Page 422 - From GMS to LTE

408 From GSM to LTE-Advanced Pro and 5G

Access
Terminal 1 Terminal 2
point
RTS

CTS CTS
(Once sent in all directions)

Data frame

ACK ACK
(Once sent in all directions)

Distance too large, devices cannot see each other‘s data frames.

Figure 6.12 Reservation of the air interface via RTS/CTS frames.

Distributing coordination to access the air interface is a completely different approach
to that taken by all other systems described in this book. The other systems use a central
logic that decides which user device is allowed to send at a certain time and for how
long. The advantage of the DCF, however, is the easy implementation in all devices. The
biggest disadvantage is the fact that no bandwidth can be reserved or guaranteed. This
is mainly a problem for real‐time applications like voice or video telephony if the net-
work is already highly loaded with other traffic. As voice and video telephony over IP
and over WLAN has become more and more popular, the IEEE has released the 802.11e
standard for devices and applications that require a constant bandwidth and a deter-
ministic medium access time. With this enhancement, devices can request a certain
Quality of Service (QoS) from the AP to get precedence over transmissions from other
devices. The enhancements also include a method to assign the air interface to a device
for a specific time and thus guarantee a certain bandwidth and a maximum medium
access time. 802.11e is backward compatible with the older 802.11b, g and a standards.
Older devices that do not support the new standard can still be used in such a network
without degrading the new QoS mechanism offered by the 802.11e standard.
Going back to the standard 802.11b DCF medium access scheme, DCF uses Carrier
Sense Multiple Access/Collision Avoidance (CSMA/CA) to detect if another device is
currently transmitting a frame. This method is quite similar to CSMA/Collision Detect
(CD), which is used in fixed‐line Ethernet, but it offers a number of additional function-
alities to avoid collisions.
If a device wants to send a data packet and no activity is detected on the air interface,
the packet can be sent without delay. If another device is already sending a data packet,
the device has to wait until the data transfer has finished. Afterward, the device has to
observe another delay time, the DIFS period, which has been described above. Then,
the device yet again defers sending its packet for additional backoff time, which is gen-
erated by a random number generator. Therefore, it becomes very unlikely that several