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USB flash drives are NAND-type flash memory data storage devices integrated with a USB (universal serial bus) connector. They are typically small, lightweight, removable and rewritable. (USB Memory card readers are also available, whereby rather than being built-in, the memory is a removable flash memory card housed in what is otherwise a regular USB flash drive, as described below.)
USB flash drives offer potential advantages over other portable storage devices, particularly the floppy disk. They are more compact, faster, hold more data, are more reliable due to their lack of moving parts, and have a more durable design. Additionally, it has become increasingly common for computers to ship without floppy disk drives. USB ports, on the other hand, appear on almost every current mainstream PC and laptop. These types of drives use the USB mass storage standard, supported natively by modern operating systems such as Windows, Mac OS X, Linux, and other Unix-like systems.
With nothing being mechanically driven in a flash drive, the name is something of a misnomer. It is called a "drive" because it appears to the computer operating system (and the user) in a manner identical to a mechanical disk drive, and is accessed in the same way.
A flash drive consists of a small printed circuit board typically in a plastic or metal casing and more recently in rubber casings to increase their robustness. This makes the drive sturdy enough to be carried about in a pocket, for example as a key fob, or on a lanyard. Only the USB connector protrudes, and it is typically protected either by a removable cap or by retracting into the body of the drive. Most flash drives use a standard type-A USB connection allowing them to be connected directly to a port on a personal computer.
To access the data stored in a flash drive, the drive must be connected to a computer, either by plugging it into a USB host controller built into the computer, or into a USB hub. Flash drives are active only when plugged into a USB connection and draw all necessary power from the supply provided by that connection. However, some flash drives, especially high-speed drives, may require more power than the limited amount provided by a bus-powered USB hub, such as those built into some computer keyboards or monitors. These drives will not work unless plugged directly into a host controller (i.e., the ports found on the computer itself) or a self-powered hub.
[edit] Technology
Flash memory is actually a combination of a number of older technologies, the low cost, low power consumption and small size being made possible by recent advances in microprocessor technology. The memory storage is based on earlier EPROM technology. The first EPROMS had very limited capacity, were very slow for both reading and writing, required complex high-voltage drive circuitry, and could only be re-written by first exposing them to short-wavelength ultraviolet light, which would erase the entire contents.
A later variant, the EEPROM, could be erased electrically without the need for UV light, but as with the original versions, it was still a total erasure. (These were widely used for storing the BIOS firmware in PCs.)
Later EEPROMS were developed where the erasure region was broken up into smaller "fields" that could be erased individually without affecting the others. Altering the contents of a particular memory location involved first copying the entire field into an off-chip buffer memory, erasing the field, and then re-writing the data back into the same field, making the necessary alteration to the relevant memory location while doing so. This required considerable computer support, and PC-Based EEPROM flash memory systems often carried their own dedicated microprocessor system. Flash drives are more or less a miniaturized version of this.
The development of high-speed serial data interfaces such as USB for the first time made serially accessed storage memory systems viable, and the simultaneous development of small, high-speed, low-power microprocessor systems allowed this to be incorporated into extremely compact systems. Serial access also greatly reduced the number of electrical connections required for the memory chips, which has allowed the successful manufacture of multi-gigabyte capacities. (Every external electrical connection is a potential source of manufacturing failure, and with traditional manufacturing, a point is rapidly reached where the successful yield approaches zero).
Modern flash memory systems are accessed very much like hard disk drives, where the controller system has full control over where information is actually stored. The actual EEPROM writing and erasure processes are, however, still very similar to the earlier systems described above.
Many low-cost MP3 Players simply add extra software to a standard flash memory control microprocessor so it can also serve as a music playback decoder. Most of these can also be used as a conventional flash drive.
[edit] History
[edit] First invention and sale
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Several companies claim to have invented the USB flash drive. Trek was the first company to sell a USB flash drive (ThumbDrive) in early 2000. However, their patent does not describe the USB flash drive; rather, it describes a very broad family of storage devices which could include the USB flash drive.
M-Systems (acquired by SanDisk in November 2006)[1] had been working on a USB flash drive since 1998. They registered the domain www.diskonkey.com on October 12, 1999,[2] indicating their intention to sell a product. In 2000 Dan Harkabi joined the M-Systems team and led the development of DiskOnKey. The industrial design was done by Ziba and the product won the IDEA award in 2001. M-Systems' patent rigorously describes the USB flash drive and its implementation.
