Design the hardware. Develop the software. Test the completed product. These are the three easy steps for building a Mobitex modem. Well, maybe they aren't so easy, but the design process for producing a Mobitex modem is very straightforward. Luckily, much of the hard work has already been done. Today there are a variety of tools and hardware and software components readily available for building a Mobitex modem. Mobitex Technology can also provide extensive support for testing and verification of the final product.

STEP ONE: Design the hardware
The radio modem is the core component in any wireless terminal device and the component that provides the network connection. In addition to network connectivity, a fully functional terminal, whether it is a wireless PDA (personal digital assistant) or a POS (point-of-sale) terminal, must provide some form of user interface that allows input and display of the data being sent and received over the network. Since the focus in this article is on radio modems, we will not be concerned with the many different types of terminal devices used on Mobitex networks. However, it is important to consider the interfaces that a radio modem provides to support terminal applications.

Typically, a radio modem provides connectors for an antenna, a power supply and a serial interface. Data is normally exchanged between the modem and the terminal via the serial interface. In addition to this interface, some radio modems, such as the M3000 series of OEM modems, employ I2 C or some other interface that is more powerful and versatile than the standard RS-232C serial interface. In most cases, the actual data being exchanged over the network is still handled by the serial interface, while the second interface is used for communication between the modem and the application. As will become clearer below, the serial interface is often called the MASC port, since it uses the Mobitex asynchronous communications protocol exclusively.

"Basically, a Mobitex radio modem implements the functionality required for the lowest levels of data communication, which are the physical, link and network layers," explains Folke Bergqvist, chief technology officer at Mobitex, Ericsson Mobile Data Design. "Primarily this is done using hardware, but the software - also known as firmware - embedded in the modem plays an important role. Some OEM modems, such as our M3000 products, are also able to host onboard applications, while others are designed to be connected to a separate data terminal."

STEP TWO : Write the software
The hardware required for a Mobitex modem can be implemented in a number of ways, partly depending on how much software will be hosted on the modem. The simplest approach is to use a special chip, such as the CMX909B from Consumer Microcircuits Limited (CML), which contains all the baseband signal processing functions required for a Mobitex modem in a single integrated circuit. Other approaches are to use a general-purpose DSP (digital signal processor) chip or a powerful CPU (central processing unit) chip, both of which can be programmed to perform the required baseband signal processing functions, as well as other signal processing. The CPU can also provide enough processing power to host onboard applications and a Java virtual machine (JVM).

An important component in the software required for a Mobitex radio modem is the protocol stack. Technically, a protocol stack is the set of protocols that work together on various levels to enable communication over the network. For a complex protocol suite, such as IP (Internet Protocol), the protocol stack must support not only the familiar higher-level protocols ftp (file transfer protocol) and http (hypertext transfer protocol), but dozens of other less familiar protocols. Mobitex is much simpler, requiring only MASC and MPAK protocols (see following sidebar) for basic functionality. It is worth noting, however, that the protocol stacks used in many Mobitex terminals also support additional protocols for POS (point-of-sale) applications, for example, as well as extensions for such purposes as data compression and encryption.

"Developing the protocol stack and associated software required for the basic radio modem functionality is a challenging task," notes Folke Bergqvist. "For this reason, many designers of wireless data terminals elect to use OEM modems for Mobitex, which are available from a variety of sources and typically provide a high-level application programming interface (API) for developers. Alternatively, several suppliers offer software packages that can be integrated with the modem hardware. This approach requires more work and involves more low-level programming, but it is sometimes the only alternative when a custom design is required."

STEP THREE : Test the device
The final step in building a Mobitex radio modem is to make sure that it works. This naturally includes verifying the design and testing the basic hardware and software functions, steps that are normally completed before a prototype is produced and further development work is begun. More importantly, however, the designers must verify that the Mobitex protocol has been implemented correctly and that the modem functions correctly on the network under a variety of traffic conditions. Some of these tests can be performed using emulation software, but the final testing requires a live network.

"We can provide a number of time-saving tools and services during the testing and verification stages to help designers get their product to market faster," notes Folke Bergqvist. "Ericsson has extensive modem expertise that our business partners can leverage. We can also assist in ROSI, MASC and Radio Performance verification. Naturally, these services are provided under strict confidentiality."

As was recently announced, Ericsson does not currently plan to develop any new Mobitex modems. Instead, the company will continue to support the M3000 series OEM radio modems for Mobitex and to assist third-party suppliers in developing new radio modem products. Companies wishing to develop Mobitex modems can thus count on Ericsson's expertise and rest assured that Ericsson is committed to helping its partners succeed in their targeted markets and to making Mobitex an even greater success.