The G.SHDSL (single-pair high-speed digital subscriber line) standard includes the transfer capabilities of HDSL, the speed variations of ADSL, and single pair support. Add to this the electrically friendly line code from HDSL2 that reduces the interference caused by other DSL technologies and a new winner emerges in the DSL environment.
G.SHDSL is an important development because it provides a symmetric DSL capability over a single copper pair at speeds from 192 kbps to 2.312 Mbps. The addition of a second pair doubles the speed capability. It is also important in that it addresses the key features and benefits found in other proprietary or standard implementations. In this sense it is viewed as a convergent technology that will promote interoperability in the DSL community.
G.shdsl addresses rate and range adaptability, spectral compatibility with adjacent services in a cable bundle, impairment tolerance, and high-speed symmetric applications. The standard was created to deal with business-based applications like multiple line voice delivery, Internet access, and remote LAN access. Over a single pair G.shdsl offers the predictability of TDM with the statistical multiplexing flexibility of packet switching.
G.SHDSL Line Code
In the development of G.SHDSL, the major obstacle to be overcome was the elimination of noise interference from other services (e.g., ADSL). The specific problems were near end and far end crosstalk. These problems were exacerbated by the fact that a cable bundle could have 25 pairs, a residential bundle, or as many as 600 pairs, a bundle near the CO. The trick was to get the information transfer in the 0–400 kHz range, which would keep the G.SHDSL transmission out of the high frequencies used by other DSL services. Moreover, since the service was designed for voice, the end-to-end latency had to be less than 500 microseconds. This meant that both the transmission technique and the digital signal processing had to be optimized.
G.SHDSL uses a one-dimensional TCPAM signal processed through a framer circuit and modulated into the analog domain by the analog front end. (The details of this process are well beyond the scope of this discussion.) This process is similar to that in ADSL. It is very different, however, from HDSL and T-1’s use of digital line codes (2B1Q and AMI respectively). The 16 level PAM uses a rate 1/2 trellis coder that protects one out of three bits per symbol by using a convolutional algorithm. The whole process has an end-to-end latency of 216 microseconds—more than adequate for voice services.
TCPAM offers the characteristics of high speeds and long distances while lowering the spectral interference seen in other “digital” line codes. This last attribute allows G.SHDSL to coexist in a bundled cable with other services. Compared to a 2B1Q system running at the same data rate, the TCPAM system uses a third of the spectrum (-120 dbmHz at 400 kHz vs. 1200 kHz).
G.SHDSL Distances and Rates
The G.SHDSL standard allows a user or provider to select a rate in 8 kbps increments between 192 kbps and 2.3 Mbps for single pair operation and a rate in 16 kbps increments between 384 kbps to 4.6 Mbps for two pair operation. G.SHDSL modems use a startup line equalization process to adapt the transmission rate to the line conditions. It is expected, however, that the provider will fix the actual line rate during installation. This allows the G.SHDSL lines to be used for DS-1, FT-1, E-1, and FE-1 services as well as normal Internet access. The line rates are symmetrical (i.e., the same rate is expected in both the upstream and downstream directions).
The distances applicable to a G.SHDSL line depend on the data rates, and vice versa. For the maximum data rate of 2.3 Mbps the nominal operating distance is about 7000 feet (a product brochure shows a maximum of 9300 meters, 28.8 kft.). The table gives some rates and distances assuming a 26AWG cable.
|Speed (Mbps)||Distance (kft.)|
The actual data rates depend on the type of twisted pair used in the outside plant and the noise characteristics of the cable plant. Wire gauge, splices, bridge taps, gauge changes, line attenuation, and crosstalk levels also contribute to the speed/distance equation. Speeds and distances increase with the use of a second pair. For long distances, the standard supports up to eight repeaters.