Consider the scenario shown in the following figure, in which there are four wireless nodes, A$,$ B$,$ C$,$ and D$.$ The radio coverage of the four nodes is shown via the shaded ovals; all nodes share the same frequency. When A transmits, it can only be heard$/$received by B; when B transmits, both A and C can hear$/$receive from B; when C transmits, both B and D can hear$/$receive from C; when D transmits, only C can hear$/$receive from D$.$ Suppose now that each node has an infinite supply of messages that it wants to send to each of the other nodes. If a message$$s destination is not an immediate neighbor, then the message must be relayed. For example, if A wants to send to D$,$ a message from A must first be sent to B$,$ which then sends the message to C$,$ which then sends the message to D$.$ Time is slotted, with a message transmission time taking exactly one time slot, e$.$g$.,$ as in TDMA, but nodes can arbitrarily choose which time slot to send a message. During a slot, a node can do one of the follo