Mobility types

4.5.3.2 Mobility types

The three transition types of significance to this standard that describe the mobility of STAs within a
network are as follows:
a) No-transition: In this type, two subclasses that are usually indistinguishable are identified:
1) Static—no motion.
2) Local movement—movement within the PHY range of the communicating STAs, i.e.,
movement within a basic service area (BSA).
b) BSS-transition: This type is defined as a STA movement from one BSS in one ESS to another BSS
within the same ESS. A fast BSS transition is a BSS transition that establishes the state necessary for
data connectivity before the reassociation rather than after the reassociation.
c) ESS-transition: This type is defined as STA movement from a BSS in one ESS to a BSS in a
different ESS. This case is supported only in the sense that the STA may move. Maintenance of
upper-layer connections cannot be guaranteed by IEEE Std 802.11; in fact, disruption of service is
likely to occur.
The FT Protocol provides a mechanism for a STA to perform a BSS transition between access points (APs)
in a robust security network (RSN) or when quality-of-service (QoS) admission control is enabled in the
ESS.
The different association services support the different categories of mobility.
4.5.3.3 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. How the information provided
by the association service is stored and managed within the DS is not specified by this standard.

Within a robust security network (RSN), association is handled differently. In an RSNA, the IEEE 802.1X
Port determines when to allow data traffic across an IEEE 802.11 link. A single IEEE 802.1X Port maps to
one association, and each association maps to an IEEE 802.1X Port. An IEEE 802.1X Port consists of an
IEEE 802.1X Controlled Port and an IEEE 802.1X Uncontrolled Port. The IEEE 802.1X Controlled Port is
blocked from passing general data traffic between two STAs until an IEEE 802.1X authentication procedure
completes successfully over the IEEE 802.1X Uncontrolled Port. Once the AKM completes successfully,
data protection is enabled to prevent unauthorized access, and the IEEE 802.1X Controlled Port unblocks to
allow protected data traffic. IEEE 802.1X Supplicants and Authenticators exchange protocol information via
the IEEE 802.1X Uncontrolled Port. It is expected that most other protocol exchanges will make use of the
IEEE 802.1X Controlled Ports. However, a given protocol might need to bypass the authorization function
and make use of the IEEE 802.1X Uncontrolled Port.
NOTE—See IEEE Std 802.1X-2004 for a discussion of Controlled Port and Uncontrolled Port.
At any given instant, a STA is associated with no more than one AP. This allows the DS to determine a
unique answer to the question, “Which AP is serving STA X?” Once an association is completed, a STA
may make full use of a DS (via the AP) to communicate. Association is always initiated by the mobile STA,
not the AP.
An AP may be associated with many STAs at one time.
A STA learns what APs are present and what operational capabilities are available from each of those APs
and then invokes the association service to establish an association. For details of how a STA learns about
what APs are present, see 10.1.4.