An IBM invention disclosure RPS8-1999-0201 (Sept. 99) by Shimon Shmueli et al. is the earliest known document to describe the USB-FD accurately and completely, and only the USB-FD. M-Systems manufactured the DiskOnKey for IBM, which in late 2000 was the first to sell the product in North America. The IBM 8 MB USB Memory Key became available December 15, 2000[3] [4]. Shmueli later founded KeyNetica, the first company to patent and develop the concept that mobile and smart storage devices are all one needs for mobile computing. Current implementors of the concept are U3 (part of SanDisk, which also owns the original KeyNetica patent) and Ceedo.
Trek Technology claims it was first to conceive and create the ThumbDrive.[5]
| “ | "When we first introduced the ThumbDrive in early 2000, we believed that this little device was set to change the way consumers across the world would store and transport information and data," Trek 2000 Chief Executive Officer Henn Tan was quoted as saying. "[Its potential] has made it essential for Trek to invest and protect its intellectual property ownership." | ” |
Trek holds patents for the ThumbDrive in Japan, Taiwan, South Korea, the United Kingdom, New Zealand and Singapore.
Nevertheless, the ownership of the patent for this device has been widely disputed. According to The Straits Times' report, other companies started marketing similar devices. M-Systems, which was listed on NASDAQ at the time, called its gadgets DiskOnKey and Diskey. Electec is M-Systems' importer, and FE Global is its sole distributor in Singapore. Lexar can also lay claim to a pioneering USB flash drive product. In 2000 they introduced a Compact Flash (CF) card having an internal USB function. Lexar offered a companion card reader and USB cable that eliminated the need for a USB hub.
Trek sued the four companies for infringing its patent. They counterclaimed, asking that Trek's patent be revoked as it was invalid.
The Singapore Court of Appeal confirmed the validity of Trek Technology's patent for its ThumbDrive, calling it "novel and inventive" in the decision published in The Straits Times. The city-state's highest court also quashed the plea of four companies—Israeli firm M-Systems Flash Disk Pioneers, Electec, FE Global Electronics and Singapore-based Ritronics Components—and ordered them to stop selling similar devices. The decision is expected to have a ripple effect on other similar lawsuits which the Trek group has pending in Britain, Japan and Taiwan. At least one decision in the UK has addressed similar points as those addressed by the Singapore Court of Appeal, but the hearing officer reached a different conclusion from that of the Singapore Court of Appeal, and Trek's patent was revoked in the UK.
Netac Technology of Shenzhen, China also holds a 1999 Chinese and 2004 US patent on USB flash technology which it has licensed to major manufacturers [6].
[edit] Second generation
Modern flash drives have USB 2.0 connectivity. However, they do not currently use the full 480 Mbit/s the specification supports due to technical limitations inherent in NAND flash. The fastest drives currently available use a dual channel controller, although they still fall considerably short of the transfer rate possible from a current generation hard disk, or the maximum high speed USB throughput.
Typical overall file transfer speeds are about 3 MB/s. The highest current overall file transfer speeds are about 10-30 MB/s. Older, "full speed" 12 Mbit/s devices are limited to a maximum of about 1 MB/s.
[edit] Components
|
| |
| 1 | USB connector |
|---|---|
| 2 | USB mass storage controller device |
| 3 | Test points |
| 4 | Flash memory chip |
| 5 | Crystal oscillator |
| 6 | LED |
| 7 | Write-protect switch |
| 8 | Space for second flash memory chip |
One end of the device is fitted with a single male type-A USB connector. Inside the plastic casing is a small printed circuit board. Mounted on this board is some simple power circuitry and a small number of surface-mounted integrated circuits (ICs). Typically, one of these ICs provides an interface to the USB port, another drives the onboard memory, and the other is the flash memory.
[edit] Essential components
There are typically four parts to a flash drive:
- Male type-A USB connector — provides an interface to the host computer.
- USB mass storage controller — implements the USB host controller and provides a linear interface to block-oriented serial flash devices while hiding the complexities of block-orientation, block erasure, and wear levelling, or wear balancing. The controller contains a small RISC microprocessor and a small amount of on-chip ROM and RAM.
- NAND flash memory chip — stores data. NAND flash is typically also used in digital cameras.
- Crystal oscillator — produces the device's main 12 MHz clock signal and controls the device's data output through a phase-locked loop.
[edit] Additional components
The typical device may also include:
- Jumpers and test pins — for testing during the flash drive's manufacturing or loading code into the microprocessor.
