If an 4G modem falls back to 3G, does it required a PPP connection to make a new dial-up?

Question

Configured to auto mode, an LTE modem will fall back to 3G/2G in bad signal conditions. When it falls back (hands over to 2G or 3G), does it need to make a new PPP dialup connection?

Answer 1

It depends upon the situation because there are two scenarios for fallback.

1. Fallback on the Radio Only
   In this case, the device falls back to 3G (HSPA or EvDO for example), but we remain in the IMS core. In this case, everything is the same, you will see no connection state change.

2. Fallback to the legacy network
   In this case, the device loses its LTE/IMS connection. For some networks, it falls back to HSPA, others back to its CDMA roots. In this case, you DROP your connection to the IMS core and LTE and fall back to the legacy network, as if you had disconnected and reconnected. In fact, you may disconnect. At this point you will see a new PPP session.