Stephen Shankland/CNET

Intel believes Thunderbolt will remake mobile computing by endowing laptops with a high-speed, versatile port.

To match Intel's mainstream ambitions for Thunderbolt, though, Intel will have to prove to hardware designers and to consumers that it's got compelling advantages over the alternatives. Today, those are chiefly USB (Universal Serial Bus) and HDMI (High-Definition Multimedia Interface. Tomorrow, another challenger could arrive in the form of PCI Express Cable, and it's got a strong ally in PC giant Hewlett-Packard.

Getting new input-output technologies to catch on is particularly hard because I/O standards only succeed with support from both ends of the communication link.

Often with input-output (I/O, as it's called in the trade), peripheral makers wait for support in computers, but computer makers wait for peripherals to justify the added expense. It's called the chicken-and-egg problem.

It gives a lot of power to incumbent standards such as USB and HDMI that have caught on widely. And it can mean battles between adherents of competing standards are particularly long and bloody.

1, 2, 3, 4, let's have a bus war

One historical I/O struggle was the drawn-out "bus wars" over PC expansion cards back in the 1980s and 1990s. MicroChannel Architecture (MCA) competed against Extended Industry Standard Architecture (EISA) and then VL Bus (VESA Local Bus), and customers had to make sure video cards and other devices worked in their machines. That battle ended only when PCI (Peripheral Component Interconnect), backed by Intel, swept the other challengers aside.

Another struggle was between Future I/O v. Next Generation I/O for server expansion; a compromise resulted the very high-end but very scarce InfiniBand. Memory interfaces, too, have been battlegrounds, for example when Intel's FB-DIMM lost out to DDR.

Often, lower-end, cheaper technologies can win even if they're slower or not as useful. One case in point is USB, whose ubiquity largely squeezed the technologically more advanced FireWire into a high-end niche. Likewise, the ATA standard (also called IDE) for attaching disk drives generally beat out SCSI in the mass market, then extended its dominance with the shift to higher-speed serial-interfaces SATA vs SAS.

Some of these battles are ancient history, but consumers today can get a flavor of them with video. Want to attach an external monitor to your computer? Better check whether to use HDMI or DisplayPort. Even 1990s-era VGA is still in widespread use.

HDMI's clout

HDMI shows the incumbent power of an I/O standard. The newest-generation MacBook Pro systems add an HDMI port, freeing new MacBook owners from reliance on a DisplayPort-to-HDMI adapter when plugging into TVs or projectors that lack DisplayPort.

"DisplayPort has big benefits on the monitor and display interface side that HDMI didn't have until version 1.4," IDC analyst Linn Huang said. "But HDMI is still embedded into every device -- set-top boxes, game consoles, some phones."

He's seen the difficulties of decision firsthand in his earlier career as a product manager for computer monitors. "In an industry where margins are slimmer than they were 5 years ago, interface choices are pretty critical," Huang said. "Often it's the difference between making money and not making money."

External monitor maker ViewSonic illustrates the chicken-and-egg problem.

"Thunderbolt is very new, and there are not many connectivity options at the current time other than a few of the new Apple iMacs and Macbooks," said Erik Willey, who markets the company's LCD monitors and PCs. "You may recall that after DisplayPort was introduced we also remained on the sidelines for several years until we felt there was a large enough installed base of sources, and began hearing requests from our key enterprise customers." DisplayPort devices can plug into Thunderbolt ports, but they can't be used as Thunderbolt hubs the way Apple's new Thunderbolt-equipped external monitors can.

Performance and versatility are on Thunderbolt's side. It can handle some of what both USB and HDMI can, bridging the divide between data and video. And that could help when it's time to pick which ports get precious real estate around a laptop's edges.

"As laptops get slimmer, interfaces are going to get squeezed out of the designs. An interface that can carry video and data signals can be more advantageous to PC OEMs," Huang said.

Intel

USB moving ahead

Intel goes to great pains to argue that Thunderbolt and USB are complementary, but in practice they do compete. If you're buying an external drive, do you want to pay the premium for a Thunderbolt model? And if you get one, you might want to make sure it's got a USB port, too, in case you have to attach it to a computer that lacks a Thunderbolt port.