- LEDs — indicate data transfers or data reads and writes.
- Write-protect switches — indicate whether the device should be in "write-protection" mode.
- Unpopulated space — provides space to include a second memory chip. Having this second space allows the manufacturer to develop only one printed circuit board that can be used for more than one storage size device, to meet the needs of the market.
- USB connector cover or cap — reduces the risk of damage due to static electricity, and improves overall device appearance. Some flash drives do not feature a cap, but instead have retractable USB connectors. Other flash drives have a "swivel" cap that is permanently connected to the drive itself and eliminates the chance of losing the cap.
- Transport aid — In some cases, the cap or the main body contains a hole suitable for connection to a key chain or lanyard or to otherwise aid transport and storage of the USB flash device.
[edit] Size and style of packaging
Some manufacturers differentiate their products by using elaborate housings, which are often bulky and make the drive difficult to connect to the USB port. Because the USB port connectors on a computer housing are often closely spaced, plugging a flash drive into a USB port may block an adjacent port. Such devices may only carry the USB logo if sold with a separate extension cable.
USB flash drives have been integrated into other tools commonly carried on one's person such as a watches, pens, and even the Swiss Army Knife; others have been fitted with novelty cases such as toy cars or LEGO bricks. The small size, robust nature and relatively low price of USB flash drives makes them an increasingly popular peripheral for case modding.
Overweight or ill-fitting flash drive packaging can cause disconnection from the host computer. This can be overcome by using a short USB to USB (male to female) extension cable to relieve tension on the port. Such cables are USB-compatible, but do not conform to the USB 1.0 standard. [7] [8]
[edit] Common uses
[edit] Personal data transport
The most common use of flash drives is to transport and store personal files such as documents, pictures and video. Individuals also store medical alert information on MedicTag flash drives for use in emergencies and for disaster preparation.
[edit] Secure Storage of data, application and software files
With wide deployment(s) of flash drives being used in various environments (secured or otherwise), the issue of data and information security remains of the utmost importance. The use of biometrics and encryption is becoming the norm with the need for increased security for data; OTFE systems such as FreeOTFE and TrueCrypt are particularly useful in this regard, as they can transparently encrypt large amounts of data.
[edit] System administration
Flash drives are particularly popular among system and network administrators, who load them with configuration information and software used for system maintenance, troubleshooting, and recovery.
[edit] Computer repair
Flash drives enjoy notable success in the PC repair field as a means to transfer recovery and antivirus software to infected PCs, while allowing a portion of the host machine's data to be archived in case of emergency.
[edit] Application carriers
Flash drives are used to carry applications that run on the host computer without requiring installation. U3, backed by flash drive vendors, offers an API to flash drive-specific functions. The Mozilla Firefox browser has a configuration for flash drives, as does Opera.[9][10]
[edit] To boot operating systems
In a way similar to that used in LiveCD, one can launch any operating system from a bootable flash drive, known as a LiveUSB.
[edit] Windows Vista ReadyBoost
In Windows Vista, the ReadyBoost feature allows use of some flash drives to augment system memory.
[edit] Audio players
Many companies make solid-state digital audio players in a small form factor, essentially producing flash drives with sound output and a simple user interface. Examples include the Creative MuVo and the iPod shuffle.
[edit] Music storing
USB key without player (called raw USB memory) can be played in a Hi-Fi or Radio-CD with a USB port. The file formats of the music in the USB key are MP3 or Ogg. In 2004, the German band WIZO made a recording industry first of releasing the "Stick EP", an album available only as a USB drive preloaded with high-bitrate MP3s. In 2007, a product known as the MU-STIK appeared in the market, offering a means of packaging music albums by containing all relevant digital audio/video data and a customizable player interface within a USB key.[11]. Other releases on thumb drive include Not Here to Please You by Hadouken!, Icky Thump by White Stripes and Good Life by Kanye West. In November/December 2007 Kylie Minogue's new album "X" was released on a USB stick. On February 5, 2008, The Mars Volta plan to release their album The Bedlam in Goliath on a USB stick the shape of a ouija planchette to coincide with the album's concept.
[edit] In arcades
In the arcade game In the Groove and more commonly In The Groove 2, flash drives are used to transfer high scores, screenshots, dance edits, and combos throughout sessions. While use of flash drives is common, the drive must be Linux compatible, causing problems for some players.
