Advanced Digital Set-Top Boxes: Making a Case for Flash File Systems

Over the past couple of years, the convergence of television and computers has taken a major step forward with the proliferation of digital TV technologies. This new environment facilitates the broadcasting of data alongside video and audio content. One of the more practical devices for accessing and using this new media is the digital set-top box. Multiple Service Operators (MSOs), such as cable TV and Internet Service Providers (ISPs), are moving aggressively to capitalize on the opportunities emanating from this new paradigm by installing millions of these types of appliances in homes across the globe.

The storage technology chosen by set-top manufacturers accounts for a substantial percentage of the overall cost of a digital set-top box. This paper offers a new insight into the technical and financial benefits of incorporating storage solutions based on flash file system technology into mid-range and advanced digital set-top boxes.

The launch of digital television services is having a profound affect on the market for set-top boxes. In many countries, service providers are retrofitting subscriber's analog set-top boxes with new digital set-top boxes. In addition, some of the more technologically developed countries are beginning to push second-generation set-top boxes to support a range of new services. The set-top box, once a relatively passive device, is now capable of handling traditional computing and multimedia applications.

This huge installed base of set-top boxes includes:

• Analog set-top boxes
  Analog set-top boxes perform the functions of receiving, tuning and de-scrambling incoming television signals. These appliances have changed very little over the past twenty years.
  
  
• Dial-up set-top boxes
  Dial-up set-top boxes allow subscribers to access the Internet from the comfort of their living room through the television. An excellent example in this category would be the NetGem Netbox.
  
  
• Entry-level digital set-top boxes
  Entry-level digital set-top boxes are capable of receiving broadcast digital television that is complemented with a pay-per-view system and a very basic navigation tool. Characteristics of this type of box include low cost, limited quantities of memory, interface ports and processing power.
  
  
• Mid-range digital set-top boxes
  Mid-range set-top boxes include a return path or back channel, which provides communication with a server located at the head-end. These types of boxes have double the processing power and storage capabilities of entry-level boxes. For example, while a basic set-top box needs approximately 1-2 Mbytes of flash memory (mostly for code storage) in order to operate, mid-range set-top boxes normally include between 4 and 8 Mbytes of flash memory for code and data storage.
  
  
• Advanced digital set-top boxes
  Digital set-top boxes in the advanced category bare a close resemblance to a multimedia desktop computer. These boxes can contain more than ten times the processing power of a low-level broadcast TV set-top box. You can use enhanced storage capabilities of between 16 and 32 Mbytes flash memory (for code and data storage) in conjunction with a high-speed return path to run many advanced services. These services include video teleconferencing, home networking, IP telephony, video-on-demand (VOD), and high-speed Internet TV services. In addition, subscribers are able to use enhanced graphical capabilities within these types of boxes to receive high definition TV signals.
  
  
• Advanced set-top boxes that include PVR (Personal Video Recorder) functionality
  The idea of putting a hard disk drive (HDD) into an advanced digital set-top box in order to provide PVR functionality is getting increasing attention from MSOs and manufacturers alike. Such receivers may come with a choice of home networking ports, which might later allow them to be used as residential gateways.

Advanced and mid-range set-top boxes incorporate the necessary hardware and software subsystems to receive digital television, the Internet, and Interactive TV services.

• Software
  There are three layers of set-top box software required to operate a digital set-top box—the operating system and device-drivers layer, the middleware layer, and the user-applications layer. The operating system and device-drivers layer keep all parts of the set-top box operating together. Vendors addressing this space include Microsoft, Wind River, various Linux vendors (such as Lineo and JNT), Microware Systems, and PowerTV. The middleware is a layer of software programs that operate below the interactive TV applications and above the operating system, providing set-top box programmers with a common API to which they may write applications. Key vendors and technologies that compete in this market include OpenTV, Liberate Technologies, Canal Plus Technologies, PowerTV, and Microsoft. Subscribers use the application-software layer to watch TV and use interactive features.
  
