Is SCDMA better for impulse noise environments while ATDMA is better for ingress?
A. SCDMA has a Burst Noise Advantage over ATDMA, because of its capability to spread transmissions out over time. Multiple codewords are sent simultaneously, which effectively interleaves codewords from different cable modems. However, SCDMA uses longer symbol times than ATDMA, and this reduces the number of errored symbols created for a given forward error correction (FEC) block. This allows those errored symbols to be corrected with the FEC information. However, these limitations for SCDMA modems must be considered in the real world: • Must perform periodic ranging for all modems every second. • Only gives throughput benefit when more than 60 percent of upstream traffic is carried in SCDMA mode. • Significant interoperability issues remain in SCDMA mode between different cable modem vendors that have not closely followed the DOCSIS 2.0 specification. Remember, cable networks are not dominated by burst noise in absence of ingress or narrow band interference. These two always occur tog