[edit] Advantages and Disadvantages
[edit] Advantages
Flash drives are nearly impervious to the scratches and dust that were problematic for previous forms of portable storage, such as compact discs and floppy disks, and their durable solid-state design means they often survive casual abuse. This makes them ideal for transporting personal data or work files from one location to another, such as from home to school or office or for carrying around personal data that the user typically wants to access in a variety of places. The near-ubiquity of USB support on modern computers means that such a drive will work in most places.
Flash drives are also a relatively dense form of storage, where even the cheapest will store dozens of floppy disks worth of data. Many can hold more data than a CD (703[12] MB). Top of the line flash drives can store more data than a DVD(4.7 GB).
The power consumption of a typical flash drive is very low, much lower than that of the common alternative, the portable hard disk, due to there being no moving parts inside a flash drive. Portable hard disks, on the other hand, require several motors or actuators to rotate the disk platter, and move the read / write head. Consequently, the USB flash drive need not be as big, or heavy, allowing greater portability.
Flash drives implement the USB mass storage device class, meaning that most modern operating systems can read and write to flash drives without any additional device drivers. The flash drives present a simple block-structured logical unit to the host operating system, hiding the individual complex implementation details of the various underlying flash memory devices. The operating system can use whatever type of file system or block addressing scheme it wants. Some computers have the ability to boot up from flash drives.
Flash drives are much more tolerant of abuse than mechanical drives, but can still be damaged or have data corrupted by severe physical impacts. Improperly wired USB ports can also destroy the circuitry of a flash drive, a danger in home-built desktop PCs.
Some flash drives can retain their memory after being submerged in water [13], even through a machine wash. Leaving the flash drive out to dry completely before allowing current to run through it has been known to result in a working drive with no future problems. Channel Five's Gadget Show cooked a flash drive with propane, froze it with dry ice, submerged it in various acidic liquids, ran over it with a jeep and fired it against a wall with a mortar. A company, specializing in recovering lost data from computer drives, managed to recover all the data on the drive. [14] All the other removal storage devices, using optical or magnetic discs or semiconductor (Secure Digital) were completely destroyed.
[edit] Disadvantages
Like all flash memory devices, flash drives can sustain only a limited number of write and erase cycles before failure. Mid-range flash drives under normal conditions will support several hundred thousand cycles, although write operations will gradually slow as the device ages. This should be a consideration when using a flash drive to run application software or an operating system. To address this, as well as space limitations, some developers have produced special versions of operating systems (such as Linux) or commonplace applications (such as Mozilla Firefox) designed to run from flash drives. These are typically optimized for size and configured to place temporary or intermediate files in the computer's main RAM rather than store them temporarily on the flash drive.
Most USB flash drives do not include a write-protect mechanism, although some have a switch on the housing of the drive itself to keep the host computer from writing or modifying data on the drive. Write-protection makes a device suitable for repairing virus-contaminated host computers without risk of infecting the USB flash drive itself.
A drawback to the small size is that they are easily misplaced, left behind, or otherwise lost. This has led to the common practice of attaching flash drives to keychains, necklaces and lanyards.
[edit] Comparison with other portable memory forms
Flash storage devices are often compared to other common, portable, swappable data storage devices, such as floppy disks, Zip disks, miniCD / miniDVD, CD-R/CD-RW and DVD-RW discs.
Following magnetic tape, the floppy disk was the first popular method of file transport; but floppies have been almost completely phased out due to their low capacity, low speed, and low durability. Virtually no new computers include floppy drives, but they do include USB ports. However, floppy disks are still being used because of their low cost, are often the easiest or only way to share files with older systems, are easily bootable, and because floppy drives can be added to new systems in external or internal forms.
Attempts to extend the floppy standard (such as the Imation SuperDisk) were not successful because of a reputation for unreliability and the lack of a single standard for PC vendors to adopt. The Iomega Zip drive enjoyed some popularity, but never reached the point of ubiquity in computers. Also, the larger sizes of Zip—now up to 750 MB—cannot be read on older drives. Unless one were to carry an external drive, their usefulness as a means of moving data was rather limited. The cost per megabyte was fairly high, with individual disks often costing US$10 or more. Because moving parts are involved and the material used for creating the storage medium in Zip disks is similar to that used in floppy disks, Zip disks have a high risk of failure and data loss compared to flash drives. Larger removable storage media, like Iomega's Jaz drive, had even higher costs for both drives and media, and as such were not pervasively adopted as a floppy alternative.
