By Hal Roberts, Benoit Legault, Mike Rude, ADC
The purpose of CableLabs® first Data Over Cable System Interface Specifications (DOCSIS) - DOCSIS 1.0 and DOCSIS 1.1, were to respectively enable residential data services and voice services over a single Internet Protocol (IP) cable infrastructure. The 1.0 specification defined the upstream and downstream physical and data link layers necessary to transmit over shared multiple-access cable IP networks. DOCSIS 1.0 specified the basic Quality of Service (QoS) features required to offer tiered services based on rate-limits, and was later enhanced to support minimum guaranteed rates. The DOCSIS 1.1 specification introduced support for constant bit rate services, which greatly enhanced the QoS feature set, and somewhat improved the robustness of the return path, which allowed twice the bandwidth, while providing full backward compatibility with the 1.0 specification. In December 2001, CableLabs® released the first version of the DOCSIS 2.0 specification. The primary objective of DOCSIS 2.0 is to enhance upstream spectral efficiency, which requires additional robustness. This paper’s objective is to investigate the new features introduced in DOCSIS 2.0, by closely examining its benefits to legacy 1.0 and 1.1 cable modems (CMs), and 2.0 CMs. DOCSIS 2.0 achieves the goal of increasing upstream spectral efficiency and robustness by enhancing the 1.x (DOCSIS 1.0 and 1.1) TDMA modulation encoding method, renaming it Advanced-TDMA (ATDMA), and by introducing a new upstream modulation encoding method known as SCDMA. DOCSIS 2.0 requires that CMs and cable modem termination systems (CMTSs) support both A-TDMA and S-CDMA, thereby leaving the choice of enabling either or both methods on the DOCSIS channels to the operator. This paper analyses both encoding schemes to help the reader better understand the how they should be enabled in the network.