A Tutorial on Two-Wire Digital Transmission in the Loop Plant

This paper explores the constraints on the design of twowire repeaterless digital subscriber loop (DSL) systems. Broadly categorized, the design depends on the technical feasibility of the approach used to achieve two-wire transmission, constraints related to compatibility with other systems sharing the same cable, and immunity to central office noise. Each of these varies With the choice of system parameters including the transmission rate, transmit power, choice of line codes, etc. Technical feasibility is evaluated by computer simulation studies. Compatibility with other systems is explored by crosstalk calculations. Noise immunity considerations, as they translate into digital line power levels, are also explored.     

Design Philosophy and Hardware Implementation for Digital Subscriber Loops

This paper presents various requirements and configurations in the information network system (INS) NTT's version of the integrated services digital network (ISDN)- such as field trials, and technologies and equipment design of the digital subscriber loop using balanced wire pairs. The main aspects characterizing digital subscriber loops, are a customer access structure, a digital transmission system on existing subscriber loops, and customer interface. A configuration of two-wire digital subscriber loops with two customer access channels, which is adopted in the field trials, will be described. The time-shared two-wire digital transmission and power feeding methods on subscriber loops are discussed. The common mode rejection ratio (CMRR) is discussed in detail, since CMRR is one of the main factors which affects the transmission ability of balanced wire pairs. A proposed self-clocked four-wire customer interface utilizes time division multiplexing technique. The digital subscriber loop operates on a call-by-call basis for link establishment to avoid excessive power dissipations as well as to avoid impairments caused by crosstalk from other lines within a cable. Its design objectives and implementation are also discussed.     

Error correction performance in digital subscriber looptransmission systems

The author describes an error correction system for digital subscriber loop transmission systems which use time compression multiplexing (TCM). An interleaved block code is used to correct the burst errors due to impulse noise from analog telephone circuits. This interleaving method requires no extra hardware and contributes no additional delay. To evaluate the transmission performance of this error correction system, the bit error rate after decoding is derived on the basis of a burst error model for 200 kb/s digital subscriber transmission using the alternate mark inversion (AMI) line code. The experimental results for a 200 kb/s TCM system show that burst errors are substantially reduced     

An Integrated Voice/Data Terminal with Simple Synchronization Circuits Using an 80 kbit/s Ping-Pong Method

With recent digital technique progress, digitalization is spreading to subscriber loop systems. In-house systems will be digitalized earlier than other systems. In in-house networks, a pingpong method, especially an 80 kbit/s ping-pong method, using an existing cable pair, is superior to other digital transmission methods due to the sample system structure. For office use, a digital subscriber terminal is required to offer integrated services. However, the already reported 80 kbit/s method is insufficient to provide simultaneous and independent integrated services. This paper presents an 80 kbit/s ping-pong method which has 72 kbit/s capacity for the voice and data communications, so as to provide such integrated services. Furthermore, an experimental integrated terminal, which has simple synchronization circuits, is described.     

A New Digital Echo Canceler for Two-Wire Subscriber Lines

A new type of digital echo canceler for two-wire digital transmission is presented. The new principle involves very simple signal processing and is thus an interesting alternative for digital transmission on subscriber lines. The principle is compared with other echo cancellation techniques, and it is shown how choice of line code, equalization, and carrier recovery are affected by the new echo canceler. A theoretical analysis of the principle is given, taking into account finite accuracy, jitter, noise, and correlated data streams. The echo canceler can be used for line attenuation up to 40 dB. At 80 kbits/s this corresponds to at least 7 km 0.6 mm cable and is sufficient to cover more than 99 percent of the existing Norwegian subscriber lines.     

An Approach to Multi-Service Subscriber Loop System Using Packetized Voice/Data Terminals

A new subscriber communication system and its design philosophy are described. In addition to telephone communication, the system is simultaneously able to offer data communication, still picture communication, etc. The system is composed of a packetized voice/data terminal, a multi-service switching equipment and the digital subscriber loop connecting between them. The system utilizes the existing subscriber line efficiently and is entirely suitable for coming telephone and data public digital network. 64 kbit/s PCM coded voice conversation and 48 kbit/s data communication were achieved simultaneously over 96 kbit/s digital subscriber line which was spanned up to 2 km.     

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.     

