G.SHDSL
G.SHDSL is an international standard for single-pair, high-speed DSL, as defined in the ITU-T Standard G.991.2.
Unlike asymmetric DSL, which was designed for residential applications in which more bandwidth is delivered downstream (to the house) than is available upstream (to the Internet), G.SHDSL is symmetrical - offering 2.3M bit/sec in both directions.
Today, U.S. business DSL lines are predominantly asymmetric DSL (ADSL) - the residential technology with deliverable data rates that cap out at 384K bit/sec for symmetrical service. North American telephone companies are evaluating G.SHDSL technology for Internet services targeted at small to midsize companies, offering data rates of 786K, 1.544M and 2.3M bit/sec. These Internet services will offer a reduced service-level agreement compared with T-1 or E-1 services, at a lower monthly cost.
Four factors are driving the interest in G.SHDSL:
Standardization: The industry needs a higher-speed digital transport service for business applications. HDSL was never adopted as an international standard. Symmetric DSL - introduced as the DSL service for businesses in the late 1990s - never became a standard and interfered with the residential ADSL service because it was spectrally incompatible (very noisy). G.SHDSL is positioned for deployment in Internet and T-1/E-1 infrastructure applications because of its international standardization.
Improved data rate: G.SHDSL offers a two-wire standard operating at 2.3M bit/sec and four-wire standard operating at 4.6M bit/sec. HDSL, when initially introduced, provided 1.544M bit/sec with four wires. G.SHDSL offers roughly three times that and, when compared with the newer HDSL2 and HDSL4 services (1.544M bit/sec over two wires or four wires), still provides significantly more bandwidth.
Improved reach: G.SHDSL generally provides 20% to 30% increase in reach over HDSL at the same deliverable data rates. Additionally, when G.SHDSL multilink technologies are used, such as four-wire, Inverse Multiplexing for ATM and permanent-virtual-circuit bonding, G.SHDSL's reach is more than double HDSL's.
Spectral compatibility: G.SHDSL is spectrally compatible with ADSL, causing little noise or crosstalk between cables. Therefore, G.SHDSL services can be mixed with ADSL in the same cables without much - if any - interference.
G.SHDSL quickly has caught on in European markets, and the major North American local exchange carriers will roll it out soon.
From G.SHDSL: It's like ADSL, only better, Network World Tech Update, 11/11/02.
Additional resources
Lighting up copper
A look at MetaLoop's G.HDSL-based technology. The Edge, 10/11/02.
Providers add business-class DSL options
Network World, 02/18/02.
Cisco White Paper: G.SHDSL
Topic: DSL
Latest DSL news and analysis from Network World Fusion.
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