xDSL Signal Encoding Efficiency (January 2008) Delaportas George
[email protected]
Abstract—Digital Subscriber Line is a data communications technology that transmits data faster over the simple copper telephone lines. The signal encoding of xDSL technology is illustrated by a set of modulation techniques. This journal will overview the xDSL technology and will discuss subjects on the efficiency of these modulation techniques. Index Terms—DSL, DSP, signal analysis, modulation, encoding, digital signal, analog signal, transmission, noise.
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I.INTRODUCTION
HIS journal will technically describe and analyze the techniques of the variations of DSL (Digital Subscriber Line) technology and their efficiency according to their encoding schemes. It will also show that the current advances of modulation techniques used on xDSL technologies offer more bandwidth while at the same time limit the Bit Error Rate (BER) in a transmitted byte stream. DSL technology has many forms and the encoding schemes for each modulation vary according to the form and the needs. DSL is a relatively new technology but it offered a lot in a small amount of time to the public as it is not very expensive. The next sections will analyze more on the xDSL technology, the requirements, the benefits and the problems.
II.TRANSMISSION LINE FUNDAMENTALS A transmission line usually can be either a wire or the air. All xDSL technologies work well on wires but recent advances as 3G telecommunication networks made xDSL technology available everywhere. The most common wired transmission lines are the unshielded twisted pair or the shielded twisted pair, the coaxial cable and the optical fiber. We usually call these lines as mediums. Early xDSL technologies were developed to work on top of simple telecommunication networks (PSTN) and so they use a simple unshielded twisted pair to transfer data. Each line suffers from a variety of Manuscript received November 29 2007. This work was supported in part by the U.S. Department of Commerce under Grant BS123456. F. A. Delaportas George is with the National Institute of Standards and Technology, Boulder, CO 80305 USA (Delaportas George, phone: +30-210-4221336; fax: +30-210-4221336; e-mail: g.delaportas@ gmail.com).
physical or technical problems. The most common problem in any medium is the attenuation, where the signals power degrades by the distance in time. Attenuation differs among different mediums. Another big problem is noise. Noise is a physical problem and exists everywhere. There are many types of noise but the most common are Gaussian (thermal noise) and white noise. Because noise exists everywhere a signal to noise ration (SNR) is required as a criterion to a lines’ capability to transfer data over a distance. There are technical problems such as input/output impedance of the line or intersymbol interference that sometimes are difficult to solve. The worst is that they depend on the physical characteristics of the line or to global constants such as propagation of speed of light. To solve the above or many more other transmission problems the engineers have found ways to modulate the signals and ways to handle errors. These solutions will be further analyzed based on the encoding techniques that xDSL technology uses in the next chapters [3], [5].
III.XDSL TECHNOLOGY OVERVIEW A.General Information The DSL is a data communications technology that enables faster data transmission over the copper telephone lines. Anyone can use an xDSL as long as having a telephone line, near to the local loop and not further than 3Km, and has an xDSL modem. The DSL is many times faster than a conventional voice band modem. DSL does that by utilizing frequencies that are not used by a simple voice telephone [1]. At the telephone exchange the line generally terminates at a DS-LAM (Digital Subscriber – Local Area Multiplexer) where another frequency splitter separates the voice band signal for the conventional phone network. Data carried by the DSL is typically routed over the telephone company's data network and eventually reaches a conventional Internet network. Finally the IP datagrams are routed through the Internet network via the ATM switches [1], [2]. B.xDSL Technical Issues There are many forms of the DSL technology. The most widespread are:
1. ADSL (Asymmetric DSL) 2. SDSL (Symmetric DSL) 3. HDSL (High-speed DSL or High bit rate DSL) 4. VDSL (Very High Speed DSL) The above, have other variations, combinations and advances such as SHDL (Symmetric High-speed DSL), MSDSL (Multirate Symmetric DSL), ADSL-2, and HDSL-2 [4], [9]. A typical DSL circuit is Full-Duplex. This means that it can transmit and receive at the same time. Full duplex xDSL communication is usually achieved on a wire pair by either FDM (Frequency Division Multiplexing) or TDM (Time Division Multiplexing). FDM uses two separate frequency bands, the upstream and the downstream bands. The upstream band is used for communication from the end user to the telephone central office. The downstream band is used for communicating from the central office to the end user. According to ITU G.992.5 (ADSL2+), the band from 25.875 kHz to 138 kHz is used for upstream communication, while 138 kHz – 12000 kHz is used for downstream communication (Fig. 1). Each of these is further divided into smaller frequency channels of 4.3125 kHz. During initial connection, the ADSL modem tests which of the available channels have an acceptable signal-to-noise ratio. The distance from the telephone exchange, noise on the copper wire or interference