Modernization of Mobile Storage

At the University of New Hampshire InterOperability Laboratory (UNH-IOL), we are fortunate to be able to follow technology developments in a number of areas, and watch as they go from sketches on a whiteboard to actual shipping products.

Sometimes these developments happen in their own bubbles, other times there are not so coincidental coincidences, where several industries are able to take advantage of technology originally developed for one community. We may be watching that happen right now as technologies originally developed for mobile are finding application in storage, and vice versa.

Mobile device makers tend to highlight high storage capacities of their devices. Lower end devices are typically in the 8-16 GB range, whereas higher end devices can have 128 GB or more. Most Android devices accept microSD cards, increasing their capacity further (currently microSD cards max out at 128 GB). But what are the technologies behind all this storage, and what’s in store for them and the future of mobile storage?

Today’s dominant mobile storage technologies

Today the dominant technologies for mobile storage are eMMC and microSD cards. Internal (i.e. non-user serviceable) storage is typically eMMC. Swappable and upgradeable microSD is what we know from use in cameras, phones, etc. The speed class on microSD refers to the read/write performance. The typical standard today is Class 10. Soon UHS (Ultra High Speed) cards will be available. Here are the key differences between eMMC and microSD:


  • Embedded, not upgradeable
  • 400 MB/s

microSD Cards:

  • 10 MB/s Class 10, up to 312 MB/s for future UHS cards
  • Easily swappable, upgradable

Most users don’t think about the differences between these technologies. But when we look at the performance expectations on mobile devices, it’s worth asking if these technologies can handle the coming performance demands for mobile devices.

What are these performance demands? Stepping through an example may be helpful. Consider a 4K mobile display operating at 60 frames per second (fps). How much bandwidth will it need?

Here’s a simplified rough calculation:

  • 4096x 2160 = 8847360 pixels
  • 24 bits per pixel x 8847360 = 212336640 bits per frame
  • 60 frames per second x 212336640 = 12,700,000,000 bits per second
  • 12,700,000,000 bits per second = 1,590,0000,000 bytes per second

That's 1.59 GB per second, nearly 4 times the throughput available from eMMC today.

To be fair, we have greatly simplified the process that a system architect would go through to determine the necessary bandwidth. You can argue whether this simplification is valid or not, but we’re not trying to show how to build such a system, simply that existing technologies are at best being stretched to enable the demanded user experience and price point, and at worst, cannot meet the demand.

It’s important to keep in mind that system designers are juggling many competing factors, and we’ve ignored the effects of compression and buffering that can be used to save bandwidth and system cost. Architects will employ combinations of high performance (but constantly powered) DDR3 RAM, on board NAND Flash, and image processing and compression hardware to enable the best performance with minimal power draw. Surely a system with a lot of RAM could meet the necessary performance specs. But it would also have constant power draw and far exceed the sub $100 BoM of many devices. An expensive mobile device capable of showing a 4K movie is of little use if the battery dies at the climax of the movie.

The point is, today’s technologies aren’t enough. We need to ask what the new technologies are that will address this need. Is there one clear winner or will new technologies provide mobile device integrators and designers with more choice? Evolutions of technologies like NVMe, PCIe, UFS, and M-PHY (Figure 1) may lead to drastic performance increases for on-device storage. Each of these and how they can be used are examined below.

NVMe / PCIe / mPCIe

NVMe is a protocol for interfacing with solid state drives (SSDs) over the PCIe bus. Existing PC drivers allow these NVMe devices to look like hard disk drives to a PC operating system, but with exceptional performance. Already NVMe devices are appearing in enterprise environments and so-called ‘all flash arrays.’ As you would expect, the cost/GB of NVMe is high relative to HDD, but that gap is narrowing quickly. Enterprise installations demanding high performance are ready and willing to absorb the increased cost/GB of NVMe SSDs and take advantage of their performance benefits. In fact, many large enterprises see savings on cooling and power when switching to all flash arrays and all flash data centers, making that choice even more compelling. Even though the enterprise is where NVMe is having an early impact, it’s a stated goal of the technology to migrate into the client space as well.

What is the client space? Desktops, workstations, laptops? These types of devices already have PCIe interfaces in them and can easily handle the standard 2.5” disk drive form factor that many early NVMe devices use. What about convertibles? Tablets? Phones? Yes, especially with the introduction of the M.2 form factor. This further reduces the space that storage will occupy in a device. Furthering the mPCIe interface could potentially enable the use of PCIe storage over a phy specifically designed for low-power mobile applications. In this case the ‘m’ stands for ‘mobile.’ It’s an adaption of the PCIe protocol onto a low power phy, namely M-PHY (more below). Remember, PCIe was initially developed as a high performance interface for the PC and server worlds, where power wasn’t a major concern. The need for a new phy stems from the need to have a low-power interface, hence M-PHY.

UFS / UniPro

UFS (Universal Flash Storage) is a protocol created by the JEDEC Mobile Forum (the same people that brought you DDR and MMC). While still a future technology, UFS is designed to be implemented in either an SD card or embedded chip form factor. The UFS protocol will use the MIPI UniPro protocol as its transport. This is important on several fronts. Having the implementation defined for multiple form factors enables design reuse. This speeds up development and drives down cost. Using UniPro may lead to design reuse as well. Within the MIPI Alliance, UniPro is targeted to be used as a transport for camera interfaces as well. Reusing the technology on multiple interfaces within a mobile device further speeds up development and drives down cost. UniPro is also designed to use that low-power electrical layer, M-PHY.


M-PHY is the technology that serves as the foundation for the coming performance boost in mobile storage. The M-PHY specification created by the MIPI Alliance allows low power transfers of up to 5.8 Gbps per lane, and multi-lane systems are defined. Future specification releases will push transfer rates even higher. The leading processor companies in both the mobile and PC arenas are key supporters of the work of MIPI Alliance, so we can expect to see M-PHY interfaces in the near future. But when? Economics will decide: when the current crop of interfaces becomes cheap enough that it’s economically viable to move to a newer, more expensive interface that enables a new set of experiences.

We can’t fully predict the types of architectures that future mobile and storage products will have. Most companies have roadmaps stretching years into the future. However, the possibilities enabled by technologies under development today are exciting, and no doubt there are forthcoming applications we haven’t even yet considered.