This leads to the problem where you, as the user, have to poke values into the NV_HDRSCP_FORCE_AT_CONFIG_I NVRAM item to manually switch between HRPD and eHRPD just to get connected. My theory here is that to maintain backwards compatibility or for some other reason, PPP data sessions using ATD#777 only allow HRPD, and thus handoffs between EVDO and LTE don’t work because the LTE side doesn’t like the older HRPD. But when you use traditional CDMA PPP via ATD#777 the session is between pppd on your computer and the packet data gateway in the network, in contrast to GSM/WCDMA/LTE where the PPP session is only between pppd and the modem itself, not over the air. Normally your device uses the eHRPD protocol when starting a data session since both the network and the modem support it. eHRPD stands for “evolved” or “enhanced” HRPD and essentially drops in pieces of the LTE core network modified to work with older EVDO protocols. EVDO (Evolution Data Only) is just the marketing name for all that. HRPD (High Rate Packet Data) is the new name for HDR (High Data Rate) which was the old name for the IS-856 standard developed by Qualcomm ten years ago for high speed 3G packet data. To allow seamless operation between the EVDO and LTE networks Verizon upgraded parts of their core network to eHRPD. Unfortunately, the PPP functionality does not support data connection handoff between the EVDO and LTE networks, so you have to break the connection and reconnect with the appropriate ATD command when necessary. Connections to the 3G EVDO network can be made with the standard ATD#777 command. You can connect to the LTE network using standard ETSI 27.007 GSM-style AT commands like AT+CGDCONT and ATD#99* and such. Of these, only the AT command and the QCDM ports are really usable in Linux.
The UML290 exposes four USB interfaces: a standard CDC-ACM AT command port which supports PPP, a QCDM port, a WMC port, and a raw IP network port. Inside is a Qualcomm MDM9600 chipset providing both CDMA 1xRTT and EVDO on the standard North American 850 MHz Cellular and 1900 MHz PCS bands, and LTE on Verizon’s Upper 700 MHz C-block band. Despite being quite large (over twice the size of a normal 3G modem) it’s not a bad device and performs quite well in speed tests.
Along with the LG VL600 this modem was the launch device for the Verizon 4G LTE network late last year.
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