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	English	French
	An important part of the CSMA/CD algorithm is the exponential backoff. To illustrate the operation of this algorithm, let us consider a cable that has a length of one kilometer. The bandwidth of the network is set to 10 Mbps. Assume that when a collision occurs, host A always selects the highest possible random delay according to the exponential backoff algorithm while host B always selects the shortest one. In this network, the slot time is equal to the time required to transmit 100 bits. We further assume that a host can detect collision immediately (i.e. as soon as the other frame arrives).
	Assume now that `R1` is using round-robin scheduling instead of a FIFO buffer. One queue is used to store the packets sent by `A` and another for the packets sent by `B`. `A` sends one 1000 bits packet every second while `B` sends packets at 10 Mbps. What is the round-trip-time measured by each of these two hosts if each of the two queues of `R1` can store 5 packets ?
	Compute the max-min fair bandwidth allocation in the network below.
	Consider a CSMA/CD network that contains hosts that generate frames at a regular rate. When the transmission rate increases, the amount of collisions increases. For a given network load, measured in bits/sec, would the number of collisions be smaller, equal or larger with short frames than with long frames ?
	Consider again a network that uses CSMA/CD. This time, the bandwidth is set to 1 Gbps and the cable has a length of two kilometers. When a collision occurs, consider that the hosts B and C retransmit immediately while host A waits for the next slot.
	Consider the following scenario for the ALOHA medium access control algorithm. Three hosts are attached to a one-kilometer long cable and transmit 1000 bits frames at 1 Mbps. Each arrow represents a request to transmit a frame on the corresponding host. Each square represents 250 microseconds in the figure. Represent all the transmitted frames and list the frames that collide.
	Consider the network below. Compute the max-min fair allocation for the hosts in this network assuming that nodes `Sx` always send traffic towards node `Dx`. Furthermore, link `R1-R2` has a bandwidth of 10 Mbps while link `R2-R3` has a bandwidth of 20 Mbps.
	Consider the network depicted in the figure below.
	Consider the simple network depicted in the figure below.
	CSMA/CD does not use acknowledgments but instead assumes that each host can detect collisions by listening while transmitting. Consider a 2 kilometers long cable running at 10 Mbps. Show in the figure below the utilization of the communication channel and the collisions that would occur. For this exercise, do not attempt to retransmit the frames that have collided.
	Fairness and congestion control
	Hosts `A` and `B` use the simple congestion control scheme described in the book and router `R1` uses the DECBIT mechanism to mark packets as soon as its buffers contain one packet. Hosts `A` and `B` need to send five segments and start exactly at the same time. How long does each hosts needs to wait to receive the acknowledgment for its fifth segment ?
	How is the maximum round-trip-time influenced if the buffers of router `R1` store 10 packets ?
	If host `A` uses a window of two segments and needs to transmit five segments of data. How long does the entire transfer lasts ?
	If hosts `A` and `B` send to `D` 1000 bits segments and use a sending window of four segments, what is the maximum throughput that they can achieve ?
	If the transmitting host is located at another position on the shared medium than one of the edges, then the geometrical pattern that represents the transmission of a frame is slightly different. If the transmitting host is placed in the middle of the cable, then the signal is transmitted in both directions on the cable. The figure below shows the transmission of one 100 bits frame at 100 Mbps by host C on the same cable.
	In a deployed CSMA/CD network, would it be possible to increase or decrease the duration of the slotTime ? Justify your answer
	In ALOHA, the hosts rely on acknowledgments to detect whether their frame has been received correctly by the destination. Consider a network running at 100 Mbps where the host exchange 1000 bits frames and acknowledgments of 100 bits. Draw the frames sent by hosts A and B in the figure below. Assume that a square corresponds to 10 microseconds and that the cable has a length of 2 kilometers.
	In a shared medium, a collision may happen if two hosts transmit at almost the same time as shown in the example below.
	In general, the links have a non-zero delay. This is illustrated in the figure below where a delay has been added on the link between `R` and `C`.