Digital Signal Processing Reference
In-Depth Information
Fig. 5
Operation of unsynchronized low duty-cycle protocols
ification of CSMA, which reduces the congestion on a channel by deferring a
transmission for a random interval (contention window). The contention window
is increased if the channel is sensed to be busy (backoff), thus allowing the MAC
to adjust to the network conditions. Still, collisions may occur due to a hidden
node problem: nodes separated by two hops may not detect each other, and their
transmissions may collide on a receiver that is located between the nodes. The
hidden node collisions can be significantly reduced by performing a Request-
To-Send (RTS)/Clear-To-Send (CTS) handshaking prior to a data transmission.
Therefore, the handshaking is defined as an option in many CSMA/CA-based
protocols. While contention-based protocols work well under low traffic loads,
their performance and reliability degrades drastically under higher loads because
of collisions and retransmissions.
In contention-free protocols, nodes get unique time slots or frequency channels
for transmissions. Ideally, collisions are eliminated. Time Division Multiple Access
(TDMA) divides time into numerous slots, where only one node is allowed to
transmit on each slot. Other alternatives are Frequency Division Multiple Access
(FDMA) and Code Division Multiple Access (CDMA), which provide contention-
free operation by separate frequency channels and spreading codes, respectively.
Contention-free protocols achieve high performance and reliability regardless of
the traffic load. Yet, the bandwidth must be reserved in advance, which increases
control traffic overhead.
5.2
Unsynchronized Low Duty-Cycle MAC Protocols
Listening (LPL) mechanism, where nodes poll channel asynchronously to test for
that is longer than the channel-polling interval. Hence, the preamble part acts like a
wake up signal. If a busy channel is detected, nodes begin to listen to the channel
until a data packet is received or a time-out occurs.
