ICT/CSC1010 Computer Networks
2021-2022 Trimester 2
Tutorial 1:
Data Link layer/MAC Sublayer - Ethernet LAN
1. To understand why we need to have a minimum frame size tframe = 2 x tprop in a CSMA/CD
network, assume we have a bus network with only two stations, A and B, in which tframe = 40 µs
and tprop = 25 µs between A and B. Given that station A starts sending a frame at time t = 0.0 µs.
a. Does the protocol allow station B to start transmitting at time t = 23.0 µs? Why and what
will happen?
b. If frames collide, are station A and B able to detect collision? Why?
2. In a bus 1-persistence CSMA/CD with tprop = 12.5 µs and tframe = 120 µs, there are two stations, A
and B. Both stations start sending frames to each other at the same time. Since the frames
collide, each station follows Binary Exponential Backoff algorithm to retransmit. Station A
randomly generates time-slot R = 0 and station B generates time-slot R = 1.
a. Do the frames collide again during the first retranmission? Why?
b. Is station B able to retransmit at time-slot R = 1? Why?
3. Consider the Ethernet LAN in Lab 2b as shown. The MAC address tables of switches S1 and S2 are
cleared and are empty.
Given that PC-A sends a frame to PC-B, briefly describe address self-learning and frame
forwarding at switches S1 and S2, and show their updated MAC address tables.
S1:
S2:
Address
Interface
Address
Interface