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This is a very ignorant question, as I am just getting into wiring diagrams and trying to understand them, but look at this picture of usb-C with usb 3.2:
With 2 high speed data paths, plus the old usb 2.0 data wires still there, how is this still considered a serial device? It seems like it's sending more than one stream of data simultaneously. Are the multiple data streams resolved on the software side?
It seems like it's sending more than one stream of data simultaneously.
Those are still serial data streams, just like in ≥gbit ethernet, etc. There can be multiple serial streams running in parallel to increase the bandwidth, but it's not parallel data transmission by any reasonable definition. The streams are individually equalized, clock recovery is done for each of them individually, etc. They are independent other than participating as transports for a higher level protocol that bonds them for a given application.
It seems like it's sending more than one stream of data simultaneously.
It is, but they're all serial.
First off, not everything is active at once. USB2 uses D+/D-; The USB 3.0 and 3.1 modes use TX1+/TX1-/RX1+/RX1-, and the USB3.2 and USB4 "x2" modes use all four SuperSpeed pairs (two in each direction). We'll consider the last one, since it's the most parallel-looking.
If this was a parallel system, we would probably have one end supplying a clock/strobe, and on a given edge of that clock, all four lines would have to have valid bits on them, so we would move two bits in each direction on every clock.
But with USB that doesn't happen. Each pair is self-clocking (8b/10b, 64b/66b, or 128b/132b, depending on the version), and independently recovered on the receiving end. The RX and TX directions are independent of one another, and while the two lanes in each direction do have a timing relationship to one another (they carry alternating bytes of the same stream, basically), this is put back together a bit later in the process, which means it can stand more timing skew than if we were trying to line things up on a bit-by-bit basis.
So, yeah, it's complicated. But folks are telling the truth when they say it's more serial than parallel.
Traditional parallel interfaces, where all signals were timed off the same clock, suffered from "skew": signals arriving on different wires at different times. This made it difficult to continue increasing the speed of parallel interfaces, particularly those that operated over longer distances.
As a result, serial interfaces with clocking embedded in the data stream rose in popularity. Parallel printer ports gave way to USB, PATA gave way to SATA, SCSI gave way to SAS.
However there is a limit (which increases over time as technology improves) to how fast a serial interface can be with affordable transceivers and wiring. The solution to this are what are known as "multi-lane serial" interfaces. Each "lane" has its own timing, but the data streams are split at the source and merged back together at the destination.
Multi-lane techniques have allowed serial interfaces to not only compete in, but come to dominate the high speed world. A single lane of PCI express 1.0 could outperform regular PCI, but not the faster variants of PCI-X or AGP. On the other hand with 16 lanes PCI express could easily outperform PCI-X and AGP.
USB was originally designed to replace a bunch of PC peripheral interfaces, none of which were particularly fast. The priority was to be cheap; people wouldn't switch to USB devices if they were significantly more expensive than their traditional counterparts. A single half-duplex serial link was the order of the day. It got a speed bump with USB 2 to better support external storage.
With USB 3, the original design had run out of steam and the solution was essentially to put two entirely independent interfaces on the same connector: the original bidirectional low/full/high speed data pair and two new unidirectional data pairs to carry the "superspeed" traffic. USB 3.1 bumped up the data rate on the "superspeed" pairs.
USB C was created to be "one connector to rule them all"; in addition to supporting all the existing USB functionality, it was also designed to carry a variety of "alternate modes" including DisplayPort, HDMI and Thunderbolt. The connector was also designed to be reversible and to support delivery of substantial amounts of power. The alternate modes necessitated having four high performance data pairs. There were also two "sideband use" pins to support the alternate modes and two "configuration and control" pins, which are used to detect, negotiate alternate modes, and negotiate power delivery.
So it all comes together, USB C had four high-performance pairs (plus the legacy pair) but two of them weren't actually used in USB mode. Furthermore thanks to the reversibility, superspeed devices with USB C ports were required to have either multiple transceivers or signal switches to direct the superspeed signals to the correct pins depending on cable orientation. Multi-lane serial was a proven technology (PCIe had been in widespread use for over a decade). The natural thing to do was to add a two-lane mode to USB, doubling throughput at little extra cost.
USB3 uses high speed link to device and another high speed link from device. It may use one or two pairs per direction.
Yes, it is still serial, as data is not sent in parallel. PCIe x16 is also still serial link, even if it uses 16 data lanes in parallel for transmission and another 16 data lanes for reception.
The other data pins are for backward USB2 compatibility, CC pins for cable flip and device presence detection, etc.
In alternate mode it supports also DisplayPort video, which uses the SBU pins.
USB Type C / USB 3.2 is not 'single-wire' serial, instead being composed of multiple serial links, plus sideband and power.
For being ‘serial’ it nevertheless uses a lot of wires.
Why? Several reasons:
However, like you, I feel that USB Type C it is a bit of a mess. With so many modes and cable variants to choose from it's a challenge to get the right combo. It suffers from classic feature-creep. Maybe down the road it will drop the legacy USB pairs and only support SuperSpeed (this is allowed for devices already.)
Nonetheless it has the might of Intel to push it along, and in furtherance of that goal they've placed the standard in the open to promote its adoption, as they have all the other USB standards before it. And despite being so complex, the USB type C connector has proven robust - even more so than micro-USB or Apple’s Lightning.
A downside to its aspirations to be, well, universal, USB Type C pin count and the complex software stack it brings with it make USB a poor choice for high-reliability and long-cable applications (not to mention extreme low cost.) There are more-specialized standards that do it better over fewer wires.
In the world of computing, parallel means "two or more things are done (more or less) at the same time". Even when processes run one at a time, controlled by a preemptive scheduler, overall they are called to be run "in parallel".
The situation is similar with USB. The difference discussed is:
parallel synchronous transmission of bits (e.g., the classic parallel port)
versus
parallel transmission of serial streams of bits - asynchronous at electrical level, but (mostly?) synchronous at higher levels (from the user's POV)
Also please note that when the data streams flow in opposite directions, we change the discussion from "parallel or not" to "bi-directional or not" (even though bi-directionality is also a form of parallelism - it avoids time multiplexing).
Usually any transmission line with a interface less than the whole data bus (or word size) is considered serial. Actually, If all bits don't go all at once, then it is considered serial. QSPI for example, has 4 serial lines while the the data bus or word size of that peripheral or chip could be 8bits; It is the maximum number of data lines on an 8bit system that is not considered parallel. The same applies for the USB.
A very important thing that people don't understand (and was missed in the rest of the responses): USB "C" is a cable, USB 3.0 is a Standard which includes cables, and different transmission systems, such as conventional and High Speed USB.
So the USB 3.x specification defines a USB-C cable, which can carry multiple different types of (serialized) protocols such as Conventional USB, USB SuperSpeed, HDMI, PCI and others. (But not all at the same time...)
