Тонкая настройка модема

Man pppd?

mtu n Set the MTU [Maximum Transmit Unit] value to n. Unless the peer requests a smaller value via MRU negotiation, pppd will request that the kernel networking code send data packets of no more than n bytes through the PPP network interface. Note that for the IPv6 protocol, the MTU must be at least 1280.

(ц) man ppp

и /proc/sys/net/ip_default_ttl

по-моему так.