Cable operators have long realized the advantages of open systems and of advanced digital technologies. CableLabs, as the cable industry’s R&D consortium, has as one of it’s objectives to serve as an incubator of new technology. With that goal in mind, the cable industry began an initiative to standardize digital cable terminal devices. Known as OpenCable™, that initiative’s primary objective is to standardize equipment functionality and interfaces, and related components and equipment used in the cable plant.
Another objective is to standardize delivery signal components in order to attain interoperable digital STBs and other advanced digital devices manufactured by multiple vendors. These devices would be capable of delivering digital video, data and interactive services to a television set. The capability also may be built into consumer electronics devices, such as digital television receivers.
In the traditional business model, cable operators lease STBs to those cable subscribers who wish to receive scrambled services, who have a very limited choice of equipment and, if they move from one service area to another, must obtain a STB from the operator serving their new area of residence. However, if specifications for signal component delivery and customer premises equipment (CPE) are standardized, that would enable multiple vendors to manufacture STBs and sell them at retail. Multiple vendor participation would create a wide choice of equipment, varying in features and functions, offering consumers the ability to buy the STB of their choice at a competitive price. The end result would be a proliferation of innovation in equipment.
In the cable plant (a generic architecture diagram is shown in Figure 1), signal content (video, Internet, and other data)—both external and local—is interfaced at the headend. Headend equipment processes those signals and delivers them via a hybrid-fiber/coax (HFC) cable network. The external contents consist of direct feed (via coaxial or optical fiber), off-air or terrestrial signals, and satellite distributed signals. Signals sent from terrestrial broadcast (direct or off-air) networks are usually in the clear. Signals sent from premium content producers are distributed via satellite or direct media, and are usually transmitted in encrypted form along with a conditional access (CA) system. All network distributed signals are demodulated to baseband signals and then de-encrypted.
Signals containing premium content need to be re-encrypted by the encryption method used by each cable system headend (selected by the operator) before being modulated over a RF carrier and transmitted over the cable network.
Figure 1. Cable Plant Generic Architecture
Analog television programs are categorized into service tiers (e.g., basic, enhanced basic, premium, pay-per-view (PPV), etc.). Basic services consist of clear channels, which do not require a STB for viewing; enhanced basic services also do not require a STB for viewing—in each case, only a cable-ready TV is required. Higher service categories (e.g., premium and PPV channels) need a STB for viewing. Premium service includes all basic channels, enhanced services, plus some additional channels. STBs are authorized from the headend to descramble additional channels and PPV channels. A STB, when authorized from the headend, demodulates and descrambles (if necessary) user-selected channels. The baseband signal then remodulates a channel 3 or 4 carrier before sending it to a coaxial connector—to view this signal, a TV must be connected to the STB’s coaxial connector and tuned to channel 3 or 4. In this case, channels are selected by the STB tuner and not the TV tuner. In a hybrid system, where both analog and digital services are available, a hybrid STB with digital decoding capability (MP@ML per MPEG-2 standard) is necessary to view digital channels. Video and audio signals are compressed and encrypted. To view digital PPV channels, the STB has to be authorized to de-encrypt the signals. Access control and de-encryption subsystems are embedded inside the STB.
The OpenCable project is working to standardize interfaces so that STBs (or other terminal devices) can be designed and manufactured by multiple vendors, yet still interoperate. In order to design interoperable and portable devices, the device’s network and output interfaces will have to be standardized and specified. The minimal functional requirements for STBs will have to be specified to leave room for creativity in hardware and software design, future enhancements, and add-on features.
Since a major portion of cable systems’ revenue derives from the delivery of premium content, access control of all digital channels (with the exception of clear channels) and individual services (if a channel carries more than one service) are of prime concern to cable operators. A digital STB, with its counterpart at the cable system headend, should be able to control delivery of the channels and services delivered through individual STBs.
Traditionally, cable system headends used analog scrambling systems. When digital services were added to existing analog services, a number of systems (cable headends) introduced a hybrid STB (in their customers’ homes) with analog and digital descrambling capabilities to manage both types of services. Replacing them with new systems would engender high costs for cable operators. To preserve their investments, these legacy digital systems will have to be supported by the OpenCable system—an important requirement in creating the OpenCable specifications. The legacy systems use one of two encryption methods: the Digi-cipher system designed by General Instrument (GI), or the Power Key system created by Scientific-Atlanta (SA). In designing an interoperable and portable STB, it would be simpler not to have to support an existing legacy system. However, to meet the FCC requirements of retail availability and portability, as well as to preserve existing digital delivery systems, the cable industry has come up with a novel idea—to de-couple the security module from the STB. This point-of-deployment (POD) security module is a type of PCMCIA smart card. The security module and the STB each will have either a male or female PCMCIA connector. When snapped together, the connectors will provide the required connectivity between the STB and the POD module. De-coupling the security module will add an additional advantage in that it is renewable. If, for some reason (e.g., security break-in, future enhancements, or expansion of security system, etc.), the card needs to be replaced, the STB or other "host" device will not be affected.
STBs will be portable from system to system. However, PODs will not be portable—when a STB is deployed in a service area, a POD module compatible with the new system will be needed to de-encrypt digital signals. It will be possible for consumers to purchase STBs, but POD modules will be available from their local cable operator when they request encrypted service.
A POD module is completely dependent upon the CA system used in the headend. If the headend currently uses, for example, SA’s Power Key CA and encryption system to support existing STBs, the same CA and encryption system will have to be used with the POD module. If an OpenCable-compliant STB is deployed in that service area, a POD compatible with the Power Key system has to be used to view premium channels. Similarly, if a headend currently uses GI’s Digi-cipher CA and encryption system, the OpenCable-compliant STB will require a Digi-cipher-compatible POD.
OpenCable is not restricted to using only GI’s and SA’s CA and encryption systems; it is designed to use systems from other vendors as well. A large number of cable systems have not yet rolled out digital services and are awaiting the availability of OpenCable-compliant STBs. These systems will have no digital CA and encryption system legacy issues, and will be able to utilize CA and encryption systems from any vendor. Of course, the POD module from the respective CA system supplier will be required for use in their service area.
The host/POD interface needs to be specified in designing a portable and interoperable STB. This is needed in addition to a network interface (OCI-N) and an interface with home digital appliances, such as a digital TV and a digital VCR. These interfaces are shown in Figure 2, where OCI-N is the network interface, C1 is the output interface, and C2 is the host/POD interface. OpenCable has prepared specifications for these interfaces, in addition to functional requirements for the STB. These are the major documents needed to design portable and interoperable STBs compliant with OpenCable specifications. The functional requirements are written for a baseline STB; manufacturers may add other features in designing a STB with added functions.
Figure 2. OpenCable Set-top Box (STB) Interfaces