Why are link-layer ACKs used in WiFi (802.11) networks? [Hint: check two of the boxes below].Group of answer choicesBecause of the hidden terminal problem, a node that is transmitting and hears no collisions still doesn’t know if there was a collision at the receiver.Hearing a receiver ACK, all other stations will stop transmitting.  This reduces collisions.The sender can used the differences in the signal strength in an ACK to infer whether the receiver is moving towards, or away from, the senderWireless links are noisier than wired links, and so bit level errors are more likely to occur, making link-layer error recovery more valuable that in less-noisy wired links.

7.3-1. Overlapping 802.11 wireless networks, with ACKs (d)Overlapping wireless senders with ACKs. Now suppose we are again in the wireless scenario, and that for every data message sent from source to destination, the destination will send an ACK message back to the source (e.g., as in TCP). Also suppose that each ACK message takes up one slot. What is the maximum rate at which data messages can be transferred from C to A, given that there are no other messages between any other source/destination pairs? [Note that successful ACKs do not count towards data message throughput].Group of answer choices1 message/slot.0.25 messages/slot (i.e., one message every four slots).0.5 messages/slot (i.e., 1 message every two slots).2 messages/slot.

The sender infers segment loss from the absence of an ACK from the receiver.

7.3-1. Overlapping 802.11 wireless networks, with ACKs (e)Overlapping wireless senders with ACKs (more). Suppose now that A sends messages to B, and D sends messages to C, and suppose that the destination will again send an ACK message back to the source (e.g., as in TCP) and that each ACK message takes up one slot. What is the combined maximum rate at which data messages can flow from A to B and from D to C?  [Hint:  this is a bit trickier than the previous questions].Group of answer choices0.66666 messages/slot (i.e., 2 messages every three slots)0.5 messages/slot (i.e., 1 message every two slots).0.25 messages/slot (i.e., one message every four slots).1 message/slot.

Which of the following statements about the characteristics of wireless links are true?Group of answer choicesThe bit error rate (BER) of a wireless channel decreases as the signal-to-noise ratio (SNR) increases.The "hidden terminal problem" happens when A sends to B over a wireless channel, and an observer, C (that can be even closer to A than B), does not detect/receive A's transmission because of physical obstacles in the path between A and B.Multipath propagation occurs when portions of the electromagnetic wave reflect off objects and the ground taking paths of different lengths between the sender and a receiver, and thus arriving at the receiver at slightly different points in time.The "hidden terminal problem" happens when A sends to B over a wireless channel, and an observer, C (that is further away from A than B), does not detect/receive A's transmission because the signal strength of A's transmission has faded significantly by the time it reaches C.The bit error rate (BER) of a wireless channel increases as the signal-to-noise ratio (SNR) increases.Path loss refers to the dropping of link-layer frames that are being relayed among wireless access points due to buffer overflow, just as network-layer datagrams are dropped at routers with full buffers.The "hidden terminal problem" happens when A sends to B over a wireless channel, and an observer, C (that can be even closer to A than B), does not detect/receive A's transmission because of physical obstacles in the path between A and C.Multipath propagation occurs when a sender sends multiple copies of a frame to a receiver, which is relayed over different by base stations or other wireless devices to the receiver.Path loss refers to link-layer frames that are corrupted due to the higher bit error rates in wireless channels.Path loss refers to the decrease in the strength of a radio signal as it propagates through space.The "hidden terminal problem" refers to the fact that many people can never seem to find their mobile phones.

7.3-1. Overlapping 802.11 wireless networks (a)Consider the scenario shown below 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. If a node hears two simultaneous transmissions at a time, the messages interfere at that receiver, even through they may not interfere at other receivers, where only one of the messages is heard.[Make sure you understand this paragraph.]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 slotted Aloha. During a slot, a node can do one of the following: (i) send a message (ii) receive a message (if exactly one message is being sent to it), (iii) remain silent. As always, if a node hears two or more simultaneous transmissions, a collision occurs and none of the transmitted messages are received successfully.You can assume here that there are no bit-level errors, and thus if exactly one message is heard at a receiver, it will be received correctly at that receiver.Suppose now that an omniscient controller (e.g., a controller that knows the state of every node in the network) can command each node to do whatever it (the omniscient controller) wishes, that is, to send a message, to receive a message, or to remain silent. Given this omniscient controller, what is the maximum rate at which messages can be transferred from C to A, given that there are no other messages between any other source/destination pairs?Group of answer choices0.5 messages/slot (i.e., 1 message every two slots).1 message/slot.2 messages/slot.0.25 messages/slot (i.e., one message every four slots).