DMT Carrier Frequency Allocation in ADSL

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

"This is done in real time as well..." seems clearly to me to say the latter.

DQo8ZW4ubnVpQGhvdG1haWwuY29tPiB3cm90ZSBpbiBtZXNzYWdlIG5ld3M6MTE2NTgwODk4MC42 NDAxOTcuOTU0ODBAMTZnMjAwMGN3eS5nb29nbGVncm91cHMuY29tLi4uDQoNCj4gSGksDQogDQo+ IEknbSBxdW90aW5nIGZyb20gQmlqdSBTdWt1bWFyYW4ncyBEU0wgZG9jdW1lbnQ6DQogDQo+ICJE dXJpbmcgdGhlIGluaXRpYWxpc2F0aW9uIHBoYXNlIG9mIHRoZSBzZXR0aW5nIHVwIG9mIGFuIEFE U0wNCj4gY29ubmVjdGlvbiwgdGhlIGNhcGFjaXR5IG9mIGVhY2ggY2FycmllciBvZiB0aGUgRE1U IGlzIGV2YWx1YXRlZCBhbmQNCj4gdGhlIG1heGltdW0gbnVtYmVyIG9mIGJpdHMgdGhhdCBjYW4g YmUgc3R1ZmZlZCBpbnRvIGVhY2ggY2FycmllciBpcw0KPiBkZXRlcm1pbmVkLg0KDQpPdGhlcndp c2UgY2FsbGVkICdTaG93dGltZScuDQoNCj4gVGhlIGJpdHMgdG8gYmUgdHJhbnNtaXR0ZWQgYXJl IHRoZW4gZGlzdHJpYnV0ZWQgYWNyb3NzDQo+IHRoZXNlIGNhcnJpZXJzLCBzdWNoIHRoYXQgdGhl IGNhcnJpZXIgd2l0aCB0aGUgYmVzdCBwZXJmb3JtYW5jZSBnZXRzDQo+IHRoZSBtb3N0IGJpdHMg YW5kIHRoZSBtb3N0IG5vaXN5IG9uZSBnZXRzIHRoZSBsZWFzdCwgb3Igbm8gYml0cy4gVGhpcw0K PiBtZWFucyB0aGF0IHRoZSBoaWdoIHBlcmZvcm1hbmNlIGNhcnJpZXJzLCB3aGljaCBhcmUgdXN1 YWxseSB0aGUgbG93DQo+IGZyZXF1ZW5jeSBvbmVzLCBhcmUgdmVyeSBmaW5lbHkgbW9kdWxhdGVk IHRvIGEgbWF4aW11bSBvZiAxNSBiaXRzLg0KPiBUaGlzIGlzIGRvbmUgaW4gcmVhbCB0aW1lIGFz IHdlbGwsIHNvIHRoYXQgdGhlIGJpdCByYXRlIHJlbWFpbnMNCj4gdW5hZmZlY3RlZCBieSB0aGUg dGltZSB2YXJ5aW5nIG5hdHVyZSBvZiB0aGUgbm9pc2UgaW4gdGhlIGNoYW5uZWwuIg0KIA0KPiBN eSBxdWVzdGlvbiBub3cgaXMsIGRvZXMgdGhlIGJpdCBhbGxvY2F0aW9uIG9mIGVhY2ggZnJlcXVl bmN5IGlzDQo+IGRldGVybWluZWQgb25seSBkdXJpbmcgaW5pdGlhbGl6YXRpb24gcGhhc2Ugb3Ig ZG9lcyBpdCBjaGFuZ2Ugb3Zlcg0KPiB0aW1lPyBJbiBvdGhlciB3b3JkcywgYWZ0ZXIgSSBoYXZl IG1hZGUgdGhlIGluaXRpYWwgQURTTCBjb25uZWN0aW9uLA0KPiBkb2VzIHRoZSBiaXQgYWxsb2Nh dGlvbnMgcmVtYWluIGNvbnN0YW50IHVubGVzcyBhIHJlc3luY2hyb25pemF0aW9uDQo+IGJldHdl ZW4gQURTTCBtb2RlbSBhbmQgRFNMQU0gaXMgbWFkZT8NCg0KRGVwZW5kcyBpZiBiaXQgc3dhcHBp bmcgaXMgYWN0dWF0ZWQgb3Igbm90LiBJdCB1c3VhbGx5IGlzLg0KDQoNCi0tIA0KTWFnaWxsYQ0K

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

Hi,

Thanks for the information! :)

Ennui

Merl> > Hi,