from AM radio stations may introduce errors on some frequencies. So, by keeping the channels small, a high bit error rate on one frequency will not reduce the total throughput of the ADSL connection. Finally, there is a direct relationship between the number of channels available and the throughput capacity of the ADSL connection. The exact data capacity per channel depends on the modulation method used [4] – [9]. High bit rate Digital Subscriber Line (HDSL) was the first DSL technology that used a higher frequency spectrum of copper, twisted pair cables. HDSL was developed in the USA, as a better technology for high-speed, synchronous circuits typically used to interconnect local exchange carrier systems, and also to carry high-speed corporate data links and voice channels, using T1 lines. American T-carrier circuits operate at 1.544 Mbit/s. These circuits were originally carried using a line code called Alternate Mark Inversion (AMI). Later the line code used was B8ZS. AMI did not have sufficient range, requiring the application of repeaters over long circuits. As with any wire circuit, they were subject to lightning and cable trouble such as inferior splices and backhoe fade. In troubleshooting these type of services, the *felt* frequency on each conductor is 772 Hz and the repeaters are usually spaced every mile to 1.2 miles depending on conductor gauge and the whim of the engineers. As in classical T-carrier, HDSL has a positive and negative polarity to the side of the repeater. In splicing this type of service the telcos placed the low voltage side of the repeater cable together and then the High voltage side together in the splice. The telcos have a powering end to the circuit path and this gives the polarity and the repeaters are typically powered up to 130 volts dc. Usually if you see 130 volts there is trouble because the repeaters are running FULL power to try to compensate for the trouble. They require 60
milliamps and if they cannot get it they try to achieve it by raising the voltage. The first attempts to use DSL technology to solve the problem were done in the USA, using the line code 2B1Q. This modulation allowed for a 784 Kbit/s data rate over a single twisted pair cable. With two twisted pair cables, the full 1.544 Mbit/s was achieved. The new technology attracted the attention of the industry, but could not be directly used worldwide, due to the differences between the T1 and E1 standards [7], [9]. A new standard was then developed by the ITU for HDSL, using the CAP (Carrierless Amplitude Phase Modulation) line code that reached the maximum bandwidth of 2.0 Mbit/s using two pairs of copper. HDSL gave the telcos a greater distance reach when delivering a T-1 circuit. It was marketed originally as a Non Repeated T-1, with a distance of 12k feet over 24 gauge cable. The cable gauge affects the distance. To allow for longer distances, a repeater can be used. The repeater actually terminates the circuit and regenerates the signal. Up to four repeaters can be used for a reach of 60k feet (about 20 km). This reduced the cost of maintenance when compared with AMI-based repeaters that had to be used at every 35 db of attenuation (about 1 mile). HDSL can be used either at the T1 (American) rate of 1.544 Mbit/s or the E1 (European) rate of 2 Mbit/s. Slower speeds are obtained by using multiples of 64 Kbit/s channels, inside the T1/E1 frame. This is usually known as channelized T1/E1, and it's used to provide slow-speed data links to customers. In this case, the line rate is still the full T1/E1 rate, but the customer only gets the limited (64 multiple) data rate over the local serial interface. HDSL gave way to two new technologies, called HDSL2 and SDSL. HDSL-2 offers the same data rate over a single pair of copper; it also offers longer reach, and can work over copper of lower gauge or quality. SDSL is a multi-rate technology, offering speeds ranging from 192 Kbit/s to 2.3 Mbit/s, using a single pair of copper. SDSL is used as a replacement (and in some cases, as a generic designation) for the entire HDSL family of protocols [1] – [3], [9]. The ITU G.993.2 (VDSL-2) on the other hand utilizes bandwidth of up to 30 MHz to provide data rates exceeding 100 Mbit/s simultaneously in both the upstream and downstream directions. The maximum available bit rate is achieved at a range of about 300 meters and performance degrades as the loop attenuation increases. First generation of VDSL standard specified both QAM (Quadrature amplitude modulation) and DMT (Discrete Multi-Tone modulation). In 2006, ITU-T standardized VDSL in recommendation G.993.2 which specified only DMT modulation for VDSL-2 [9]. VDSL-2 is an access technology that exploits the existing infrastructure of copper wires that were originally deployed for POTS (Plain Old Telephone Services). It can be deployed from central offices, from fiber-fed cabinets located near the customer premises, or within buildings. It is the newest and most advanced standard of DSL broadband wire line communications. Designed to support the wide deployment of Triple Play services such as voice, video, data, high definition television (HDTV) and interactive gaming, VDSL-2 enables
operators and carriers to gradually, flexibly, and cost efficiently upgrade existing xDSL infrastructures. VDSL-2 deteriorates quickly from a theoretical maximum of 250 Mbit/s at 'source' to 100 Mbit/s at 0.5 km (1640 ft) and 50 Mbit/s at 1 km (3280 ft), but degrades at a much slower rate from there, and still outperforms VDSL. Starting from 1.6 km its performance is equal to ADSL2+ (Fig. 2) [1] – [4].
