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

204 Use of Smart Antennas in Ad Hoc Networks
Figure 7.3. The effect of omni-directional / directional transmissions of control messages with
the 802.11 MAC Protocol
by C. C then updates its NAV and defers its transmission to D until the communication
between A and B is complete. However, clearly, C could have initiated
its communication to D without interfering with the communication between A
and B. This shows that the receipt of the RTS message by C did not necessarily
imply that C should not indulge in transmissions.
In the second scenario, we assume that the RTS messages are transmitted
directionally. Thus, A does not hear the RTS message sent out by node C while it
is in communication with node B. In the meantime, C has begun the transmission
of its data packet to node D. Once node A completes its communication with
node B, it initiates a new communication with node E. This causes a collision
at node D. Note that in this case, despite the fact that node A did not hear an
RTS message, its new handshake caused a collision.
7.3.3 Directional Medium Access Control with
Omni-Directional Receptions
In [16], Ramanathan considers two approaches to deal with this problem. The
first approach which is called the conservative approach precludes a node from
performing transmissions upon the receipt of any control message. The second
approach which they call the aggressive approach, allows a node to initiate new
transmissions in spite of hearing control messages sent by other nodes. RTS and
CTS messages are assumed to be transmitted and received omni-directionally.
The RTS and CTS messages are assumed to contain location information of
both the sender and receiver; this in turn helps transmit (or receive) the DATA
and ACK messages directionally. Neither of the two schemes overcome the