CD-R and CD-RW are swappable storage media alternatives. Unlike Zip and floppy drives, DVD and CD recorders are now common in personal computer systems. CD-Rs can be written to only once. But CD-RWs are rated at up to 1,000 erase/write cycles, and modern NAND-based flash drives often last for 500,000 or more erase/write cycles. Optical storage devices also usually are slower than their flash-based counterparts. And, compact discs with a 12 cm diameter can be inconveniently large and, unlike flash drives, cannot fit into a pocket or hang from a key chain. There are smaller CD-R media such as business card CD-Rs, which have the same dimensions as a credit card, and (slightly less convenient but have more storage) 8 cm CD-Rs. But these are harder to obtain and generally more expensive than the standard 12 cm version. The small CDs also require a tray-loading drive; they can cause damage if inserted into a slot-loading drive. There also is no standard file system for rewritable optical media. Packet-writing utilities like DirectCD and InCD exist but produce discs that are not universally readable, despite their claiming to be based on the UDF standard. The upcoming Mount Rainier standard addresses this shortcoming in CD-RW media, but it still is not supported by most DVD and CD recorders or major operating systems. As a result, CDs/DVDs are a good way to record a great deal of information cheaply and also has the advantage of being able to put it into any console/optical disk player for playback, but not good for making ongoing small changes to a large collection of information; flash drives' ability to do this is their major advantage.
[edit] Security
Some flash drives feature encryption of the data stored on them, generally using full disk encryption below the file system. This prevents an unauthorized person from accessing the data stored on it. The disadvantage is that the drive is accessible only in the minority of computers which have compatible encryption software, for which no portable standard is widely deployed.
Some encryption applications allow running without installation. The executable files can be stored on the USB drive, together with the encrypted file image. The encrypted partition can be accessed on any computer running the correct operating system. Other flash drives allow the user to configure secure and public partitions of different sizes. Executable files for Windows, Macintosh, and Linux may be on the drive, depending on manufacturer support. Some security software may require administrative rights on the host PC to access data.
Newer flash drives support biometric fingerprinting to confirm the user's identity. As of mid-2005, this was a relatively costly alternative to standard password protection offered on many new USB flash storage devices. Most fingerprint scanning drives rely upon the host operating system to validate the fingerprint via a software driver, often restricting the drive to Microsoft Windows computers.
Some manufacturers deploy physical authentication tokens in the form of a flash drive. These are used to control access to a sensitive system by containing encryption keys or, more commonly, communicating with security software on the target machine. The system is designed so the target machine will not operate except when the flash drive device is plugged into it. Some of these "PC lock" devices also function as normal flash drives when plugged into other machines.
All such forms of data protection security involve an increased risk of loss of access to the data by the legitimate owner/user.
Flash drives present a significant security challenge for large organizations. Their small size and ease of use allows unsupervised visitors or unscrupulous employees to smuggle confidential data out with little chance of detection. Equally, corporate and public computers alike are vulnerable to attackers connecting a flash drive to a free USB port and using malicious software such as rootkits or packet sniffers. To prevent this, some organizations forbid the use of flash drives, and some computers are configured to disable the mounting of USB mass storage devices by ordinary users, a feature introduced in Windows XP Service Pack 2; others use third-party software to control USB usage. In a lower-tech security solution, some organizations disconnect USB ports inside the computer or fill the USB sockets with epoxy.
[edit] Naming
Recently, "USB flash drive" or simply "UFD" has emerged as the de facto standard term for these devices. However, the myriad different brand names and terminology used, in the past and currently, makes UFDs more difficult for manufacturers to market and for consumers to research. Some commonly used names are actually trademarks of particular companies. Some examples of this are Cruzer, TravelDrive, & Disgo.
[edit] Future developments
Semiconductor corporations have worked to reduce the cost of the components in a flash drive by integrating various flash drive functions in a single chip, thereby reducing the part-count and overall package cost.
Flash drive capacities on the market are continuously increasing. As of 2007, 64 MB and smaller capacity flash memory has been largely discontinued, and 128-256 MB capacity flash memory is being phased out. High-speed is now a standard for modern flash drives and capacities of up to 64 GB are available [15].
Lexar is attempting to introduce a USB FlashCard [16] [17], which would be a compact USB flash drive intended to replace various kinds of flash memory cards. Pretec introduced a similar card, which also plugs into every USB port, but is just one quarter the thickness of the Lexar model [18]
SanDisk has introduced a new technology to allow controlled storage and usage of copyrighted materials on flash drives, primarily for use by students. This technology is termed FlashCP.
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