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1138 | 
| What is Thunderbolt 4 and is it the successor to Thunderbolt 3? | Mar-01-21 |
| Is Thunderbolt 4 the successor to Thunderbolt 3?
You may expect this to be the case. After all, TB was 10Gb, TB2 was 20Gb, and TB3 was 40Gb. But the speed stops there. Thunderbolt 4 is also 40Gb.
Thunderbolt 4 is not a successor to, nor is it a replacement of Thunderbolt 3. Both versions are intended to live together harmoniously in the Thunderbolt ecosystem. Notably, Thunderbolt 3 and Thunderbolt 4 each can do something that the other cannot, so we expect a long life for each.
The Similarities
Thunderbolt 3 and 4 are intended to be interoperable, and they use the same protocol. Both versions use the USB-C connector; both versions have 40Gbps line bandwidth; both versions support one USB 3 Gen 2 port; both versions support DisplayPort tunneling; both versions support PCIe 3 tunneling; both versions support six devices total. You can intermix Thunderbolt 3 and Thunderbolt 4 cables and devices in any order (up to six TB devices maximum on one port).
Let's explore the difference between TB3 and TB4 in a device; and in a computer.
The Differences in a Device
Thunderbolt 4 is perfect to use in a Dock or a Hub, because Thunderbolt 4 can support three downstream Thunderbolt ports. However, Thunderbolt 4 support of three downstream Thunderbolt ports comes at a cost--a Thunderbolt 4 peripheral controller supports only one lane of PCIe. This is OK in a Dock. In a Dock, the Thunderbolt 4 controller connects to the three downstream Thunderbolt ports plus to an internal USB 3 Hub--which in turn connects to everything else--the external USB 3 ports, USB-to-Audio, USB 3-to-gigabit ethernet, USB 3-to-SD, etc. The single lane of PCIe is not used.
A Thunderbolt 3 device supports only one downstream Thunderbolt port, but it also supports four lanes of PCIe. Sonnet's professional Thunderbolt 3 products--including Thunderbolt 3 Echo PCIe Expansion Chassis, External GPU enclosures, 10Gb Networking, and Pro Media Readers--all rely on having the bandwidth of four lanes of PCIe. It makes no sense for Sonnet to migrate our professional Thunderbolt 3 product line (except for Docks) to Thunderbolt 4, and of course, we have no intention of doing so.
The lock-step progression of Thunderbolt and PCIe speed has ended. Because Thunderbolt 1 devices supported PCIe 2 @750MB/s, Thunderbolt 2 devices supported PCIe 2 @1350MB/s, and Thunderbolt 3 devices supported PCIe 3 @2800MB/s, one may logically expect that Thunderbolt 4 devices may support even faster PCIe--but this is not the case. In order to accomodate the hub function, Thunderbolt 4 PCIe support in a device has been reduced to one lane at 750MB/s. A Thunderbolt 4 hub or dock can pass through 4 lanes of PCIe to its downstream Thunderbolt ports, but you have to have a Thunderbolt 3 device to use all of it.
The Differences in a Computer
Thunderbolt 3 was a loose standard. 15W Power Deliver (PD) was the goal, but you could provide only 7.5W if you didn't have enough power. Connecting 4 lanes of PCIe was the aspiration, but you could connect only 2 lanes if you were short. Two 4K monitor support was the goal, but you could get by with supporting only one 4K if your dGPU wasn't up to the task. 40Gb TB3 cables were the ideal, but you could sell a 20Gb cable if you wanted to go cheap.
Thunderbolt 4 is a tight standard. 15W PD; 4 lanes of PCIe; 40Gbps cables; two 4K monitor support--no exceptions. If you want exceptions, you can call your port USB4.
OK, so what is USB4?
Let's start by talking about what USB4 is not. USB4 is not USB 3 Enhanced. Think of USB4 as Thunderbolt 4 Lite. USB4 uses the same protocol as TB3 and TB4. If an aspiring TB4 computer port can support only one monitor: call yourself USB4; if you connect at only 20Gb: your port name is USB4; If you connect only two lanes of PCIe: you are a USB4 port. Get the picture? Under TB3, exceptions remained in the TB3 tent; under TB4 exceptions are kicked to the USB4 wigwam.
Do I need new Thunderbolt 4 cables for my Thunderbolt 4 computer?
No. Thunderbolt 3 and Thunderbolt 4 cables will work interchangably with Thunderbolt 3, Thunderbolt 4 or USB4 ports between computers, devices or monitors. |
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1150 | 
| On my M1 Mac, the write speed to my Thunderbolt peripheral is significantly less than the read speed. Can I fix this? | May-13-22 |
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