From man pppd:
Set the async character map to <map>. This map describes which control characters cannot be successfully received over the serial line. Pppd will ask the peer to send these characters as a 2-byte escape sequence. The argument is a 32 bit hex number with each bitrepresenting a character to escape. Bit 0 (00000001) represents the character 0x00; bit 31 (80000000) represents the character 0x1f or ^_. If multiple asyncmap options are given, the values are ORed together. If no asyncmap option is given, no async character map will be negotiated for the receive direction; the peer should then escape all control characters. To escape transmitted characters, use the escape option.