USB 3.0 has taken years to arrive, but with Intel's Ivy Bridge generation of computers, it's finally a standard part of PC hardware, and millions of existing USB devices already on the market work with it already. Thunderbolt has more muscle, but USB 3.0 will reach 100 percent of PCs in 2016, IDC forecasts, the same year Thunderbolt reaches less than a sixth of laptops and an even smaller percentage of desktop PCs.

USB is moving forward, too, with new projects from Intel and USB allies. One vision is running external displays over USB 3.0, something its 5Gbps data transfer rate is better suited for than USB 2.0's 480Mbps. That's well short of Thunderbolt's 10Gbps -- a speed it can sustain over two channels, in two directions simultaneously for each channel -- but it's enough for some video and for tasks such as using external drives or taking photos with an SLR tethered to a computer with a cable.

Another development is USB PD (power distribution), in which you could plug your laptop into an external monitor that's plugged into a wall power socket. The monitor would supply the PC with up to 100 watts electrical power to run and to charge the battery. The monitor could also serve as a hub to distribute power to USB external hard drives.

Intel

Speaking of storage, USB allies also are working on USB Attached SCSI Protocol (UASP), designed to lift some bottlenecks that currently slow USB data transfer rates for USB storage.

Higher-end competition: PCI Express Cable

Thunderbolt won't stop at its current data rate of 10Gbps per channel, though. When Intel showed it off as an optical technology, the company promised eventual speeds of 100Gbps. It can get much of the way there with the electrical connections of copper wires, though.

"We're doing a lot of research looking forward to higher speeds both on the electrical and optical side," Ziller said. "We expect at this point we could probably get at least halfway to 100Gbps with electrical. But getting all the way to 100 gigabits -- right now our thinking is you'd need optical for it."

Another technology in the offing, PCI Express Cable, also is geared for high performance. Its advocates don't think of it as a USB replacement, but they do see it as a viable alternative in the high-end market where Thunderbolt is just now gaining a foothold.

One big selling point is its data transfer rate: 32Gbps initially, then 64Gbps when PCI Express 4.0 arrives, said Michael Krause, chairman of the PCI Express Cable Workgroup (and a Hewlett-Packard employee, which just so happens to like PCI Express Cable's advantages).

Another selling point is that PCI Express Cable is an external variation of the ubiquitous PCI Express (PCIe) technology already used internally within every PC today. Even better, PCIe is built directly into newer processors, which come with a number of communication pathways called lanes that can be devoted uses such as connecting video cards or storage subsystems. For PCI Express cable, you don't need to buy a proprietary Thunderbolt controller chip from Intel -- though if a PC designer wants to pipe DisplayPort or HDMI data across it, which is possible, extra silicon is necessary.

A third advantage is more theoretical but potentially important: The storied SATA standard for connecting hard drives tops out at 6Gbps, and that's the end of the line for the technology, Krause said. For next-generation storage -- flash memory or the more radical possibilities such as HP's memristors -- something faster is needed.

Riding the storage interface coattails

"SATA stopped at 6 gigatransfers, and that's it. High-seed storage wlll go a lot faster," Krause said. PCI Express Cable, used internally, "has the potential to replace the SATA cable." That could be a great way to get around the chicken-and-egg problem, especially with major backers such as HP backing it.

The technology also can carry up to 10 watts of power to run external devices, supports cheaper passive cables that don't have Thunderbolt's active chips inside, and could let devices be daisy-chained with the use of small switches. "It isn't magical," Krause said.

And, he added, PCI Express has been updated so it's easier to add and remove devices while a computer is running. "We enhanced PCI express with new error containment strategy that allows surprise removals. If pull cable out, you won't get a bluescreen," he said.

PCI Express Cable, though, remains a plan rather than a product for now.

"The goal is to finish version 0.9 [of the specification] by the end of the year, then 1.0 in the first quarter of 2013, then products toward end of 2013 and 2014," Krause said.

That's a lot of time for Thunderbolt to spread across Macs, Windows PCs, and peripherals. But Krause thinks there's time.

"We believe the market is so new, even though people are shipping stuff, because these new markets -- especially with the slow economy -- have an opportunity to evolve," he said.

Ultimately, though, PCI Express Cable faces the same high-end pigeonholing challenge that Thunderbolt does, said Insight 64 analyst Nathan Brookwood.

"I think the PCI Express Cable will end up being a lot more nichey," Brookwood said.