Competing for throughput in the local loop

The voice-grade telephone channel is the communication medium most widely available throughout the world. Its limitations, a relatively narrow bandwidth and a moderate SNR figure, have challenged telecommunication researchers and engineers over several decades to develop technologies able to pass higher and higher data rates. The sophisticated combination of trellis coding and quadrature amplitude modulation increases the bandwidth utilization close to the Shannon limit of 10 b/s/Hz for the 3.1 kHz bandwidth voice channel. However, the resultant data rate of about 30 kb/s does not even begin to approach the throughput needed by multimedia applications. Fortunately, twisted pair bandwidth reaches up to several megahertz, so multilevel signaling implemented as in the xDSF family of modems enables data rates of several megabits per second to be reached. Standard ADSL modems are able to deliver 8 Mb/s to a subscriber in the best case. VDSL modems can deliver 52 Mb/s, but only up to 300 m, so their mass implementation will only become possible when fiber migrates into an access network closer to the user premises. Both broadband xDSL solutions approach the Shannon limit and the related spectral efficiency as well, so there is not much room left for a transmission improvement     

Performance Evalution of 2B1Q Data Echo Canceller in Digital Subscriber Access Networks

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SDSL — it is more than just a simple voice/web access

SDSL (Symmetrical Digital Subscriber Line) is a future-proof wideband access technology for small business and residential users via twisted pair. The original motive for development of new xDSL technologies was the desire on faster Internet access for classic telephone users via copper line. Especially the development of SDSL with its symmetrical nature presents new possibilities itself. For instance SDSL copes excellently with the growing demand for carrying several voice connections via a single copper line always called VoDSL (Voice over DSL), the new generation of voice services for small enterprises and the small office/home office (SOHO) market. We will summarize the features of SDSL and its implementation and have a short look at the actual standardization items. Different Voice over DSL approaches and the use of SDSL as in-house installation will be discussed.      

Microprocessor application for numerical ECG encoding and transmission

Over the past few years a number of hospitals have established computer facilities for routine ECG interpretation. The combined workload of several hospitals could be processed by a "regional ECG interpretation center," if ECG data could be conveniently and efficiently transmitted to this center. ECG transmission over the public telephone network is common practice today, using FM techniques. However, because of the limited bandwidth of the telephone network and of occasionally severe noise, the signal quality achieved is only marginally adequate for computer interpretation. Transmission of binary-coded ECG's could overcome this problem but the data rates required greatly exceed the available channel capacity of telephone lines. Digital transmission may still be possible if ECG are preprocessed in order to eliminate redundant information and thereby lower the data rate. In this paper the operational characteristics of digital ECG communication are discussed in the context of computer-aided interpretation. Encoding methods proposed by other authors are briefly reviewed, and finally, a prototype system based on an 8-bit microprocessor is described. This system provides real-time transmission of 3 encoded ECG leads, sampled at a rate of 300 per second per lead and digitized with a resolution of 8 bits, using a 2400-Bd synchronous MODEM on standard (unconditioned) telephone lines. Some preliminary results are shown.     

Technical and economic impact of the use of standard-B earth stations in the intelsat system

More than 20 years after the INTELSAT system started up, satellite technology developments have led to considerable changes in the world-wide international satellite transmission system. This evolution and the present capabilities offered by digital technologies make it possible to reassess the potential cost advantages of standard-B earth stations over a wider portion of international telephone transmission routes. This paper addresses the technical and economic issues raised by the characteristics of the present and planned INTELSAT satellites, together with the current capabilities of digital techniques (forward error correction, low rate encoding, time and frequency division multiple access). The foreseeable impact of the use of such techniques on overall satellite telephone transmission costs is also presented, and compared with that forecasted for fibre optic cable circuits.     

Equalization for DMT based broadband modems

Faced with fierce competition from cable operators, telephone companies are now aggressively investing in digital subscriber line technology with the objective of providing an economic broadband access alternative. However, high-bit-rate transmissions over the metallic access network is far from trivial. The subscriber loop predominantly consists of interconnection of twisted pair copper sections. This causes severe attenuation of the high-frequency components of the transmitted signal. Equalization refers to the signal processing in the transceiver to combat the effects of the dispersive channel on the transmitted signal. This article overviews equalization methods for discrete multitone modulation (DMT). The interference structure created by a non-ideally equalized channel is analyzed. Time modulation as well as frequency domain equalization methods are addressed     

Broadband to the home (BTTH): architectures, access methods, andthe appetite for it

