ANSI T1.413 Issue 2
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ADSL
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ADSL2
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| ADSL2+ |
ANSI T1.413 defines the requirements for the single Asymmetric Digital Subscriber Line (ADSL) for the interface between the telecommunications network and the customer installation in terms of their interaction and electrical characteristics. ADSL allows the provision of voiceband services (including POTS and data services up to 56 kbit/s) and a variety of digital channels. In the direction from the network to the customer premises, the digital bearer channels may consist of full-duplex low-speed bearer channels and simpler high-speed bearer channels; in the other direction, only low-speed bearer channels are provided.
The American National Standards Institute (ANSI) Telecommunications Committee created the first standardized ADSL specification. It defines the minimum requirements for satisfactory performance of ADSL systems using the Discrete Multi-Tone (DMT) line code. DMT divides the useful bandwidth of the standard two wire copper medium used in the PSTN, which is 0 to 1104 kHz, into 256 separate 4.3125 kHz wide bins called sub-carriers.
Up to 254 sub-carriers are used; each of these 254 sub-carriers can support the modulation of 0 to 15 bits. The data frame rate (baud rate) is 4,000 frames per second, the maximum theoretical downstream data rate of an ADSL system is 15.24 Mbit/s. However, because the data is split up into packets (actually Reed-Solomon encoded codewords) of 255 bytes, the maximum achievable downstream data rate is 8.128 Mbit/s (including other overheads). It is possible to interleave two Reed-Solomon codewords and obtain one logical codeword of 510 bytes. If this is done, then the maximum theoretical download speed goes back up to around 15 Mbit/s.
In the upstream direction, a maximum of 30 sub-carriers can be used, again each frame modulated with up to 15 bits. Taken with the frame rate of 4,000 per second, the maximum throughput is a just over 1.5 Mbit/s.
In order to combine the PSTN service with download and upload ADSL signals the bandwidth is split into discrete parts using frequency-division multiplexing (FDM). In this case: 0–4 kHz is POTS, 26–138 kHz is the upload band and 138–1100 kHz is the download band.
