[Unit]
Description=Start ppp0 interface
BindsTo=dev-ttyUSB0.device
After=dev-ttyUSB0.device

[Service]
Type=forking
RemainAfterExit=yes
ExecStart=/sbin/ifup ppp0
ExecStop=/sbin/ifdown ppp0

[Install]
Alias=startppp0.service
WantedBy=multi-user.target