  
• Hardware
  Advanced digital set-top boxes comprise three separate subsystems—TV, conditional access (CA), and PC components. The TV subsystem includes a number of tuners and video decoders that are responsible for processing streams of digital information. The CA system provides MSOs with unprecedented control over what their subscribers watch and when they watch it. The PC subsystem itself is modular-based, which means that set-top designers can add and subtract various components depending on user requirements. For instance, MSOs that want to offer Internet services to their subscribers will incorporate some type of storage solution into their PC subsystem.

There is a considerable amount of uncertainty as to how the overall digital set-top box market will develop in the coming years. Most analysts are predicting the set-top box will evolve into a residential gateway and the primary access point for subscribers connecting to the Internet. Such a move from the relatively low-level set-top boxes that exist today to powerful home-networking centers demands a flexible, reliable, secure, and scalable embedded-storage solution. Manufacturers of set-top boxes have two main options when selecting storage solutions for their set-top box designs—solid-state flash-memory storage and the mechanical hard disk.

Flash Memory Storage
A flash-memory chip is essentially a type of non-volatile memory (like EEPROM). Flash-memory components offer some very attractive features for storing data and software code. Flash chips are non-volatile, so data is retained without supplying any power to the flash components. Flash memory consumes very little power and may take up very little space. It uses solid-state technology and has no moving parts, so it can work in any living-room conditions where mechanical hard disks might prove unsuitable in the longer run.

There are two general categories of flash solutions, local (or embedded) flash storage and removable flash storage. Removable flash storage, such as compact flash, includes a dedicated hardware controller used to manage the flash memory (which naturally differs greatly from the physical characteristics of a hard disk). This dedicated hardware controller, along with the extra packaging and socket involved, make the compact flash a problematic component from a cost-structure point of view when considering it as an alternative to embedded local storage within a set-top box.

The local flash memory market can be further divided into two broad categories, based on its two dominant technologies—NAND and NOR. Both technologies have unique features and are aimed at fulfilling different market needs.

A NOR-based flash solution, such as Intel's StrataFlash, has a faster read-cycle characteristic than a NAND-based flash product. The NOR-based code execute-in-place (XIP) capability, albeit far slower than code execution in RAM, makes it a suitable technology for storing small amounts of executable code in a very similar way to storage in a primitive ROM. These characteristics make NOR-based products an ideal fit for entry-level set-top boxes, which only need software code storage and execution capabilities. For mid-range and high-end set top boxes, which require all sorts of data storage on top of the software code storage, the paradigm is completely different.

Manufacturers such as Toshiba and Samsung have optimized NAND-based storage solutions for data storage operations. This technology offers write/erase cycle-response times over 15 times faster than equivalent NOR solutions. In addition, NAND solutions also have an increased ability to withstand rigorous write/erase cycles over long periods of time. These unique storage characteristics of NAND flash make it an ideal solution for MSOs that want to extend the functionality of their digital set-top box platforms. NAND is also far more cost effective than NOR when higher capacities are involved, making it the perfect technology for mid-range and high-end set-top boxes.

However, NAND flash on its own is unable to support the increasing number of services that are available from MSOs. In addition to the silicon itself, a file system is required to interact with the actual NAND flash-memory array in order to provide the functionality of a mechanical hard drive on a solid-state silicon chip. Typically, a flash file-management system is a piece of software code, used to make flash-memory components emulate a disk drive. The world standard in flash file management systems is the TrueFFS software originally patented by M-Systems over eight years ago and later adopted by Microsoft, Wind River, and many other vendors.

This approach allows set-top box designers to use a common, well-understood mechanism for storing data on non-volatile media.

Required functions for a set-top box flash file-management system include:

• Mapping the file structure of the real-time operating system (RTOS) to the physical flash system
• Increasing the endurance and lifecycle of the flash memory
• Detecting and correcting data errors on the fly.

