Figure 6.11(a) also shows a potential problem occurring in T-MAC. Assume that messages
flow from top to bottom, that is, node A sends only to node B, node B sends to node C,
etc. Every time node C wants to send a message to node D, it must contend for the medium
and may lose to either node B (which may transmit an RTS before C does) or to node A
(node C overhears a CTS transmitted by node B). While node C stays awake after overhearing
node B’s CTS message, its intended receiver (node D) is not aware of C’s intention to
transmit data and therefore returns to the sleep mode after TA has expired. This problem is
referred to as the early sleeping problem, and one possible solution to this problem is shown
in Figure 6.11(b). In the future-request-to-send technique, a node with pending data can
inform its intended receiver by transmitting a future-request-to-send (FRTS) packet immediately
after overhearing a CTS message. Node D, upon receiving the FRTS message, knows