Electronic – USB “Serial” Transceivers (PHY)

The terminology is vague at best. There are no hard and fast rules, and nobody playing police when people don't use the term correctly.

Usually, but not always, a transceiver is a simple buffer that has different signal standards on the inputs and outputs. For example, an RS-232 transceiver usually interfaces TTL signals to RS-232 signals. Also, transceivers are similar to "drivers" and "receivers" except that a transceiver usually does both driver and receiver functions in the same chip.

A Phy is similar to a transceiver in that there is usually different signal standards on "both sides of the chip". With Ethernet it is MII/GMII/etc on one side and, well, Ethernet on the other. There are other Phy's that talk USB, PCIe, and many others. Phys usually incorporate some sort of SERDES (SERializer-DESerializer) function and line encoding. More sophisticated PHYs contain a mini-DSP in them to do all sorts of crazy communication things like advanced baseband wander correction.

A transceiver does not have to be associated with a serial data-- they can be used for parallel data as well. A PHY usually involves some sort of serial data stream. But this is more of a convention than a rule, and I am sure there are chips out there that are exceptions to this.

What you are looking at is a problem I like to call bus hierarchy.

You have high speed devices that need a fast low-latency path to the CPU (like the LTE modem) and slow devices like the temperature sensor. Connecting a full fledged LTE modem over I2C will seriously bottleneck the system. And connecting a temperature sensor on USB is a serious overkill. You need to prioritize.

What devices are to be accessed most often? Which ones need to respond quickly? And which don't need that much bandwidth?

Feel free to edit your question.

Edit:

USB may not be such an overkill after all. Only in the latest versions of it (2.0, 3.0) it has gotten the notion of being targeted to fast peripherals only. Before that, it was simply a Universal Serial Bus.

This doesn't change anything on the fact that it's implementation cost is very often too high.