Asymmetric Digital Subscriber Line 2
The Asymmetric Digital Subscriber Line 2 (ADSL2) family of standards increases performance over standard ADSL. The ADSL2 G.992.3 (full-rate ADSL) and G.992.4 (splitterless ADSL) standards come from the International Telecommunications Union (ITU) and were approved in the Fall of 2002.
The performance increase takes the form of increased speeds and loop lengths plus the ability to do dynamic rate adaptation and diagnostics. With ADSL the maximum downstream rate is between 8 Mbps and 10 Mbps depending on a number of factors, most notably loop length. ADSL2 has a maximum downstream rate of 12 Mbps while at the same time extending the loop length by roughly 600 feet beyond that offered by ADSL. Furthermore, ADSL2 is backward compatible with the previous ADSL standards so it will run both ADSL and ADSL2. This is essential for service providers who need to recoup their investment in existing equipment while being able to upgrade to near technologies.
The reason ADSL2 can increase its rate and reach over ADSL is primarily due to improved modulation efficiency, reduced framing overhead, faster initialization, and better signal processing algorithms.
ADSL2 equipment provides for the collection of diagnostic information such as loop attenuation, noise, and signal-to-noise ratio. This information can be collected by test equipment when the line is put into a special diagnostic and testing mode. Furthermore, ADSL2 allows for real-time performance monitoring which could be used for troubleshooting problems, preventative maintenance, or determining whether higher data rates could be provisioned for the customer.
ADSL2 allows for real-time rate adaptation, called seamless rate adaptation (SRA), meaning that the line rate can change during operation based upon line quality.
ADSL2 supports bonding of multiple physical layers to achieve higher data rates. To accomplish this the standard specifies the use of the ATM Forum's inverse multiplexing for ATM (IMA). So ADSL2 can combine two or more copper pairs and treat them as a single channel to create higher data rates.
One last interesting enhancement is that ADSL2 can operate in what is called All-Digital Mode meaning that it can use the bandwidth that is normally allocated to analog voice for upstream data. This would increase the upstream rate by about 256 kbps and could be used in customer environments where they do not use the analog voice channel.
ADSL2+ is ITU standard G.992.5 and was approved in January 2003. It more than doubles the data rate of ADSL2 by providing a maximum of 25Mbps downstream on loops as long as 5,000 feet. It accomplishes this by increasing the spectrum used from 1.1 MHz to 2.2 MHz. ADSL2+ can also bond channels in the same way ADSL2 does to achieve even higher data rates. In Annex M, an option for shifting the upstream/downstream frequency split from the normal 138kHz to 276kHz is included. This makes it possible for upstream bandwidth to be increased from the normal 1 Mbps limit to as much as 3.5 Mbps.
The main application for ADSL2+ will be in provisioning voice, video, and data services to customers serviced by a digital loop carrier (DLC) system. In other words, ADSL2+ would be used as the local loop in fiber-to-the-node (FTTN) or fiber-to-the-curb (FTTC) applications.
Specified in Annex L G.992.5, RE/ADSL or Reach Extended ADSL2 increases loop length to about 23,000 feet by increasing power. It was approved by the ITU in October of 2003. Also know as ADSL2 Annex L, it defines an increased power spectral density (PSD) that allows for the longer loop lengths while at the same time being compatible with the ANSI Standard T1.417 that specifies spectrum compatibility requirements in North America.
This increase in reach is about a 20% increase in coverage area which equates to a carriers ability to offer service to new customers who previously did not qualify due to their loop length being longer than around 18K ft.
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