Recently I tested support for the Verizon UML290 and Sprint U600 USB Cellular modems in an embedded Linux system. Both modems support 3G and 4G networks, but only the 3G modes were tested due to lack of 4G coverage at the testing location.
Fortunately, both modems function very similar to previous modems, so with the drivers available in the Linux kernel, and standard pppd support in OpenEmbedded, they worked fine.
The Verizon UML290 modem provides a challenge in that it must be manually switched between 4G and 3G modes. Typically this is done automatically by the vzaccess program Verizon supplies with the modem that runs on Windows. The solution for this system was to manually set the modem to 3G mode as detailed on the following page:
It appears that some embedded systems such as the Cradlepoint routers have implemented automatic 3G/4G switching support for the UML290, so this is no doubt possible with a little effort.
The Sprint U600 modem appears to default to 3G, or automatically switch inside the modem.
The same pppd scripts can be used with both modems:
# /etc/ppp/peers/verizon_um290 user a password v connect "/usr/sbin/chat -v -f /etc/ppp/peers/verizon_um290_chat" defaultroute usepeerdns ttyACM0 921600 local usepeerdns debug -detach
# /etc/ppp/peers/verizon_um290_chat '' 'ATZ' 'OK' 'ATDT#777' 'CONNECT' ''
To initiate a connection:
pppd call verizon_um290
With Verizon cellular modems, it appears that port 22 is often blocked, so if you need to access a remote device via ssh, you may need to run ssh on a higher port number. With 4G networks, it appears that the networking setup may be different in that a public IP address may not be assigned. From the above evdoinfo.com page, we find the following text:
This fix will also work for users looking to use their device for remote based applications because it assigns a public facing IP address (3G ONLY). With eHRPD you’re assigned a private IP in either 3G or 4G mode, which has prevents UML290 users from accessing remote applications.
Perhaps the Rev A HDR Service mode will also work in 4G mode, but its seems as cellular networks become more complicated, there will be more issues to deal with in using USB Cellular modems for remote access in embedded systems.
Past articles on USB Cellular modems: