Digital Signals over RF Channels

Cable specifications are defined by a document known as Data-over-Cable Service Interface Specifications (DOCSIS). DOCSIS is an international standard developed by CableLabs, a nonprofit organization and development consortium dedicated to cable-related technologies. Founded in 1988, CableLabs is essentially charged with the testing and certification of cable technology access equipment such as cable modems and CMTS. The organization makes decisions on standardization and grants for DOCSIS certification and qualification.

The core of DOCSIS defines the manner in which individual components communicate in the cable network. The specification for data-over-cable defines high-speed data transfer over an existing CATV system. Cable operators use DOCSIS to implement Internet access over their existing HFC infrastructure.

Cable transmissions are highly similar to wireless transmissions, with the obvious exception of the presence or absence of copper. DOCSIS defines the frequency plan to be used as well (6 MHz for DOCSIS, 7 MHz and 8 MHz for Euro-DOCSIS). As discussed, cable transmission uses the RF bands. The RF band is composed of the frequencies above audio and below infrared.

Within DOCSIS are the OSI Layer 1 and Layer 2 requirements for connectivity between cable devices:

■ Physical layer (Layer 1)—Definition of data signals to be used by cable operators. DOCSIS specifies bandwidths for each channel. These channel widths are 200 kHz, 400 kHz, 800 kHz, 1.6 MHz, 3.2 MHz, and 6.4 MHz. Additionally, DOCSIS defines the manner in which these signals are modulated.

■ MAC layer (Layer 2)—Definition of a deterministic access method depending on DOCSIS version: time division multiple access (TDMA) for version 1.0, 1.1, and 2.0 or synchronous code division multiple access (S-CDMA) in version 2.0. The MAC layer protocol controls access to the return path. The DOCSIS MAC protocol uses a request/grant system for transmissions. This means that there is little or no use of contention for bandwidth as in Ethernet networks (and no collisions).

Like many other standards and specifications relating to technology, DOCSIS is evolving. DOCSIS version 1.0 was released in March 1997, followed by version 1.1 in April 1999. Version 2.0 came about in January 2002 as a result of increased demand for symmetric, real-time services and applications such as IP telephony. This release enhanced the technology by augmenting upstream speeds and putting QoS capabilities in place.

DOCSIS 3.0 was released in August 2006. Expected enhancements may include IPv6 support and channel bonding. Channel bonding allows the use of multiple downstream and upstream channels together, at the same time, by the same subscriber to increase overall bandwidth. In fact, through the use of the Wideband architecture pioneered by Cisco, current expectations would allow the offering of 100+ Mbps services to the subscriber. In fact, DOCSIS 3.0 expects capabilities reaching 160 Mbps downstream with 120 Mbps upstream.

With new products on the horizon from Cisco's Linksys and Scientific Atlanta business units, speeds and services will most likely continue to evolve well beyond current imagination.

More information regarding DOCSIS can be found at CableLabs' website: http:// www.cablemodem.com/specifications/.

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