USB 3.0: What's the Fuss?


USB 3.0 is here. It’s fast and it has the potential to reshape the PC as we know it

Today, we’re starting to see the first motherboards with USB 3.0 support. That support exists in the form of a discrete controller chip, typically the NEC uPD720200; it will likely be late 2010 or sometime in 2011 before we see USB 3.0 integrated into motherboard chipsets. Still, USB 3.0 is a major leap beyond USB 2.0, so peripheral manufacturers are already announcing products to support the new standard.

First, let’s clarify some terminology. USB 1.0/1.1 was typically just called USB, and supported throughput up to 12Mb/s. When USB 2.0 arrived, with its 480Mb/s speed, the USB Working Group ( ) needed a distinguishing name, hence Hi-Speed USB. USB 3.0 will be called SuperSpeed USB. Got that?

Where Old Meets New

SuperSpeed USB supports maximum throughput of up to 5Gb/s—roughly 10x the speed of USB 2.0. However, USB 3.0 is fully backward compatible with USB 2.0, so you won’t need to toss your old peripherals as SuperSpeed USB–capable motherboards and systems arrive on the scene.

USB 3.0 manages its backward compatibility with a dual-bus architecture, which operates concurrently with USB 2.0 signaling. It achieves this by adding pins to the USB connector. Note that USB 3.0 is a bidirectional architecture, whereas Hi-Speed USB is unidirectional.

The connector shell for the host system (that familiar flat connector) looks the same. If the host detects no USB 3.0 connections, it reverts to USB 2.0; otherwise it will run at full USB 3.0 speed.

The interface on the peripheral side—the taller, D-shaped connector—has grown a bit, gaining a slight bulge at the top. Now it looks a little like a skinnier version of the typical Ethernet connector. However, that connector will accept the old-style D-shell connector for USB 2.0 devices. Micro connectors—those pesky, tiny interfaces built into cameras, digital media players, and other smaller devices—are more problematic. The compatibility issue is solved by adding additional real estate to the connector itself, effectively turning it into a double-connector—one for USB 2.0 devices and one for USB 3.0 hardware.

There will be two types of micro connectors for USB 3.0, down from a seeming multitude of small connectors for USB 2.0.
When you start running at 5Gb/s, signaling integrity becomes paramount. So, USB 3.0 will move away from the unshielded twisted-pair cabling used for older USB versions, to shielded  differential-pair cabling. This enables the cable lengths needed for useful peripheral interconnects while maintaining the signal integrity needed for SuperSpeed USB. Cable lengths up to three meters (10 feet) will be supported.

Unsurprisingly, the signaling itself is somewhat similar to PCI Express and Serial ATA, with two differential pairs. Typical USB hot-plug capability will still be supported—a must in the world of easily detachable devices.

Power Management

One feature that’s going away is the need for polling, which will be replaced by a more interrupt-driven model in which the peripheral will ping the host system, which can then initiate a transfer request. The bus is only active when actually moving data. This improves overall power efficiency, so that battery life remains robust, even at the higher throughput.

In fact, SuperSpeed USB maintains power management at all levels, from the host, through hubs, and down the actual physical layers. There’s no broadcasting of packets—they’re only sent when requested. This is managed by asynchronous notifications as to when a device or host link is ready to receive or transmit data. One of the key design parameters was that USB 3.0 links enter a low-power state whenever the bus is idle.

More sophisticated power management meant that hubs had to become smarter. Don’t think of a USB 3.0 hub as just a way to add more ports. Hubs will need to monitor upstream and downstream packets to ensure that data is routed to the correct device. Data will need to be buffered until sleeping devices and ports are woken.

A key feature that facilitates power management is Latency Tolerant Messaging (LTM), which allows systems to go into deep sleep states while still maintaining active links to connected peripherals. It’s likely that some peripherals will be more “tolerant” of message delays than others, so the device can actually notify the host system as to what its maximum latency tolerance will be.

One last key feature of note is power delivery. USB 3.0 is rated to deliver 900 milliamps per connection, almost double the current 500mA. That means devices that require higher power now won’t require power bricks. Remember that external optical drive that needed two USB connections—one for supplemental power, one for data and power? That peripheral can now get both power and data off a single connection. More robust USB chargers are also possible.

A Look Ahead

At 5Gb/s, USB 3.0 is faster than Serial ATA 3Gb/s and almost as fast as SATA 6Gb/s. That makes it plenty fast for external storage, including very fast SSDs. Latencies are likely to be a little higher, but not so much that it will make a difference. That might mean that eSATA’s life is limited, since USB 3.0 will likely be more flexible and offer better power efficiency.

A single USB 3.0 connection will also be roughly the same speed as a single-lane PCI Express 2.0 connection. So external networking, audio, and high-definition video devices that operate in real time become possible. In fact, the first publicly announced USB 3.0 peripheral was the Point Grey USB 3.0 camera, capable of streaming full 1080p video in real time over a SuperSpeed USB connection.

At the same time, don’t expect miracles. If you chain a bunch of devices to a single USB 3.0 port, overall throughput per device will naturally be reduced. Today, we’re seeing systems with eight or 10 USB 2.0 ports; it wouldn’t be surprising to see demand for ports increase in the future. USB 3.0 will also likely become the nail in the coffin of FireWire connections for PCs.
Of course, your usual peripherals will continue to work as expected. Keyboards, mice, card readers, and other USB 2.0 devices will still function. You’ll get faster throughput, better power delivery and robust efficiency. It’s no wonder that motherboard and peripheral manufacturers are jumping on the bandwagon.

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