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08/05/2009
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How the latest USB specification boosts multimedia data transfer rates.
The USB 3.0 Promoter Group was announced at the 2007 Intel Developer's Forum. The six member companies – HP, Intel, Microsoft, NEC, NXP and Texas Instruments – developed the initial specification, then contributor companies provided input, creating a pool of nearly 200 industry experts to ensure wide acceptance upon release. The specification was finalised in October 2008 and, by mid November 2008, was available to any company wanting to adopt it.
But what is SuperSpeed USB 3.0 and what can be done with it?
The most obvious difference with SuperSpeed USB is the more than tenfold speed increase: from 480Mbit/s to 5Gbit/s. While USB 2.0 high speed is more than adequate for many applications, the USB connection can become a bottleneck for others. Additionally, SuperSpeed USB continues to inspire creative minds to develop new applications and end equipment that benefit from higher performance. The 5Gbit/s data rate should provide headroom for the next five years.
However, the increased speed presents issues for existing USB 2.0 cables. SuperSpeed USB requires a shielded differential pair cable to ensure signal integrity and to minimise EMI. And the maximum cable length is reduced from 5m for USB 2.0 to 3m for SuperSpeed USB.
The physical layer electrical signalling has also changed from the simple two wire system to a dual simplex data path. This is done over a completely different set of connections than the existing USB 2.0 two wire interface, that remains untouched.
The goal was to support a 3m cable length at 5Gbit/s, but that any changes in the electrical signalling scheme would not drive the need for a new form factor plug or receptacle on the host and, if possible, on the peripheral. To minimise risk, the promoter's group determined the new signalling scheme should be similar to PCI-Express. The connection retains sideband functionality without added wires and allows receiver termination to detect connects/disconnects.
Significant differences exist between the SuperSpeed USB and PCI-Express signalling schemes. SuperSpeed USB does not use a common clock architecture, requiring a spread spectrum on both sides and receiver equalisation. Jitter is also handled differently and there are no sideband pins for connect/disconnect detection or for link reset, power management state control or wake up.
Another key change is improved bus power efficiency. This is ideal for extending the battery life for portable devices, whether hosts or peripherals. The specification defines excellent power characteristics, especially for idle links. Both upstream and downstream ports can initiate lower power states of the link. Local power management control uses multiple link power states to further improve power efficiency. Another power saving method eliminates all polling of devices.
The key to the SuperSpeed USB standard is backwards compatibility with USB 2.0 as much as possible. But, because SuperSpeed USB signalling is done over separate conductors to those used for USB 2.0, this presented a serious challenge to the cable and connector working group:
Add five new conductors to the receptacle and plug while maintaining the ability to allow existing USB 2.0 plugs to be inserted into SuperSpeed USB receptacles, and SuperSpeed USB 3.0 plugs be inserted into USB 2.0 receptacles and enable an operational link in all cases!
The solution was to add five new conductors on the insert side of existing plugs and receptacles. This allows for the same mechanical interface as USB 2.0 and enables full backwards compatibility.
The existing Micro-B receptacle has insufficient room to add five new conductors – requiring a major change with a side by side solution. This connector should be used only on small portable devices. SuperSpeed USB Micro-A and -AB connectors will be identical to the Micro-B connector, except that keying and profiles will be different to prevent incorrect insertion.
Along with backwards mechanical compatibility, the goal was to maintain the extensive device driver infrastructure. The same data transfer types – interrupt, bulk and isochronous – were maintained. Finally, this standard preserves the existing USB ease of use expectations.
Applications and benefits
The increased speed reduces wait time when transferring data from computers to portable consumer devices. End users desire an excellent Sync-n-Go experience as they use their content rich portable devices, such as music players, video players, digital still cameras (DSC) and/or camcorders. The increase in rich content size also drives an increase in storage capacity. Richer content can be separated as richer capture and richer playback.
Ten years ago, the average DSC was 1Mpixel. Today, many entry level models are 16Mpixel and camera phones can often capture 8Mpixel images. Neither trend can be expected to slow. Another change in the mainstream DSC market is the less expensive digital zoom versus bulkier, more expensive, optical zooms.
Both trends are driving an increase in the individual image file size. This, as well as the desire to store a week's worth of vacation photos, for example, is driving increased storage capacity. Camcorders are following suit. They have a still functionality (typically lower resolution than the average DSC, but growing), with higher quality video that requires more storage. High definition (HD) Mpeg2 camcorders are available. Both capture types have a high 'turnaround' rate – content needs uploaded to free space that will be refilled quickly.
Simultaneously, there is a transition to playing back richer content 'on the go'. DVDs are transitioning from standard definition (5.7Gbyte storage) to HD (25Gbyte storage). Flash based music players can play 60minutes of music, but HDD based players feature 80Gbyte of storage and room for 20,000 songs. When storing an entire music library in something smaller than your wallet, anywhere/anytime is compelling.
The transition continues to portable media players (PMP) capable of playing videos 'on the go', with the next generation capable of playing HD movies. Finally, new business models (for example, local caching of new video releases in a PVR-like device that can be downloaded to a portable player) require high speed connectivity.
Where can SuperSpeed USB be used? Anywhere that high capacity storage is needed to perform a fast data Sync'n'Go. The goal is for Sync'n'Go operations to take less than 90s – the 'pain threshold' for many end users. Many portable devices use flash memory for storage and their capacity will continue to increase, due to customer demand and cheaper memory. Therefore, flash based peripherals will benefit and will, ultimately, require SuperSpeed USB data rates. This includes the most commonly used USB peripheral – the ubiquitous flash drive!
In summary, any multimedia application that wants to move lots of files or just large files quickly to or from the pc will benefit greatly from SuperSpeed USB. It delivers the bandwidth to satisfy user needs to sync multimedia files with portable consumer products today and the headroom needed to do the same for flash based products in the next five years.
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Author
Dan Harmon
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