Prelude

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 that are 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. The following 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.

Digital Set-top boxes - a new computing paradigm

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. Additionally, 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.

Categories of set-top boxes

This huge installed base of set-top boxes can be broadly classified into the following categories:

• Analog set-top boxes
• Dial-up set-top boxes
• Entry-level digital set-top boxes
• Mid-range digital set-top boxes
• Advanced digital set-top boxes
• Advanced set-top boxes that include PVR functionality

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 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 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 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 MB of flash memory (mostly for code storage) in order to operate, mid range set-top boxes normally include between 4MB and 8MB of flash memory for code and data storage.

Digital set-top boxes from the advanced category bare close resemblance to a multimedia desktop computer. They can contain more than ten times the processing power of a low-level broadcast TV set-top box. Enhanced storage capabilities of between 16MB and 32 MB of flash memory (for code and data storage) in conjunction with a high speed return path can be used to run a variety of advanced services such as video teleconferencing, home networking, IP telephony, video-on-demand (VOD) and high-speed Internet TV services. Additionally, subscribers are able to use enhanced graphical capabilities within these types of boxes to receive high definition TV signals.

The idea of putting a hard disk drive (HDD) into the 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 a residential gateway.

This paper primarily focuses on the mid-range and advanced digital set-top box categories.

Advanced and Mid-Range Digital Set-top Box Technologies

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

Software: There are three layers of set-top box software required to operate a digital set-top box, namely, 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 operates below the interactive TV applications and above the operating system and provides 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 of three separate subsystems, namely 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. Another important subsystem included with a digital set-top box is the CA system. This subsystem provides MSOs with unprecedented control over what their subscribers watch and when. The PC subsystem itself is modular based, which means that set-top designers can add and subtract various components depending on the user requirements. For instance, MSOs who want to offer Internet services to their subscribers will incorporate some type of storage solution into their PC subsystem.

Storage solutions for advanced digital set-top boxes

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 that set-top boxes will evolve into a residential gateway and the primary access point for subscribers connecting to the Internet. Such a move from 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, namely the solid-state flash memory storage and the mechanical hard disk.

a) About 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 storage of data and software code. They are non-volatile, so the data is retained without 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, namely the local (or embedded) flash storage category and the removable flash storage category. Removable flash storage, such as compact flash for example, includes a dedicated hardware controller used to manage the flash memory (which naturally differs greatly from the physical characteristics of the hard disk). This dedicated hardware controller, and 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, namely NAND and NOR. Both technologies have unique features and are aimed at fulfilling different market needs. The faster read cycle characteristic of a NOR based flash solution, such as Intel's StrataFlash, coupled with its code execute in place (XIP) capability - albeit far slower than code execution in RAM, make it a suitable technology for storing small amounts of executable code, in a very similar way to the primitive ROM. These characteristics make NOR based products an ideal fit for the entry-level set-top boxes, which only need its 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.

NAND based storage solutions have been optimized by manufacturers such as Toshiba and Samsung for data storage operations and thus, have write/erase cycle response time over 15 times faster than equivalent NOR solutions. To top that, 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 who want to extend the functionality of their digital set-top box platforms. Cost-effective storage capacity is yet another NAND advantage over NOR: NAND is far more cost effective 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 software code, which is 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 8 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 from a set-top box flash file management system include:

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

b) About hard disk storage solutions
The popularity of PVR (Personal Video Recorder) 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 as described below:

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, etc).
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 the 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 + hard disk) design, when the hard disk fails, the flash disk kicks in running various utilities to mark the damaged areas on the hard disk and get it up and running again - when software correction is possible. 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 (temperature-wise) operation of the set-top box in the living room environment.
• The file system is able to cache files, store attachments, images, sound files, finally waking the hard disk up 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 same basic set top box design can be supplied 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 future-proofs the rollout of digital television services in a cost-effective manner. One should 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 frequent writing/updating is required. 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, less susceptible to failure, overall subscriber experience.

Compelling reasons for using flash disks in advanced digital set-top boxes

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 MB of memory per user profile - used to store cookies, favorites, cached Web pages, chat buddy lists, 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 and error correcting and detecting algorithms is a must 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. Therefore manufacturers must consider the implications of usage variance and 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 e-mail in the home. The client e-mail 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 e-mail 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, 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 provides 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 that is used to authenticate subscribers and check 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 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 which 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 (sans 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 KB to 1-2MB in size. Nowadays, when a combined OS, middleware and applications image file may take up 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 re-loaded into the set-top box. From the MSOs point of view, this operation is hazardous as it is susceptible to power or communication outages (of the kind folks in California have been experiencing lately). 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 only be 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.

Set-top box storage capacities

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 MB 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 64MB and above. Naturally, as previously explained, only NAND flash technology is a cost-effective player in these capacity ranges (in fact, NAND flash has already reached 64MB on a single chip solution, while NOR is left behind with a costly 16MB component).

