The RTS packet in the MACA scheme has a similar function as that of the packet preamble in
the RI-BTMA scheme. The RI-BTMA scheme does not have the CTS packet because it uses a
busy tone to notify the communication initiator. Since the CTS packet may suffer from packet
collisions, the notification from CTS packets is not as safe as that from the busy tone in the RI-
BTMA scheme. An example is the reception failure of CTS packet at some hidden nodes
because of transmissions from other nodes. These hidden nodes, without receiving any CTS
packet notification, may transmit new RTS packets when the CTS packet sender is receiving its
data packet. This leads to data packet collisions. It is clear that additional continuous
notification is necessary to protect data packets.
2.2 The
MACAW
scheme
Bharghavan [Bha94] suggested the use of the RTS-CTS-DS-DATA-ACK message exchange
for a data packet transmission in the MACAW protocol. Two new control packets were added
to the packet train: DS and ACK packets. When the transmitter receives the CTS packet from
its intended destination, it sends out a DS (Data Sending) packet before it transmits the data
packet. The DS packet notifies neighbor nodes of the fact that a RTS/CTS dialogue has been
successful and a data packet will be sent. The ACK packet was implemented for immediate
acknowledgment and the possibility of fast retransmission of collided data packets instead of
upper-layer retransmission.
A new back-off algorithm, the Multiple Increase and Linear Decrease (MILD) algorithm, was
also proposed in the paper to address the unfairness problem in accessing the shared channel.
In the MILD back-off algorithm, successful nodes decrease their back-off interval by one step
and unsuccessful nodes increase their back-off interval by multiplying them with 1.5. Back-off
interval is also put into the header of the transmitted packet, so that the nodes overhearing
successful packet transmission can copy the back-off interval on the packet into a local
variable and use it (back-off copy mechanism). Compared with the Binary Exponential Back-off
algorithm, the MILD algorithm has milder oscillation of the back-off intervals. Additional features
of the MILD algorithm, such as multiple back-off intervals for different destinations, further
improve the fairness performance of MACAW. The drawback of the MACAW scheme is
inherited from the MACA scheme: the RTS/CTS packet collisions in a network with hidden
terminals degrade its performance.
2.3 The
Floor Acquisition Multiple Access (FAMA)
schemes
In [Ful94], Fullmer and Garcia-Luna-Aceves proposed the Floor Acquisition Multiple Access
(FAMA) scheme. In FAMA, each ready node has to acquire the channel (the “floor'') before it
can use the channel to transmit its data packets. FAMA uses both carrier sensing and
RTS/CTS dialogue to ensure the acquisition of the "floor'' and the successful transmission of
the data packets. FAMA performs as well as MACA, when hidden terminals are present and as
well as CSMA otherwise. In [Ful97], FAMA was extended to FAMA-NPS (FAMA Non-persistent
Packet Sensing) and FAMA-NCS (FAMA Non-persistent Carrier Sensing). FAMA-NPS requires
nodes sensing packets to back off. FAMA-NCS uses carrier sensing to keep neighbor nodes
from transmitting while the channel is being used for data packet transmission. The length of
the CTS packet is longer than that of the RTS packet, maintaining the dominance of CTS
packets in the situation of collisions. Nodes can sense the carrier of the CTS packet when
there is a collision between an RTS and a CTS packet and keep quiet; hence the data packet
is protected at the receiver.