Bridge Architecture: revolutionizing dual-mode 4G cellular-modem dongle design

Figure 1: Worldwide data card forecast for 2005-2012
(Click on image to enlarge)

The devices that enable cellular connectivity of laptop computers are known by a variety of names, including mobile broadband devices and laptop connect cards, among others, depending on which carrier you talk to. For the purpose of this article, we will group them all under the umbrella of Cellular Modem Dongles (CMD).

As data revenues become increasingly larger in each carrier's revenue pie, so has the growth in popularity of CMDs. Currently, every major cellular carrier offers a complete line-up of such devices for sale. These modem dongles range from the simplest 3G connectivity to modems with additional features, such as SD expandability, and even to dual-mode modems.

The simplest 3G modem dongle available in the market today is the single-mode GSM or CDMA modem dongle. These modem dongles offer only the capability to connect to the cellular network for data needs. These devices are dedicated data transmitting and receiving devices which mainly consist of an RF chip, a baseband modem, and a power management IC.

With the market for modem dongles heating up, manufacturers have been trying to differentiate their products. The easiest differentiation is with the addition of an SD card slot for storing data, as this feature is usually included with the modem's baseband processor and thus minimal design work is needed, Figure 2. However, beyond the simple addition of SD cards, today's modem designer is looking to add new features to cellular modem dongle designs, particular having dual mode modem capabilities.

Figure 2: Traditional modem architecture
(Click on image to enlarge)

When it comes to dual-mode modem dongles, the possibilities are:

1. GSM + CDMA modem
2. 3G + 4G Cellular modem
3. Cellular modem + WiFi

2. 3G + 4G Cellular
Another popular CMD design today is a combination of 3G and 4G modems. These designs leverage the high-profile 4G cellular data service and the more widely available 3G data service. With most carriers today either choosing WiMax or LTE as their preferred 4G implementation, modem dongles are starting to incorporate these into their design while supporting more prevalent 3G connectivity.

3. Cellular Modem + WiFi
A third design option is the cellular modem and WiFi dongle, Figure 3.

Figure 3: Dual-mode modem architecture
(Click on image to enlarge)

Although WiFi might come as a standard feature on many laptop PCs today, the new 802.11n standard is not yet integrated into PCs. Furthermore, due to carrier specifications, many carrier-linked netbooks will not have WiFi capability. This will create a need for an additional device capable of providing WiFi connectivity when available and yet still keep the carrier's data revenue stream intact.

Given the clear need and demand for such dual-mode CMDs, one might question why is it that we have yet to see such products readily available. One of the main reasons for the delay to market is the complexity of designing such a device. For example, a dual-mode CMD has to have the following specifications:

1. Both modes available
2. 500 mA operating power consumption limits
3. Fast switching time between modes
4. USB performance

2. Operating power consumption limits
Since USB is the most prevalent connectivity option to a mobile PC or almost any PC today, many modem dongles are also adopting this standard. As such, there is a limitation to the amount of current the modem can draw from a USB port. The USB 2.0 standard allows for 500 mA to be drawn by a device connected to a single USB port. Thus, when operational, a modem dongle only has availability of 500 mA from the USB port.

This is a major concern for many modem-dongle designers, as each of the main processor alone can potentially draw upwards of 400 mA when operational. In addition, engineers must also take into account the interconnect between the two processors and the USB port when computing the total power consumption of the system in full operation. Under such a scenario, where both modems are operational, the modems themselves will consume currents of beyond 800 mA which is higher than the possible supply of USB 2.0.

However, many designers have understood that there is no scenario where both modems are working at full power simultaneously, and thus the highest power consumption will be with a modem operating, with the addition to the interconnect chip.

This is where using a bridge chip provides a significant advantage over the use of a hub. Assuming only 400 mA power consumption from a modem, any addition of more than 100 mA will cause the device to fail, thus ruling out the use of hubs in dual-mode cellular-modem dongles. Bridge chips that consume only around 80 mA to 100 mA will be able to keep the system within the specified current range.

3. Fast switching time between modes
Another important factor for dual-mode modems is the capability to switch between modems as fast as possible. Depending on designer requirements, some designs require switching within microseconds and others milliseconds. A dual-mode modem with a hub usually has a much slower switching time compared to a modem with a bridge chip.

This is because hubs behave as a "pass through" and all instructions have to come from the PC to which the modem. Similarly, all actions from one of the modems need to be passed along through the hub to the PC, and relayed back to the other modem.

In bridge architectures, the switching time is minimal due to the ability of the modems to communicate directly, without needing to send data to the PC first. Even if data is to be sent to the PC, all communications can happen simultaneously and seamlessly. This minimizes the switching time between both modes.

4. USB performance
USB performance is another key factor in dual-mode modem design. As USB is the predominant interconnect between the modem and PC, it is essential that the USB capability of the system operates at full capacity. After all, why have a modem that is capable of delivering more than 100 Mbps of data if the USB capability of your system is limited to around 80 Mbps?

This is another vital factor that put bridge chips in the forefront of these systems. Unlike a hub that just passes through the USB data coming from the modems, bridge chips can connect to other higher-performance interfaces of the modems (for instance, the memory interface). This enables the modem to output the maximum data capable and not be gated by the integrated USB performance of the modem.

Apart from satisfying the main requirements of a dual-mode cellular-modem dongle, bridge architectures also bring additional benefits to modem designs. They are:

1. Independent datapaths
2. Flexible interfaces
3. Size

2. Flexible interfaces
Bridge chips also provide more flexible interfaces within a design, allowing developers to add an additional SD card or switch memory interfaces. This flexibility gives designers the freedom to readily swap features on their system while maintaining a fast time to market for their design.

3. Size
Many bridge chips are specifically designed for mobile devices. Their size is much smaller compared to hubs which are typically designed for computer peripherals. This is an added benefit to a mobile device like a USB modem dongle as size does matter. The smaller a CMD is, the easier it is to carry around.

The availability of cost-effective and power-efficient dual-modem dongles promises to meet the requirements of both consumers and carriers. Through the use of bridge architectures, developers can enable dongle to satisfy all the needed specifications of a dual mode cellular modem dongle while enabling additional enhancements and functionality.

About the author
Ming Hoong Chong is a Product Manager at Cypress Semiconductor's Data Communication Division (West Bridge BU). He is responsible for wireless technology market research, and business development. He has a bachelor's degree in electrical engineering from the University of Illinois–Urbana-Champaign.