The latest iteration, DOCSIS 3.1, has raised the bar once again with downstream speeds up to 10 Gbps and upstream speeds up to 2 Gbps. Speeds in this range had previously only been possible using fiber optic technology, so this breakthrough with DOCSIS 3.1 technology has provided service providers flexibility in maintaining existing coax to home connections without significantly impacting performance.
DOCSIS 3.1 has incorporated multiple advancements to make cable a viable player in the ultra-high-speed arena. The improvements engendered by DOCSIS 3.1 merit a closer look at the basic components of DOCSIS, and the opportunities for optimized speed, bandwidth and reliability that have been realized within them.
The physical layer (PHY), as the name suggests, refers to the visible hardware elements of the system, including the equipment and wiring, as well as the frequencies used for transmission. Utilizing carriers in the range of 25 – 50 kHz wide, thousands of signals can now occupy the same cable that once housed a single television connection. These signals take the form of sub-carriers, disseminating the signal into discrete elements that are later recombined by the receiver, thereby optimizing density and throughput.
To put skinny carriers to work, improved technology to minimize or eliminate the guard-banding or spacing between signals was necessary. In DOCSIS 3.1, this has been accomplished through orthogonal frequency division multiplexing (OFDM). This technology takes the existing concept of channel bonding, first seen in DOCSIS 3.0, and builds upon it through the principle of mathematical orthogonality. Essentially, subcarrier signals placed side by side are transmitted orthogonally to one another, thereby enabling the receiver to accurately demodulate the individual signals. This concept is graphically equivalent to the peak of one wave aligning with the low point of an adjacent wave of the same frequency.
Forward Error Correction
Forward error correction (FEC) is a technique through which a receiver can detect errors in redundant signals, then correct them without retransmission. A new feature of DOCSIS 3.1 is a method of FEC known as low density parity check (LDPC). Although FEC existed in past DOCSIS versions as well, improvements in encoding have led to nearly 100% correctable LDPC codeword errors. In turn, this improvement has led to greater noise resiliency and a higher level of modulation.
DOCSIS 3.1 Frequency Range
The DOCSIS 3.1 frequency range has been expanded in stages. This broad overall range is an important element in achieving exceptional upstream and downstream speeds. The current 3.1 frequency range extends from 5 MHz to 1218 MHz, with the upper limit reaching 1794 MHz for Full Duplex DOCSIS 3.1. The upper frequency limit for DOCSIS 3.0 was a bit lower, at 1002 MHz. Channel bandwidths within the 3.1 spectrum can reach up to 96 MHz for upstream and 192 MHz for downstream.