Page 418 - From GMS to LTE

404 From GSM to LTE-Advanced Pro and 5G

ESS and APs of other networks. To change to a new AP, the client device changes to the
send/transmit frequency of the new AP and sends a reassociation request frame. This
frame is similar to the association request frame and only contains an additional IE
which contains the ID of the AP to which the client device was previously connected.
The new AP then informs the previous AP via the wired Ethernet (distribution system)
that the user has changed its association. The previous AP then acknowledges the
operation and sends any buffered packets for the device to the new AP. Later, it deletes
the hardware address and association ID from its list of served devices. In the future, all
packets arriving for the client device via the wired distribution system will be ignored
by the previous AP and will only be forwarded to the client device by the new AP. In the
last step of the procedure, the new AP sends a Reassociation Response message to the
client device.
At first, only the message exchange between the client device and the AP was stand-
ardized for the reassociation procedure. For a long time, however, no standard existed
for the wired network between the two APs that are part of the procedure. Therefore,
manufacturers developed their own proprietary messages to fill the gap. As a result, it
is preferable to use only APs from the same manufacturer to form an ESS to ensure
flawless roaming of client devices. To tackle this shortcoming, the IEEE later standard-
ized the procedure in the 802.11f Inter‐Access Point Protocol (IAPP) recommendation.
Implementation of the 802.11f standard, however, is optional.
The 802.11 standard also offers a PS mode to increase the operation time of battery‐
driven devices. If a device enters PS mode, the data transmission speed is decreased to
some extent during certain situations. This is only a small disadvantage compared to the
substantial reduction in power consumption that can be achieved.

Access Ethernet Access
Terminal
point (new) (Distribution point (old)
system)
Reassociation request
(ID of the old access
point)
Data exchange from the old
to the new access point. Protocol
either proprietary or according to
802.11f (IAPP)

Reassociation response
(successful,
association ID,
capability information)

Figure 6.9 Reassociation (acknowledgment frames not shown).