A System on Chip (SoC) usually has many functional blocks which need to exchange signals in order to operate. Communication of these signals can be grouped into either on chip or off chip to other chips or the external environment. 

For on chip communication how signals are transmitted and received are determined by standard protocols using a bus architecture. The Advanced Microcontroller Bus Architecture (AMBA).is a open protocol for designing SoC bus architectures.  For more complex many core devices Network On Chip (NoC) protocols are adopted for communication between various components. 

Some systems functions are best performed outside of the System on Chip using different technology both in terms of CMOS technology nodes or different semiconductor material systems. It can also be simply due to economics of fabrication or specialist IP. For off chip communication there are a broad variety of serial and parallel protocols and connection methods using a variety of media such as wireless, wired and optical which best suit the specific SoC application.