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

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Achievable information rates on digital subscriber loops: limitinginformation rates with crosstalk noise

The capacity and cutoff rates for channels with linear intersymbol interference, power dependent crosstalk noise, and additive white noise are examined, focusing on high speed digital subscriber line data transmission. The effects of varying the level of additive white noise, crosstalk coupling gain, sampling rate, and input power levels are studied in detail for a set of simulated two-wire local loops. A closed-form expression for the shell constrained Gaussian cutoff rate on the crosstalk limited channel is developed and related to the capacity, showing that the relationship between these two rates is the same as on a channel without crosstalk noise. The study also projects achievable rates on a digital subscriber line, inside and outside of a carrier serving area, with a sophisticated but realizable receiver     

A Time Compression Multiplexing System for a Circuit Switched Digital Capability

The time compression multiplexing (TCM) technique offers a solution to the problem of achieving digital penetration of the two-wire loop plant. TCM is planned to provide the digital loop transmission in the new circuit switched digital capability of the Bell System, so that a full-duplex 56 khit/s digital connection is obtained between subscriber's premises and the serving central office. Herein, the concept of TCM and its parameters are described. Tradeoffs leading to determination of system parameters such as burst frequency, burst length, guard time, and delay as they affect the range of the system are discussed. Also, the performance of the digital loops in terms of eye diagrams, system range limitations due to crosstalk and impulse noise, and percentage of loops covered are summarized. In addition, the circuit implementation using both analog and digital technologies is described. Experiences with various digital architectures to implement TCM and tradeoffs leading to a low-power digital architecture suitable for VLSI are discussed.     

A line terminating LSI using echo cancelling method for ISDNsubscriber loop transition

A 144-kb/s digital subscriber loop (DSL) transmission system based on hybrid transmission with an echo cancelling method is described. It incorporates advanced LSI technology to obtain compactness, low cost, and high reliability. An echo canceller (EC) LSI has been developed using CMOS technology. Combined with the multiplexing processor (MXP) LSI, the EC LSI provides basic DSL equipment functions. A specially arranged frame format with a newly developed digital phase-locked loop (DPLL) circuit for stable timing extraction, an automatic balancing network, and a two-stage echo canceller characterize the system. Using this line termination circuit, the DSL equipment showed a reach of over 6 km when used with 0.5 mm diameter cable for 160-kb/s bidirectional digital transmission     

Modeling and Analysis of Error Probability Performance for Digital Transmission Over the Two-Wire Loop Plant

We describe a methodology for the characterization of the 2-wire twisted-pair subscriber loop plant for high-speed digital communications typical of evolving integrated service digital networks (ISDN's). Through use of this analytical/numerical approach it is possible to predict the resulting error probability performance as a function of signal-to-noise ratio for a given set of input parameters representing a particular subscriber loop and its noise environment, including such effects as intersymbol interference, crosstalk and impulse noise. The results are useful in penetration assessment studies of the loop plant population at a fixed grade of service measured objectively in terms of operating bit error probability. This approach is in contrast to simulation-based methodologies leading to subjective performance assessment criteria, such as eye opening measurements. In the present paper we describe the modeling assumptions together with the analytical/numerical techniques underlying this approach. Attention is restricted to time-compression multiplexing (TCM) schemes with bipolar line coding, although the approach is much more general. Numerical results are illustrated through sample graphical output of an extensive computer program implementing this approach. Finally, we illustrate the application of this approach to objective penetration assessment of the GTE subscriber loop plant utilizing data collected as part of an extensive recent survey.     

The error performance of digital subscriber lines in the presenceof impulse noise

This paper describes the error performance of the ISDN basic access digital subscriber line (DSL), the high bit rate digital subscriber line (HDSL), and the asymmetric digital subscriber line (ADSL) in the presence of impulse noise. Results are found by using data from the 1986 NYNEX impulse noise survey in simulations. It is shown that a simple uncoded ADSL would have an order of magnitude more errored seconds than DSL and HDSL     

Positioning the Subscriber Loop Network for Digital Services

The need to enhance the digital transmission capability of the local loop network to provide for new services is widely recognized in the telecommunications industry. Companies around the world [1]-[3] have been working towards this goal and plan to use digital loop carrier (DLC) to meet this demand. Although this approach is generally accepted, there has been little information presented on how to plan for the evolution of the local plant. To address these pressing issues, the Bell System is introducing a concept called fundamental subscriber carrier planning (FSCP). This paper discusses the impact that digital services will have on the subscriber loop network, including loop plant design requirements necessary to position the network for up to 64 kbit/s digital services. Existing cable plant has the capability to support most digital services, but there are specific design requirements that are quite different from those required for the "typical" analog voiceband service. This paper reviews the need to position the loop for digital services, and substantiates the major conclusion that digital loop carrier technology offers a significant advantage when provisioning for digital services.     

