USB问题

luoqs2003

---PHY mode
In this mode, an external serial transceiver is connected to the port. This is used for off-board
USB connections
----TLL mode
In TLL mode, internal logic is enabled to emulate the functionality of 2 back-to-back connected
transceivers. This mode is typically used for on-board USB connections to USB-capable
peripherals

哪个兄弟帮我看看，这2个是要说明什么意思？