What’s All the Buzz About Thunderbolt?


The new Apple / Intel wired external interface, Thunderbolt, is all the buzz these days. Developers are already wondering whether Thunderbolt is going to replace USB 3.0 as the main I/O of choice on PCs. But before diving into the USB 3.0 versus Thunderbolt discussion, let’s take a quick review of Thunderbolt.

Thunderbolt’s basic concept is to take the DisplayPort and PCIExpress (PCIe) interfaces, multiplex them, and send out a combined transmission from the host system. The peripheral system takes the Thunderbolt signal in, de-multiplexes the PCIe and DisplayPort content and provides those for use by the peripheral system.

One implication of this has been lost in the buzz: The source of the DisplayPort and PCIe data is not changing and therefore the performance remains identical. If the source is DisplayPort 1.1, then the throughput is still at DisplayPort 1.1 rates (2.7 Gbps).If the source is DisplayPort 1.2 capable, then the performance bumps up to the 5.4 Gbps of DisplayPort 1.2. For PCIe, the assumption is that it will be an x1, Gen 2 link,which drives 5 Gbps on both Tx and Rx lines. If you add the bandwidth of PCIe x1 Gen2 to that of DisplayPort 1.2, you get approximately 10.4 Gbps versus the Thunderbolt transmission speed of 10.3 Gbps. This means that the mux chip must be doing more than simply time-slicing the two signals together. It must be eliminating some of the protocol overhead and replacing it with Thunderbolt-specific transmission overhead.

So if you are not getting better DisplayPort or PCIe performance, what is the value of Thunderbolt? What is likely is form factor and ergonomics.

Thunderbolt allows various connectors –DisplayPort, eSATA, FireWire, HDMI and even USB – to be replaced by a single Thunderbolt receptacle. This makes slim form factors possible while still enabling multiple high-bandwidth I/O. But what will this cost to system implementers? How much more will the additional chip and receptacle cost than existing connectors? Are end users willing to pay the premium for sleeker form factors, but no additional performance?

Getting back to Thunderbolt versus USB 3.0,these technologies appear to be complementary rather than conflicting. USB will not go away as a key interface for PC systems – it has become too ubiquitous. For most portable PCs, Thunderbolt would be an ideal interface for a new docking paradigm – a cabled dock. With Thunderbolt, a single cable passes both the monitor interface (DisplayPort) as well as the data interface (PCIe).Inside the dock, the PCIe interface can be used to add any other typical data I/O, such as USB 3.0, Firewire IEEE-1394 or eSATA via the use of PCIe packet switch and various PCIe-based host controllers. These docks also can offer the full user I/O experience via additional downstream devices such as flash media readers or stereo audio, as well as ports for thumb drives.

Finally, there are legal regulations that mandate that USB continues to be available for the foreseeable future. The Chinese government and the European Union Commission have mandated that all mobile phones use the micro-B USB receptacle for charging and cannot have any other type of dedicated power receptacle. While this does not mean that a PC must have a USB connector as a dedicated wall charger, USB in the host system offers a simple means to charge a mobile phone and exchange data with the PC. These governmental rules mean that other portable consumer products (personal navigation devices, portable music players, portable media players, tablets, eBooks, etc.) have chosen to use the same micro-B receptacle as their only charging port as well.

At first glance, it may appear that Thunderbolt is a new I/O standard that could replace SuperSpeed USB; in actuality, both technologies will work together to deliver high-speed capabilities to the end user. Thunderbolt and Lightpeak will continue to rely on underlying protocols such as DisplayPort, PCI Express or USB for the real data transfer between PCs and peripherals.



Dan Harmon is product marketing manager for consumer and computing interfaces at Texas Instruments. He also serves as TI’s USB-IF representative and chair for TI’s USB 3.0 Promoter’s Group. He earned a BSEE from the University of Dayton and a MSEE from the University of Texas at Arlington. You can reach Dan at danharmon@ti.com

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