Since its creation in 1997, the Data Over Cable Service Interface Specification (DOCSIS) has become the worldwide standard for data transmission over cable television infrastructures. Announced in December 2001, DOCSIS 2.0 significantly enhanced previous versions by defining an improved upstream channel physical layer (PHY). The upstream channel provides a data link from the cable subscriber’s customer premises equipment (CPE) to the cable modem termination systems (CMTSs) at the cable operator’s headend or hub site. DOCSIS 2.0 made no changes to the downstream channel PHY. It continues to provide the link between the CMTS and CPE at raw data rates up to 42.88 Mbps (55.62 Mbps for Euro-DOCSIS). The upstream PHY defined by DOCSIS 2.0 allows raw data rates up to 30.72 Mbps over a single channel. Compared to DOCSIS 1.0, this represents a 50 percent increase in spectral efficiency and 300 percent increase in throughput of a single carrier. DOCSIS 2.0 enhancements increase system capacity and provide cable operators with the potential to reduce the number of upstream ports at the CMTS and thereby improve statistical multiplexing performance. The upshot of these developments is that cable operators are able to reduce their cost per bit. New Applications DOCSIS 2.0 also makes it possible for cable operators to offer new, revenue-generating applications that require a robust upstream data path such as video conferencing, voice over Internet protocol (VoIP), peer-to-peer networking and on-line gaming. In addition to providing a broader upstream path, the DOCSIS 2.0 PHY is less susceptible to channel impairments such as additive white Gaussian noise (AWGN), impulse noise and burst noise. The DOCSIS 2.0 PHY incorporates two multiple access techniques: advanced time division multiple access (A-TDMA) and synchronous code division multiple access (S-CDMA). Both technologies use quadrature amplitude modulation (QAM) as the underlying modulation format and provide enhancements to DOCSIS 1.0 and 1.1 upstream PHYs by providing additional throughput and improved robustness. A-TDMA is an evolution of the DOCSIS 1.x PHY, which uses TDMA. S-CDMA is a different approach in which up to 128 symbols are transmitted simultaneously using 128 orthogonal codes. Though there may be cases in which either technique may perform better than the other, both A-TDMA and S-CDMA provide the same maximum throughput. Both techniques also provide tools to mitigate various channel impairments. Market drivers—Development DOCSIS 2.0 provides cable operators with two primary benefits: The latter element has played a more dominant role in driving the development of DOCSIS 2.0, as explained below. Work on DOCSIS 2.0 began in August 2001. CableLabs and the industry’s leading silicon and cable modem equipment manufacturers worked to complete the detailed specifications for the enhancements to the standard. When DOCSIS 2.0 was conceived, the most popular applications for subscribers with broadband cable connections were Internet browsing and downloading of large files. Both applications required more downstream throughput than upstream throughput. At that time, cable operators also were focusing much of their capital and engineering resources on upgrading their networks from legacy one-way plants to new DOCSIS-compliant, two-way plants, with the goal of extracting as much value as possible out of their networks. Because applications such as Internet browsing and file downloading offered little opportunity to increase the average revenue per user (ARPU), cable operators were focused on earning a return on their network upgrade investments by increasing the capacity of their networks to increase the number of subscribers per network node. Therefore, the desire to create a technology that would add capacity and reduce the cost of the network on a per-subscriber basis appeared to be the dominant driver of DOCSIS 2.0. Fortunately, the standard’s ability to support throughput-hungry applications has come along for the ride. Market drivers—adoption While DOCSIS 2.0’s creation may have been driven by a desire to increase capacity of the network, the key drivers behind adoption of the standard also are worth a closer look. Times have changed quite dramatically since the days when DOCSIS 2.0 was conceived and today’s broadband reality. In North America, it is clear that the cable industry is adopting DOCSIS 2.0 and is actively implementing new upgrade policies. On the CPE side, many cable operators require all cable modems deployed in their network to be DOCSIS 2.0 compliant. DOCSIS 2.0 cable modems are interoperable and backwards compatible with older modems and CMTSs and carry little cost difference. On the CMTS side, cable operators are gearing up to deploy DOCSIS 2.0-compliant equipment, as well. They are likely to begin by greenfield installations followed by a gradual upgrade of existing CMTSs. Other regions of the world are expected to follow North America’s lead in the near future. However, market dynamics outside North America vary and include elements such as stiff competition from rival technologies such as digital subscriber line (xDSL) that are deployed with very high data rates provisioned for users. In these regions, DOCSIS 2.0’s increased upstream throughput provides a strong answer for cable companies. From the applications and services aspect, several applications available today will continue to move the market toward the trend of requiring a more symmetrical pipe. Video conferencing, on-line gaming, peer-to-peer and file sharing applications, personal Internet servers and corporate virtual private networks (VPNs) are all upstream-intensive applications. Although the market is not yet at the stage where the asymmetrical nature of the existing pipe is severely limiting their use, the fact that these upstream-dependent broadband applications exist and are in use today reflects a clear trend towards an increased demand for symmetrical data capacity. What’s next…? With cable operators upgrading their networks to gain the capability to handle very high throughput data transfer in both directions, it is important to envision what next-generation services will look like and examine whether or not they will exploit the true potential of next-generation networks. Looking at the dynamics of the PC market as an analogy, Moore’s law predicts that, every 18 months, silicon devices double in their performance for a given price point. This steep silicon curve goes hand-in-hand with a comparably steep software curve. While it is sometimes difficult to figure out which is the driving factor, it is obvious that one cannot economically exist without the other. Applying the PC industry model to the cable industry, it is likely that new and existing applications and services will evolve over time to take advantage of the capabilities that DOCSIS 2.0 enables. For instance, imagine a grandmother in Ohio using a broadband cable connection to engage in a two-way video conversation with her grandson in California. At the same time, grandson number two could be using that same broadband connection to shoot down aliens from Mars as he plays his favorite interactive computer game in the family room. All parties could freely engage in their broadband pursuits without having to endure the “choppiness” and delay that subscribers have become all too familiar with today when using their broadband networks. In fact, game console manufacturers are considering including DOCSIS 2.0 cable modem modules in their next-generation products. Tomorrow’s set-top boxes and interactive TVs are likely to include Web cams and support high-speed data connections. New devices will support applications such as personal video chat and broadband telecommuting. With more users needing corporate-like connections to their work servers, demand for DOCSIS 2.0-compliant end devices is likely to increase sharply. The previous examples are only a partial list of next-generation services that have been conceived. There are many other new and exciting applications and services waiting to be invented that will make good use of the benefits offered by DOCSIS 2.0 and upgrades of the specification yet to come. The DOCSIS versions available today provide the promise of future growth and increased revenue opportunities for service providers as the subscribers around the world connect to one digital universe. Eran Eshed is product marketing manager at Texas Instruments Cable Broadband Communications Group. Email him at [email protected]. Oded Melamed is director of program management at Texas Instruments Cable Broadband Communications Group. Email him at [email protected]. Did this article help you? Send comments to [email protected]. Bottom Line: Move to Symmetry
The upstream physical layer channel defined by DOCSIS 2.0 allows raw data rates of up to 30.72 Mbps over a single channel. These enhancements increase system capacity and provide operators with the potential to reduce the number of upstream ports at the CMTS and thereby improve statistical multiplexing performance, resulting in reduced cost per bit. Today’s apps are increasingly symmetrical. Video conferencing, on-line gaming, corporate VPNs and P2P applications are all upstream-intensive. Reflecting a clear trend towards an increased demand for symmetrical bandwidth, which can be satisfied with DOCSIS 2.0.

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