Quiz 7 Key CNT 5106C 12 April 2011 1. Why is reliable data transfer easier at the link layer than it is at the transport layer? 2. a) Why is Slotted ALOHA more efficient than pure ALOHA? b) Why is CSMA more efficient than Slotted ALOHA? c) Why is Slotted ALOHA still used in some situations? d) How does CSMA/CD improve over pure CSMA? Explain. e) Why is pure CSMA still used in some situations? KEY 1. At the link layer, a frame sent by a sender must either be lost or received within a predictable amount of time (i.e., there are no routing options queuing delays), since the sender and receiver have a direct link. Therefore the sender can accurately determine a tight time-out value for acknowledgments, and the receiver does not have to deal with old frames suddenly appearing on the medium or even frames changing the order of arrival. 2. a) In S-ALOHA frames may only be sent within slots, so the period of vulnerability is half that of pure ALOHA.
This is the end of the preview.
access the rest of the document.