A Line Termination Circuit for Burst-Mode Digital Subscriber Loop Transmission

This paper describes a line termination circuit for burst-mode bidirectional digital subscriber loop transmission. It incorporates the most advanced LSI technology to obtain compactness, low cost, and high reliability. Two CMOS LSI's have been developed; one is a line termination LSI (LT) and another is a circuit termination LSI (CT). LT LSI adopts a novelRCactive filter-type equalizer and decision feedback bridged tap equalizer suitable for incorporation in LSI and provides high performance. By using these LSI's, a line termination circuit realizes a reach of over 5 km at 88 kbit/s bidirectional digital transmission. This paper describes each LSI and shows total performance characteristics in detail.     

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.     

Evaluation of 16 kbit/s digital voice transmission for mobile radio

In this paper 16 kbit/s digital voice transmission with conventional channel spacing of 25 kHz, employing a 16 kbit/s adaptive delta modulation (ADM) coder-decoder (CODEC) is evaluated. The main characteristics of narrow-band digital FM modulation schemes, such as tamed FM, Gaussian filtered minimum shift keying (GMSK), four-level FM and phase locked loop-quaternary phase shift keying (PLL-QPSK), are compared by laboratory tests. Digitized voice quality in a digital channel incorporating a 16 kbit/s ADM CODEC and GMSK coherent detection was compared with voice quality of a conventional analog FM channel. Bit error ratio (BER) performance is shown to depend primarily on demodulation schemes. Digital voice quality is inferior to that of analog voice with an opinion score difference of about 0.5 in fading environments. This kind of digital voice transmission will be applicable for those systems that require high security at an expense of speech quality.     

Impulse generation with appropriate amplitude, length,inter-arrival, and spectral characteristics

This paper proposes a suitable method for simulating impulses with appropriate amplitude, spectral, and inter-arrival characteristics. The statistics used to develop the parameters of this model are based on statistics derived from observations of impulse noise on the telephone networks of British Telecom (BT) and Deutsche Telekom (DT). This paper initially reviews the former DT approach to impulse noise generation for testing digital subscriber line systems, so called xDSL systems. Some problems are highlighted and an alternative technique is suggested that is capable of generating impulses with both appropriate amplitude an spectral characteristics     

A finite precision analysis of the block-gradient adaptivedata-driven echo canceller

The block-gradient (block LMS) algorithm's finite precision performance in the data-driven echo canceller application is analyzed. From both the analysis and simulation results, it was found that the block LMS requires significantly less precision than the standard LMS algorithm. The analysis also shows how higher convergence and tracking speeds may be traded for an improvement in precision requirements. The authors derive formulae that can be used to accurately compute echo rejection levels as a function of precision, signal powers, step-size, block length, and echo canceller length. The utility of the formulae is demonstrated by showing performance levels for a typical V.32bis full-duplex voiceband modem operating at a transmission rate of 14.4 kbit/s (7200 Hz sampling rate), and for a high-speed digital subscriber line echo canceller operating at a sampling rate of 400 kHz     

Timing Jitter Effects on Digital Subscriber Loop Echo Cancellers: Part II--Considerations for Squaring Loop Timing Recovery

The performance of an adaptive echo canceller at the looptimed subscriber end of a digital subscriber loop has been shown to be sensitive to jitter arising from the timing recovery subsystem. We evaluate this degradation for the common timing recovery subsystem consisting of a prefilter, a squarer, and a second-order phase-locked loop. The evaluation shows the influence of equalization, prefilter shape, and phaselocked loop parameters. A narrow-band accurately tuned prefilter, line equalizer, and a narrow-band phase-locked loop are found to be necessary for adequate performance in a 144 kbit/s bipolar-coded digital subscriber loop system employing echo cancellation.