At some point in my life, I used to run the USB business for big semi company. The best result I remember was NEC SATA controller capable of pushing 320Mbps actual data throughput for mass storage, probably current sata drives are capable of this or slightly more. This was using BOT (some mass storage protocol runs on USB).
I can give a technical detailed answer but I guess you can deduce yourself. What you need to see is that, this is ecosystem play, any significant improvement would require somebody like Microsoft to change their stack, optimize etc, which is not going to happen. Interoperability is far more important than speed. Because existing stacks carefully cover the mistakes of slew of devices out there because when the USB2 spec come out probably the initial devices didn't really confirm to the spec that well since the spec was buggy, the certification system was buggy etc. etc.. If you build a home brew system using Linux or custom USB host drivers for MS and a fast device controller you can probably get close to the theoretical limits.
In terms of streaming, the ISO supposed to be very fast but controllers do not implement that very well, since 95% of the apps use Bulk transfer.
As a bonus insight, for example, if you go and build a hub IC today, if you follow the spec to the dot, you will practically sell zero chips. If you know all the bugs in the market and make sure your hub IC can tolerate to them, you can probably get in to the market. I am still amazed today, how well USB is working given number of bad software and chips out there.
- Sata-Express uses 4 differential pairs for signaling, composing two PCI-e lanes.
It also requires some auxilliary connections (ground, etc), but the primary signaling is done using the 4 differential pairs.
The USB-3.1 spec defines a number of different connectors: However, all but the USB-c connector clearly have insufficent pins to be applicable.
The USB-c connector's pinout:
The connector does have enough differential pairs to properly carry two PCI-e lanes (and by effect, it could probably theoretically carry Sata-Express. However:
- This connector is, by design, not "keyed". This means it can be inserted in two different orientations. When it's used for USB-3, the matching pairs are connected together (e.g. A1-B1, A2-B2, etc...) so reversing the connector is harmless. For PCI-e, this could be a problem, as it would either completely reverse the wiring, or (with some clever design) swap the ordering of the lanes in the PCI-e bus.
I confess I do not know enough about PCI-e to tell you if swapping the physical lanes would be a problem here.
- From an implementer's standpoint, using the USB-c connector for something other then USB-c is a terrible idea, as it will lead to people plugging USB-3 devices into their hypothetical PCI-e-over-USB-c motherboard, and plugging PCI-e-over-USB-c devices into USB-3 ports. There is a reason we have different connectors for different things.
Realistically, the USB-c connector could be fairly easily modified to make it incompatible with USB-3 connectors, at which point you'd basically just have yet another PCI-e connector. Considering that one of the major design decisions for the current Sata-Express connector is backwards compatibility with normal SATA, it's not likely to happen.
Furthermore, the SATA-express interface definition provides a lot of further connectivity intended for enterprise use (take a look at SFF-8639). There are specialized versions that have four PCI-e lanes, and an additional optional plain SATA channel. This is physically compatible with the normal device-end SATA-express connector (if you connect a SFF-8639 device to a SATA-express interface, it simply falls back to SATA-express). There is no physical way that you could route all the required connectivity for a SFF-8639 interface over a USB-c connector.
The current SATA standard has a lot of things in it for enterprise use that you may have not seen. In particular, there are SAS drives, which use the SFF 8482 connector, which is, again, physically compatible with current SATA connectors (and will safely interoperate at the slower device's transfer speed if interconnected, just like SFF-8639).
The design decisions for the SATA-express connector are very clearly inline with the design decisions for the SATA & SAS connectors.
TL;DR - Theoretically, it could work. Realistically, no one is likely to do it.
Best Answer
Added mid 2022:
A lightly edited version of a comment by @LittleWhole
In 2022 the world is moving towards the far more robust and convenient USB-C connector. While there are still issues with USB-C (including even mechanical incompatibilities), things are slowly being addressed (i.e. USB4 standard on the protocol side) and I have only ever encountered one USB-C cable that wouldn't plug into a USB-C receptacle in my life. Adoption of USB-C is definitely picking up the pace - not just in consumer electronics, but a motor controller for my school's robotics club has even adopted USB-C
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A major flaw:
A major factor in abandoning mini-USB is that it was fatally flawed mechanically. Most people who have used a mini-USB device which requires many insertions will have experienced poor reliability after a significant but not vast number of uses.