Set-top box manufacturers face a changing industry
The recent slump in the global demand for digital set-top boxes is concentrating people's mind on designing products that are adjustable to this shifting demand: Future-proof set-top box designs. During the design process, and even during the marketing phase, a design may change and one OS may be replaced for another. Only a mature and reliable flash file management system, not of 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.

Complementing existing international standards

At the moment 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 standard bodies such as ECCA (European Cable Communications Association) and the U.S. based CableLabs are producing sets of specifications to help define the next 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. As an 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.

Flash file system solutions

There are various implementation issues to be addressed before the construction of a mid-range or high-end advanced digital set-top box is initiated. 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 that is 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. The market for NAND flash disk storage solutions is growing at a phenomenal rate and is led by a U.S company based in the Silicon Valley in California, M-Systems Inc.

During my extensive investigation into the realm of evolving flash storage solutions, it became apparent very early on that M-Systems is playing a major role in the advanced digital set-top box market. Companies such as Motorola, Microsoft's WebTV, Scientific Atlanta, Sony, NetGem and many others have all selected M-Systems' flash disk data storage products for use with their advanced set-top box designs.

To strengthen its leadership position, the company has just launched a unique 16-bit 32MB NAND flash disk solution in TSOP-1 package for mid-range and high-end set-top boxes called the DiskOnChip Millennium Plus. The main features of this innovative product are briefly covered below:

• Protection, Identification and Security - Set-top box manufacturers invest a significant amount of effort into protecting their products against fraud, IP theft and other threats. The M-Systems DiskOnChip Millennium Plus product is much more than a flash disk in that respect as it incorporates some clever security features such as a non-changeable unique serial number, a One Time Programmable area, and hardware-lockable read/write protected memory areas where sensitive data such as X.509 V3 compliant static certificates as well as software code, registry data, etc., can be stored.
  
• Boot ROM chip replacement capability - Due to M-Systems' singular design of 1KB of programmable XIP-capable SRAM into the monolithic 0.16µm silicon die which constitutes the DiskOnChip Millennium Plus, the M-Systems product can provide an alternative to the classic, separate, boot device, and get the boot loading process started. That is truly an interesting concept in itself as it can play a major role in getting rid of yet another onboard real-estate occupant - the boot ROM chip - and cut costs for set-top box designers and MSOs.
  
• Compatibility - Set top box manufacturers must be as platform flexible as possible if they do not want to get stuck with their designs and lose on their investments. The M-Systems DiskOnChip Millennium Plus through its unique hardware implementation of M-Systems' own world standardized TrueFFS flash file system, supports the widest variety of set-top box operating systems available today. These include all MS Embedded Windows OS flavors, Wind River's VxWorks and pSOS, BeIA, QNX, Linux, ATI Nucleus, Micorware's OS9 DAVID and many others. M-Systems products also support the widest range of hardware platforms and CPUs, including National Semiconductor, Cyrix, Intel, AMD and ST's x86 CPU families as well as Motorola and IBM's PowerPC, MIPS, Hitachi SH-x, Intel StrongARM and ARM's RISC CPUs and many others.
  
• Capacity - The 16-bit DiskOnChip Millennium Plus is available as a 32MB (256Mbits) flash disk, and much like its 8MB DiskOnChip Millennium family member, it too is an extremely scalable standard building block. For example the Millennium Plus capacity can be easily expanded to 64MB, 96MB or 128MB by cascading up to four chips in total, without any additional glue logic. This is an ideal solution for set-top box manufacturers who wish to future-proof their existing designs and place them in the pole position to evolve into a form of residential gateway, as it allows for an extremely easy capacity upgrade design path.
  
• High Reliability - Users of Internet-centric computing devices such as digital set-top boxes are an entirely different audience from PC users. Television viewers will not put up with what the average PC user has come to accept as reality - low levels of reliability. For example, sophisticated computer users are familiar with the various ways desktop PC's can hang, whereas a typical television viewer will never get used to such problems. Therefore, the challenge manufacturers are facing is to design and build reliable set-top boxes for the consumer products world of digital television. The new DiskOnChip helps manufacturers achieve those levels of reliability through advanced error correction and detection features unavailable in most other flash memory solutions.

Conclusion

In conclusion, 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. The market for flash disks is growing at a phenomenal rate and is led by M-Systems, a California based company, utilizing advanced NAND flash technology from Toshiba and Samsung. The company has recently launched an attractive solution for mid-range and high-end set-top boxes called the DiskOnChip Millennium Plus.

The DiskOnChip Millennium Plus is for all intents and purposes a standard building block for designers of digital set-top boxes. The successful deployments of advanced digital set-top boxes will not only depend on the cost, reliability, robustness and security features of a well designed box, but also on the designers' ability to future-proof their present design. In my opinion, the DiskOnChip Millennium Plus total storage solution from M-Systems meets all of these pre-requisites and manufacturers should seriously consider the inclusion of this solution in their existing and next generation set-top box products.

Gerard O'Driscoll is the author of the two best selling books on digital TV set-top boxes and home networking technologies. Additionally, O'Driscoll has 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 .