Keydrive
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A keydrive is a small removable data storage device that uses flash memory and a USB connector. Keydrives are also known as keychain drive, pen drive, pocket drive, thumb drive, jump drive, USB flash drive, USB flash memory drive, USB key, USB memory key, USB stick, Piripicho (primarily in Spanish), and Kikinou (primarily in French)
Keydrives are typically small, lightweight devices, around 50 mm long and weighing only around 100 grammes. A keydrive consists of a small printed circuit board encased with a robust plastic casing, making the drive sturdy enough to be carried around in a pocket, as a keyfob, or on a lanyard around the owner's neck. Only the USB connector protrudes from this plastic protection, and this is often covered by a removable plastic cap. Keydrives are active only when powered by a connection to a computer, and require no external power source or battery power when not in use. To access the data stored in a keydrive, it must be connected to a computer, either by direct connection to its USB port or via a USB hub. Most keydrives feature the larger type-A USB connection, although some feature the smaller "miniUSB" connection. There is no separate power connection; keydrives are run from the limited supply afforded by the USB connection.
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Composition of a USB keydrive
One end is fitted a single male type-A USB connector. Inside the plastic package is a small, highly cost-engineered, printed circuit board. On this is mounted some simple power circuity and a small number of surface-mounted integrated circuits (ICs). Typically one of these devices provides an interface to the USB port, another drives the onboard memory, and the other devices provide the flash memory storage themselves. Semiconductors corporations have striven to radically reduce the cost of the components in a keydrive, doing so by integrating various keydrive functions in a single chip, and thus reducing the part-count and overall package cost. As of 2004 a typical keydrive consists largely of a single unified I/O chip and a generic flash memory part, and some manufacturers are planning yet more integrated devices, in which the storage and logic/communications functions are packaged in a single, ultra-low-cost device.
Additionally many keydrives feature:
- A light-emitting diode (or two) which flash to indicate that the device is connected, and which often flash a different pattern to indicate a write or read operation is underway.
- Some supporting electronic components, needed to glue together the integrated circuits and to manage the power needed for USB communications and for writing to (and particularly erasing) flash memory.
- A "write protect" switch, which stops the keydrive from being written to.
A number of manufacturers have chosen to make the appearance of the electronics within the keydrive a design feature, and so produce versions with transparent or translucent cases.
Typical uses for keydrives
Keydrives are impervious to the scratches and dust that plagued previous forms of portable storage like compact discs and floppy disks, and their durable solid-state design means they often survive casual insults (impacts, being dropped or crushed, run through a washing machine, or even dropped in coffee). 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 and that problems with device and driver incompatibilities are unlikely.
Keydrives are also a relatively dense form of storage - even the cheapest will store more than dozens of floppy disks, and a moderately priced one will carry as much as a CD. Historically, keydrive capacity has ranged from a few megabytes in size up to a few gigabytes, although some computers have trouble reading and writing to devices that have more than 2 GB of storage. In 2003 most keydrives ran at the USB 1.0/1.1 speeds of 1.5 Mbit/s or 12 Mbit/s. 2004 saw the release of newer USB keys featuring USB 2.0 interfaces. Although USB 2.0 tops out at 480 Mbits/s, these keydrives are limited by the bandwidth of the underlying flash memory device, with maximum read speeds of around 100 Mbits/s and write speeds a little slower. In ideal conditions, the flash memory in the drives can retain data for 10 years.
Keydrives implement the USB mass storage device class, meaning that most modern operating systems can read and write to keydrives without any additional device drivers. Instead of exposing the complex technical detail of the underlying flash memory devices, the keydrives export a simple block-structured logical unit to the host operating system. This way the OS can use whatever type of filesystem or block addressing scheme it wants. Some computers have the ability to boot up from keydrives, but that capability must be supported in the computer's BIOS, and (like other mass storage devices) the keydrive must be set up to do so and loaded with a bootable disk image (rather than a conventional filesystem image).
Like all flash memory devices, keydrives can sustain only a limited number of write/erase cycles before failure. In normal use, mid-range keydrives currently on the market will support several million cycles, although write operations will gradually slow as the device ages. This should be a consideration when using a keydrive as a hard drive to run software or an operating system. To address this (and the space limitations common on keydrives) some developers have produced versions of operating systems (such as Linux) or commonplace applications (such as the Firefox browser) designed to run from keydrives. These are typically optimized for size and set up so as to place temporary or intermediate files in memory rather than nonvolatile storage (to avoid excessive writing to the flash memory in the keydrive).