The original mini-USB had an extremely poor insertion lifetime - about 1000 insertions total claimed. That's about once a day for 3 years. Or 3 times a day for one year. Or ... For some people that order of reliability may be acceptable and the problems may go unnoticed. For others it becomes a major issue. A photographer using a flash card reader may expend that lifetime in well under a year.
The original mini-USB connector had sides which sloped as at present but they were reasonably straight. (Much the same as the sides on a micro-A connector). These are now so rare that I couldn't find an image using a web search. This image is diagrammatic only but shows the basic shape with sloped but straight sides.
Efforts were made to address the low lifetime issues while maintaining backwards compatibility and the current "kinked sides" design was produced. Both plug and socket were changed but the sockets ("receptacle") will still accept the old straight sided plugs. This is the shape that we are all so used to that the old shape is largely forgotten.
Unfortunately, this alteration "only sort of worked". Insertion lifetime was increased to about 5,000 cycles. This sounds high enough in theory but in practice the design was still walking wounded with respect to mechanical reliability. 5,000 cycles is a very poor rating in the connector industry. While most users will not achieve that many insertion cycles, the actual reliability in heavy use is poor.
The micro-USB connector was designed with these past failings in mind and has a rated lifetime of about 10,000 insertion cycles. This despite its apparent frailty and what may appear to be a less robust design. [This still seems woefully low to me. Time will tell].
Latching Unlike mini USB, Micro USB has a passive latching mechanism which increases retention force but which allows removal without active user action (apart from pulling). [Latching seems liable to reduce the plug "working" in the receptacle and may increase reliability].
Size matters:
The micro and mini USB connectors are of similar width. But the micro connector is much thinner (smaller vertical dimension). Some product designs were not able to accommodate the height of the mini receptacle and the new thinner receptacle will encourage and allow thinner products. A mini-USB socket would have been too tall for thin design. By way of example - a number of Motorola's "Razr" cellphones used micro-USB receptacles, thus allowing the designs to be thinner than would have been possible with a Mini-USB receptacle.
Wikipedia on USB - see "durability".
Connector manufacturer Molex's micro USB page
They say:
Micro-USB replaces a majority of Mini-USB plugs and receptacles currently in use. The specification of the Micro-USB supports the current USB On-The-Go (OTG) supplement and provides total mobile interconnectivity by enabling portable devices to communicate directly with each other without the need for a host computer.
... Other key features of the product include high durability of over 10,000 insertion cycles, and a passive latching mechanism that provides higher extraction forces without sacrificing the USB's ease-of-use when synchronizing and charging portable devices.
All change:
Once all can change, all tend to. A significant driver to a common USB connector is the new USB charging standard which is being adopted by all cellphone makers. (Or all who wish to survive). The standard relates primarily to the electrical standards required to allow universal charging and chargers but a common mechanical connection system using the various micro-USB components is part of the standard. Whereas in the past it only really mattered that your 'whizzygig' could plug into its supplied power supply, it is now required that any whizzygig's power supply will fit any other device. A common plug and socket system is a necessary minimum for this to happen. While adapters can be used this is an undesirable approach. As USB charging becomes widely accepted not only for cellphones but for xxxpods, xxxpads, pda's and stuff in general, the drive for a common connector accelerates. The exception may be manufacturers whose names begin with A who consider themselves large enough and safe enough to actively pursue interconnect incompatibility in their products.
Once a new standard is widely adopted and attains 'critical mass" the economics of scale tend to drive the market very rapidly to the new standard. It becomes increasingly less cost effective to manufacture and stock and handle parts which have a diminishing market share and which are incompatible with new facilities.
I may add some more references to this if it appears there is interest - or ask Mr Gargoyle.
Large list of cellphones that use micro-USB receptacle
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A few more images allowing comparisons of a range of aspects including thickness, area of panel, overall volume (all being important independently of the others to some for various reasons) and retention means.
Large Google image samples each linked to a web page
and more
Useful discussion & brief history Note: they say (and, as Bailey S also notes)
Accomplished by moving leaf-spring from the PCB receptacle to plug, the most-stressed part is now on the cable side of the connection. Inexpensive cable bears most wear instead of the µUSB device.
Maybe useful:
USB CONNECTOR GUIDE — GUIDE TO USB CABLES
USB connections compared
What is Micro USB vs Mini USB