DOCSIS - Features

Features

DOCSIS provides great variety in options available at Open Systems Interconnection (OSI) layers 1 and 2, the physical and data link layers.

Physical layer

• Channel width: All versions of DOCSIS utilize either 6 MHz channels (e.g. North America) or 8 MHz channels ("EuroDOCSIS") for downstream transmission. In the upstream, DOCSIS 1.0/1.1 specifies channel widths between 200 kHz and 3.2 MHz. DOCSIS 2.0 also specifies 6.4 MHz, but can use the earlier, narrower channel widths for backward compatibility.
• Modulation: All versions of DOCSIS specify that 64-level or 256-level QAM (64-QAM or 256-QAM) be used for modulation of downstream data, utilizing the ITU-T J.83-AnnexB standard for 6 MHz channel operation, and the DVB-C modulation standard for 8 MHz (EuroDOCSIS) operation. Upstream data uses QPSK or 16-level QAM (16-QAM) for DOCSIS 1.x, and it uses QPSK, 8-QAM, 16-QAM, 32-QAM, 64-QAM for DOCSIS 2.0 & 3.0. DOCSIS 2.0 & 3.0 also support 128-QAM with trellis coded modulation in S-CDMA mode (with an effective spectral efficiency equivalent to that of 64-QAM).

Data link layer

• DOCSIS employs a mixture of deterministic access methods for upstream transmissions, specifically TDMA for DOCSIS 1.0/1.1 and both TDMA and S-CDMA for DOCSIS 2.0 and 3.0, with a limited use of contention for bandwidth requests. In contrast to the pure contention-based MAC CSMA/CD employed in older Ethernet systems (there is no contention in switched Ethernet), DOCSIS systems experience few collisions.
• For DOCSIS 1.1 and above the MAC layer also includes extensive quality-of-service (QoS) features that help to efficiently support applications that have specific traffic requirements such as low latency, e.g. voice over IP.
• DOCSIS 3.0 features channel bonding, which enables multiple downstream and upstream channels to be used together at the same time by a single subscriber.

Throughput

All of these features combined enable a total upstream throughput of 30.72 Mbit/s per 6.4 MHz channel, or 10.24 Mbit/s per 3.2 MHz channel. All three versions of the DOCSIS standard support a downstream throughput with 256-QAM of up to 42.88 Mbit/s per 6 MHz channel, or 55.62 Mbit/s per 8 MHz channel for EuroDOCSIS. (see table below)

Network layer

• DOCSIS modems are managed via an IP address.
• DOCSIS 3.0 adds management over IPv6.
• The 'DOCSIS 2.0 + IPv6' specification also allows support for IPv6 on DOCSIS 2.0 cable modems (via a firmware upgrade)