Loop Test System: A New Maintenance Feature for the Distributed Integrated Digital Network

The introduction of the digital Class 5 switch offers unique solutions to the requirements of growth and demands for improved service. For example, remote subscriber switches (RSS's) and subscriber carrier terminals (SCT's) can now be located near subscribers and digitally connected to the Class 5 switch via T1 repeatered lines or radio links. The result is shorter subscriber loops, improved transmission, and deferment or elimination of feeder plant additions. Traditionally, interposing electronics between the subscriber loops and the Class 5 switch resulted in isolating these loops from the office loop testing facilities. Remote testing of RSS and SCT assigned loops is an important step toward minimizing maintenance costs. Loop testing is accomplished in TRW Vidar's ITS5 switching system by locating loop test equipment in the remote RSS and SCT cabinets. This equipment is available to the test operator for real-time testing, and to the switch for automatic line testing. Some of the requirements that the loop test equipment must meet are: low cost, no adjustments, low power consumption, small size, operation in a harsh environment, and centralized testing capability from a test console. The data returned to the office must be in a quantitative form to allow further processing by the console. It must also be compatible with the direct distance dialing (DDD) network to allow transmission not only to the Class 5 switch serving the RSS and SCT, but also to distant service centers. This paper describes a newly developed system that meets the above objectives.     

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.     

Digital optical CATV system using hubbed distribution architecture

A hubbed distribution architecture based on a bus-star configuration, three-level single single-mode fiber transmission using M-TPC line code, and high-speed IC technology is proposed. The architecture is cost-effective and compact. It is well suited to metropolitan area CATV systems and would allow a gradual transition to a broadband ISDN (integrated-services digital network) subscriber loop in the future. The proposed architecture is applied to a digital optical CATV system prototype. The equipment making up this system consists of a transmitter, hubs including video selectors, and subscriber unit. Two video channels are selected for each subscriber from eight video channels at hub stations. The transmission speed is 900 Mb/s, three-level for the trunk line and 200 Mb/s, three-level for each subscriber loop. Very compact and low-cost equipment is realized by using recently developed high-speed ICs. Experimental results show that the system satisfies requirements and that its commercial application is feasible     

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.     

The Loop Coverage Comparison Between TCM and Echo Canceller Under Various Noise Considerations

This paper describes loop coverage evaluations of two-wire digital subscriber transmission systems to support the CCITT recommended 1-430 interface. Several range-limiting factors are discussed and a cable utility design for two representative two-wire digital subscriber transmission systems is presented. The two transmission systems are time compression multiplexing (TCM) and echo canceller (EC), utilizing the AMI line code and full cosine rolloff line equalization for both systems. A new impulse noise evaluation is applied to cable utility design, because impulse noise is one of the most stringent range-limiting factors, especially for TCM in utilizing existing subscriber loops. Comparison of TCM with EC is also carried out according to various range-limiting factors. As a result, it is noted that the 320 kbit/s line bit rate TCM is more applicable in NTT's exchange area coverage than the 160 kbit/s EC to achieve the2B + Dbasic interface transmission.     

Loop survey in the Taiwan area and feasibility study for HDSL

About 2800 lines out of a total of 8,500,000 subscriber lines in Taiwan have been sampled and characterized. Various kinds of physical compositions and transmission characteristics were investigated and measured. It was found that the near-end crosstalk (NEXT) loss of Taiwan's subscriber loops is about 10 dB worse than that of US local loops. A corresponding new foam-skin PIC cable is proposed to reduce the NEXT loss. The new type of cable will be used to replace the paper cable and old foam-skin PIC cable step-by-step. Based on the measured data, the feasibility of HDSL (high bit rate digital subscriber line) service in Taiwan is studied. To improve the HDSL service distance via the existing subscriber lines, several cable models are discussed and suggested     

Subscriber Line Equipment for an Elastic Carrier Switching System

This is the third of three papers [l], [2] dealing with an elastic carrier-concentrator system. The functioning of the subscriber line equipment is described herein. The system serves 256 telephone subscriber lines by digital concentration switching and transmission of multiplexed message channels over a singleT1transmission line. The concentration switching function is nonblocking. Message channels are the individual communication links between terminals and are created on demand at the request of active subscriber lines. They are not merely inactive without service requests; they do not exist unless service is requested. The created message channels serve for their normal communication function and, in addition, carry for each line the ancillary functions of dialing, ringing, and pay-station coin disposal. Message channels are coded by delta modulation, which permits flexible digital concentration and provides a smooth tradeoff of channel quality versus the instantaneous traffic load. Each of two complementary subscriber line interface units, one per line at each end, provides the delta-modulation encoding and decoding functions for the analog voice-band signal input and output and, by overriding the delta codec, provides the desired signaling functions of dialing, ringing, etc. The line interface units provide the major equipment interface between the subscriber line and the digitalT1line. An integral test facility permits remote loop-back of any selected subscriber line unit via a test channel to the office terminals and at the same time connects the subscriber loop to an auxiliary programmed test facility which may transmit pertinent loop-test results to the office terminal.     