Hard-Disk Storage Solutions
The popularity of PVR technology and the promise of its related services are posing huge challenges to set-top manufacturers and MSOs alike. At the heart of a PVR-centric set-top box is a hard-disk drive. However, prior to enabling HDD technologies in new product designs, set-top manufacturers and MSOs should seriously and carefully consider the drawbacks of deploying a hard-disk-only storage solution. One of the main risks has to do with a design decision to use the hard disk for all set-top box functionality, described as follows:

1. Storing the set-top box's software code
2. Storing system and user data (such as user profiles, configuration, the system registry, updateable system files, and so on)
3. Storing video streams (the actual PVR functionality).

Such a set-top box design, which uses the hard disk to store all code and data is very risky—if the hard disk develops physical errors after deployment, or fails altogether, the set-top box may completely cease to operate and the subscriber is left unable to access services. This will directly lead to loss of revenue and customer loyalty for the MSO. For this reason, careful thought should be devoted to storing the PVR subsystem functions on the hard disk and storing critical code and data-storage subsystems on a safer storage device, such as the far more reliable solid state flash disk. In a hybrid (flash disk and hard disk) design, when the hard disk fails, the flash disk kicks in and runs various utilities, marking the damaged areas on the hard disk and—when software correction is possible—getting it up and running again. If, however, the hard disk is beyond software repair, only PVR functionality is lost, but the other subsystems continue to function.

Other benefits of deploying a hybrid flash-disk and hard-disk storage solution include:

1. Extension of hard disk's life cycle
   A flash disk allows the hard disk to rest (power-down) for long periods of time, eliminating the life shortening on-and-off toggling of the hard disk when PVR operation is not required. The main benefits of such a design approach include:
   • Prolonging the HDD's life span
   • Allowing a smoother, quieter, and cooler operation of the set-top box in the living-room environment
   • The file system is able to cache files, storing attachments, images, and sound files, finally waking the hard disk only upon real system demand for mass storage space.
   
   
   
2. Valuable marketing tool
   Set-top box designs that couple a flash disk with a hard disk provide MSOs with an extremely valuable marketing tool. The MSO can supply the same basic set-top box design with or without the hard disk, and an easy non-subsidized hard-disk upgrade path could later be offered to consumers who originally opted for the HDD-less set-top box. Thus, the hard disk would not impede on the MSOs' efforts to penetrate the market with a low-cost, aggressively priced solution: having a set-top box design that includes both flash-disk storage and a hard disk helps assure the rollout of future digital television services in a cost-effective manner. Also note that many MSOs might soon opt for network server side storage (virtual PVRs), taking advantage of their broadband infrastructure and VOD capabilities, rendering the HDD upgrade unnecessary. Operator-subsidized set-top-box business models should carefully consider these issues.
   
   
3. Improved overall subscriber experience
   In the context of a digital-television environment, the hard disk is less appropriate as a data storage device where you need frequent writing/updating. Keeping the hard disk solely for storage of video streams and utilizing the fast-write characteristics of the NAND flash disk for all other storage demands results in a smoother overall subscriber experience that is less susceptible to failure.

The acceptance of using flash and a supporting file system in advanced digital set-top boxes is rapidly growing. The motivating factors that are encouraging manufacturers and MSOs to extend the functionality of flash-storage devices within the set-top box include:

1. TV Internet Browsing
   Including an enhanced Internet browser in a set-top box design demands between 2 and 5 Mbytes of memory per user profile. This memory stores cookies, favorites, cached Web pages, chat buddy lists, and updateable plug-ins—without which no Internet browsing experience is complete—and other Web originated data. Such patterns of data storage require a series of rigorous write and erase operations to the storage device. A flash disk, equipped with sophisticated wear leveling along with error correcting and detecting algorithms is necessary for adding this type of functionality to a set-top box.
   
   
2. Targeted Advertising
   The ability of MSOs to determine consumer habits and trends of their subscriber base is proving to be invaluable in generating new streams of revenue through advertising and t-commerce. For MSOs to implement advanced personalization techniques, digital set-top boxes need to log, store, and protect user identification data. Advanced flash disks can stimulate targeted advertising campaigns through support for sophisticated features such as non-changeable unique identification data and designated read-protected areas.
   