The demand for high-volume data transfer to the home for interactive multipurpose residential multimedia applications (such as tele-education, interactive on-demand entertainment, home shopping, browsing the World Wide Web to retrieve up-to-date information) and telecollaboration, including telecommuting to work, are continuously increasing. Technological advances and the 1996 Telecommunications (Deregulation) Act allow both telephone and television service providers to compete in each other's territory for providing broadband services to the home (BTTH). BTTH refers to broadband data services which may or may not use a high bandwidth link. It essentially gives the end user an impression or feeling that megabit-rate data transmission is occurring to/from the terminal, so the benefits of broadband services are enjoyed at the most convenient time within the comforts of home. It can be achieved for example, by using twisted pair copper wire telephone line using various types of digital subscriber line (xDSL) modems installed at both ends of the line. Issues and challenges of BTTH are discussed, in addition to describing future position(s) of the traditional service providers     

HDSL的技术与应用

ＨＤＳＬ（高比特率数字用户线）技术是在铜线上传输高速数据的新技术，它对于提高现存的电话用户接入网的利用率有着重要意义。本文分析了ＨＤＳＬ技术产生的背景，详细讨论了应用于ＨＤＳＬ中的两种线路传输码：２Ｂ１Ｑ和ＣＡＰ，最后，阐述了ＨＤＳＬ的最主要性能，优点及其应用。     

VDSL中调制方式的比较

甚高速数字用户线(VDSL)是数字用户线(DSL)家族中速率最高的接入方式.新一代VDSL速率最高可到100Mbit/s以上,包括对称传输和非对称传输以及多种不同的传输速率.由于VDSL的传输速率高,其对应的传输距离和其他DSL技术相比较短.调制方式是VDSL中的核心技术,目前有两种并存的方式,离散多音(DMT)与正交/调幅和无载波调幅/调相(QAM/CAP).文章主要介绍这两种调制方式,并从多个方面对其优、缺点进行比较.     

The ISDN U interface and transceiver

This paper describes the integrated services digital network (ISDN) basic rate interface (BRI) two-wire U interface and the transceiver necessary for the digital transmission of 160 kb/s of information over unshielded twisted pair transmission media. Topics discussed include the U interface structure and framing as described in the American National Standards Institute (ANSI) T1.601 specification, and the VLSI implementation of the ISDN U transceiver. Pertinent transceiver design issues addressed include the type of line code used, such as 2B1Q or 4B3T, echo cancellation, near end crosstalk, equalization and sampling rates.     

An overview of broad-band access technologies

The provision of broad-band services based on either the Internet or the asynchronous transfer mode (ATM) technique requires a new generation of access networks. In the short term, solutions such as x-digital subscriber line (DSL) and hybrid fiber coaxial (HFC), allowing the reuse of existing infrastructures look very promising. For the longer term, new infrastructures based either on radio or on optical access links seem preferable. Three x-DSL techniques aiming at high-bit-rate transmission over twisted pairs are presented: high-bit-rate DSL, asymmetrical DSL, and very-high-bit-rate DSL. An extension of existing cable television networks known as HFC is also described. Two other prospective approaches, wireless in the loop (WITL) and fiber in the loop (FITL), are then presented. Several techniques are considered for WITL: digital enhanced cordless telecommunication local multipoint distribution service, wireless ATM, and low Earth orbit satellite constellations. The various architectural alternatives for the FITL approach are discussed, a special interest being dedicated to the synchronous digital hierarchy self-healing loop and the ATM over a passive optical network. We mention the main experiments and the standardization activities inherent to the domain     

High-bit-rate digital subscriber lines

High-bit-rate digital subscriber line (HDSL) technology provides a simple, cost-effective solution for the rapid provision of a new digital telecommunication services direct to a customer's premises over existing twisted copper pair telephone lines. It makes full use of the existing network infrastructure and hence removes the need for the costly replacement of copper twisted pairs by alternative transmission media     

Time-Shared Two-Wire Digital Subscriber Transmission System and Its Application to the Digital Telephone Set

Adopting digital techniques in the local network area will enhance the possibility of extending digital operation capability to the subscriber's equipment. The problem is to convey digital signals at a bit rate of at least 64 kbits/s in both directions over existing two-wire subscriber loops. This paper describes a time-shared two-wire digital subscriber transmission system in which the directional separation of two signals (GO and RETURN) is accomplished by the time division bidirectional transmission method in burst-mode, and also reports results of investigations on the feasibility of an all digital telephone subscriber loop system.     

