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

210 Use of Smart Antennas in Ad Hoc Networks
node A’s signal now reaches node E and this would cause a collision at node E
(between the data from B and the DRTS from A).
The third problem that was identified in [6] was the problem of deafness. We
once again refer to Figure 7.5. Consider the case wherein node D is sending
data to node E via node B. The directional exchange of control messages might
not be heard by node C. During the time that B is transmitting the message from
D to E, node C might attempt to transmit a DRTS message to node B. However,
since node B has beamformed in the direction of E, it is unable to receive the
RTS. Hence, C does not receive a CTS response. In accordance to the IEEE
802.11 MAC protcol policy, node C would then back off. If node D were to
have a continuous stream of packets destined for node E, this problem might
repeat itself. Node C would continue to experience RTS failures and would
increase its back-off interval. This phenomenon, referred to as deafness, could
therefore cause false link failures (C believes that the link to B has failed even
if it has not) and unfairness in channel access.
Finally, due to the higher gain of directional antennas, the shape of the regions
where transmissions are blocked (referred to as silenced regions in [6]) are
different for omni-directional and directional communications. When both are
used, the silenced regions vary depending upon the traffic and the network topology.
The authors of [6] do not examine this in detail in the paper. Quantifying
the trade-offs while using hybrid directional/omni-directional communications
has still not been explored in detail.
7.3.6 The Multi-hop RTS MAC Protocol (MMAC)
Roy Choudhury et al attempt to to exploit the increased directional range via
the Multi-hop RTS MAC protocol (MMAC) in [6]. The basic problems with
hidden terminals and deafness still exist with the MMAC protocol. However,
the authors claim that the benefits due to the exploitation of the increased range
somewhat compensates for the other negative effects. To recap, if both the
sender and the receiver are beamforming (i.e, both directional transmissions and
directional receptions are invoked) the antenna gain can be potentially much
higher than in the case where they use directional transmissions but omnidirectional
receptions or vice versa. In Figure 7.6 if all the nodes were listening
omni-directionally, node A would be able to communicate (with a directional
transmission) with only nodes D and B. However, if node E were to be receiving
directionally, node A could communicate with node E.
The basic idea in MMAC is to route an RTS message via multiple hops to the
intended recipient asking the recipient to beamform in the direction of the originator
of the RTS message. The neighbors of a node are divided into two types:
(a) The Direction-Omni (DO) Neighbors are those neighbors of a node that can
receive transmissions from the node even if they are in the omni-directional