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A quick look at Thunderbolt on the PC

There's thunder, but lightning?

When Intel's Thunderbolt interconnect was formally unveiled last year, we learned that Apple would have first dibs on the technology. Device makers would be free to build Thunderbolt-compatible accessories right away, of course, but the first computers to employ the uber-fast interconnect would be Macs.

The enthusiast community met this news mostly with a collective shrug. We already had USB 3.0 ports with oodles of bandwidth for external storage devices that were affordable, backward compatible, and readily available. Thunderbolt hardware? Not so much. We also knew that Light Peak, the precursor to Thunderbolt, was conceived as an optical interconnect. Apple's implementation ditched optical fibers for copper wires, which simply aren't as cool.

More than a year after Thunderbolt ports made their Mac debut, PCs are getting their first taste. Select Ivy Bridge notebooks are destined to feature the technology, and Thunderbolt-equipped motherboards are already selling online. A closer look is warranted, so we've rounded up a couple of motherboards and an external storage box to see what's what.

You've been... Thunderstruck
Before we dip into the PC-specific gear, we should familiarize ourselves with the underlying technology. From a hardware geek's perspective, it's pretty slick. Thunderbolt is a point-to-point interconnect with an obscene amount of bandwidth and the ability to transmit both DisplayPort and PCI Express signals along a remarkably thin cable. How much bandwidth? 10Gbps in each direction, per channel. Each Thunderbolt connector carries two channels, bringing the pipe's aggregate bandwidth up to a staggering 40Gbps, or 20Gbps in each direction.

To put that into perspective, a single USB 3.0 port offers 5Gbps of bidirectional bandwidth. One PCI Express 3.0 lane has 10Gbps of full-duplex connectivity. Thunderbolt beats them both with ease. And all of its bandwidth is usable thanks to a low-overhead packet format, Intel says.

Offering gobs of bandwidth is one thing, but Thunderbolt's real crown jewel is its ability to map DisplayPort and PCI Express signals to a single "meta-protocol." Translation is handled entirely by the Thunderbolt controller, allowing the interconnect to remain invisible to the host PC, which sees only the connected devices. Software isn't required for the controller, but Thunderbolt devices will have their own drivers.

Source: Intel

Intel hasn't revealed exactly how bits are encoded within the Thunderbolt protocol, but we do know there's a flexible QoS framework tasked with multiplexing the PCIe and DP streams. Depending on the configuration, encapsulated signals may be routed through multiple Thunderbolt controllers before being decoded. As many as seven Thunderbolt controllers can be daisy chained together, and the switch-based architecture supports other topologies, as well.

A domain-based time synchronization mechanism keeps attached devices within eight nanoseconds of each other. That feature will be key to Thunderbolt's appeal among audio and video content creation professionals. Intel claims Thunderbolt's low latency as another benefit, although it doesn't offer any specific numbers on that front.

Source: Intel

The Thunderbolt controller chip does all the work associated with the interconnect. It contains a Thunderbolt switch, a PCI Express switch, and adapter ports for DisplayPort and PCI Express. Controllers can come with one or more Thunderbolt ports, and they connect to the host system using a PCI Express 2.0 x4 link. DisplayPort devices hook into the controller via the associated adapter ports.

Although PCI Express is now in its third generation, four gen-two lanes still offer plenty of bandwidth for a whole range of devices, including the I/O peripherals one might expect to find in a Thunderbolt companion hub for notebooks. A hub could easily offer a collection of Serial ATA, USB, and FireWire ports courtesy of auxiliary controller chips. Heck, you could even throw a discrete graphics processor in there. Indeed, we saw a notebook with a Thunderbolt-connected external GPU at the Consumer Electronics Show earlier this year.

The final component of the Thunderbolt equation is the physical interface: the connector and its associated cabling. Mini DisplayPort has been adopted as the connector, in part because it's interoperable with existing DisplayPort monitors, but also because it's small enough for the sort of ultra-slim notebooks that seem ripe for Thunderbolt expansion. Electrical cables can span distances up to 3 m and carry as much as 10W of power for connected devices. Optical cables promise lengths in the "tens of meters," though the device at the other end will need its own power source. Both types of cables use the same MiniDP connector.

If you're wondering how Thunderbolt can get away with using an electrical port interface for optical cables, the answer is that all Thunderbolt cables are smart or "active." They include termination electronics at each end to assist with communications. The optical cables will convert electrical impulses into light at one end and back into electrical signals at the other. The reason not all Thunderbolt cables are optical now? Copper Thunderbolt cables are less expensive.

Despite its considerable potential, Thunderbolt still has some limitations. In addition to the seven-device cap on daisy chaining, no more than two DisplayPort screens can be connected at a given time. Those need to be DisplayPort 1.1a or newer displays, by the way. With older DP screens, users may be limited to one display per chain.

A bigger concern is the lack of hot-plugging support in Windows. If you want a Thunderbolt device to be accessible, it needs to be connected when the system is booted. Intel says hot-plugging support is coming in driver updates for Thunderbolt devices. We've heard those drivers may arrive as early as this month and that some cooperation is required from the motherboard firmware. Any Thunderbolt device that's certified for Windows should support hot plugging, Intel says.