When computer networks were developed decades ago, organizational rules and protocols were designed for wired connections. Networks were organized into 5 (sometimes 7) "layers" of functionality. They are:

• Application: This is the stuff the end-user sees. For example, when you open up your email program, you're using the application layer.
• Transport: This layer provides a mechanism for transferring data to/from end-users.
• Network: This is where packets of information are routed through many devices.
• Link: This is the lowest level of "1s" and "0s."
• Physical: This is the part where electromagnetic signals travel through space or material from transmitter to receiver.

In general, a layer does not "know" anything about the internal workings of another layer. The application layer, for example, does not know how information is actually sent, it simply knows that it can communicate with others by passing data to the transport layer. It's kind of like sending a letter to someone. You don't actually know how the letter gets there, you just know that if you put an address on it and place it in your mailbox, it gets there. You don't need to worry how the post office actually routes it.

Under this paradigm, it's easy to see how the application layer would have no business getting involved in the physical layer. In wireless networks, however, strict layering breaks down. Since nodes are mobile and transmit through space instead of a wire, layers must pass information to each other in a "cross-layer" architecture.

While wireless networks and cross-layer architecture present many challenges, Xandem views the wireless channel as a fertile ground for innovation. Specifically, there are many opportunities that arise when the application and physical layer are connected. By measuring how strong the physical layer signals are, for example, technologies like radio tomographic imaging are possible (see figure below).

A photo of a simple radio tomographic imaging network with its corresponding result.