We often take high-speed Internet service for granted. Today, it's easy to send e-mails with huge attachments, catch up on the latest music, or watch the hottest videos all instantly and online.
Yet these broadband connections aren't as prevalent in the U.S. as they are in some parts of Asia and Europe, despite the fact that the technology is here to make it happen. High capital-expenditure investments, ruthless competition, and self-serving government regulations have limited some broadband access.
Over the past several years, though, we've seen those logjams loosen as deregulation increases the number of people who can get affordable services. The driving force behind it all comes from the increased rollout of digital subscriber line (DSL), which commanded 47% of the U.S. broadband market in 2006 (see "DSL Takes Second Place In U.S. Broadband Wargames,").
A PHYSICS BREAKTHROUGH
Thanks to continuous development, innovative process technology, and some great new DSP algorithms, DSL can transmit data over a plain-old-telephone-system (POTS), twisted-pair telephone line designed for up to 4-kHz voice at 100 Mbits/s up to several thousand feet. And DSL speed continues to incrementally rise as new standards and chips arrive.
DSL transmits data through discrete multitone (DMT), a wired version of orthogonal frequency-division multiplexing (OFDM) that's widely used in wireless standards like Wi-Fi and WiMAX. The data signal to be transmitted is divided up into many parallel low-speed data paths. Those paths are modulated on hundreds or thousands of adjacent but orthogonal carriers over a broad spectrum.
Such a feat is achieved by implementing an inverse fast Fourier transform (IFFT) with DSP at the transmitting end. The resulting wideband signal then is put on the twisted-pair line. A receiver at the other end uses FFT to recover and rejuvenate the data into its original fast serial form.
The medium itself is simple twisted-pair telephone cable made with #24 or #26 gauge copper wire. Generally known as a local loop, that wire connects the telephone to a telephone central office (CO). It can be from a few thousand feet long to over 20,000 feet in some rural areas. Typical lengths measure 5000 to 18,000 feet. Just think of the resistance and capacitance on that line—talk about your big distributed low-pass filter!
Some of that cable has been underground or strung overhead for decades. Unfortunately, its attachments are a bane to DMT. Loading coils that extend voice service to rural areas are a real no-no.
Also, there are quite a few bridge taps—unused and unterminated extensions to the local loop—that act like transmission line stubs. These produce lossy segments in the spectrum at the frequency where the stub is a quarter-wavelength long. Crosstalk from adjacent lines makes transmission even more difficult.
The U.S.'s spectrum for DSL is divided into segments for downstream (carrier to modem) and upstream (modem to carrier) to minimize interference (Fig. 1). All kinds of fancy equalization, crosstalk minimization, and echo-cancellation techniques help move the DMT signals over the longest possible distance and yet be recovered at the receiver.
The original asynchronous DSL (ADSL) used 256 channels (bins or tones) that were 4.3125 kHz wide up to 1.1 MHz to achieve a downstream rate reaching 8 Mbits/s. The upstream employs up to 31 carriers that can deliver a 384- to 768-kbit/s data rate. ADSL2+ uses 256 additional channels to hit speeds that are 24 Mbits/s over the shorter reaches.
The real limiting factor is the length of the local loop. Short loops provide the highest, most reliable data rate, while long loops make it tougher. Since each subscriber has a different length, maximum speeds vary widely. Wire size also makes a difference, with the larger #24 gauge offering higher rates over longer distances than the usual #26. But the extra cost of larger wire becomes a major factor.
In the early days of DSL, a local loop longer than about 10,000 feet automatically disqualified you from DSL service. Telcos expanded by installing DSL access multiplexers (DSLAMs) or gateways in selected neighborhoods. These big boxes of line cards are connected back to the CO by a fiber link, but they also connect to thousands of new subscribers within the 18,000-foot range of most DSLAMs. The DSLAMs aggregate all individual lines and connect to the Internet back at the central office.
There are several versions of ADSL, with differing rates and ranges (see the table). The original, G.Lite, brought a maximum of 1.5 Mbits/s if you were lucky—or very close to the CO. Later versions boosted speeds and ranges.
ADSL2 and ADSL2+ improved data rates at longer distances. They're now being rolled out in many parts of the U.S., increasing the number of potential subscribers while giving present subscribers faster service. Though ADSL2+ is being widely deployed in upgrades, it isn't the best standard for Internet Protocol TV (IPTV). It can manage one HDTV channel over short loops, but that's it. To the rescue comes next-generation VDSL2.
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