Steve:
Thank you for describing interleaving. Here is a EEtimes article dated Jan 20th 1997....
"ADSL modems speed infobahn access
By John M. Cioffi, Chief Technology Officer, Amati Communications Corp., San Jose, Calif.
Fierce competition among providers of Internet-access services has fueled interest in asynchronous digital subscriber line (ADSL) to a state of near frenzy. Compared with existing Internet access through dial-up modems, ADSL offers a 100x to 500x increase in speed and provides simultaneous POTS, real-time video and Internet access.
Existing dial-up modems transmit at 14 to 28.8 kbits/second between a customer and the Internet, while ISDN modems transmit approximately four times faster. Most Internet users dislike the long delays in World Wide Web file access caused by the relatively slow speed of the voiceband or ISDN modem. A file of a few megabytes can take minutes to transfer, which can annoy and discourage a customer. ADSL trials with publicly reported results demonstrate full transfer at speeds to 8 Mbits/s, enabling the same transfer in just seconds.
Also, users of existing dial-up systems cannot use their telephones when accessing the Internet. ADSL preserves existing POTS while adding to it much faster ADSL access to the Internet. No new phones are necessary (unlike ISDN, which requires digital telephones), so customers get value-added ADSL service without disruption or replacement of their present, reliable POTS.
Simultaneous video is a standardized feature of ADSL. The feature lets several people in a single-phone-line household simultaneously use their phone service in the same way they always have, watch one or more television signals, and still have high-speed ADSL Internet access.
In the basic ADSL system, the telephone-company central office may connect phone lines to both Internet and video switches/routers through the ADSL access multiplexer.
The ADSL modems can connect the access mux selectively to the phone lines of those individual customers who have requested ADSL service (and only those who are willing to pay for such service). The connection is labor free, without any need for laying fiber or coax to the customer. The ADSL modems format the digital data into analog signals that can be carried on the phone line, using a process known as discrete multitone (DMT) modulation. The POTS splitter combines and separates the ADSL signals with normal phone signals present on the phone line without need for modification of that service or the equipment used to provide it. A similar splitter provides an identical function at the customer premises.
The remote ADSL modem converts the DMT signals back into digital bits and routes the various signals to such application/display devices as television sets and personal computers. Data rates of up to 8 Mbits/s flow to the customer; data rates of up to 800 kbits/s (or up to 1 Mbit/s) flow in the opposite direction. A user can still use an old modem through the POTS connection, if so desired.
The designers of ADSL originally anticipated that data transfers in the growing use of the Internet would be asymmetric. Typical down:up data-rate ratios of information packets in TCP/IP protocols are 10:1 or less. The down:up ratio in the worldwide ADSL standard is 8:1, thus meeting the needs for Internet service. The maximum downstream data rate is currently about 8 Mbits/s; the maximum upstream is about 1 Mbit/s. A minor revision to the ADSL standard appears likely to extend the upstream speed to 2 Mbits/s, with relatively minor performance/cost penalties.
Such asymmetry is nicely and intentionally matched to the Internet data transfer. The asymmetry also nearly doubles the distance the data rates can travel on a phone line compared with symmetric transmission, in which both directions would carry the same data rate.
ADSL's rate adaptation allows the variation of ADSL's data rate according to the quality of the phone line. The rate-adaptation concept is analogous to voiceband modems that connect at the highest possible data rate. ADSL's rate adaptation is far more robust via the use of an ADSL-specific invention of Stanford University that is now incorporated in the worldwide standard. Shorter phone lines (i.e., about 1.5 miles or less) exhibit less attenuation with increasing frequency, and are more immune to noise. Longer lines (1.5 to 5 miles) can carry only lower data rates (down to about 1.5 Mbits/s at 5 miles). The users of Internet service need not know the actual data rate but may be willing to pay for premium service that provides them the fastest service possible.
