The graphs on the right summarise the speeds obtainable for each ADSL standard based on line length and attenuation. The second graph is of more importance since it is attenuation which is the governing factor for line speed because attenuation rate over distance can vary significantly between various copper lines due to their quality and other factors. ADSL2 is able to extend the reach of extremely long lines that have around 90 dB attenuation. Standard ADSL is only able to provide a service on lines with an attenuation no greater than about 75 dB.
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This Recommendation provides a variety of bearer channels in conjunction with one of three other services dependent on the environment:1 ADSL transmission simultaneously on the same pair with voice band service;2 ADSL transmission simultaneously on the same pair with G. This Recommendation has been written to help ensure the proper interfacing and interworking of ADSL transmission units at the customer end ATU-R and at the network operator end ATU-C and also to define the transport capability of the units.
Proper operation shall be ensured when these two units are manufactured and provided independently. The ADSL transmission units must deal with a variety of wire pair characteristics and typical impairments e. An ADSL transmission unit can simultaneously convey all of the following: downstream simplex bearers, duplex bearers, a baseband analogue duplex channel, and ADSL line overhead for framing, error control, operations and maintenance.
Systems support a minimum of 6. This Recommendation includes mandatory requirements, recommendations and options; these are designated by the words "shall", "should" and "may" respectively.
The word "will" is used only to designate events that take place under some defined set of circumstances. Two categories of performance are specified. Category I performance is required for compliance with this Recommendation; performance enhancement options are not required for category I equipment.
Category II is a higher level of performance i. Category II performance and characteristics are not required for compliance with this Recommendation.