DSL Dominates Broadband Worldwide

March 29, 2007
Surprisingly, cable still edges out DSL in the U.S. But more carriers are turning to DSL as they move to IPTV and the triple play.

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,").

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.

Also known as very high data-rate DSL 2 or ITU standard G.993.2, VDSL2 is the latest and perhaps greatest version of DSL. Though approved back in mid-2005, only lately have chips and other products become available. Adoptions are just beginning, but you can expect it to reign as the great new broadband technology.

VDSL2 boosts the data rate up to where it can compete with fiber in certain applications. Several versions or profiles of the standard extend the reach and push the data rate over the older VDSL(1) version, which used QAM instead of DMT. It does this by greatly extending the bandwidth of the DMT signal on the cable (Fig. 2). It also uses a larger number of channels for both upstream and downstream.

ITU Profiles 8a and 12a give VDSL1-like speeds to up to 55 Mbits/s downstream. The longer-reach profile 17a version uses spectrum up to 17 MHz, while the short-reach profile 30a version extends the bandwidth to 30 MHz utilizing up to 4096 bins or channels. Profile 30a employs twice the bandwidth for each channel (8.625 kHz) to boost data rates to 100 Mbits/s symmetrical.

The really good news is that all VDSL2 versions are fully backward-compatible with most other versions of DSL, which simplifies upgrades and equipment adaptation. Many new products will eventually adopt the VDSL2 standard. For now, ADSL2 and ADSL2+ maintain the focus of most network operators who are still upgrading from plain old DSL.

VDSL2's first usage will come in multi-dwelling units with closely clustered subscribers, such as apartment buildings, condos, and townhouses. Carriers will run fiber to those establishments and then use VDSL2 over the installed base of telephone cable to get the signals to the subscribers. VDSL2 also will be the choice of IPTV distributors that run fiber to the neighborhood DSLAM and then distribute to subscribers over their local loops.

Infineon Technologies' Amazon PSB 50510 ADSL2+ chip set suits customer premise equipment (CPE) like modems, home gateways, and routers. It complies with all major DSL standards through the latest ADSL2+ upgrades. So, it can support up to 24 Mbits/s downstream and 2 Mbits/s upstream.

The PSB 50510 comes with 10/100 Ethernet, UART, SPI, PCI, CardBus, and JTAG interfaces. It's a good match with Infineon's GEMINAX MAX PEF 55016 and PEF 55017 central-office chips, which support all ADSL2+ features.

The Infineon VINAX chips were among the first products to fully support the most recent VDSL2 and ADSL2+ standards. They offer long-range loop reach and ultra-high data rates over short loops. Their sophisticated quality-of-service features include dial latency, dual interleaving, and pre-emption, all of which are needed to meet the rigorous demands of the triple play—VoIP, IPTV, and Internet access.

On the CO side, the two-chip set provides for line cards with 48 full-rate VDSL2 ports or remote terminal applications. On the CPE side, a single-chip solution addresses the need for both simple bridge models or full-access gateways.

Imran Hajimusa, senior director of marketing for Infineon's COM wireline business unit, says his company's VDSL2 products are incorporated in one of the new standard's first rollouts. Hyundai Network Systems of Korea will supply VINAX-based VDSL2 systems to a major Korean telecom carrier.

The primary market driver is IPTV and HDTV. The short-reach (350 m) version of VDSL2 is ideal for Korea's high-density living arrangements. It also can deliver full 100-Mbit/s symmetrical service, for which the chips were designed. Germany's Deutsche Telekom uses the new chips in its hyperspeed communications and entertainment services network as well.

Texas Instruments' UR8 DSL residential gateway chip set exemplifies the latest trends and features. Designed for home gateway boxes, the chips cover all of the latest DSL offerings, including the newest VDSL2 standard. They feature a multimedia gateway processor, a programmable DSL physical layer (PHY), a high-performance DSP-based voice subsystem, and an extended set of local-area-network (LAN) interfaces. A well-defined application program interface (API) reduces time-to-market by enabling hardware and software reuse across all DSL platforms.

The UR8 features four Ethernet 10/100 ports (Fig. 3). A built-in SLAC/SLIC handles two VoIP phones. The serial peripheral interface (SPI) and time-division-multiplexing (TDM) ports connect to the Voice over Internet Protocol (VoIP) processor. There are home-networking ports for a wireless LAN (WLAN), Multimedia over Coax Alliance (MoCA), HomePlug, or Ethernet, plus a USB port. And, the DSL PHY is implemented on the programmable DSP.

Five chips make up the UR8 family. The TNETD8430 (VDSL2) and TNETD7530 (ADSL2+) don't support voice, while the TNETD8431 (VDSL2), TNETD7531, and TNETD7501 (ADSL2+) do. TI's Telogy and PIQUA extensive VoIP software support voice operations.

As for end equipment, 2Wire makes a range of modems and gateways for DSL systems. DSL carriers still supply most home gateways, but some can be obtained retail. Also, 2Wire's 3000 series residential gateway is VDSL2-capable (Fig. 4).

In addition to Ethernet or a USB port to a PC, the 3000 offers home-networking capability (e.g., 802.11g wireless). Some units also feature VoIP ports and interfaces for MoCA or HPNAv3 home-networking systems. Furthermore, 2Wire's iNID 3800 gateway targets outside installation. The company's gateways are widely used in AT&T/Yahoo systems throughout the U.S.

A recent amendment to the ITU 993.2 VDSL2 standard caught the attention of new company Aktino. The change enhances VDSL2's functionality by giving it newly defined flexible band plans. These will allow the modem to be configured for either asymmetric or symmetric rate services.

Michail Tsatsanis, chief scientist at Aktino, says that this revision positions VDSL2 as the optimal means for converging both residential- and business-access networks equipment into unified platforms. In turn, this change can greatly simplify the CO side of the system and generate sizable savings.

Aktino builds carrier-end networking equipment that implements the recent amendment. The company also has developed a system that applies wireless MIMO processing techniques to DMT. The primary performance barrier, other than loop distance, is crosstalk.

Both near-end crosstalk (NEXT) and far-end crosstalk (FEXT) on twisted pairs limit range and speed. If access is available to multiple pairs in a cable, the MIMO multichannel processing techniques can substantially improve all aspects of performance. The AK3000, AK4000, and AK5000 series of CO and remote platforms implement this extended VSDL2 system with MIMO.

Finally, ZyXEL's VOP1248G targets the company's widely used IES5000 DSLAM (Fig. 5). This POTS/VoIP line card eases the carrier's transition to VoIP networks by eliminating the need to install special VoIP phones or an analog telephone adapter (ATA) on customer premises. The card's media gateway converts analog voice to VoIP, so service providers can offer next-generation voice services while saving costs. The IES5000 DSLAM is a multiservice access platform with a nonblocking Ethernet backplane that supports ADSL2+, G.SHDSL, and VDSL2.

For more, see "Check Your Broadband Speed".

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