Competing for throughput in the local loop

The voice-grade telephone channel is the communication medium most widely available throughout the world. Its limitations, a relatively narrow bandwidth and a moderate SNR figure, have challenged telecommunication researchers and engineers over several decades to develop technologies able to pass higher and higher data rates. The sophisticated combination of trellis coding and quadrature amplitude modulation increases the bandwidth utilization close to the Shannon limit of 10 b/s/Hz for the 3.1 kHz bandwidth voice channel. However, the resultant data rate of about 30 kb/s does not even begin to approach the throughput needed by multimedia applications. Fortunately, twisted pair bandwidth reaches up to several megahertz, so multilevel signaling implemented as in the xDSF family of modems enables data rates of several megabits per second to be reached. Standard ADSL modems are able to deliver 8 Mb/s to a subscriber in the best case. VDSL modems can deliver 52 Mb/s, but only up to 300 m, so their mass implementation will only become possible when fiber migrates into an access network closer to the user premises. Both broadband xDSL solutions approach the Shannon limit and the related spectral efficiency as well, so there is not much room left for a transmission improvement.