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     

Performance of a 51.84-Mb/s VDSL transceiver over the loop withbridged taps

This paper presents a study of the effects of bridged taps on very high-speed digital subscriber line (VDSL) transmission systems. It is shown that the short bridged taps, which affect VDSL signals, are much more damaging than the longer bridged taps, which affect other xDSL signals, such as high-rate digital subscriber line (HDSL) and asymmetric digital subscriber line (ADSL). Specifically, short bridged taps introduce a lot of linear distortion in the loop's transfer function. In addition, they also tend to introduce more overall propagation loss than longer bridged taps. The peak-power-to-average-power ratio (PAR) of the signal increases for the loop with short bridged taps, and thus more A/D bits are needed in the implementation of a digital VDSL transceiver as compared to the loop with long bridged taps. The performance of various equalizer structures is discussed in the presence of a severe channel distortion caused by the bridged tap     

Investigation of Synchronization Parameters in a Digital Subscriber Loop Transmission System

The degree of echo reduction achieved in a digital subscriber loop (DSL) transceiver employing adaptive echo cancellation has previously been shown to be very sensitive to timing jitter. This paper reports the experimental study, by computer simulation, of a 144 kbit/s DSL timing recovery system incorporating a prefilter, nonlinearity, and second-order phase-locked loop. The simulations were carried out for a wide variety of line lengths, bridged-tap conditions, loop and filter parameters, and choice of nonlinearity. The results showed that a properly designed timing recovery system with a 20 khzwide prefilter allows satisfactory echo reduction performance, even for long loops with bridged taps. For shorter loops the achievable echo reduction is less, but is still sufficient to yield satisfactory performance.     

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     

Performance evaluation of a fast computation algorithm for the DMTin high-speed subscriber loop

The discrete multitone (DMT) modulation is considered to be a viable transmission scheme for high-speed subscriber loop. In this paper, the fast algorithm for computing the equalizer settings derived in [1] is extended and applied for the DMT in high-speed subscriber loop. The channel pulse response is assumed to be given by the channel identification method, and then the equalizer filter settings are computed. In simulations, a fast algorithm for the symbol spaced equalizer in a colored noise channel is used. Simulation results performed in various CSA loops indicate that the fast algorithm yields the near-optimum settings for the DMT system     

Emerging high-speed xDSL access services: architectures, issues,insights, and implications

High-speed asymmetric and symmetric digital subscriber line loop access systems, designed by numerous computer networking and telecommunications equipment companies, are currently the focus of technical trials, market trials, and commercial services deployment around the world. This is to provide millions of residential and business customers worldwide with high-speed always-on connectivity to the Internet, corporate intranets, and various online services. It is estimated that over 460 million copper loops worldwide could potentially be equipped end-to-end with xDSL access systems. Large-scale deployment of broadband access systems offering always-on connectivity to the global Internet and corporate intranets is seen as a key enabler of the bold vision articulated worldwide to utilize open Internet protocols and emerging technologies such as Java to create a unified software-defined (IP) global networking infrastructure. Such a networking platform would serve as a de facto medium for generation and delivery of integrated multimedia content, and revolutionize the way people around the world learn, do business, and interact with each other     

A Field Trial of Fiber Optic Subscriber Loop Systems Utilizing Wavelength-Division Multiplexers

Future subscriber loops will provide various services including broad-band video. Fiber optic technology is the key for introducing these services. The Electrical Communication Laboratories (ECL) of Nippon Telegraph and Telephone Public Corporation (NTT) have been promoting research and development on basic technology of fiber optic subscriber loops, and studying the ways to use fiber optic subscriber loops for new services. As one of the programs, a field trial of fiber optic subscriber loops was planned and conducted in the Yokosuka area from April 1980 until May 1981. This paper first describes fiber optic transmission technologies for subscriber loops. Then, the system configuration and experimental results of the Yokosuka field trial are presented.     

Flexible synchronous broad-band subscriber loop system: opticalshuttle bus

A flexible, synchronous, broadband subscriber loop system called the optical shuttle bus is discussed. The emphasis is on the proposed design architecture and performance of a 1.8-Gb/s prototype system. The optical shuttle bus is constructed of multigigabit/second intelligent optical shuttle nodes (OSNs). The software-defined OSNs allow the bus to be configured as a synchronous transmission network with any topology and provides flexible channel allocation by means of a drop/insert/cross-connect function. The OSN can be configured as a highly reliable network by virtue of its self-healing functions such as line switch, bypass, and loop back. The proposed bus architecture has been successfully applied in a 1.8-Gb/s prototype in which three OSNs are connected by one 10-km and two 5-dm single-mode fibers in a ring configuration. Very compact and inexpensive equipment is realized by using high-speed ICs. Experimental results show the proposed architecture to be a powerful one for the realization of synchronous broadband subscriber loop systems     

Passive optical subscriber loops with multiaccess

Deployment of optical fiber in the subscriber loop may eventually lead to a full-scale deployment of the broadband network. A network architecture that can be introduced inexpensively to meet the near-term demand and upgraded smoothly to support future needs is desired. The feasibility of applying multiaccess architectures to subscriber loops is studied, and five architectures for passive optical subscriber loops that meet this need are presented. All of these architectures use a double-star topology and dense wavelength division multiplexing in the downstream direction; however, various topologies and multiaccess techniques are used in the upstream direction. Limitations on node size, frame synchronization, cost, privacy and security, and standards are discussed     

The HDSL environment [high bit rate digital subscriber line]

The author presents a tutorial on the physical environment in which high bit rate digital subscriber line (HDSL) transceivers will have to evolve and succeed. Special attention is given to the most damaging impairments that are encountered in subscriber lines, such as propagation loss, linear distortion, crosstalk, bridged taps, and impulse noise. Somewhat less important impairments, such as change of gauge, temperature variation, and thermal noise, are also briefly described. The author concludes with a discussion of the capacity of a twisted-pair channel in a crosstalk-dominated environment     

OEIC technology and its application to subscriber loops

Recent progress in GaAs- and InP-based optoelectronic integrated circuits (OEICs) for use in optical fiber transmission systems is reviewed. OEIC technology and system demonstrations for short-haul and high-speed transmission are described, and the problems arising in practical applications are discussed. Approaches to resolution of these problems are presented, and future OEIC applications, especially with regard to subscriber loops, are predicted     

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

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