Image Processing Reference
In-Depth Information
DIFS
Random
back-off
DIFS
Random
back-off
Random
back-off
DIFS
Random
back-off
SIFS
SIFS
SIFS
DIFS
Data
ACK
AP
Data
Data
ACK
MS 1
Data
MS 2
ACK
MS 3
FIGURE .
Principle time behavior of IEEE . under the distributed coordination function; note that the
random back-off timer from units
not
transmitted next continues after the next DIFS with the remaining number
of slots. (From Matheus, K.
Industrial Information Technology Handbook
. Ed. R. Zurawski, CRC Press, Boca Raton, FL,
. With permission.)
.. Nevertheless, this specification allows QoS in a single network only. Restriction owing to
interference of coexisting networks cannot be compensated for by this QoS approach.
The IEEE . MAC concept includes an optional mechanism to solve the hidden terminal prob-
lem. Whether this “ready-to-send/clear-to-send” (RTS/CTS) packet exchange saves more bandwidth
(due to avoided retransmissions) than it adds (due to additional overhead) depends on the termi-
nal density and payload packet length [Bia]. As the RTS/CTS mechanism does not have a large
visibility,itwillbeassumedinthefollowingthattheRTS/CTSmechanismisnotused.
IEEE . has a significantly larger power consumption than Bluetooth. Note that this is not only
due to the higher transmit power ( dBm in Europe, dBm in the United States) but also due to
the CSMA concept. IEEE . units not specifically in sleep status have to listen to the channel
all
the time (and not like Bluetooth just at the beginning of the receive slot). Simultaneously though the
higher transmit power allows for a larger range of about m indoors (with dBm).
Originally, six different physical layer implementations has been specified. The IR mode, the FH
spread spectrum mode, and the Packet Binary Convolution Code mode have not found real market
acceptance though. hey will therefore be excluded from further discussions.
DSSS: The Direct Sequence Spread Spectrum mode is (like Bluetooth) used in the . GHz ISM-
band. The nominal bandwidth of the main lobe is MHz. The transmit power reduction in
theirstandresidualsidelobesissupposedtobeanddB,respectively(seealsoFigure
. for a measured spectrum). In principle / (United States/Europe) center frequencies
are available for the DSSS system. Nevertheless, using several systems in parallel requires a
spacing of / MHz (United States/Europe), which consequently only allows three systems
to be used simultaneously.
The DSSS mode comprises the original version (specified in IEEE .) and a high rate
extension(speciiedinIEEE.b).Intheoriginalmodethechippingofthebasebandsignal
is performed with MHz, employing an -chip pseudorandom code (Barker sequence). For
the Mbps modulation rate a one bit DBPSK symbol is spread with the Barker sequence, for
the Mbps modulation rate a two bit DQPSK symbol is spread with the same sequence.
∗
he
introduction of the high rate extension IEEE .b substantiated the market breakthrough of
IEEE .. For backward compatibility reasons, the PHY-header IEEE .b uses the same
and Mbps modulations as the plain DSSS mode. Note though that a shortened header of
∗
Note that in contrast to a typical CDMA system like UMTS,
all
users use the same spreading code.
