a) Layered protocol structures have been adopted to construct networked systems.
(i) Give one advantage and one disadvantage of using layered protocol structures.
(ii) In a layered protocol structure, both network services and network protocols are defined. Should the network services and the network protocols be coupled or decoupled? Explain your answer.

b) ALOHA, CSMA/CD, and CSMA/CA are mechanisms of random access control.
(i) Give the main change made from ALOHA to CSMA, and also the main change made from CSMA to CSMA/CD
(ii) CSMA/CA, instead of CSMA/CD was adopted by IEEE 802.11. Give two reasons why collision detection is not used by IEEE 802.11.

c) A network system, shown in Figure 1-1, consists of a network device X, a sending host A and a receiving host B. Suppose the transmission rates (bandwidth) between the host A and the network device X is R1 bps (bits per second), and the bandwidth between the network device X and the receiving host is R2 bps. Further suppose the distance of the link between the host A and the network device is D1 meters, and the distance of the link between the network device X and the host B is D2 meters, and the propagation speed for both links is S meters per second. Assuming that the network device X uses store-and-forward packet switching, what is the total end to end delay to send a packet of length L bits? (ignore queuing delay and
processing delay)