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Thunderbolt and USB 3.0

May 25, 2013 by Steve Modica

Steve ModicaApple has always been on the leading edge of connectivity for their systems.

Back in 2004, before we had formed Small Tree as a company, I can recall drooling over a Power Book laptop with an integrated Gigabit port. That was a crazy thing to have on a laptop at the time. Gigabit was still a little weird, very expensive, and not common as a drop at anyone’s desk. Yet here apple was putting it on a laptop.

Thunderbolt is a similarly aggressive move. It puts a great deal of IO horsepower on some very small systems.

Firstly, let’s consider what Thunderbolt is.  Thunderbolt is a 4X (4 lane) PCIE 2.0 bus. It’s equal in performance (and protocol) to the top two slots of a traditional tower Mac Pro. Along with that 4X pipe, there’s a graphics output pipe for a monitor. These pipes are not shared! So using a daisy-chained monitor will not impinge on any attached IO devices.

Thunderbolt is capable of moving data at 10Gbits/sec FULL DUPLEX, meaning data can move in two directions at the same time, giving the pipe a total bandwidth of 20Gbits/sec.

As I read through the forums and opinion articles on Thunderbolt, one of the themes that pops up is “It’s Apple proprietary and expensive.  Just use USB 3.0.”  This is a reasonable point. USB 3.0 is capable of 4.8Gbits/sec (about half of the speed of Thunderbolt).  Further, there are plans to speed up USB 3.0 to 10Gbits/sec to match Thunderbolt. So given these factors (and the low cost of most USB devices), it seems like an obvious choice.

However, there are some reasons that Thunderbolt may win the day for external high-speed connectivity (and relegate USB to it’s traditional low-end role).

First of all, most IO chips (Ethernet, SATA, SAS) are manufactured with a native PCIE backend.  The chips are natively built to sit on a PCIE bus. So not only will you save the overhead of an additional protocol, the guys writing the code to support these devices only need to write one driver (PCIE).  It just works whether the device is on a card or in a Thunderbolt toaster.

Another advantage of Thunderbolt is its power budget. Often, devices are powered by the port itself (very common with USB).  USB can provide 4.5Watts of power to attached devices, whereas Thunderbolt offers a full 10Watts of power.

Lastly (and this is probably the most interesting thing about PCIE and Thunderbolt) is that Thunderbolt is a switched/negotiated protocol that is extremely flexible. Cards that want a 16X slot can work in a 4X slot.  PCIE switches can (and do) exist to allow multiple machines to talk to one PCIE based device (like a RAID). So imagine a time in the future when devices can be connected to a “switch” in a back room and multiple systems can see them. Imagine those systems can have multiple connections to boost their bandwidth.

Thunderbolt may not be everywhere yet, but it’s really the first imaginings of a new way to handle IO outside of the “tower” type machines. I think it is easily the best choice for Mac users and will likely offer some amazing benefits in the next generation.


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