Intel Thunderbolt 5: Up to 120 Gbps with PAM3

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If there’s one constant in modern computing, it’s that the standards of the pluggable accessories are constantly changing. From parallel to serial, from USB 2.0 to USB 3.0 and USB4, then came along Thunderbolt (and whatever Apple Lightning is). With each new generation, the goal has been to increase capabilities as well as increase speed - some of this is enabled through new silicon and tighter signalling tolerances, other is just simple ideas for features. 

It wasn’t until the industry moved from the ‘you-always-insert-it-wrong’ USB Type-A port to the reversible Type-C port that we also saw a massive jump in bandwidth and support, through both the USB standards and Thunderbolt standards. Both sets of standards have technically come out of Intel, but USB has been around forever, with Thunderbolt only recently getting the royalty free status since Thunderbolt 4.

The benefit of Thunderbolt over USB is that Thunderbolt is a PCIe derived protocol. While USB has a root and a tree of devices, Thunderbolt acts like a PCIe switched protocol, and since Thunderbolt 4, acts as a superset of the USB standard, supporting more features and functionality as standard, some of which is optional over USB. This allows support for PCIe graphics over Thunderbolt, one key use-case, as well as daisy chained devices with full bandwidth. Thunderbolt supports point-to-point networking as well (something often overlooked), and dual monitors. Since the integration of Thunderbolt into Intel silicon a couple of generations ago, there are now over a million capable Thunderbolt processors in the market today.

However, today Intel is announcing the new Thunderbolt 5 specification. 

The key highlights of Thunderbolt 5 over the previous generation include a peak 3x point-to-point unidirectional bandwidth, support for up to 240W charging over 140W, double speed networking, and double speed PCIe. Thunderbolt 5 is backwards compatible with Thunderbolt 4, using the same Type-C connector, and is a super-set of the USB4 v2 standards.

What does all this mean?

Let’s start with the bandwidth. TB4 supports 40 Gbps bi-directional. That means 40 Gbps in either direction, or what you might call a unidirectional port in each direction, each capable of 40 Gbps. TB5 doubles this to 80 Gbps bi-directional, with two unidirectional ports in each capable, or 2 x 40 Gbps. Rather than two ports one way and two ports the other, TB5 can support three ports in one way, boosting that speed up to 120 Gbps. 

Why does 120 Gbps matter? Intel points to storage use cases, or high speed video use cases (8K+), where traffic might be mostly in one direction.

Intel states that they achieve this increase in speed in two ways. One is the native frequency of the communications - that’s usually linear as long as the bit-error rates are equivalent. However the second way the speed increases is through the use of PAM-3 signalling. 

Most people are aware of PAM-2, usually called NRZ or Non-Return-to-Zero. Even if you’ve not heard the name before, NRZ is simply binary - bits that are transmitted down the line are either a zero or a one. This means you are transmitting one bit of information per transfer.

Usually the step above PAM-2 is PAM-4: with tighter tolerances, you can send a voltage down a wire that could be interpreted as one of four states: 00, 01, 10, or 11. This means you are transmitting two bits per transfer.

PAM-3 is an odd medium. With the tolerances, the voltage you send down the wire is either a -1, a 0, or a 1. The combination of two of these transfers gives you one of nine different formats, which is interpreted as a set of three bits. This means you are transmitting three bits per two transfers:

From AnandTech

Overall, the benefit is speed and efficiency. With PAM-3, to hit 80Gb/sec over the previous generation, a doubling of transmit frequency isn’t needed. USB4 v2 also has this requirement.

We did ask about backwards compatibility. Yes, any TB3 or TB4 device will be supported by a TB5 host. When a Thunderbolt 5 host is talking to a Thunderbolt 5 device, it will be in PAM-3 mode, regardless of what the connection rate actually is. If a Thunderbolt 4 device is being used, the host will come back down to a PAM-2 mode.

It’s also worth noting that at 40 Gbps, Thunderbolt 4 was able to act as a PCIe 4.0 x4 device over the connection. Thunderbolt 5 going up to 80 Gbps means that the link looks like two PCIe 4.0 x4 devices, not a single PCIe 4.0 x8 (because PCIe cannot be combined that way, at least not easily). This means graphics attach will still be on an x4 connection, but users will be able to install a full speed PCIe 4.0x across a TB5 connection at the same time. Even at 120 Gbps, this is still multiple PCIe 4.0 x4 devices. 

On cables, Intel is again going for 2m passive cables according to our slide decks, however in our briefing it was stated that 1m passive cables would be rated for 80 Gbps, with longer cables requiring optical or optical with redrivers. 

Today’s announcement from Intel isn’t just the standard, but an announcement that they have a Thunderbolt 5 controller - early silicon in prototype devices with partners today. This controller has the internal codename Barlow Ridge, and has a total four ports, three of which are external for attachment (and one for the CPU). As with the previous generation, Thunderbolt 5 will initially be distributed with controllers with the expectation that it may be integrated with a future generation Intel processor. Which one? They haven’t specified at this time. 

Thunderbolt 5 will again be royalty free, as was Thunderbolt 4, however validation and certification will be needed in order to use the official logos. Independent testing facilities unaffiliated with Intel are available globally, and when asked about cycle time, we were told that it depends on the number of ports per device what the test time would be - up to two weeks for lots of ports. For OEMs that have common platforms but different chassis, devices can get a ‘similar certification’ without the need for additional testing.

The first computers and accessories with Thunderbolt 5 support are expected in 2024, with technical resources available to partners in Q4 this year. The TDP of the chip is not final at this time as it is still early silicon (and the fab wasn’t disclosed).

Overall thoughts is that as always, this is usually a step in the right direction. Given my experience with connected devices and the technology, I’m not sure that this explicitly opens up new use-cases - Thunderbolt was always about the convenience to me, never about the new. We already know that Intel’s upcoming Meteor Lake will support Thunderbolt 4, not Thunderbolt 5 (the timing doesn’t work for TB5), so we’ll likely have to wait a couple of CPU generations to see it integrated onto the platform. Financially, it’s not a driver in the sense that it’s a must-have feature for customers - there will be the rare exception, but from Intel’s perspective it pushes onwards to support wider, faster, IO.

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