Fig. 1. The ADSL2+ Spectrum. The ADSL2+ technology uses a wider range of frequencies to achieve better performance. The downstream bandwidth can now reach the 24Mbit/s. The upstream is still limited at 1Mbit/s.
1024 Kbit/s upstream. DMT is used widely today as it is the de facto xDSL encoding and it will be briefly analyzed on the next section [5], [6].
Fig. 2. Comparison of the ADSL variants. Each ADSL has a different attenuation over the distance. ADSL-2+ transmittion degrades earlier because it transmits more data than ADSL and ADSL-2.
IV.XDSL MODULATION TECHNIQUES The xDSL technology uses mainly 2 modulation techniques to transmit digital data over analog signals. The first and oldest one is Carrierless amplitude phase modulation (CAP) and the newest one is Discrete Multi-Tone (DMT). The International Telecommunication Union (ITU) later introduced a new revised standard of DMT, called G.DMT, for more efficient xDSL signal modulation and therefore more bandwidth [9]. CAP is a non-standard variation of Quadrature Amplitude Modulation (QAM). Instead of modulating the amplitude of two carrier waves, CAP generates QAM signal by combining two PAM signals filtered through two filters designed so that their impulse responses form a Hilbert pair. CAP used for ADSL divides the available space into three bands. The range from 0 to 4 kHz is allocated for POTS transmissions. The range of 25 kHz to 160 kHz is allocated for upstream data traffic and the range of 240 kHz to 1.5 MHz is allocated for downstream data traffic. CAP was the de facto standard for ADSL deployments up until 1996, deployed in 90 percent of ADSL installs. Now it is deprecated in favor of Discrete MultiTone Modulation (DMT), but it is still used for some variants of HDSL [4] - [6]. Discrete Multi-Tone (DMT) describes a version of multicarrier DSL modulation in which incoming data is collected and then distributed over a large number of small individual carriers, each of which uses a form of QAM modulation. DMT creates these channels using a digital technique known as Discrete Fast-Fourier Transform. By varying the number of bits per symbol within a channel, DMT carriers can also be rate-adaptive. Both ITU 992.1 (G.DMT) and ITU 992.2 (G.LITE) use a form of DMT modulation for data transmission. The DMT Technical specification is outlined ANSI Standard T1.413, and it is capable of transmission speeds of up to 8064 Kbit/s downstream and
V.TECHNICAL ANALYSIS OF G.DMT Discrete Multi-Tone (DMT) is the most widely used modulation method. DMT together with a set of other techniques can transmit lots of data in a matter of seconds. DMT separates the ADSL signal into 255 carriers, called bins and are centered on multiples of 4.3125 kHz. DMT has 224 downstream frequency bins and up to 31 upstream bins. Bin 0 is at DC and is not used. When voice (POTS) is used on the same line, then bin 7 is the lowest bin used for ADSL. The center frequency of bin N is (N x 4.3125) kHz. The spectrum of each bin overlaps that of its neighbors. The orthogonality of COFDM (Specialized FDM) makes this possible without interference. Up to 15 bits per symbol can be encoded on each bin on a good quality line [6], [9]. The frequency layout can be summarized as: 1. 0-4 kHz, voice. 2. 4-25 kHz, unused guard band. 3. 25-138 kHz, 25 upstream bins (7-31). 4. 138-1107 kHz, 224 downstream bins (32-255). Typically, a few bins around 31-32 are not used in order to prevent interference between upstream and downstream bins either side of 138 kHz. These unused bins constitute a guard band to be chosen by each DS-LAM manufacturer [7], [9]. The use of bins produces a transmission system which exhibits a form of Frequency Division Multiplexing (FDM) known as Coded Orthogonal FDM (COFDM) [5], [6]. In the context of G.992.1, the term "Discrete MultiTone" (DMT) is used instead, hence the alternative name of the standard, G.DMT. Using DMT is useful since it allows the communications equipment (modem/router and DSLAM) to select only bins which are usable on the line thus effectively obtaining the best overall bit rate from the line at any given moment in time (Fig. 3). With COFDM, a
combined signal containing many frequencies (for each bin) is transmitted down the line. Fast Fourier Transform (and the Inverse-FFT) is used to convert the signal on the line into the individual bins [9]. A type of QAM or Phase Shift Keying (PSK) is used to encode the bits within each bin. This is a complex and mathematical subject and will not be discussed further here. However, much research has been done on these modulation techniques and they are used for transmission because they allow the Signal to Noise Ration (SNR) to be improved, thus beating the noise floor and enabling more reliable transmission of a signal with fewer errors. The gain obtainable above the noise floor can be anything from 0.5-1.5 dB and these small amounts make a vast difference when sending signals over long distance copper lines of 6 km or more [8], [9]. The quality of the line at the frequency of the bin determines how many bits can be encoded within that bin. As with all the transmission lines, the maximum capacity depends on the attenuation and signal-to-noise ratio [8]. SNR may differ for each bin and this plays an important factor for deciding how many bits can be encoded reliably on it. Generally speaking, 1 bit can be encoded reliably for each 3 dB of available dynamic range above the noise floor within a transmission medium so, for example, a bin with a SNR of 18 dB would be able to accommodate 6 bits [9]. Furthermore, echo cancellation can be used so the downstream channel overlaps the upstream channel, or vice versa, meaning simultaneous upstream and downstream signals are sent. Echo cancellation is optional and is typically not used [8]. In DMT, up to 15 bits may be assigned to each channel. The default is 2, but as channels suffer interference and attenuation, those bits are swapped onto other channels. If bit swapping is disabled then this does not happen and the line rate suffers. There are 2 competing standards for DMT ADSL - ANSI & DMT; ANSI T1.413 is the North American standard, G.992.1 (DMT) is the ITU (United Nations Telecom committee) standard. There is a difference in framing between the two, and selecting the wrong standard can cause frame alignment errors every 5 or so minutes. Error correction is done using Reed-Solomon encoding and further protection can be used if Trellis encoding is used at both ends. Interleaving can also increase the robustness of the line but increases latency [9].
Fig. 3. ADSL BIN - bits. Each ADSL BIN – Channel encodes different bits according to the SNR and the attenuation at the given time.
VI.CONCLUSIONS The needs of modern communication networks have lead to clearly abandon any obsolete low bit rate encoding techniques. The G.DMT encoding scheme is the most suitable and currently covers all the needs in low BER, fast data rate, lower latencies and fairly big transmission distances. It is a modulation that ITU has cleverly defined and integrated in all xDSL technologies. REFERENCES [1] [2] [3] [4] [5] [6] [7]
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Andrew S. Tanenbaum. Computer Networks. ISBN 0-13-066102-3. Douglas E. Comer. Internetworking with TCP/IP - Principles, Protocols and Architecture. ISBN 86-7991-142-9. Bahai, A. R. S., Saltzberg, B. R., Ergen, M. (2004)., Multi Carrier Digital Communications: Theory and Applications of OFDM, Springer, 2004. K. Fazel, S. Kaiser (2003), Multi-Carrier and Spread Spectrum Systems, John Wiley & Sons, 2003, ISBN 0-470-84899-5. Burstein, Dave (2002). DSL. John Wiley and Sons, New York. ISBN 0-471-08390-9 Leon W. Couch III, "Digital and Analog Communication Systems, 6th Edition", Prentice-Hall, Inc., 2001. ISBN 0-13-081223-4. Couch, Leon W. III (1997). Digital and Analog Communications. Upper Saddle River, NJ: Prentice-Hall. ISBN 0-13-081223-4. John G. Proakis, "Digital Communications, 3rd Edition", McGraw-Hill Book Co., 1995. ISBN 0-07-113814-5. ITU-T Recommendation G.992.1: Asymmetric digital subscriber line (ADSL) transceivers.
Delaportas George became an IEEE Fellow (F) in 2004. Delaportas George was born in Corfu at 1984, Greece. He is a Computer Engineer and currently an MSc student. He has a long experience as a SYSTEM’S and NETWORK DEVICES ADMINISTRATOR, APPLICATIONS and SYSTEMS ENGINEER and a NETWORK SECURITY PROFESSOR. He published an IEEE paper entitled: Application of a Robotic Arm to an Automated Recording Process (Halkida, Greece: Inderscience Books, 2004). His researching interests are basically on A.I, robotics, digital signals analysis, cryptography, coding in information theory, optimization algorithms and dynamic architectures (FPGA), distributed systems, networks architecture analysis and QoS.