Many companies make solid-state MP3 players in a small form factor, essentially producing keydrives with sound output and a rudimentary user interface. Others produce small solid-state mp3 players which contain a removable keydrive.
Keydrives for network administration
Keydrives are particularly popular among system and network administrators, who load them with configuration information and with software used for system maintainance, troubleshooting, and recovery. The write protect feature on the keydrive is particularly useful for such uses, as it allows the system administrator to plug a keydrive containing anti-virus, spyware-removal, or trouble diagnosis software onto a suspect machine without risking the transmission of a virus or worm.
Keydrives and computer security
Some keydrives feature encryption of the data stored on them, generally using an encrypted filesystem rather than a conventional one. This prevents an unauthorized person (who has found or stolen the drive) from accessing confidential data stored on it. The disadvantage of this is that the drive is accessible only in the minority of computers which have the same encryption software (for which no portable standard is widely deployed) unless the encryption software is stored unencrypted on the drive, and the user must move the (large, and frequently impossible-to-remember) cryptographic key around by some other means.
Some manufacturers deploy physical authentication tokens in the form of a keydrive. These are used to control access to a sensitive system, whether by containing encryption keys or (more commonly) by communicating with security software on the target machine. The system is designed so the target machine will not operate except when the keydrive device is plugged into it. Some of these "PC lock" devices also function as normal keydrives when plugged into other machines.
Keydrives also pose large organisations a significant security problem. 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 keydrive to a free USB port and uploading hacking software such as rootkits or packet sniffers. To prevent this some organisations (particularly government departments and larger corporations) forbid the use of keydrives, and some computers are configured to disable the mounting of USB mass storage devices by ordinary users (a feature found only belatedly on Microsoft Windows XP, being introduced only in its second service pack). In a lower-tech security solution, some organizations disconnect USB ports inside the computer or fill the USB sockets with epoxy.
The internal components of a typical keydrive
|
The internal components of a typical keydrive | |
| 1 | USB connector |
|---|---|
| 2 | USB mass storage controller device |
| 3 | test pins |
| 4 | flash memory device |
| 5 | crystal oscillator |
| 6 | LED |
| 7 | write-protect switch |
| 8 | unused space for second flash memory device |
This photograph shows both sides of the printed circuit board inside a typical keydrive (circa 2004), in this case an inexpensive 64 Mbyte USB2.0 device. The plastic clamshell case has been removed.
In practice the keydrive consists of only two significant components. The first is the flash memory part (item 4 in the diagram), a generic device that might as easily be found in a digital camera's memory card. The second (item 2 in the diagram) is a device which implements the USB networking and mass-storage interface, and which knows how to make a chunk of generic flash memory appear like a normal disk drive. It is the high degree of integration in this latter part that makes small, inexpensive keydrives possible. <p> The parts of the device are as follows:
- A male type-A USB connector.
- An Ours Technology Inc. OTi-2168 USB 2.0 mass storage controller. This implements the USB 2.0 host controller, and provides a seamless linear interface to block-oriented serial flash devices, while hiding the complexities of block-orientation, block erasure, and wear balancing. It contains a small RISC microprocessor and a small amount of ROM and RAM. This communicates with the Hynix device over an 8-line unified address/data bus. This version is a 7x7mm 48-pin LQFP (Low Profile Quad Flat Pack) surface-mount device (info).
- JP1 and JP2: two unpopulated 10-pin connectors, used for testing during the keydrive's manufacture.
- A Hynix Semiconductor HY27USxx121M series NAND Flash memory device, featuring 4096 independently erasable blocks each providing 16 Kbytes of storage, yielding a total of 64 Mbytes of usable storage. The version used in this keydrive is a 20x12mm 48-pin TSOP1 (Thin Small Outline Package) surface-mount package (info) (datasheet).
- An SKC Shin Chang Electronics 12.000 MHz crystal oscillator (XTAL). The OTi device runs the output of this through a phase-locked loop to produce its main 12 MHz clock signal.
- A single yellow light-emitting diode (run from a pin on the OTi device) which flashes to indicate activity.
- A simple two-position switch, used to indicate whether the device should be in "write-protect" mode. It is shown here in the make position, indicating write-protect is off.
- An unpopulated space for a second TSOP1 memory package. The OTi device is capable of driving up to eight such devices. Having this second space allows the manufacturer to choose (generally on a cost basis) whether to use one or two TSOP flash parts.
See also
External links
- operating system designed for USB Keydrives
- http://www.meritline.com/usbpendrive.html
- http://www.ncsu.edu/resnet/runt/
- http://wiki.debian.net/index.cgi?Keydrive
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