Want to know Tunnel concept then look at the document

Tunnel: A tunnel refers to the passage that conveys bearer messages over the Mc and Nc interfaces. The tunnel is defined by the Bearer Control Tunnel Protocol (BCTP). A tunnel packet is defined by the BCTP protocol, but the content of the tunnel packet is defined by the IPBCP protocol, including the IP address, port number, and IPBCP primitive (Request, Reject, Accept, Confused).