To: Scrapps who wrote (3228 ) 4/8/1998 1:24:00 AM From: flickerful Respond to of 9236
another brilliant technoblurb... Sorting the various x's in xDSL PC Week A huge issue confronting small and large businesses is how to make Internet connections faster over plain old copper-wire telephone lines. Various telecommunications companies, RBOCs and, most significantly, cable television companies have experimented with ways to overcome this challenge. The phone companies' ISDN has been a rather dismal failure overall. Cable television companies are having only minor success with their high-speed cable modems. Nearly all of these solutions require the homeowner, business owner or telco to rewire their buildings--an expensive proposition. The more compelling solution may lie in xDSL, the family of digital subscriber line technologies. For example, there is HDSL (High-bit-rate DSL), S-HDSL (Single-pair High-bit-rate DSL), SDSL (Symmetric DSL), ADSL (Asymmetric DSL), RADSL (Rate Adaptive DSL) and VDSL (Very High-bit-rate DSL). These terms refer to the way transmission bandwidth is configured and used to support the customer's bandwidth needs. You can find even more variations on xDSL by following some of the links in the accompanying chart. Despite what you might hear or read about xDSL being new and different from ISDN, it is basically another name for an ISDN-BRI channel operating at the Basic Rate Interface, with two 64K-bps circuit-switched channels and one 16K-bps packet-switching and signaling channel. This circuit can carry voice and data in both directions at the same time. xDSL refers to those various arrangements in which advanced modulating techniques are imposed on the local channel in order to derive higher throughput in one or both directions. The various types of xDSL are described in the following sections. HDSL is one of the xDSL technologies that may interest IT managers in LAN-based organizations. It has become a reliable and cost-effective means for providing repeaterless T-1 and E-1 services over two twisted-pair loops. S-HDSL/SDSL (Single-pair or Symmetric High-bit-rate DSL) operates on a single copper pair, compared with the two-pair HDSL described above. S-HDSL/SDSL allows easy implementation of applications that require symmetric data rates on a single local loop while maintaining the existing phone wiring on the same loop. As only one pair is needed, the overall capacity of the entire local loop infrastructure is greatly magnified. ADSL is comparatively high-bandwidth in one direction, with low bandwidth in the opposite direction. ADSL uses a single phone line for transmission. Internet service providers and other telecommunications companies already support or at least recognize its potential to support a range of data applications. RADSL, an extension of ADSL, is used to encompass and support a wide variety of data rates depending on the line's transmission characteristics. VDSL provides very high bandwidth asymmetrically, up to 52M bps in one direction and 2M bps in the other. Copyright c 1998 ZDNet. All rights reserved. Reproduction in whole or in part in any form or medium without express written permission of ZDNet is prohibited. ZDNet and the ZDNet logo are trademarks of Ziff-Davis Publishing Company.