Chapter 6. Single-Pair High-Speed Digital Subscriber Line (SHDSL)
HDSL applications have traditionally targeted provisioning of T1 (1.544 Mb/s) service in North America and E1 (2.048 Mb/s) service in Europe. As we have seen in Chapter 4, HDSL in North America was provisioned on two pairs with first generation HDSL that used 2B1Q technology. In Europe, first-generation HDSL used 2B1Q technology for provisioning 2.048 Mb/s service on 1, 2, and 3 wire pairs and CAP technology for provisioning 2.048 Mb/s service on 1 or 2 pairs. The higher number of pairs allows deployment to a greater distance because the bit rate on each line is lower, whereas deployment on a single pair allows a shorter maximum distance because the bit rate on the wire pair is larger. HDSL service was typically deployed to business customers addressing high-capacity access applications. The advantage to the service provider in deploying HDSL was lower provisioning costs and faster provisioning times compared with the original repeatered AMI or HDB3 access technologies. In 1996, a proposal was made in ETSI TM6 to begin work on a multirate symmetric DSL transmission system to address business access applications delivering fractional E1 rate services, that is, n x 64 kb/s service up to the maximum of 2.048 Mb/s. The deployable distance would be a function of the bit rate: the lower the bit rate, the greater the distance; the higher the bit rate, the shorter the distance. Although there was general interest for this new work, the project did not begin if full force until mid-1998. The project was entitled SDSL for symmetric digital subscriber line in ETSI TM6. During this same time, T1E1 was developing second-generation HDSL also known as HDSL2 (see Chapter 4). HDSL2 targeted the transmission of T1 service on a single wire pair. Although there was not an equivalent HDSL2 project in ETSI TM6, the ETSI SDSL project covered the transmission of the E1 (2.048 Mb/s) payload on a single wire pair in its definition. During the discussion of the scope of ETSI SDSL, there was debate on the possible operation over baseband POTS. The result of the debates was that ADSL would target residential access applications with operation over baseband POTS and that SDSL would target business access applications without the use of baseband POTS. Although never standardized, there were significant deployments of symmetric DSLs using 2B1Q and carrierless AM/PM (CAP) technologies. Primary deployments were by competitive access providers on local loops that were leased from the incumbent operators. As described in Chapter 4, the architecture of the CAP-based solution is closest to that of SHDSL. The 2B1Q SDSL solutions were extensions of the core first-generation HDSL technology, where the transceivers were configured to operate at select bit rates that were integer multiples of 8 kb/s. These solutions targeted business customers with high-capacity access applications. In January 1999, a companion project to ETSI'S SDSL and to Committee T1's HDSL2 was begun in the ITU-T Study Group 15 Question 4 (i.e., ITU-T Q4/15) called single-pair high-speed digital subscriber line (SHDSL) and the corresponding Recommendation was referred to as G.shdsl. As with ETSI SDSL, SHDSL defines operation at bit rates ranging from 192 kb/s to 2312 kb/s in increments of 8 kb/s. The system defines transmission of only symmetric payloads, that is, the upstream and downstream bit rates are the same. Business applications are the primary target for SHDSL. The core technology for SHDSL and ETSI SDSL is that of HDSL2, namely, 16-level trellis coded pulse amplitude modulation (16 TC-PAM). This chapter provides a description of SHDSL transceivers, some of their driving applications, and the manner in which SHDSL addresses these applications. |
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EAN: 2147483647
Pages: 154