Page 421 - From GMS to LTE

Wireless Local Area Network (WLAN) 407

Access
Terminal 2
Terminal 1 point

Data frame
ACK frame SIFS

DIFS
Data frame
ACK frame

Data frame
ACK frame

Data frame
ACK frame

Figure 6.11 Acknowledgment for every frame and required interframe space periods.

space (SIFS). All other devices have to delay their transmission by at least a distributed
coordination function (DCF) interframe space (distributed coordination function
interframe space, or DIFS for short).
Optionally, devices can also reserve the air interface prior to the transmission of a
data frame. This might be useful in situations where devices can reach the AP but are
too far away from each other to receive each other’s frames. Under these circumstances,
it can happen that two stations might attempt to send a frame to the AP at the same
time. As the two frames will interfere with each other, the AP will not be able to receive
either of the frames correctly. This scenario is also known as the ‘hidden station prob-
lem’. To prevent such an overlap, a device can reserve the air interface as shown in
Figure 6.12 by sending a short RTS (Ready to Send) frame to the AP. The AP then
answers with a CTS (Clear to Send) frame and the air interface is reserved. While the
RTS frame might not be seen by all client devices in the network because of the large
distance between them, the CTS frame can be seen by all devices because the AP is the
central point of the network. Both RTS and CTS frames contain a so‐called Network
Allocation Vector (NAV) to inform other devices regarding the period of time during
which the air interface is reserved. If a device uses an RTS/CTS sequence before send-
ing, a frame can be configured in the driver settings dialog box of the network card.
However, RTS/CTS sequences slow down the throughput of a device. Therefore, this
mechanism should be used only if a very high network load is expected and the client
devices are dispersed over a wide area.
As in a wired network, there is no central instance controlling which device is allowed
to send a frame at a certain time. Every device has to decide on its own when it can send
a frame. To minimize the chance of a collision with frames of other devices, a coordina-
tion function is necessary. In WLAN networks, the DCF is used for this purpose.