DMT Carrier Frequency Allocation in ADSL
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Posted by on December 10, 2006, 9:46 pm
Please log in for more thread options Hi, I'm quoting from Biju Sukumaran's DSL document: "During the initialisation phase of the setting up of an ADSL connection, the capacity of each carrier of the DMT is evaluated and the maximum number of bits that can be stuffed into each carrier is determined. The bits to be transmitted are then distributed across these carriers, such that the carrier with the best performance gets the most bits and the most noisy one gets the least, or no bits. This means that the high performance carriers, which are usually the low frequency ones, are very finely modulated to a maximum of 15 bits. This is done in real time as well, so that the bit rate remains unaffected by the time varying nature of the noise in the channel." My question now is, does the bit allocation of each frequency is determined only during initialization phase or does it change over time? In other words, after I have made the initial ADSL connection, does the bit allocations remain constant unless a resynchronization between ADSL modem and DSLAM is made? Thanks, ennui | ||||||||||||||||
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Posted by on December 10, 2006, 10:49 pm
Please log in for more thread options Hi, I'm quoting from Biju Sukumaran's DSL document: "During the initialisation phase of the setting up of an ADSL connection, the capacity of each carrier of the DMT is evaluated and the maximum number of bits that can be stuffed into each carrier is determined. The bits to be transmitted are then distributed across these carriers, such that the carrier with the best performance gets the most bits and the most noisy one gets the least, or no bits. This means that the high performance carriers, which are usually the low frequency ones, are very finely modulated to a maximum of 15 bits. This is done in real time as well, so that the bit rate remains unaffected by the time varying nature of the noise in the channel." My question now is, does the bit allocation of each frequency is determined only during initialization phase or does it change over time? In other words, after I have made the initial ADSL connection, does the bit allocations remain constant unless a resynchronization between ADSL modem and DSLAM is made? Thanks, ennui | ||||||||||||||||
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Posted by Ian Stirling on December 11, 2006, 4:04 am
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In uk.telecom.broadband en.nui@hotmail.com wrote: "This is done in real time as well..." seems clearly to me to say the latter. | ||||||||||||||||
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Posted by Magilla on December 11, 2006, 5:26 am
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Posted by Merlin on December 13, 2006, 5:16 pm
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> Hi,
> > > I'm quoting from Biju Sukumaran's DSL document: > > "During the initialisation phase of the setting up of an ADSL > connection, the capacity of each carrier of the DMT is evaluated and > the maximum number of bits that can be stuffed into each carrier is > determined. The bits to be transmitted are then distributed across > these carriers, such that the carrier with the best performance gets > the most bits and the most noisy one gets the least, oro bits. This > means that the high performance carriers, which are usually the low > frequency ones, are very finely modulated to a maximum of 15 bits. This > is done in real time as well, so that the bit rate remains unaffected > by the time varying nature of the noise in the channel." > > My question now is, does the bit allocation of each frequency is > determined only during initialization phase or does it change over > time? In other words, after I have made the initial ADSL connection, > does the bit allocations remain constant unless a resynchronization > between ADSL modem and DSLAM is made? > > Thanks, > ennui > This is done in real time via the Bitswap mechanism. If for some reason, noise at a particular frequency becomes to great for that tone to support the number of bit in that tone, then the bits will move from that tone to another tone where the noise margin is able to support more bits. This is only done if the DSLAM, CPE conbination support this. -- Merlin | ||||||||||||||||
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>
>
> I'm quoting from Biju Sukumaran's DSL document:
>
> "During the initialisation phase of the setting up of an ADSL
> connection, the capacity of each carrier of the DMT is evaluated and
> the maximum number of bits that can be stuffed into each carrier is
> determined. The bits to be transmitted are then distributed across
> these carriers, such that the carrier with the best performance gets
> the most bits and the most noisy one gets the least, or no bits. This
> means that the high performance carriers, which are usually the low
> frequency ones, are very finely modulated to a maximum of 15 bits.
> This is done in real time as well, so that the bit rate remains
> unaffected by the time varying nature of the noise in the channel."
>
> My question now is, does the bit allocation of each frequency is
> determined only during initialization phase or does it change over
> time? In other words, after I have made the initial ADSL connection,
> does the bit allocations remain constant unless a resynchronization
> between ADSL modem and DSLAM is made?