The growing popularity of handhelds is driving the need for secure mobile storage, but a plethora of standards means that available solutions are incompatible with each other.

Storage capacity in mobile clients such as handhelds is one of the most critical factors affecting their usefulness for enterprises. Available storage space has a direct impact on the kind of applications that can be run, and how much data can be stored locally. Disk-based media is typically impractical for handhelds because of power and size restrictions and, unfortunately, alternative technologies can rarely match the storage density of disk Ã,­ and tend to cost much more per megabyte of capacity.

Most handhelds, such as Palm OS-based devices or Pocket PC palmtops like Compaq's iPaq, rely on built-in RAM to meet the initial storage needs of users. Such devices are often fitted with 32MB or less of RAM, and this is divided by the operating system into memory space for running applications and storage space for files. This carries the risk of any files held in RAM being lost if the handheld's batteries run flat, so some extra form of storage is usually desirable.

Flash in the palm

Most storage solutions for handhelds use Flash memory cards. Flash memory is a silicon-based technology like RAM, but the physical structure of the chip means that the electrical charge representing data can be retained almost indefinitely. This structure means that Flash is better suited to storage purposes rather than as a replacement for a device's working memory, as data has to be written and erased in blocks, rather like disks.

There are some disk-based alternatives to Flash memory. PC Card hard drives have been around for many years, and now feature capacities of over 1GB. For smaller client devices, IBM's Microdrive squeezes 1GB of storage onto a 1in disk in a CompactFlash housing.

Optical technologies also hold some promise, particularly the miniature disks that will be available later this year from mobile storage specialist DataPlay. DataPlay disks are similar to a CD-R, as they hold up to 500MB and are write-once. But DataPlay disks require an additional drive to read them, whereas Flash slots are typically built into devices.

For this reason, Flash memory is likely to be the main type of storage used with PDAs and handhelds for the time being. There are many different formats available, the most common being CompactFlash. Others such as Secure Digital look set to grow in popularity because of their small size and security features.

PC Card was the first widely adopted expansion option to emerge for mobile computers. The standard, created by the Personal Computer Memory Card International Association (PCMCIA) which includes companies such as Toshiba and IBM, defines a credit-card sized device and a matching slot. PC Card slots not only provide storage expansion, but peripheral connectivity for hardware such as Ethernet adapters and modems. Three types are defined: Type I is 3mm thick and mainly used for memory cards; Type II is 5mm thick and commonly used for I/O, such as LAN adapters; Type III is 10.5mm thick and often used for small hard disks.

PC Card slots are common on notebooks, but are generally too large to be built into handhelds, apart from bigger models such as HP's Jornada 680. Because of their larger size, Flash memory PC Cards can have capacities of up to 1GB.

Like PC Card, CompactFlash cards can be used to add storage or I/O capability to a mobile device. However, CompactFlash devices are much smaller: about the size of a book of matches. The slots are also correspondingly smaller, and are common on many makes of PDA.

The vast majority of CompactFlash products are Flash memory cards, with capacities ranging from 8MB to 512MB. Ethernet adapters and modems are also available in a CompactFlash format, as is IBM's Microdrive miniature hard disk, which offers up to 1GB of storage capacity. Even though IBM has worked hard to keep power consumption down, the Microdrive may still draw too much electrical current for extended use with a handheld device Ã,­ 250mA when writing data, as compared with 35mA for a typical Flash card.

SmartMedia is widely used in digital cameras and digital audio players, but does not seem to have been used in any PDAs. Developed as a standard by Toshiba under the auspices of the PCMCIA, its main advantage is its very small size; SmartMedia cards are just 45x37mm and as thin as a credit card. SmartMedia generally costs less per MB of storage capacity than other Flash formats, although CompactFlash is almost as cheap. The highest capacity SmartMedia card is currently only 128MB.

Seen by many as SmartMedia's successor, the MultiMediaCard (MMC) standard is backed by the MultiMediaCard Association, which is made up of more than 80 members. Many MMC-compatible products are already available, including Casio's EM500 Pocket PC range.

MMC Flash memory cards are the size of a large postage stamp and almost as thin as SmartMedia cards, so can be used in even the smallest of devices. Their size is kept down by the use of a serial interface which requires just seven pins.

The current maximum capacity of an MMC is currently only 128MB, and the cards are also more expensive than SmartMedia or CompactFlash. Prices are expected to come down as MMC cards become more widespread.

Secure Digital (SD) is the latest standard to appear on the market, and is a superset of MMC. It already has the backing of more than 200 companies, including Palm, whose m500 and m505 PDAs feature SD slots. SD cards are almost identical in size and shape to MMC cards, and are backward compatible with them. Devices fitted with SD slots can also accept MMC cards, but an SD card cannot be used in an MMC slot.

The main difference between SD and MMC lies in SD's built-in support for the Content Rights Management (CRM) system, an industry standard for preventing the pirating of copyright material. Similar content security facilities are also being developed for other Flash memory formats.

SD cards can also be used for adding I/O capabilities, despite their small size. Palm and Toshiba recently announced a Bluetooth SD adapter for the Palm m500 and m505, to be available later this year.

Among these well-supported standards, Memory Stick is unusual in that it is a proprietary Flash memory format. Developed by Sony for use in its digital cameras and digital audio players, Memory Stick slots are now found in Sony's Vaio range of PCs and notebooks, and also in the firm's Clié Palm-compatible PDA.

Each Memory Stick is about the size of a stick of chewing gum, measuring 21x50x2.8mm, and capacities are available up to 128MB. Memory Stick's size has also been kept down by using a serial interface to reduce the number of required pin connections. Sony has recently announced a smaller version called the Memory Stick Duo. At 20x31x1.6mm, it is roughly half the length of the standard version.

The biggest drawback to Memory Stick is that it is only available from two sources: Sony itself and Lexar, which manufactures it under licence. Memory Sticks are more expensive than other Flash standards, with a 64MB Memory Stick costing Ã,£86 from Sony. No other equipment manufacturers seem to have adopted Memory Stick so far, so its interoperability is limited.

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