This documentation gives some background information that is helpful in understanding the 802.11 Wireless LAN model developed by Bellum Software.

Layers and Interfaces

The diagram below is shows the relationships of the Operating System, Application software, and the OSI layers. The Service Access Points LLC-MAC and SME-MLME (marked with the red numerals) are of particular interest since these are where transactions are initiated by either the LLC (data transfers), or by the SME (control functions).

The 802.11 Wireless LAN specification deals with how the transactions listed under 1. LLC-MAC SAP and 2. SME-MLME SAP are initiated from these SAPs and how they are handled by the MAC and the Physical Layer.

Message Paths

The 802.11 model created by Bellum Software is organized with two main parts:

1. the intra-layer portion models the messages as being passed conceptually within the Data Link layer,
2. the inter-layer portion models the messages moving down to the Physical Link layer before being passed to the far end.

Note: v1.1 of the 802.11 model contains an infrastructure for adding a model of the Physical Link, but does not yet model it.

The intra-layer description is really concerned with the purpose and reason for messages to be passed between entities. On the other hand, an inter-layer transfer through the Physical Link layer is not really concerned with the meaning and content of a message. A message is simply a list of bytes that needs to be sliced into manageable chunks and transmitted over a channel. The intra-layer and inter-layer paths are shown in the diagram below.

One thing that can be confusing to new readers of the 802.11 (and similar) specifications is that the text will freely mingle intra-layer and inter-layer descriptions. In other words, the text may say something like, "The STA will send the AP an Authentication message with these parameters...", making it sound like a direct communication. But this is simply a conceptualization used to simplify the discussion. In reality all messages go through the physical layer and are broken into fragments for transmission over a particular channel.

The "generals on the hill" analogy may helpful. When reading a history of World War I, one may read how "General A on one hill ordered General B on another hill to attack". This is really a conceptual summary of how the message was communicated. Given the reality of communication at that time, what really happened was that Major C copied down the order, handed it to Lieutenant D, who gave it to Sargent E, who gave it to Private F to run over to the hill. The passing of the message down through the different ranks to cause its actual delivery is similar to the working of the OSI Layers.

To push the analogy a bit further, in chapters where the history author is concerned about why things happened, and what decisions caused which affects, the conceptual summary of the message will be used. But when the author wants to show the difficulties of communication during battle, the actual physical relaying of the message will be described. The authors of communication specifications will switch between these two types of message description for essentially the same reasons.

The 802.11 model from Bellum Software is built to allow a message to pass through both the intra-layer path and the inter-layer path. The model extract below shows a rule on the LLC which initiates a Data request into both parts of the model:

When a message is received at either the LLC or SME, match/mismatch rules are constructed which can compare the fields of the message to provide a self-check capability. The model extract below shows rules which check for match or mismatch when the LLC receives a Data indication:

Note: Oftentimes the number of parameters for the message going through the conceptual part of the model will be less than the number of parameters for the message going through the physical part of the model. This is seen in the model extracts shown above. This happens because physical parameters such as RoutingInfo, ReceptionStatus, Channel, etc. are not applicable in the conceptual part.