Page 462 - From GMS to LTE

448 From GSM to LTE-Advanced Pro and 5G

received, the next frame is sent after an SIFS period. In addition, the sender and receiver
can agree to use a block ACK. Here, the sender bundles a number of frames and only
expects an ACK once all frames have been sent. If the other device has received the
frames correctly, a single ACK frame is sufficient to confirm the reception. The ACK is
sent either immediately (Immediate Block ACK) or somewhat later (Delayed Block
ACK) to allow the receiver to check for errors in the received data.
The AP indicates support for block ACK by including a capability information param-
eter in the beacon frames. Client devices indicate their block ACK capabilities to the AP
during the association procedure. The 802.11n packet aggregation feature, as described
in Figure 6.16, can be used in combination with packet bursting and block ACK.
Therefore, several methods are now available in order to efficiently use the air interface
when transferring large chunks of data compared to the initial specification.
In addition to the original PS mode and the PSMP extension that has been specified
in 802.11n, an additional PS method, referred to as Automated Power Save Delivery
(APSD), was introduced with 802.11e. Again, the feature has a number of options. Using
Unscheduled‐Automated Power Save Delivery (U‐APSD), which is optionally sup-
ported by WMM, the client device and AP negotiate that the client can enter a dormant
state during which all incoming frames are buffered by the AP. During the negotiation
phase, it is also specified which priority classes the algorithm is applied to and which
frames continue to be delivered after wakeup with the normal PS mode. In addition, a
Service Period (SP) is negotiated in which the device is active before it automatically
enters the dormant state again.
The U‐APSD negotiation sets no interval value after which the device has to return
to the active state. Instead, a device transmits a trigger frame to the AP as soon as it is
available again. Frames of QoS classes for which U‐APSD has been activated will then
be delivered automatically during the SP. The device automatically reenters the dor-
mant state at the end of the SP. Frames belonging to QoS classes for which U‐APSD
has not been activated have to be retrieved with the standard PS methods by polling
for each buffered frame. If an AP supports U‐APSD, it is broadcast in beacon frames
in the WMM parameter. From a client device’s point of view, U‐APSD operation can
be negotiated during the association procedure via the QoS capability parameter or
later by transmitting a TSPEC message. In addition, the 802.11e specification also
contains a Scheduled‐Automated Power Save Delivery (S‐APSD) operation mode,
which, however, is not used in WMM. Instead of trigger frames, a cyclic activity inter-
val is used.
While most applications use a wireless network to establish a connection to a server
on the Internet, there are a growing number of applications in homes and offices that
require local connectivity, for example, video streaming or transferring large amounts
of data between wireless devices in the network. The default method of exchanging data
between two wireless devices in a network is to send the data to the AP, which then
forwards it to the other device. In other words, the data is transmitted twice over the air
interface and the overall throughput of the network is therefore cut in half. To improve
the performance of data transfers between devices in the wireless network, 802.11e
contains an extension referred to as the DLP. When two devices wish to communicate
with each other directly, one of them sends a request to the AP. The AP in turn forwards
the request to the other device. If the other device is within range of the first device and
supports the DLP protocol, it returns an answer to the AP, which returns it to the