Ad Hoc Networks : Technologies and Protocols - University of ...

Medium Access Control with Directional Antennas 207
for the duration indicated in the message. The reason that this was deemed
appropriate was that the nodes simply performed omni-directional receptions.
However, if directional receptions were possible, one could potentially examine
directional channel availability. Thus, a node, upon the receipt of an RTS or
a CTS message, precludes transmissions only in those directions in which it
interferes with the communications related to the received RTS or CTS message.
The mechanisms proposed in [14], like in other work discussed earlier, are
extensions to the IEEE 802.11 MAC protocol to enable its use in the presence of
directional antennas. In the following paragraphs, we discuss these mechanisms
in brief.
First, for directional virtual carrier sensing to work, each node would need
to cache estimated angles of arrival (AOAs) from neighboring nodes whenever
it hears any signal from these nodes. This is done regardless of whether the
communication was intended for the node under discussion. The AOA is an indicator
of where the node is located. Thus, when a node wishes to communicate
with a particular neighbor, it uses the cached AOA information with regards
to that neighbor and sends a directional RTS message in the the direction of
the neighbor. Additional attempts are made if the node fails to receive a CTS
response from the neighbor. If the node does not get a CTS response after
4 directional RTS attempts, it resorts to omni-directional transmissions of the
RTS message. The cached AOA information is purged after the failure of the
directional RTS attempts. In order to comply with the IEEE 802.11 standard
where the total number of RTS attempts for a particular packet are limited to
seven, the node will make three omni-directional attempts before it drops the
packet and reports a link failure to the higher layers.
The authors in [14] assume that a steerable beam antenna is used. Upon the
receipt of an RTS message, the receiver is assumed to lock the receive beam
pattern for maximizing the received power. Similarly, the orignal transmitter
does the same upon the receipt of the CTS message from the receiver. The beam
patterns are then used both for transmission and reception. Upon the completion
of the communication (through the transmission of the ACK message) the beam
patterns are unlocked. The locking prevents the nodes from listening to other
transmissions for the duration of the communication.
Finally, each node that overhears a control message exchange, uses a directional
network allocation vector (DNAV) as opposed to the network allocation
vector (NAV) used in the original IEEE 802.11 MAC protocol. The
DNAV allows a node to specify each direction (the direction of the main lobe
of the communication as determined from the control message) and the angular
beamwidth in each particular direction that is to be avoided so as to remove
conflicts with ongoing communications. When a node wishes to initiate a new
transmission (or respond to a new request for communications) it checks its
DNAV to see if the particular requested direction is open and transmits in the