Chapter 4. HDSL and Second-Generation HDSL (HDSL2)
The origin of the high-rate digital subscriber line (HDSL) began in the late 1980s after the successful demonstration of the feasibility and manufacturability of ISDN basic rate (160 kb/s) transceivers. The driving application of HDSL was the provisioning of T1 (bit synchronous 1.544 Mb/s) service to end customers via the local loop plant. The T1 signal itself is also referred to as DS1, which is the first-level digital signal in the North American network hierarchy. The objective of HDSL is to transport a bit synchronous DS1 signal from the CO to the customer premises (CP) without repeaters on loops that met the carrier serving area (CSA) requirements [4]. This same concept was also adopted in Europe for the transport of G.703/704 [1],[2] 2.048 Mb/s bit synchronous payload. The 2.048 Mb/s signal, often referred to as E1, is the first level digital signal in the European network hierarchy. The E1 signal in the Europe is analogous to the T1 signal in North America. The elimination of repeaters on most loops greatly reduced the cost to provide DS1 services, and also enabled more rapid turn -up of service. HDSL's benefits are largely due to the elimination of midspan repeaters. Each repeater site must be custom engineered to assure that each section of the line remains within the limits for signal loss. The repeated signals can cause severe crosstalk to other systems; thus special care must be taken in the design of repeatered facilities to avoid excessive crosstalk to other transmission systems. The repeater is placed in an environmentally hardened apparatus case in a manhole or on a pole. The apparatus case must be spliced into the cable. The apparatus case costs far more than the repeaters it holds. A repeater failure results in a field service visit. Repeaters are usually line powered ; this requires a special line feed power supply at the CO. Most of the power fed by the CO power supply is wasted due to loop resistance and power supply inefficiencies . HDSL also is preferred over a traditional T1 carrier because HDSL provides more extensive diagnostic features (including SNR measurement) and HDSL causes less crosstalk to other transmission systems because HDSL's transmit signal is confined to a narrower bandwidth than traditional T1 carrier. The first generation of HDSL was an extension of the 2B1Q technology used in ISDN [3], operating at a higher bit rate to support the transport of a DS1 payload. Other technologies considered for HDSL include carrierless amplitude and phase (CAP) modulation and discrete multitone (DMT) modulation. A standard for first-generation HDSL was never developed in North America; however Committee T1 published a technical report describing the three different line codes for HDSL. In 1994, Bellcore (now Telcordia) published a technical advisory (TA) on HDSL that described the preferred 2B1Q approach for HDSL. Although it was never fully completed, this TA was the closest to defining an interface specification for HDSL. From 1994 through 1996, Committee T1 working group T1E1.4 was developing a more complete specification for HDSL [6], which included definitions of 2B1Q and CAP line codes. This draft specification, although very thorough in scope and completion, was never balloted for publication by Committee T1 (i.e., neither as a technical report nor a standard). Contrary to the activity in North America, in 1993 ETSI started the development of an HDSL technical report, which was later converted to an ETSI technical specification TS 101 135 [7], with the driving application of transporting an E1 payload on subscriber lines. The ETSI specification defines two normative approaches to the implementation HDSL. The first approach defines HDSL using the 2B1Q line code. The TS contains definitions of the 2B1Q approach for E1 signal transport on three wire pairs, two wire pairs, and a single wire pair. The second approach for HDSL uses the CAP line code; however, the CAP specification is defined for operation on two wire pairs and a single wire pair. In 1998, the ITU-T published an HDSL Recommendation based on the contents of ETSI TS 101 135 [7] and Committee T1 TR-28 [4]. The ITU-T Recommendation on HDSL is G.991.1 [9]. Also in 1996, T1E1.4 began work on studying the feasibility of transporting a DS1 payload on a single wire pair for deployment on CSA loops. In other words, the goal was to investigate a modulation approach that would transport a DS1 payload on a single wire with similar performance to that of a 2B1Q HDSL transceiver operating on a single wire pair. The result is the generation of the second generation HDSL (HDSL2) standard T1.418 in the year 2000. This chapter provides a description of second-generation HDSL and shows the improvements achieved over the first generation. We also provide a brief description of first generation HDSL as well as the background of HDSL. |
| Top |
EAN: 2147483647
Pages: 154