Other Parts Discussed in Post: SN75DP126

The new Apple / Intel wired external interface, Thunderbolt, is all the "buzz" these days.  I am getting asked a lot of questions on how Thunderbolt is going to replace USB 3.0 as the main I/O of choice on PCs sooner rather than later.  Before I dive into the USB 3.0 versus Thunderbolt discussion, let’s take a quick review of what Thunderbolt really is.

If we go to the Intel Thunderbolt brief, we can see that the basic concept is to take your DisplayPort and PCI-Express (PCIe), multiplex them, and then send out a combined transmission from the host system. The "peripheral" system will then take Thunderbolt in and extract either the PCIe or DisplayPort signal as required for the peripheral of choice. 

One implication of this has been lost in the buzz.  The "source" of the DisplayPort and PCIe are not changing and, therefore, the performance remains identical for those interfaces.  In other words, if the source is DP 1.1, then the throughput across is still at DP 1.1 rates (2.7 Gbps).  If the source is DP 1.2 capable, then the performance for the DisplayPort throughput bumps up to the 5.4 Gbps of DP 1.2.  For PCIe, the assumption is that it will be a 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 DP 1.2, you get ~ 10.4 Gbps vs. 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 really are not getting any better DP or PCIe performance than what an existing system is capable, what is the value of Thunderbolt?  My opinion is that it is about form factor and ergonomics.  Thunderbolt (and Lightpeak in the future) can enable the elimination of various connectors on a system to be replaced by the single Thunderbolt receptacle.  This opens up the possibility of some very slim form factors while still enabling multiple high-bandwidth I/Os.  The follow-on to this becomes, What will this cost to the system implementers: How much more will the additional chip and the new receptacle cost than the existing connectors?  Are end users willing to pay the premium for sleeker form factors, but no additional performance?

Getting back to the initial question of Thunderbolt versus USB 3.0,  I do not see these as conflicting technologies. I see them as complimentary.  USB just will not go away as a key interface for PC systems - it just has become too ubiquitous.  One way to think of this is to consider that for most portable PCs, ultimately there is a docking solution for "fixed" usage scenarios such as in the office.  Thunderbolt would be an ideal interface for a new docking paradigm - a cabled dock. With Thunderbolt, a single cable passes both the monitor interface (DP) as well as the data interface (PCIe).  Then inside the dock the PCIe interface can be used to add any other data I/Os that are typically available in docks today such as USB 3.0, Firewire IEEE-1394, or eSATA via the use of PCIe Packet Switch and various PCIe-based host controllers, such as the TUSB7340 for USB 3.0 or the XIO2213B for 1394. These docks also can offer the "full" user I/O experience via additional downstream devices such as flash media readers or stereo audio. Also, fundamentally, we need a place to plug our thumb drives into!  The DisplayPort signal can be mapped directly to a DP receptacle or, if the dock wants to support HDMI in addition to DP, then a simple 1:2 switch with level shifting capabilities, such as the SN75DP122A (DP 1.1++) or soon to be available SN75DP126 (DP 1.2++) can be used.

Finally, there are legal regulations that mandate USB continue to be available for the foreseeable future.  The Chinese Government and the European Union Commission both have mandated that all mobile phones use the micro-B USB receptacle for charging and can not have any other type of dedicated power receptacle.  While this does not necessarily mean that a PC must have a USB connector as a dedicated wall charger with a  micro-USB plug can be used to charge the devices.  USB in the host system offers a simple means to not only charge your mobile phone, but to exchange data with the PC.  With the passing of these governmental rules, most 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.

Tell me what you think about Thunderbolt and if you plan on implementing it in future products?