20Gbps Thunderbolt isn't twice the bits
The current Thunderbolt architecture has two bi-directional channels, each operating at 10Gbps. Each channel can transmit and receive data concurrently at 10Gbps.
That means the cross-sectional bandwidth, the total theoretical bandwidth with each channel operating at full capacity, is 40Gbps. No bits move at 40Gbps, but if all four channels were fully loaded, 40Gbits of data would move.
20Gbps Thunderbolt
It wasn't easy to pin the Intel guy down about the new controllers, but after repeated questions I finally got the story straight. The key differences are:
Dedicated DisplayPort channel goes away: DisplayPort is still supported, but if you aren't using it — say you're connecting to a very fast storage device instead — the entire bandwidth of the Thunderbolt link is available for that one application.
Still 40Gbps bandwidth: Since there is only one channel (shareable across DisplayPort and PCIe protocols) the number of bits across the cable — the maximum cross-sectional bandwidth in other words — remains at 40Gbps.
Faster: But those 40Gbits will move at the higher signaling rate of 20Gbps.
The Storage Bits take
Leave it to Intel marketing to fumble good news. The reality is that the 20Gbps Falcon Ridge Thunderbolt controllers due next year will offer more usable bandwidth than the current parts.
Why? Because the DisplayPort's bi-directional bandwidth is never used in practice. Displays only receive data and send nothing back. A fully loaded Thunderbolt cable today will max out at 30Gbps, despite the theoretical 40Gbps cross-sectional bandwidth.
But a single high-performance SSD array could max out next year's Thunderbolt in both directions concurrently, actually moving 40Gbps — 20 in and 20 out — on the cable. And, of course, the maximum data rate is doubled: a 20GB file would transfer in 10 seconds rather than today's 20 seconds.
Bottom line: the 20Gbps Thunderbolt will give creative professionals more of what they want — speed — if they need it. Launching Thunderbolt into the headwinds of a global PC market meltdown is challenging, but will help differentiate pro products going forward.
Comments welcome, of course. I'll have more on Thunderbolt at NAB '13 next week.