Cellular Modems for M2M Systems

By leveraging the cellular infrastructure that carriers have already put in place, designers can use this network rather than Wi-Fi or an alternative network interface for collecting data or providing remote management. Ideal for small- to medium-size data packets, the cellular network can reduce the hardware budget for large, distributed systems since no local Wi-Fi hotspot or repeater is needed for each device that connects to the cellular network. Additionally, cellular modems have a longer reach (the signals can go further) than Wi-Fi or other wireless network interfaces.

There are multiple cellular options depending on the data transfer rate you need and the carriers available in the area where you need connectivity. The various standards now in use include GSM/GPRS, CDMA2000, HSPA, EVDO/CDMA, EDGE, and LTE. In some areas 2G cellular service can provide low-data-rate connections up to about 40 kbits/s, for faster connections, 3G services such as EDGE and CDMA2000 and HSPA+ can deliver data rates up to about 3.1 Mbits/s and 21 Mbits/s, respectively, while for the highest data-transfer rates, 4G (LTE) services, EVDO/CDMA, and HSPA+ can deliver data-transfer rates of up to 100 Mbits/s, depending on the carrier. Also, companies now are working on what is referred to as the 5G network, which will offer another tenfold increase in data-transfer speeds when it becomes real, probably in the 2016/2017 time frame or later.

Luckily, for machine-to-machine (M2M) communications most data transfers do not involve large quantities of data, but, instead use short bursts of a few hundred to a few thousand bytes. However, transmission speed is still important — short, high-speed bursts of data will often consume less power than a slower, continuous stream of data. The short bursts allow the baseband processors to spend more time in their power-down modes, thus greatly reducing power consumption.

At the heart of any cellular modem is the baseband processor, which, thanks to the high volume of cell phones produced over the last few years, has been integrated down to a single chip, thus simplifying the design of the modem. A typical modem module in the Skywire family from NimbeLink (Figure 1) is squeezed into an XBEE module format (29 x 33 x 6.5 mm) that was created by Digi International. Such modules are also in widespread use on development boards, such as the various ARM-based Arduino processor boards.


Figure 1: Providing all the basic functions for a 3G cellular embedded modem, a small plug-in module from NimbeLink (left) provides designers with a 3G CDMA cellular interface, a GPS/GLONASS transceiver based on a Telit baseband subsystem (right). The XBEE module format provides a standard interface for NimbeLink’s Skywire family of modules with different cellular interfaces. (Courtesy of NimbeLink)

However, unless the unit volume exceeds a project-specific cost crossover point, it is probably more economical and much faster to purchase a modem module than to build it from the chip level. Additionally, purchasing a module bypasses the need to get FCC certification since the modules are usually pre-certified. The crossover point for build-vs-buy depends on multiple factors — the quantity needed, the features required, the delivery timeline, and the cost of the bill-of-materials versus the module cost. Where to draw the line will vary on a case-by-case basis, but a good rule-of-thumb puts the crossover point at a minimum of 10,000 units, and possibly even higher.

Ready-made embeddable cellular solutions are available in two basic forms — small circuit-board modules such as the Skywire 3G CDMA module from NimbeLink that can either plug into a socket on the main circuit board (as shown in Figure 1), or a more industrial-strength metal-encased module (such as the CDMA modem from Multi-Tech Systems, shown in Figure 2) that can be mounted somewhere in a system cabinet and tie into the host using an RS-232 port, a USB cable, or some other interface. Modules from NimbeLink are available in 2G (Skywire GSM/GPRS), 3G, and 4G versions and all use the XBEE form-factor to provide a universal footprint. Data-transfer rates range from as little as 40 kbits/s for the older 2G GSM/GPRS modules, to 153.6 kbits/s for 2G CDMA2000 modules. Moving to 3G versions, data rates of up to 3.1 Mbits/s are possible with CDMA/GPRS units and jump up to 21 Mbits/s using HSPA+ or UMTS standards. The 4G implementations provide data rates of up to 100 Mbits/s using LTE and the HSPA+ standards. All the modules include a UART port, a USB port, an A/D converter, and several general-purpose I/O lines.


Figure 2: Housed in a sturdy metal case, this CDMA modem from Multi-Tech would be at home on the factory floor or mounted in a cabinet.

Digi International offers their own 4G LTE Cat 1 modem, which is ideal for M2M connectivity. Since it is LTE Cat 1, the operating costs are much lower than the traditional Cat 3 or Cat 4 plans (designed for smart phones). This device is FCC and Verizon end-device certified which results in faster time-to-market and the potential savings of the thousands of dollars typically required for product certification. Designers can see if this modem is right for there projects by using the associated development kit (Figure 3). This kit includes everything needed to start developing a cellular connectivity interface for most M2M applications.


Figure 3: This development kit from Digi International provides everything needed to evaluate the use 4G LTE Cat 1 connectivity for M2M applications.

Modules such as the Skywire units from NimbeLink consume about 580 mA when active, about 12 mA while idling, and just 1.2 mA in their power-saving mode. They operate from a 3.5 to 4.3 V power source, and are rated for an industrial temperature range of -40 to +85⁰C. Whether a module or metal-cased industrial grade modem is selected, designers have to determine the required data rate and which cellular standard to employ. In some cases the modem may incorporate multiple cellular standards to handle a variety of applications.

Of course, with any of the cellular modems, a SIM card is needed for identification and to link into a specific carrier — AT&T, Sprint, Verizon, among others; and when the system is activated, it must also have a service plan with a carrier. That service plan can be set for just simple text messages, or various data plans depending on how much data the system designer expects the system to transfer on a monthly basis.

For more information about the parts discussed in this article, use the links provided to access product information pages on the DigiKey website.