基于xDSL技术群与远程监控传输方式的研究

对现行、主要的二种远程通讯方式进行了比较,指出了光纤传输方式的缺点和采用视频电缆传输方式的不足。重点介绍了基于普通双绞线的xDSL技术群的特点,并结合视频监控系统和移动通讯系统的实际情况,给出了笔者初步研发成功的系统级模型。实践表明:在现行条件下,基于普通铜质双绞线的xDSL技术群是远程监控更为现实、可行的传输方式。     

Video communications and services in the copper loop

There are several technologies available for providing access using the embedded copper loop plant. However, none of these technologies can provide a high-bit-rate digital transport capability (1.544 Mb/s) over the existing loop plant. The authors describe continuing advances in both VLSI implementation and signal processing algorithms that are making it possible to provide the high-bit-rate digital subscriber line (HDSL) for a repeaterless T1 capability within carrier serving areas (CSAs); asymmetric digital subscriber lines (ADSL) for a 1.5-Mb/s transport capability over the nonloaded copper loop plant; and the next-generation ADSL-II for a 3 to 4 Mb/s transmission rate over CSA ranges. It is argued that, coupled with advances in video compression techniques and recent standards activities in CCITT, these bit rates will allow local exchange carriers (LECs) to provide video communication and services using highly compressed digital video by allowing the LECs to use their existing embedded loop plant to enter the video market and establish a base of customers     

SunSet xDSL用于数字用户线测试

Ｓｕｎｒｉｓｅ公司推出的数字用户线测试仪ＳｕｎＳｅｔ ｘＤＳＬ综合了数字多用表、时域反射计、频谱分析仪功能以及内置ＡＤＳＬ／ＨＤＳＬ调制解调器模块,仅重１．３千克。利用它可通过单端测量、主从机测量和ＡＴＵ－Ｃ／Ｒ仿真等方法,进行开通前线路质量确认性测试、传输质量测试以及故障定位测试等。     

High bit rate digital subscriber lines: a review of HDSL progress

The author gives an overview of progress made in the evolution of technology to provide DS1 rate telephone access in a restricted segment of the loop plant without intermediate repeaters, loop conditioning, or pair selection in assignment. This technology is called the high bit rate digital subscriber line (HDSL). Discussed are background information on electronics in the loop plant and characterization of the tranmission environment in the relevant frequency band. The progress of HDSL study project of the American National Standards Institute (ANSI) is outlined. Analytical and theoretical studies to determine the limits on the transmission capabilities of the loop plant, motivated by the need to determine the feasibility limits of HDSLs, are reviewed. Also discussed is progress in technical work on suitable transmission formats. The possibility of an asymmetrical digital subscriber line (ADSL), transmitting at the DS1 rate from the central office to a remote distribution point, through the entire nonloaded loop plant is discussed     

xDSL技术及其应用

ｘＤＳＬ技术是一种调制技术，在双绞铜线的两端分别接入ｘＤＳＬ调制解调器，即可利用其高频带特性传送高速数据，本文全面阐述了实现用户线高速比的ｘＤＳＬ技术及其应用。     

Comparison of Redundancy Reducing Codes for Facsimile Transmission of Documents

A comparison and a valuation of different redundancy reducing coding techniques for the digital transmission of facsimile documents on telephone lines is presented. Especially taken into account are those codes which have been submitted to the International Telegraph and Telephone Consultative Committee (CCITT) for standardization of digital group 3 facsimile machines. The reduction factor and the sensitivity of channel errors of these one- and two- dimensional codes have been investigated by computer simulations using the CCITT test documents and burst error patterns of real telephone lines. For a resolution of 1728 picture elements per line and 3.85 lines per mm one-dimensional run length coding proves to be the most economical solution. Using a higher vertical resolution of 7.7 lines per mm the effects of transmission errors are less visible in the received document and two-dimensional codes become more efficient. To achieve a transmission time of 1 min or less for a size A4 document a transmission bit rate of 4800 bits/s is required. For the higher vertical resolution a transmission time of 1 min cannot be guaranteed for all types of documents with this bit rate even if two-dimensional coding is used.     

Echo Cancellation in Speech and Data Transmission

This tutorial paper reviews echo cancellation techniques as applied to both voice and data transmission applications. The echo control problem in telephone voice transmission is described, and the several measures taken to counteract echo are outlined. The problem of full-duplex data transmission over two-wire lines is described, with application to both digital subscriber loop and voiceband data transmission. The unique characteristics of each of these applications, more advanced methods recently proposed, and approaches to implementation are described.