   
3. Family Profiling
   Unlike PC usage patterns, which show PC use to be a distinctively personal experience, TV viewing is a multi-person experience. As the use of set-top boxes increases, more subscribers are likely to demand personalization features similar to those offered on a PC. For instance, viewing habits of children will vary greatly from the viewing preferences of their parents. Manufacturers must consider the implications of usage variance and the personalization needs of different family members during the development of new set-top-box platforms. Fortunately for manufacturers, there are new commercially available flash disks that are able to reliably handle any number of family member profiles no matter how many times these profiles are accessed, appended, and written over.
   
   
4. TV Mail
   The set-top box with its interactive capability is seen by many as a natural environment for email in the home. The client email application is normally integrated with the digital set-top box and uses the TCP/IP protocol to communicate with the MSOs head-end. Modern set-top email applications fulfill a wide range of functions and let all family members in the subscriber's premises do more than just send or receive e-mail messages. Flash disk solutions are a perfect answer to locally storing mail messages in multiple folders, as well as full-blown address books, under separate user profiles within the set-top box, as well as file attachments.
   
   
5. Cost Savings
   To date, MSOs have been reluctant to deploy interactive services because of the high costs associated with manufacturing advanced digital set-top boxes. Using a file-system-equipped flash disk for managing storage resources eliminates the need to install a hard disk where PVR functionality is not a pre-requisite, thereby reducing overall system cost and supporting new revenue-generating applications and upgrades.
   
   
6. T-Commerce
   Set-top boxes that include a flash-disk storage solution enable digital TV viewers to use their remote controls to access information about the show they're watching, check news headlines, answer polls, make purchases, request more information, including discount coupons, and even check the status of previous purchases. Some flash disks allow set-top box designers to store and manage all personal and commercial data that is associated with the t-commerce subsystem into separate and protected logical partitions. The flash disk's ability to store and protect information through hardware-locked protection mechanisms is critical for the safety of any t-commerce system.
   
   
7. Security and Authentication
   Every set-top box system deployed today has a need for security. Set-top-box security levels are normally maintained by a CA subsystem that gives MSOs control over what types of digital-TV services their subscribers are entitled to receive. Most conditional-access systems use a smart card to store various types of information for subscriber authentication and for checking access rights. This will continue to be the case for the foreseeable future. However, CA providers and MSOs alike are beginning to look at the benefits of complementing their existing dynamic public-key security mechanisms with the security features of advanced flash disks offering a combination of hardware-software protection and security features. CA systems tend to focus on applying security measures on a per-subscriber basis alone whereas a capable flash disk can primarily focus on protecting data stored inside a specific set-top box.
   
   
8. Improved Response Time from iTV Applications
   Consumers are reluctant to wait for an iTV application to download from the network into the set-top box. In fact, set-top box manufacturers have already started to utilize powerful flash disk solutions to build iTV capabilities into their new designs. For instance, MSOs are increasingly showing interest in using flash-disk storage mechanisms as a medium to store sections of local electronic-program guides.
   
   
9. Enhanced TV and Timed Trickle-Fed Advertising
   Enhanced television is a new paradigm that merges the appeal and mass audience of traditional television viewing with the interactivity of the Web. It is an integral part of the new television experience. An enhanced TV program can vary between a very simple production that includes links to related Web sites on the Internet and highly involved forms of interactivity. High interactivity can merge the TV image with menus, advertising, rich multimedia components, and supporting text, all timed to appear in synchronization with a particular show. A flash disk's file system can be used to trickle-feed and pre-store the incoming ads, Internet links, and multimedia components.
   
   
10. Downloading Files from the Internet
    It is clear to most manufacturers and MSOs that set-top boxes have started to evolve into a form of residential gateway. One of the basic features of such appliances is the capability to download certain types of data files from the Internet. Only NAND-based flash-disk solutions are suitable for this type of functionality because of their ability to flawlessly and speedily emulate the functions and features of a mechanical hard-disk drive (without the mechanical failures, of course).
    
