Thee DocKtor's Blog

All About Docks

Thunderbolt 3 USB Type-C

Thunderbolt 3 can also supply up to 100W of power on the USB Type-C connection, but doesn’t have to support power at all.  Thunderbolt 3 brings Thunderbolt to USB-C at speeds up to 40 Gbps, creating one compact port that simultaneously supports high video and data bandwidth and power on a single cable.  A Thunderbolt 3 port on a PC can support all generations of Thunderbolt (including Thunderbolt 3), all generations of USB data (including USB 3.1 10 Gbps), PCIe Gen3 data, and DisplayPort 1.2, but not all at the same time. Thunderbolt 3 bandwidth is shared bandwidth which means all connected devices will use the bandwidth for all protocols that transfer through the same Thunderbolt 3 cable (DisplayPort over USB-C traffic and PCIe traffic such as USB, Ethernet, and Audio).

Thunderbolt 3 is a bi-directional, dual-protocol consisting of up to 4 lanes of PCIe Express Gen 3 with up to 8 lanes of DisplayPort 1.2, see https://thunderbolttechnology.net/sites/default/files/Thunderbolt3_TechBrief_FINAL.pdf .  Thunderbolt 3 ports can be configured with 2 or 4 PCIe lane designs. Intel’s Alpine Ridge uses separate inputs for Display Port 1.2 data (DP stream is not part of PCIe traffic).  And a given host’s can implement Multi-Stream Transport (MST) support available in the DP1.2 stream, or not.

Because the USB-C connector supports alternate modes, a Thunderbolt 3 port on a host (i.e. PC or MacBook) is a superset solution that supports both Thunderbolt and USB.

Figure 1:  USB and Thunderbolt 3 Maximum Data Rates.

The latest ultra-high resolution displays create a mass of data that needs to be sent from the host to the screen.  Uncompressed 1080p content consumes about 4 Gbps of data while uncompressed 4K2K (3840 x 2160 @ 60 Hz and 24 bpp color) consumes nearly 15 Gbps or 16 million more pixels than 1080p.  By combining USB and display data on a single cable, data rates rise dramatically.

Rising Data Rates

Figure 2:  A comparison of the bandwidth consumption in various use case scenarios.  Note the bandwidth required for 4K resolution displays and fast data transfers in Use Case 3.

Figure 2 shows a common use case where the expectation of dual 4K video plus USB 3.1 data exceeds exceed 40 Gbps.  In this case the video is given the highest priority and their full allotment of bandwidth.  The remaining bandwidth is allocated to the USB data.

For High Bit RTE (HBR) DisplayPort 1.2, the maximum bandwidth on the Thunderbolt interface is 17.28 Gbps per two (four lane) DisplayPort Interfaces.  For a 40 Gbps Thunderbolt link, up to two DisplayPort interfaces can be connected.  After the link is established, Thunderbolt transmits only the display traffic as demanded.  If the screen resolution is set to 1080p then about 4 Gbps of the Thunderbolt link is used for display.  If it is a 4K2K display then about 16 Gbps will be consumed.  In Figure 3 we assume two 4K displays are consuming 16 Gbps each, or 32 Gbps total, leaving 8 Gbps available for data.  Note that DisplayPort and PCIe have some encoding overhead that is not carried over the Thunderbolt link.

Figure 3:  Thunderbolt 3 bandwidth allocation in various use cases.  Video will always take priority over data and will get the full bandwidth allocation it needs. The remaining bandwidth is allocated to data transfers from the PC to a dock or display. The inbound data back to the PC is at full Thunderbolt 3 data bandwidth of 22 Gbps.

PCI Express and data traffic in general, is more variable than display traffic, in which a fixed resolution and color depth is equal to a fixed bandwidth.  PCI Express devices will therefore continue to function with a variable rate of bandwidth, but certain performance levels may not be achieved if two high-resolution displays are being used on the same port.  However it is key to understand that the Thunderbolt interface is bi-directional, while display traffic is mostly outbound from the host PC.  If the Thunderbolt link is using a PCI Express device to ingest traffic to the host system, that PCI Express bandwidth will be largely unaffected by outbound display traffic (outside of some flow control impacts).

NOTE: Systems with the 2 lane design will only support up to 20gbps data transfers over PCIe devices such as external hard drives or video cards that use PCIe. Systems with 4 lane design will support up to 40gbps of data transfer over PCIe depending on the device in use.

A Thunderbolt 3 host system, if designed to support this by the system manufacturer, can also request to charge over the connector from a device or adapter designed for delivering power to the system, up to 100W, via the USB-PD specification.  In this way, the Thunderbolt 3-enabled USB-C connector can support host power and charging, display, data, or all three, at the same time in various configurations.

Every USB-C port enabled with Thunderbolt 3 supports the basic USB 2.0 and USB 3.0/3.1 power delivery modes where power is sent from the system to a device.  Thunderbolt 3 also guarantees the ability of the host system to send at least 15W (5Vdc @ 3A) of power on at least one port for a Thunderbolt 3 device that requires bus power when connected (Figure 4).  This power delivery allows Thunderbolt devices to support additional performance and capabilities.  Also, just as a two-port Thunderbolt device supports the same capabilities on its downstream port as the Thunderbolt host, the same holds true for power delivery.

Figure 4:  Thunderbolt 3 provides 15W of power from the host to a device, and if enabled by the host, it can accept up to 100W of system charging from an external device such as a dock.

Notes: 

On some tablets, 15W power delivery won’t be provided.

Intel technologies’ features and benefits depend on system configuration and may require enabled hardware, software or service activation. Performance varies depending on system configuration. No computer system can be absolutely secure. Check with your system manufacturer or retailer or learn more at intel.com.

As compared to other PC I/O connection technologies including eSATA, USB, and IEEE 1394 Firewire, performance will vary depending on the specific hardware and software used.

Thunderbolt and the Intel logo are trademarks of Intel Corporation (not Apple as many believe) in the United States and other countries.

* Other names and brands may be claimed as the property of others.

 

Categories: White Paper

Tags: , , , , ,

1 reply

Trackbacks

  1. The Only Universal USB Type-C Device – Thee DocKtor's Blog

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s