One thing that works really well in Linux is support for USB Broadband modems. While it usually takes a little fiddling with PPP scripts, it is generally not too difficult to get working. One of the reasons for this is most modems implement a USB serial interface, and then the modem is controlled with standard AT commands. A customer who currently supports a Verizon USB modem in their product wanted to add support for a Sprint modem. After a little research, we settled on the 598U from Sierra Wireless. Below is a picture of the 598U plugged into an industrial terminal.

Why are USB Broadband modems useful in Embedded Systems?

As systems become more complex, and time to market is shrinking (thus less testing), it is becoming increasingly important to be able to remotely diagnose problems. With a broadband modem like the Sprint 598U, developers can remotely log into a device using SSH, and debug problems. This is a huge benefit. The cost of the modem and a 1 year data plan becomes fairly insignificant when you consider the cost to fly a developer on-site to debug issues. While there are other ways to remotely access an embedded system, such as reverse ssh, the simplicity of the broadband modem is attractive as it bypasses the firewall issues. Even with reverse ssh, I’ve run into paranoid sys admins who block the outgoing port 22 on their networks, which makes reverse ssh difficult. Once you have the IP address of the broadband connection, you can connect to it directly with ssh from anywhere in the world. I’ve accessed systems that were being field tested in New Zealand from the United States, and it worked great.

Increasingly, the value of complex embedded systems is shifting from control to data. While the control aspect is as important as it ever was, access to data is what is giving many products and businesses the competitive edge. Being able to collect data and visualize how the system is performing is becoming increasingly important to maximize performance. And sometimes this data must be collected in real-time. Scenarios where USB broadband modems might make sense:

• remote sites that do not have internet connectivity
• mobile applications such as vehicle terminals
• temporary installations where the system is only located in one place for a limited time
• situations where it is very difficult to get administrators of the local network to cooperate in adding the embedded system to their network

Finding PPP Config information

I usually develop as much as I can on x86 systems (vs. an embedded target) as it is a lot more convenient, so I plugged the modem into a Ubuntu 8.10 laptop. While Ubuntu 8.10 recognizes and configures a UM150 Verizon modem, it does not automatically configure the 598U. Typically, the next step is to google around for information on various support forums on how to set up the PPP config files, as most manufactures don’t provide support for Linux. As I could not find any existing information for the 598U, I decided to see if Sierra Wireless published any information. Was I ever suprised when I found this page. Sierra Wireless provides fairly comprehensive information on using the modem under Linux, and their PPP scripts actually worked — great job! But in the end, when I plugged the 598U into the embedded device that already supported the Verizon modem, it just worked with my existing PPP scripts. This is another indication of the value of using common code and interfaces in Linux.

The Linux Advantage

When you run Linux in an embedded system, using a USB broadband modem becomes fairly simple. It is actually a little bit deceiving as to what all has to happen. Different modems use different chipsets, and thus different drivers. While they all eventually expose a USB serial interface, there are enough differences to require separate drivers. As an example, the 598U uses the “sierra” USB-serial driver. This driver is 782 lines, plus it utilizes a common USB serial framework that is many times this size. On the application side, the pppd framework is used to manage the modem and the network connection. Starting the connection is as simple as running “pppd call cdma” from your application. The pppd application manages getting an IP address, setting up nameservers in resolve.conf, etc. Because all broadband modems are supported by a common application interface and very similiar drivers, supporting multiple modem vendors is actually quite easy. Contrast this to the Windows driver scenario where every vendor provides their “own” driver, and their “own” application user interface, and their “own” way to initiate an internet connection, and it is not obvious to me how you could easily control all this in a consistent, automated fashion in an embedded device. The way we set it up is you plug the modem in, and a few seconds later the system is automatically connected to the internet. There is nothing for the user to configure, initiate, etc. Printing from Linux in embedded applications is similiar.

This is another example of how it is possible to leverage the work of many to add advanced functionality and features to a device with very little development cost. Getting more done with less should be an attractive (and perhaps critical) way to operate for any company seeking to remain competitive.