USB-C Alt Mode
While shopping for a new monitor with USB-C I found out that a lot of information about DisplayPort on USB-C was missing or incomplete online, especially if you want to combine this with Multi-stream Transport (MST) for daisy-chaining multiple monitors.
USB-C alt mode
The USB-C connector has besides USB 2.0 data pins, four high speed differential pairs for high speed transport. These pins can be reconfigured using alt-mode, if using DisplayPort your options are two of the lanes to DisplayPort and the other two for USB3.2 Gen 1x1 of USB3.2 Gen 2x1 or all four lanes to DisplayPort and only having USB2.0 for the data transport.
A normal DisplayPort cable has also 4 lanes ,the bandwidth depends on the version, for DisplayPort 1.2 this will be 5.4Gbit/s and for DisplayPort 1.3 and 1.4(a) this can be up to 8.10Gbit/s however almost all current laptops only support the 1.2(a) standard, this greatly reduces the total link bandwidth on the USB-C alternative mode pairs. If for example we still want bandwidth for external hard drives or a USB gigabit ethernetcontroller in the USB-C dock or monitor we can use at most 2 high speed differential pairs for the DisplayPort signal, this results in a total available bandwidth of 10.80 Gbit/s for the image displays.
With multi-stream Transport we can encode multi display streams on the same link if we have enough bandwidth and are not using compression because there is almost no support for this. Given the 10.80Gbit/s limit we have with USB3 and DisplayPort on the same cable how many monitors and what resolution can we drive first we need to remove the encoding overhead and we are left with around 8.6Gbit/s for display data. We want to have at least 60Hz for the refresh rate given that we can’t configure this lower on most monitors. This means that for example a 1080p signal, is around 3.2Gbit/s and this means that we can drive a maximum of two 1080p (or 1200p) monitors on a two lane USB-C DisplayPort alternative mode cable. The maximum single display resolution that is possible on a two lane USB-C alternative mode setup is a 3440 by 1440p ultra wide monitor.
Order of displays
Because the longer you are down the chain the less bandwidth is needed it is sensible if you have displays that support DisplayPort 1.3 or 1.4 to have this as the first device devices in the chain, this greatly increases the amount of chaining possible because if you display adapter support DisplayPort 1.3 or 1.4 you double the bandwidth, this makes it possible to have a larger resolution in the first displays or more devices in the total chain.