Issue 139 February 2002
What Good is IrD, Eh?
Part 2: Wireless Communication

Start • IrLAP/IrLMP • IrLAP?IrPHY Frames • Who Am I/Who Are You• IrComm • Short Stories • Continuing Education • Sources & PDF

Once upon a time, I looked at the layering of protocol stacks as being unnecessarily complicated. Why divide a simple communication task into so many separately defined tasks? Although I don’t consider myself to be anymore than an adequate software programmer, I can appreciate dividing a programming task into smaller more manageable routines. Generally it isn’t necessary to go back and rewrite a working routine just because another part of the application has been modified (unless it is directly effected by the change). This idea of reusable code can be applied to protocols, as well. It is most easily seen at the bottom level, the physical interface.

Figure 1—This is the layer stack between your application and the infrared transceiver components, which make up the IrDA protocol architecture. The service layers define how data is passed between layers.

Figure 1 should be familiar from last month, when I started discussing using the IrDA standard for infrared data communications. The IrDA stack is built on the firm IrPHY foundation layer. Advances in technology (improved hardware) can be implemented without requiring changes in any of the upper protocol layers. As shown last month, the IrPHY layer defines what constitutes good communication via infrared transmission and reception, but does not define how that is accomplished with any specific hardware.

With the physical layer (IrPHY) previously defined, the link access layer deals directly with the IrPHY layer and handles the bottom level communications tasks. The IrLAP layer provides services to the IrLMP layer. These services include,link start-up/ shutdown, address discovery, address conflict resolution, connection establishment, sniff open, information exchange, connection reset, and disconnection. The IrDA standard defines these service primitives and parameters, but not how the IrLAP must implement them.

The IrLMP layer consists of the link management information access and link management multiplexer. The IAP maintains an information base of IR devices within range and their capabilities. The MUX provides services to both the IAP and the transport entities. An application must deal with one of the transport protocols, such as IrCOMM. This transport layer takes care of defining the emulated connection, which includes how the port handshaking will be implemented along with the data. An intermediate TINY-TP layer is used to break outgoing data and control into chunks, reassemble incoming data and control, and keep track of multiple transports that may be trying to use the IrDA connection simultaneously.

Of the three mandatory IrDA layers, the IrLAP layer may have the most important job. This layer is responsible for taking the requests for and confirming the results of a service from the IrLMP layer. It automatically discovers and establishes connections to those devices within range. Let’s look closer.