Bridge Architecture – Revolutionizing Dual Mode 4G Cellular Modem Dongle Design

In the continuous drive to keep consumers connected wherever they are, the cellular network has been an invaluable enabler. 3G and 4G connectivity, initially restricted to cellular phones, is now also widely available for portable computers. Initially a niche market for the business person on the move, cellular connectivity for laptops is currently experiencing an explosive growth due to college students and the new generation of latte sipping employees.

The device that enables 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. As data revenues become increasingly larger in each carrier’s revenue pie, so has the growth in popularity of Cellular Modem Dongles. 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, like SD expandability to even 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.

When it comes to dual mode modem dongles, a few possibilities come to mind. They are:

1. GSM + CDMA modem

2. 3G + 4G Cellular modem

3. Cellular modem + WiFi


In most cellular markets today, there are a mix of standards. The two main cellular standards of GSM and CDMA might work for carriers by tying subscribers to limited networks. However, this usually limits consumers, especially in developing countries where different carriers have better coverage in their own locality. Thus, just like the rise of dual mode cell phones in such countries, dual mode modem dongles will experience the same growth.

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.

Cellular Modem + WiFi

A third design option is the cellular modem and WiFi dongle. Although WiFi might come standard on many laptop PCs today,the new 802.11n standard is not yet integrated into PCs. Furthermore, due to carrier specification, 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.

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 100Mbps of data if the USB capability of your system is limited to around 80Mbps?

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

Independent datapaths

This feature is useful for dual mode cellular modem dongles due to the tight power consumption restrictions. Given the 500mA power limitation, it is vital that the modems are able to constantly toggle different power modes to keep the system in balance.The possibility of independent datapaths allow for communication and data transfer to happen simultaneously without needing to store data in a queue. This enables the possibility of constant toggling without upsetting the system’s fragile balance and interrupting data flow.

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.


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.

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