Very high-speed Digital Subscriber Lines (VDSL) John M. Cioffi Information Systems Laboratory Stanford, CA P: ; F:

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1 Very high-speed Digital Subscriber Lines (VDSL) John M. Cioffi Information Systems Laboratory Stanford, CA P: ; F: Very high-speed Digital Subscriber Lines (VDSL) is overviewed with an emphasis on the basic architecture, applications, and data rates, as well as the technological challenges of the design. A discussion of the telephone line environment, radio interference implications, impulse noise, symmetric and asymmetric multiplexing, and concentration is also included, with the consequent description of a popular implementation. 1. Introduction Very high-speed Digital Subscriber Lines (VDSL) is the latest transmission method for carrying highspeed digital service on twisted-pair phone lines. VDSL allows speeds from a few hundred kilobits per second on long phone lines to tens of megabits per second on shorter phone, depending on the length of the twisted pair. VDSL modems can be programmed to carry symmetric or asymmetric data rates over a variety of phone line types. This paper reviews the VDSL transmission problem and solution. Ordinary twisted-pair phone lines were not originally designed to carry high-speed digital signals. Nonetheless, it is theoretically possible to achieve high data rates on a phone line, even when all the physical impairments of the telephone loop plant are considered. However, the designs are not easy, and sophisticated signal processing methods are necessary for reliable transmission. These methods further need be implemented with low power and cost, mandating VLSI solutions for VDSL. This paper summarizes the impairments of the telephone line loop plant in Section 2, illustrating just how difficult telephone line transmission can be. Section 3 describes the SDMT VDSL transmission method and how it achieves the performance and cost/power requirements of VDSL. 2. VDSL Transmission Environment Figure 1 illustrates the phone-line transmission environment. Phone lines can run from a central office, or from an Optical Network Unit (ONU), to the customer. Telephone lines may share the same cable and typically are 24- or 26- gauge twisted pair. The twisted pair may have bridge taps, which are branches of Centra l VTU-O fiber VDSL twisted pair VTU-R ONU VTU-O VDSL twisted pair VTU-R Figure 1 Illustration of VDSL transmission line placement.

2 the twisted pair to other phones or service points. Normally, phone signals occupy the lower 4000 Hz, where the phone line has relatively mild transmission. On 3 to 4 mile phone lines, theoretical limits suggest that a few megabits per second of transmission capacity is possible. Figure 2(a) plots some standardized 3- mile phone line characteristics (see ANSI [1],[2]). Signals on such loops could use up to 1 MHz of bandwidth. The range of attenuation over this frequency band is from 20 db at low frequencies to over 100 db at higher frequencies, a range of 8 orders of magnitude. Such a large dynamic range of signal levels with frequency makes digital transmission very difficult, requiring precision levels in analog and digital hardware well-beyond those normally used in other digital transmission problems. The rippling of some of the curves is caused by bridge taps (the reflected and delayed signals from the tap may add constructively or destructively to unreflected signals, depending on frequency, thus the ripple ). Figure 2(b) shows a 1 km loop with a very common 10 m bridge tap, which causes a ripple of 6 orders of magnitude at 4 MHz in the passband of this channel. VDSL signals on such a channel should use frequencies to at least 10 MHz to perform well. The huge range of important signal levels in VDSL makes the design very challenging. Nominal VDSL transmit signal voltages may be a a few volts. With the attenuation shown, received sigal levels can be 10 s of microvolts. 0 ADSL Canonical Loops # 5, 6, 7, and 8 Insertion Loss 0 VDSL Line PSD (db) Insertion loss (db) (Hz) x (Hz) x 10 6 Figure 2(a) 3-5 mile phone lines Figure 2(b) 1 km phone line with 10m tap phone line 1 NEXT FEXT phone line 2 Figure 3 Illustration of Crosstalk. Figure 3 illustrates crosstalk. Crosstalk is noise on a phone line that is caused by electromagnetic radiation of other phone lines in close proximity, i.e., within the same cable. Such coupling increases with frequency and can be caused by signals traveling the opposite direction, called Near-End crosstalk or NEXT, and by signals traveling the same direction, called Far-End crosstalk or FEXT. If N is the number of crosstalking phone lines, f is frequency, d is length in feet, H(f) 2 is the channel insertion loss (transfer function - 6 db), and PSD disturber is the power spectral density the signals on a crosstalking line, then the accepted models for such noise in VDSL are:

3 ( ) f 1.5 PSD NEXT = PSD disturber N /49 PSD FEXT = PSD disturber H( f ) 2 N/49 ( ).6 ( ) d f 2 These noises can be very large, especially if the crosstalking signal is another VDSL signal at the same time and frequency. Radio noise also electromagnetically couples into phone lines. AM band radio signals are particularly strong and occupy 10 KHz wide frequency slots between 560 khz and 1600 khz. They appear like continuous narrow band interferers to VDSL signals. Amateur radio operators (HAMs) use 2.5 khz wide frequency slots intermittantly in the bands around 2 MHz, 3.5 MHz, 7 MHz, and 10 MHz. Such signals can couple into phone lines at levels as much as 1 milliwatt, db (7-9 orders of magnitude) higher than other noise signals on the line. They must be rejected by the VDSL receivers. The dual of radio noise is the electromagnetic radiation from phone lines carrying VDSL, which can cause very audible interference for radio receivers, particularly HAM receivers [3]. VDSL must avoid transmission in the known HAM bands. An audio tape of such interference will be played at the presentation of this paper, along with the solution described in Section 3 that eliminates the audible noise. Impulse noise is a temporary signal that can be narrowband or wideband and is essentially unpredictable in terms of its occurrence. Impulse noise can be caused by a variety of electronic and electro-mechanical devices. The impulse can be 10 s of millivolts in amplitude and can last as long as a few milliseconds. 3. SDMT VDSL Transmission SDMT combines two technologies, DMT for line transmission features and ping-pong for simple programmable symmetry level. Atten 4(a) TWISTED-PAIR 4(b) TWISTED-PAIR with TAP, AM/RF, and XTALK Atten AM xtalk Figure 4 Illustration of DMT, matching spectrum to the channel 3.1 DMT Transmission Figure 4 illustrates the basic Discrete MultiTone (DMT) transmission method, which is the only standardized method for DSLs [3] above 160 kbps. DMT uses a number (typically 256) of QAM signals, each modulated on a separate center frequency, and each with equal bandwidth. The tones are adjacent and precise receiver decoupling of tones is achieved through the use of the methods in [4] and [5]. The receiver measures the quality of each tone and then suggests to the transmitter a combination of energy and information to be carried on each tone. This message to the transmitter from the receiver is carried through a reliable low-speed control channel on a periodic basis in DMT. In Figure 4(a), a channel with transformer DC-notching and general attenuation is shown with corresponding resultant DMT information distribution on the right. The more attenuated higher frequencies carry less information with DMT. Figure 4(b) shows an example with bridge-tap notching, radio interference, and crosstalk noise. The resultant

4 information distribution then follows the channel, allocating the most information to the channel frequencies with highest signal-to-noise ratios. 3.2 Emission Masking A spectral mask that can be used in DMT to avoid transmission in frequency bands that correspond to amateur radio transmissions. A tape will be played at the presentation that shows the reduction in noise in a HAM receiver 30 meters away from a VDSL phone line when the emission masking is used. The frequencies in the nulled bands (which typically carry 20 db less energy than the non-nulled bands, in [1], - 60 dbm/hz and 80 dbm/hz respectively) can be programmed with a DMT transmission system. The QAM/CAP system in the tape cannot notch the same bands without serious equalization loss and increase in complexity, leading to a more expensive system that does not work as well, whence the high audible noise in the tape. 3.3 Impulse Noise Mitigation Figure 5 depicts an impulse. Traditional transmission methods exhibit impulse errors during those time periods when the impulse magnitude exceeds a threshold. DMT methods spread the impulse energy near equally over all frequencies, leading to a 24 db (for 256 tones) advantage in the threshold level. In practice, the spreading is not quite uniform and so the advantage is typically 10 db, depending on the impulse. amplitude errors with time-domain (AMI,2B1Q, QAM/CAP, VSB, amplitude no errors with DMT (AMI,2B1Q, (time) (freq) errors with time-domain Figure 5 Illustration of how DMT mitigates impulse noise. 3.4 Synchronized DMT (SDMT) Synchronized DMT (SDMT) layers time-domain ( ping-pong ) processing on DMT, while loop-timing all lines to the same network clock (so that lines in the same binder ping and pong at the same time to eliminate crosstalk). The 20-symbol frame is supported by 10 major telecom vendors [6] for VDSL standardization. Asymmetric transmission in ratio 8:1 is achieved by using 16 symbols downstream, 1 silent, 2 upstream, and 1 silent. All lines in the same cable are synchronized, thus avoiding NEXT. Symmetric transmission is achieved by the ratio 9:1:9:1 of down/off/up/off. Figure 6 illustrates these two most popular transmission formats. The 20-symbol frame typically requires 500 microseconds, corresponding to tones of width approximately 40 khz (if there are 256 of them). On longer loops, the tone width can be narrowed or more equalization can be used. Asymmetric DMT1 DMT2... DMT16 OFF UMT1 UMT2 OFF Symmetric DMT1 DMT DMT9 OFF UMT1 UMT2. UMT9 O. F Figure 6 Illustration of SDMT framing for asymmetric and symmetric transmission

5 4. Conclusion VDSL is an emerging standard for programmable symmetry levels and data rates in the area of xdsl. The transmission of digital data rates on phone lines over long distances or at high speeds requires a sophisticated adaptive transmission design. SDMT is such a design, which is being integrated by a number of major manufacturers into cost-effective low-power realizations. VDSL may be the long-awaited best answer to the x in xdsl. 5. References [1] American National Standard - T1.413, ADSL Metallic Interface Specification, 1995, New York, NY. [2] VDSL System Requirements Report, ANSI Document T1E1.4/97-133R1, June 1997, Chicago. [3] K. Foster and D. Standley, A preliminary experimental study of the RF emissions from dropwires carrying pseudo-vdsl signals and the subjective effect on a nearby amateur radio listener, ANSI Contribution T1E1.4/96-098, April 22, 1996, Colorado Springs, CO. [4] J.S. Chow and J.M. Cioffi, A Cost-Effective Maximum-Likelihood Receiver for Multicarrier Systems, IEEE ICC 92, June 16, 1992, Chicago, paper no [5] H.Y. Kwon and Y.S. Chun, Performance Projections of DMT and CAP/QAM VDSL on ANSI Test Loops, ANSI Contribution T1E1.4/96-107, (Source Kangwon U and Samsung), April 22, 1996, Colorado Springs, CO. [6] A. Ruiz, J.M. Cioffi, and S. Kasturia, Discrete Multiple-Tone Modulation with Coset Coding for the Spectrally Shaped Channel, IEEE Transactions on Communications, May 1992, pp