Over the past few years, cable operators’ hybrid fiber/coax (HFC) networks have gone through a major evolution. The upgrading of the infrastructure to enable a return signal path has created a 2-way network. This has paved the way for the delivery of broadband services over the cable network infrastructure. These services have resulted in new revenue streams for the network operators, from simple high-speed Internet access for Web surfing and e-mail, through to a whole portfolio of new data-oriented offerings.
Figure 1. A cable data system consists of cable modems (CMs), located in subscribers’ homes, as well as the cable modem termination system (CMTS), which can be located within the cable headends or in the distribution hubs. The CMTS communicates with the CMs via the existing cable infrastructure.
Enabling broadband over cable requires the introduction of new network elements on top of the existing cable infrastructure. A cable data system consists of cable modems (CMs) located in subscribers’ homes, and the cable modem termination system (CMTS). The CMTS can be located within the cable headends or located in the distribution hubs. The CMTS communicates with the CMs via the existing cable infrastructure. The CMTS may be in either a fixed or a modular chassis-based form factor. (See Figure 1.) The CMs and the CMTS make use of the data-over-cable system interface specifications (DOCSIS1), which have become the standard for data communications over HFC cable networks.
The DOCSIS specifications have been optimized for the slightly different spectrum plan of European cable operations, in the form of EuroDOCSIS. While the 5MHz to 42MHz spectrum is reserved for the DOCSIS upstream channel from the CMs to the CMTS, for EuroDOCSIS it is 5MHz to 65MHz. Using the 120MHz to 860MHz spectrum for downstream transmission from the CMTS to the CMs, DOCSIS makes use of a 6MHz downstream channel width, while EuroDOCSIS uses 8MHz channels. This means that some of the downstream spectrum becomes unavailable for video transmission, potentially reducing the maximum channel count.
This may have to be factored into business decisions. In the conventional CMTS receiver architecture, the received upstream signal is fed to an analog front-end that selects the desired channel, amplifies it and converts it down to baseband. The frequency band for upstream is subject to spurious impulse noise and interference from various sources, including amateur radio, which can present challenges to broadband deployment if not addressed. Spectrum management techniques must be used to navigate around these defects in the upstream channels, or fully digital front-end architectures and noise canceling techniques can be used to mitigate against the effects of such ingress noise. Multiple modulation schemes are used within DOCSIS.
The downstream makes use of 64-quadrature amplitude modulation (64-QAM) and 256-QAM, while quadrature phase shift keying (QPSK) and 16-QAM are used on the upstream. The quality of the network infrastructure and the CMTS implementation can restrict deployments to the lower bit-rate modulation schemes, limiting the total available bandwidth.
The overall DOCSIS architecture gives rise to an asymmetry between the upstream and downstream bandwidths, with more bandwidth being available for the downstream transmissions. This closely meets the needs of activities such as Web surfing, because users will typically receive more data from the network than they send into the network.
The upstream data traffic usually consists of simple requests and receipt acknowledgements, while the downstream traffic is often rich media content, including graphics, audio and video. Symmetric traffic capacity can be created, if required, by allocating multiple upstream channels for each downstream.
Initial broadband over cable deployments using DOCSIS 1.0 have focused on ‘best effort’ services such as Internet access, which provide additional income through a monthly flat-rate subscription charge.
The roll-outs have also laid the foundation for interactive TV offerings. The computer, entertainment and communications industries are converging on a common digital infrastructure, delivered over broadband networks, and the industry is betting that the delivery of new services over this infrastructure will increase user-demand for services. The data bandwidth can be used for secondary video services, such as video conferencing, or TV and Web browsing in a merged environment using integrated digital set-top devices that include a Web browser. URLs can be hidden inside commercials, enabling a single button push to allow users to follow links to more content, providing interactive TV. These services continue to come to maturity, while Web content from the Internet, delivered at broadband bandwidths, can already include medium-quality video delivered to the PC.
The newer DOCSIS 1.1 includes enhancements to enable quality of service-based (QoS) offerings. This support for QoS enables tiered data services, based on different bandwidths or subscription rates. Broadband cable offerings can now include business services such as remote access for teleworkers, voice over IP (VoIP), gaming and other applications that have strict traffic management requirements.
Broadband over cable has a continued evolution path, including services based around delivering more symmetric bandwidth for business users, the delivery of video, enabling true video-on-demand, as well as near video-on-demand. The IP-based broadband infrastructure also supports technologies such as multicast, allowing the new infrastructure to mimic the transport efficiencies of the more traditional video delivery methods, resulting in new business models and revenue streams for the operator.
DOCSIS 2.0 includes two new modulation techniques, advanced time-division multiple access (A-TDMA) and synchronous code-division multiple access (S-CDMA). The specification includes increased channel bandwidth, several additional modulation schemes, including 64-QAM, and an improved forward error correction scheme to help address RF quality issues. All of this, however, will require the deployment of new CMs, which means techniques to extend the life of current CMs must be more cost-effective.
While many of the cable plants in Europe have already been upgraded to support broadband services, this is still an ongoing process. In today’s capital-constrained environments, service continues to be rolled out in incremental fashion, based on the revenue opportunities. As the number of cable broadband users increases, the RF performance of the network becomes more of an issue, because the ingress noise increases at the same time as the requirement for more bandwidth.
The bulk of the operational issues around deployment of the technology have been addressed. The future challenges are deploying new services to further grow the revenue. In the meantime, broadband on cable networks is providing affordable high-speed Internet access to millions of users across Europe.
Data-over-cable service interface specifications - RF interface specification, SP-RFI-I05-991105, Cable Television Laboratories, November 1999, Louisville, Colorado.
Benjamin M. J. Ellis is product marketing manager (EMEA) for Juniper Networks.
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