Image Processing Reference
In-Depth Information
The RTS/CTS protocol attacks the hidden terminal problem using only a single channel. Here
we describe the variant used in the IEEE . WLAN, there are other ones. Consider the case that
station A has a data packet for B. Ater A has obtained channel access it sends a short request-to-send
(RTS) packet to B. his packet includes the time duration needed to finish the whole packet exchange
sequence including the final acknowledgement. If B receives the RTS packet properly, it answers with
a clear-to-send (CTS) packet, again including the time needed to inish the packet exchange sequence.
Station A starts to transmit its data packet immediately after receiving the CTS packet. Any other
station C, hearing the RTS and/or the CTS packet, defers its transmissions for the indicated time,
this way not disturbing the ongoing packet transmission. It is a conservative choice to defer on any
of the RTS or CTS packets, and in fact the exposed terminal problem still exists. One solution could
be to let C defer only on reception of a CTS frame but to allow C a packet transmission if it hears
an RTS without corresponding CTS frame.
∗
he RTS/CTS protocol described here does not prevent
collisions of RTS packets, it has significant overhead and it is still susceptible to subtle variants of the
hidden terminal problem [].
A significant problem of wireless transceivers is their inability to transmit and receive simul-
taneously on the same frequency band. Hence, a fast collision detection procedure similar to the
CSMA/CD protocol of Ethernet is impossible to implement. Instead, collision detection has to resort
to other mechanisms like the busy tone approach described above (rarely used) or the use of MAC
layer acknowledgments (used frequently). Unfortunately, there are fieldbus systems relying on such a
feature, for example, the CAN fieldbus [] with its priority arbitration protocol. In this class of pro-
tocols each message is tagged with a priority value and this value is used to deterministically resolve
collisions. In the CAN protocol, all stations are tightly time synchronized and the priority field is
always at the start of a packet. All contending stations start packet transmission at the same time.
Each contender transmits its priority field bit-by-bit and reads back the signal from the medium.
If the medium state is the same as the transmitted bit, the station continues, otherwise the station
gives up and waits for the next contention cycle. his protocol requires not only the ability to simul-
taneously listen and receive on the same channel, but channel shall also produce meaningful values
from overlapping signals. Alternative implementations are sketched in Section ...
Even receiver-based collision detection may not work reliably due to the near-far effect. Consider
two stations A and B transmitting packets in parallel to a station C. For simplicity let us assume
that both stations use the same transmit power. Station A is very close to C, whereas station B is far
away but still in reach of C. Consequently, A's signal at C is much stronger than B's. In this case it
may happen that C successfully decodes a packet sent by A despite B's parallel transmission. This
situation is advantageous for the system throughput but disadvantageous for MAC protocols relying
on collision detection or collision resolution.
24.5.2 Methods for Combating Channel Errors and Channel Variation
A challenging problem for real-time transmission is the error-prone and time-varying channel. here
are many possible control knobs for improving the channel quality, for example, transmit power,
bitrate/modulation, coding scheme/redundancy scheme, packet length, choice of retransmission
scheme (automatic repeat request [ARQ]), postponing schemes and timing of (re-)transmissions,
diversity schemes [], and adaptation as a meta method []. In general, adaptation at the
transmitter requires feedback from the receiver. This feedback can be created by using immediate
acknowledgment packets after each data packet.
The variation of transmit power and of the bitrate/modulation scheme are both equivalent to
varying the energy per bit, which in turn influences the bit error rate [,]. Roughly, higher
∗
Clearly, if C receives a distorted CTS packet it should defer.
