attenuated more at higher frequencies. Log in. Install the app. JavaScript is disabled. For a better experience, please enable JavaScript in your browser before proceeding.
You are using an out of date browser. It may not display this or other websites correctly. You should upgrade or use an alternative browser. Thread starter rajujayaraman Start date Sep 7, Replies 1 Views 17, Hi, I am using D-Link u wifi modem router.
Is the adsl modulation is dependent on the line, i mean, the connection cable wire that gets you the phone signal and broadband. What is the modulation of adsl. I now use g. Is this have anything to do with the speed that you are getting from the paid plans. For eg. Or the setting is connected with the actual wire that is used for connecting the telephone.
How to find the ideal module settings. I did find some change , when i tick adsl with g.
0コメント