What is the term that is used to describe the process of converting a signal from digital to analog？（）

A. to eliminate the need for VoIP gateways
B. to scale a VoIP installation with multiple gateways
C. to provide call admission control and limit the number of simultaneous calls on the network 
D. to meet the requirement to control H·323 gateways on a LAN
E. to accept call requests and reduce the load on the gateway

A. codec
B. link speed
C. Layer 2 encapsulation
D. packet overhead （IP/UDP/RTP）
E. link delay
F. total number of simultaneous calls that are allowed