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\head{CSG250}{Wireless Networks}{Spring 2007}{12 January 2007}{3}

\begin{center}
\Large{\bf Medium Access Control Protocols}
\end{center}
\section{Introduction}
A defining feature of wireless networks is the shared nature of the
communication medium.  Consequently, an important component of any
wireless network is a medium access control protocol.  In this
lecture, we focus our attention on random-access protocols.

The key ingredients of random access MAC protocols for wireless
networks are the following:
\begin{itemize}
\item
{\em Carrier sense (CS)}: The hardware is capable of sensing whether
transmission taking place in vicinity.
\item
{\em Collision detection (CD)}: The hardware is capable of detecting
collisions. 
\item
{\em Collision avoidance (CA)}: This refers to the protocol for
avoiding collisions.
\item
{\em Acknowledgments}: When collision detection is not possible,
link-layer mechanism for identifying failed transmissions.
\item
{\em Backoff mechanism}: This refers to the method for estimating
contention and deferring transmissions.
\end{itemize}

\section{Aloha protocols}
One of the earliest MAC protocols is the Aloha protocol.  In this
protocol, each node attempts to transmit whenever it has a packet; if
the result is a collision, the node waits for a random period of time,
determined by a geometric distribution.  Aloha does not have carrier
sense, but assumes that collisions can be detected in some way.  Since
each node attempts a packet transmission as soon as a packet is
available, the throughput of the system is good when the load is low
but bad when the load is high.  This can be quantified as follows.

Let $G$ be the average number of packets that arrive during a packet
period $T$ (time taken to transmit a packet).  If we assume a Poisson
arrival process, then the probability that a given packet transmission
is successful is the probability that no other transmission is
initiated during a span $2T$ time units containing the start of this
packet transmission at the center.  The latter probability is given by
$e^{-2G}$ (using the Poisson arrival process definition).  Therefore,
the throughput is given by $Ge^{-2G}$.  This value decreases very
quickly once $G$ exceeds one, indicating that Aloha cannot handle
a high workload well.

In the slotted version of the Aloha, time is divided into discrete
slots, and packet transmissions are initiated at slot boundaries only.

\section{Carrier-Sense Multiple Access (CSMA) protocols}
CSMA protocols all employ carrier sense.  In every CSMA protocol, a
node waiting to send a packet senses another transmission waits at
least until the end of the current transmission to transmit its own
packet.  The different protocols differ in how the wait is implemented
and when the actual transmission is attempted.  In 1-persistent CSMA,
when a node has a packet, it waits until the medium is free, and then
transmits; if a collision occurs, the station waits a random amount of
time to retry.  In non-persistent CSMA, when a node has a packet, it
transmits if the medium is free; otherwise, it waits a random amount
of time to retry.  Finally, for $p$, $0 < p < 1$, we can define a
$p$-persistent CSMA slotted protocol as follows.  When a node has a
packet, it waits until the medium is free; then it transmits packet
with probability $p$; with probability $1-p$ it waits for next slot.

It is easy to see that non-persistent protocols (or $p$-persistent
protocols with low $p$) work well when the load (the term $G$) is high
since the random waiting times (or low transmission probabilities)
keep collision probability extremely low, and enable high throughput
since there are a number of packets waiting to be transmitted.  The
key drawback of nonpersistent protocols is the high average delay they
incur.

\section{Wireless MAC protocols}

\subsection{MACA}

\subsection{MACAW}