Thunderbolt Promises Lightning Speed

by George Lawton

Intel's Light Peak optical interconnect technology has been integrated into Thunderbolt, a new electrical port that uses a mini-DisplayPort (mDP)-compatible plug to support 40 gigabits per second of aggregate bandwidth over 3 meters. A future active optical version will extend the range past 100 meters.

This should help bridge the gap between native bus technologies with low processor overhead, such as PCI, and external communications techniques with higher overhead, such as USB and 10 Gbps Ethernet, said Richard Doherty, a communication design analyst with the Envisioneering Group.

Doherty expects to see Thunderbolt used initially in very high end applications that can directly benefit from the performance improvements. For example, a video transcoding job might be completed eight times faster over Thunderbolt than PCI or SCSI. It will also enable higher performance video systems that support 4,000 and 8,000 lines of resolution compared with the 1,080 lines on existing high-definition video.

Introducing Thunderbolt

Prior to Thunderbolt, Intel demoed Light Peak, a technology with similar performance but over optical cables, which are more fragile. In contrast, Thunderbolt uses an electrical connection like USB or FireWire. Although Thunderbolt ports are backward-compatible with mDP, it will require special cables and peripherals to reach the faster speeds.

The technology combines the ability to carry video and data on a single cable and to daisy-chain up to seven devices together. It also carries up to 10 watts of power, eliminating the need for external power sources for some peripherals.

Speeds in Perspective

In contrast with Thunderbolt's 40 Gbps, PCI buses top out at 8 Gbps and can be bonded together to support up to 16 Gbps using two cables. Other theoretical interconnect speeds include SATA (6 Gbps), DisplayPort 1.2 (17.28 Gbps), USB 3.0 (4.8 Gbps), FireWire 800 (0.8 Gbps), PCI Express v3.0 (16 Gbps), ATI XGP (40 Gbps), Shuttle GXT (5 Gbps), and Express Card (2.5 Gbps).

Not only is the theoretical rate faster than most other interconnects, but Thunderbolt also applies more efficient signaling mechanisms than protocols such as USB do. Actual throughput of USB technologies can suffer a performance hit of 50 percent or more, depending on the number of devices. USB 3.0 improves on this somewhat with special signaling modes. Intel has demonstrated Thunderbolt throughput of 62.5 percent, and higher performance might be possible as the peripherals improve.

USB is cheap in part because it relies on the CPU to do most of the heavy lifting. Doherty expects Thunderbolt to add only about 2–3 percent overhead to the CPU, whereas a high-speed USB connection can generate 10–20 percent overhead on a CPU and 10 Gbps Ethernet adds about 7–10 percent. The additional CPU load not only burdens other processes but also makes the communications function dependent on the CPU: the greater the dependency on the CPU, the greater the effect CPU processing bottlenecks have on throughput.

Preparing for the Future

To promote the use of Thunderbolt, Intel has set up a free interoperability verification program.

The technology is now available on Apple’s new line of MacBook Pro laptop computers, and it's expected in a variety of peripherals later this year. PC-based versions are expected in 2012.

A variety of vendors have announced plans to support Thunderbolt in new products, including disk drives from La Cie and Western Digital and video production equipment from Aja, Apogee, Avid, and Black Magic.

Doherty expects USB to remain the dominant external interconnect in the near future because of its low cost and wide adoption. But he expects Thunderbolt to usurp USB and native PCI Express in high-end applications owing to its performance improvements and Intel’s backing.

For background on Light Peak, see www.computer.org/portal/web/computingnow/archive/news037.

George Lawton is a freelance writer from Guerneville, California. Contact him at glawton@glawton.com.