Premium service
A standard-compliant DMT modem computes information about the telephone line and reports the highest possible data rate to the network-maintenance center. The process repeats periodically. Thus, the service provider can allow those users who have paid for premium service (which may include video-entertainment or video-telephony services) to receive it when the line allows it. The network maintenance center records those lines that have limited capacity, providing highly accurate and up-to-date records of the service provider's largest asset-their installed base of copper phone lines.
The achievable data rate with DMT on a phone line depends on many complicated factors, including line length, number of other high-speed services within the vicinity, such noises as impulses or radio-frequency interference, and/or bridge taps (old, unconnected phone-extension ports on the same line)
Allowing for those factors, we realize downstream rates of approximately 8 Mbits/s at line distances to 1.5 miles; 6 Mbits/s to distance of 2.5 miles; 4 Mbits/s to distances of 3.5 miles; and 1.5 to 2 Mbits/s at distances of 3.5 to 5 miles.
The highest data rates-8 and 6 Mbits/s, up to 2 or 2.5 miles-allow super-fast Internet connectivity at nearly 500x voiceband-modem speeds, while simultaneously allowing some of that speed to be used for broadcast digital television-of higher quality than is available conventionally-or for receipt of several VCR-quality video-on-demand movies. At the longer line lengths (about 20 percent or less of the existing phone lines), only very fast Internet or VCR-quality video is possible.
The ANSI/ETSI standards committees have selected discrete multitone (DMT) for ADSL transmission. The reason for the single standard is to ensure interoperability and, thus, price competition between the various suppliers of phone equipment. Growing numbers of vendors have announced plans to supply standards-compliant DMT ADSL modems. Service providers throughout the world are committed to using, or have used, standardized DMT modems.
In DMT line coding, up to 256 4-kHz-wide "tones" are sent from the network to the customer; each tone can carry data rates comparable to voiceband modems (up to 60 kbits/s each). Each tone carries as much data as it can, which is determined adaptively by the DMT loading algorithm. Aggregate sum data rates can be as high as 8 to 9 Mbits/s in practice, depending on the quality of the tones. The DMT modem then reports this maximum data rate and connects at whatever data rate (below the maximum) the network maintenance center directs. Such transmission is theoretically optimum, and it avoids the expensive and decidedly suboptimal "equalizers" used in earlier transmission methods at the expense of the need for a channel-characterizing handshake between the transmitter and receiver.
Lines and slopes
With a relatively noise-free telephone line, the line attenuates transmitted tones, increasing with frequency-the longer the line, the greater the slope of the decrease in tone amplitude with frequency. DMT then finds those tones best-suited for transmission and uses them. At low frequencies, the POTS splitter prevents ADSL signals from traversing the line, so they are also then silenced by the loading protocol of DMT.
In a more typical phone line with a bridge tap, crosstalk noise (which decreases with frequency) and AM radio interference, DMT also adapts for the best results that can be achieved.
T1.413 ADSL DMT continuously updates the bit-loading of the tones to correspond to continuously measured tone quality. For fixed data rates, the DMT modem computes a distribution of information across the tones that maximizes performance at a given data rate.
Another major advantage of DMT is its well-known immunity to impulse noise. Impulses are temporary noises caused by a variety of electronic devices, such as refrigerators, hair dryers, switching equipment and such. DMT takes a typical impulse and smears its energy equally among the tones, allowing for maximum immunity. ADSL also adds forward error correction to DMT in a way that creates a robust performance in the presence of impulses-a characteristic that has been well-documented in lab tests.
DMT has been tested by several neutral testing facilities, including Nynex, Bellcore, GTE and NTSL, with publicly reported results. The 1993 results with DMT, in an "Olympics" held at Bellcore, showed DMT significantly outperforming QAM and CAP systems, which led to DMT's unanimous selection as a standard. A single group has continued to pursue a nonstandard CAP technology, but, at the time of this writing, some three years after they were promised, has been unable to provide modems at speeds above 1.5 to 2 Mbits/s (only 64 kbits/s upstream), despite a worldwide marketing campaign behind the technology. Test results show that the performance domination of DMT has increased, despite competitors' claims to the contrary. |
