Supporting hubs and repeaters in the Thunderbolt 3 standard

Thunderbolt 3 USB-C port on Dell XPS 13 Ultrabook

The same connector being used for Thunderbolt 3 and USB-C may lead to confusion in more sophisticated setups

Increasingly Intel’s Thunderbolt 3 very-high-speed data connection standard has come on the scene as a product differentiator for computer products.

This standard works over the USB-C physical connection, thus allowing for a logical Thunderbolt 3 or USB-C data transfer setup.

But USB-C, like the preceding USB connection standards allows for a “tree-like” connection from the host computer device. This is facilitated through self-powered or bus-powered hubs which allow multiple device to be effectively connected to the same physical connection on the host computer or previous hub, subject to certain conditions like power budget.

On the other hand, the Thunderbolt connections can only be connected in a “daisy-chain” manner where only one device can be connected to another. This is also limited by the fact that you can only have six devices connected in a Thunderbolt data bus.

A situation that can easily crop up with the Thunderbolt 3 connection is the fact that there could be an expectation to run a connection setup for multiple devices in a “tree-like” approach. This is along with an expectation to have more than six devices on a Thunderbolt 3 data bus. It is aggravated through some of the devices that have their own power supplies being expected to be USB hubs along with these devices being equipped with Thunderbolt 3 over USB-C connections.

The classic example would be a Thunderbolt 3 RAID direct-attached-storage array along with an external GPU module and, perhaps, a Thunderbolt 3 dock (expansion module) as part of a workstation setup.

But there can be the desire to hang off more than six Thunderbolt 3 devices or establish a “tree-like” approach. This can happen where there is a desire to connect multiple storage or interface devices or you are dealing with low-tier Thunderbolt 3 devices that only have the one connection for the host computer.

In the audio recording studio environment, the Thunderbolt 3 connection can appeal with analogue-digital interfaces or digital mixers where there is the desire to connect many microphones, musical instruments and speakers to a digital-audio workstation. This can extend to the video sphere with ultra-high-definition cameras connected to a suitable AV interface for digital video production.

Similarly, Thunderbolt 3 offering support for a virtual “PCI Express” card bus may appeal to computing users running with multiple “card-cage” devices like the external graphics modules. Here, it may be about increased input-output abilities or working with high-performance graphics cards. Such a setup will become relevant with portable, all-in-one and small-form-factor desktop computers which don’t have the necessary support for the traditional interface cards that were the norm for regular computers.

A situation that can easily crop up with these devices is attempts to connect Thunderbolt 3 peripherals to other USB-C connections on upstream peripherals. This can lead to error messages and the whole setup not performing as expected.

What needs to be looked at is an extension to the Thunderbolt 3 specification to cater towards different bus layouts. This is more so to allow a peripheral to effectively reiterate one or more Thunderbolt 3 buses as if it is the equivalent of an Ethernet switch. It can also lead to the possibility of implementing active repeaters for a Thunderbolt 3 connection, something that could appeal to longer connection runs like the obvious stage-based applications.

It could be simply facilitated through a hardware-software device class for this specification that addresses “hub and repeater” behaviour. This can also include the ability for these devices to work as USB-C hubs including support for different power-supply paths and power budgets for the Power Delivery device class.

The same issue also includes a requirement for the host computer to identify where each Thunderbolt 3 peripheral is and map the bandwidth in a similar way to a city’s road system.

But it will be something that Intel will have to approach when they revise Thunderbolt 3.

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