    
11. Software Updates and Upgrades
    In the fast-changing world of digital television, MSOs are continuously adding new software components and upgrades to set-top boxes connected to their networks. Set-top box software used to be compiled into a single image file, which was a few hundred Kbytes to 1-2 Mbytes in size. Nowadays, when a combined OS, middleware, and applications image file may be several megabytes in size and require several minutes to load into the set-top box, the single image file is no longer a safe option. When an MSO updates the software with some new modules or parameters, a completely new image file needs to be compiled and reloaded into the set-top box. From the MSO's point of view, this operation is hazardous as it is susceptible to power or communication outages. Such technical misfortunes may lead to total set-top box failure when the transferred image file is damaged. From a subscriber's perspective, this is both time consuming and frustrating. That is why most MSOs are insisting that set-top box software is compiled in a modular format (several modules of software image files). Only advanced NAND flash disks are able to quickly and reliably store and protect modules of software in dedicated locations on the memory chip. Each module may range from a few hundred kilobytes to a single megabyte in size, and updating specific operational parameters is easily managed in the field with minimal risk to the MSO and minimal disruption for the subscriber.

The amount of flash memory included with set-top boxes is growing to meet the increasingly complex and sophisticated software programs that are emerging in the industry. To meet the increasing subscriber and MSO demands, set-top boxes are now designed with 16 to 32 Mbytes of flash code and data-storage space. Most analysts are predicting the next storage capacity transition will occur in 2002/3 and will include set-top box platforms that incorporate local storage capacities of 64 Mbytes and above. As previously described, only NAND flash technology is a cost-effective player in these capacity ranges (in fact, NAND flash has already reached 64 Mbytes in a single-chip solution, while NOR is left behind with a costly 16 Mbyte component).

The recent slump in the global demand for digital set-top boxes is focusing people's minds on designing products that are adjustable to this shifting demand: upgradeable set-top box designs. During the design process, and even during the marketing phase, a design may change and one OS may be replaced with another. Only a mature and reliable flash file-management system, not the kind written for a specific OS or a specific set-top-box design, can truly enable that flexibility and minimize investment risks for set-top box manufacturers and network MSOs alike.

Currently, every MSO has unique set-top box requirements and therefore manufacturers are forced to have distinct designs for each customer. Although set-top boxes are customized for MSOs' needs, most of the core functionality of these devices is modeled after a number of international reference designs. International standards bodies such as ECCA (European Cable Communications Association) and the U.S.-based CableLabs are producing sets of specifications to help define future generations of digital set-top boxes and other digital devices to be deployed by MSOs around the globe. Both standard bodies will obviously not include details of preferred storage solutions. However, further detailed investigation does reveal a number of compelling reasons for including a flash-disk storage solution into set-top-box designs that comply with these standards. For example, when a certain standard calls for support of user-preferences management through the middleware layer, a flash disk is the natural data-storage solution for such a requirement.

There are various implementation issues to be addressed before initiating the construction of a mid-range or high-end advanced digital set-top box. A great deal of thought needs to be given to the type of storage solution to be selected out of the variety of storage solutions currently available. Each alternative has unique economic benefits and tradeoffs that will impact the overall cost of the set-top box. NAND flash disks seem to be better fitted for the future of advanced digital set-top boxes than their NOR counterparts.

Service providers are demanding advanced functionality from their set-top box manufacturers. Designing and building these advanced set-top box platforms is a significant challenge that requires a combination of standard hardware components with more specialized storage components. To meet this demand and future-proof their new hardware designs, manufacturers and MSOs need to seriously consider the many benefits of building set-top boxes around file-system-based flash disks.

About the Author Gerard O'Driscoll is the author of the two best-selling books on digital TV set-top boxes and home networking technologies. He has also published numerous papers for industry conferences and technical trade journals. O'Driscoll is currently working as a Senior Technology Strategist at Chorus in Ireland. He can be reached at godriscoll@set-tops.com