802.11 Logical Services

In studying for the Certified Wireless Analysis Professional (CWAP) exam I found several instances of concepts that I didn’t readily understand or that were not clear to me from reading the study guide. One of those concepts is the reason why logically some things were in the station service (SS) and some were in the distribution system service (DSS). Just so you’re familiar with what I’m talking about here is a summary of them from my study notes;

Station Service (SS)
Is used by all 802.11 client stations including APs
An AP is always also a station
Station Services includes;
• Authentication
• Deauthentication
• Encryption
• MSDU delivery
• Dynamic Frequency Selection (DFS)
• Transmit Power Control (TPC)
• Higher-layer timer synchronization (QoS only)
• QoS traffic scheduling (QoS only)
• Radio Measurement

Distribution System Service (DSS)
Are MAC services that are provided by the distribution system (DS)
These are services that are accessed by client stations
Distribution System Services includes;
• Association
• Reassociation
• Disassociation
• Distribution
• Integration
• QoS traffic scheduling (QoS only)
• Internetworking with the DS

Integration Service (IS)
Part of DSS (See Integration above)
Enables the delivery of MSDUs between the DS and a non 802.11 network via a portal.
Also called a frame format transfer method. Basically translates an 802.11 frame into an 802.3 frame.

Specifically I wondered, why is association, reassociation and disassociation part of the DSS. I went to look at what the 802.11-2012 standard said;

4.5.1 General
There are many services specified by IEEE Std 802.11. Six of the services are used to support medium
access control (MAC) service data unit (MSDU) delivery between STAs. Three of the services are used to control IEEE 802.11 LAN access and confidentiality. Two of the services are used to provide spectrum management. One of the services provides support for LAN applications with QoS requirements. Another of the services provides support for higher layer timer synchronization. One of the services is used for radio measurement.

4.5.2 Distribution of messages within a DS Distribution
This is the primary service used by IEEE 802.11 STAs. It is conceptually invoked by every data message to or from an IEEE 802.11 STA operating in an ESS (when the frame is sent via the DS). Distribution is via the DSS.

How the message is distributed within the DS is not specified by IEEE Std 802.11. All IEEE Std 802.11 is required to do is to provide the DS with enough information for the DS to be able to determine the “output” point that corresponds to the intended recipient. The necessary information is provided to the DS by the three association related services (association, reassociation, and disassociation).

4.5.3 Services that support the distribution service General
The primary purpose of a MAC sublayer is to transfer MSDUs between MAC sublayer entities. The information required for the distribution service to operate is provided by the association services. Before a data message can be handled by the distribution service, a STA is “associated.” Association
To deliver a message within a DS, the distribution service needs to know which AP to access for the given IEEE 802.11 STA. This information is provided to the DS by the concept of association. Association is necessary, but not sufficient, to support BSS-transition mobility. Association is sufficient to support notransition mobility. Association is one of the services in the DSS.

Before a STA is allowed to send a data message via an AP, it first becomes associated with the AP. The act of becoming associated invokes the association service, which provides the STA to AP mapping to the DS. The DS uses this information to accomplish its message distribution service.

My reading of this implies that the important difference between the DSS and the SS is that the DSS is all about how to get the data message from one point to another, whereas the SS is about connecting the STA and is similar to plugging in an ethernet cable. We can check this by looking at the particulars of some station services in the 802.11-2012 standard; Authentication
IEEE 802.11 authentication operates at the link level between IEEE 802.11 STAs. IEEE Std 802.11 does not provide either end-to-end (message origin to message destination) or user-to-user authentication. Data confidentiality
In a wired LAN, only those STAs physically connected to the wire can send or receive LAN traffic. With a wireless shared medium, there is no physical connection, and all STAs and certain other RF devices in or near the LAN might be able to send, receive, and/or interfere with the LAN traffic. An IEEE 802.11-compliant STA can receive like-PHY IEEE 802.11 traffic that is within range and can transmit to any other IEEE 802.11 STA within range. Thus, the connection of a single wireless link (without data confidentiality) to an existing wired LAN may seriously degrade the security level of the wired LAN.
To bring the security of the WLAN up to the level implicit in wired LAN design, IEEE Std 802.11 provides the ability to protect the contents of messages. This functionality is provided by the data confidentiality service. Data confidentiality is an SS.

There you have it!

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