Introduction
This document presents the results of testing conducted to determine the bandwidth capabilities of USB 3.0 Type-C to Type-C cables when used to connect a Thunderbolt 3 host to a Thunderbolt 3 dock. A primary motivation for this testing was to investigate whether less expensive USB 3.0 cables could be used in place of Thunderbolt 3 cables, and to quantify any resulting performance differences. The objective was to measure the bandwidth available to connected devices under various cabling configurations.
Testing Methodology
The following equipment was used for testing:
- Host: 2017 MacBook Pro 15-inch, running macOS 13.7.1, equipped with an Alpine Ridge Thunderbolt 3 controller (including a USB 3.1 host controller).
- Dock: ThinkPad Thunderbolt 3 Dock incorporating a Fresco Logic FL1100 USB 3.0 controller.
- Storage Device: Samsung T5 512GB SSD.
- Display: Monitor configured for 4K resolution at 60Hz with 8-bit color depth.
- Cables:
- Thunderbolt 3 cable (40Gb/s rated).
- 3-meter USB 3.0 Type-C to Type-C cable.
Link speeds were verified using the macOS System Information utility. The USB 3.0 cable connection between the host and dock reported a link speed of “Up to 20 Gb/s x1”. The Thunderbolt 3 cable reported “Up to 40 Gb/s x1”. Bandwidth testing focused on measuring the read and write speeds of the SSD under various connection scenarios.
Results
The table below presents the measured bandwidth data. “Direct to Mac” indicates the SSD was connected directly to the host’s Thunderbolt 3 port. “USB3” and “TB3” in the “Host Interface” column denote the cable used between the host and the dock. The “SSD Interface” column indicates the USB controller to which the SSD was connected.
| Host Interface | SSD Interface | External Display | SSD Read (MB/s) | SSD Write (MB/s) | SSD Bandwidth (Gbps) | Display Bandwidth (Gbps) | Total Bandwidth (Gbps) | Storage Slowdown (%) |
|---|---|---|---|---|---|---|---|---|
| Direct to Mac | TB3 (APPLEUSBXHCIAR) | No | 484.8 | 509.1 | 7.951 | 0.00 | 7.951 | 0.00% |
| USB3 | TB3 (APPLEUSBXHCIAR) | No | 474.4 | 502.5 | 7.815 | 0.00 | 7.815 | 1.71% |
| USB3 | TB3 (APPLEUSBXHCIAR) | Yes | 293.1 | 501.2 | 6.354 | 14.93 | 21.284 | 20.08% |
| USB3 | USB3 (AppleUSBXHCIFL1100) | No | 320.2 | 349.1 | 5.354 | 0.00 | 5.354 | 32.66% |
| USB3 | USB3 (AppleUSBXHCIFL1100) | Yes | 278.7 | 350.2 | 5.031 | 14.93 | 19.961 | 36.73% |
| TB3 | USB3 (AppleUSBXHCIFL1100) | No | 317.2 | 348.2 | 5.323 | 0.00 | 5.323 | 33.05% |
| TB3 | TB3 (APPLEUSBXHCIAR) | No | 473.3 | 500.7 | 7.792 | 0.00 | 7.792 | 2.00% |
| TB3 | TB3 (APPLEUSBXHCIAR) | Yes | 472.6 | 498.6 | 7.770 | 14.93 | 22.700 | 2.28% |
| TB3 | USB3 (AppleUSBXHCIFL1100) | Yes | 318.7 | 348.9 | 5.341 | 14.93 | 20.271 | 32.82% |
Observations
The data indicates that the type of cable used between the host computer and the Thunderbolt 3 dock has a measurable impact on the available bandwidth for connected peripherals.
- USB 3.0 Cable Connection:
- When the SSD was connected to the dock’s FL1100 USB controller, a reduction in SSD bandwidth of 32.66% to 36.73% was observed, relative to a direct connection to the host.
- When the SSD was connected via the dock’s Thunderbolt 3 pass-through (utilizing the host’s controller), a bandwidth reduction of up to 20.08% was observed when a display was also connected.
- Thunderbolt 3 Cable Connection:
- The SSD connected through the dock’s Thunderbolt pass-through exhibited a minimal reduction, of up to 2.28%.
- The SSD connected to the FL1100 USB Controller showed a similar 32.82% to 33.05% reduction.
Conclusion
The use of a USB 3.0 Type-C cable between a Thunderbolt 3 host and a Thunderbolt 3 dock results in a reduction of available bandwidth for connected devices compared to using a Thunderbolt 3 cable. This reduction is most pronounced when devices are connected to the dock’s internal USB controller. When connecting an SSD to the dock’s Thunderbolt 3 pass-through port (utilizing the host’s APPLEUSBXHCIAR controller), the difference in performance between using a USB 3.0 cable and a Thunderbolt 3 cable as the uplink to the computer was minimal when no display was connected. However, connecting a display introduced a more substantial performance reduction when using the USB 3.0 uplink.
While a USB 3.0 cable connection proved sufficient for operating a display along with low-bandwidth peripherals such as keyboards and mice, it may present issues if high-bandwidth devices are used concurrently. The performance of devices connected to the FL1100 is consistently reduced, irrespective of the host interface cable used. For applications requiring maximum bandwidth to external devices, a Thunderbolt 3 cable connection between the host and dock is necessary.