1. ECE 486/586: Computer Networks
Fall 2021
9 Problems, 11 pts each, you get 1 pt for free.
1. If a binary signal is transmitted through a 32kHz noiseless channel, what is the
maximum data rate? If the SNR of the channel is 30dB, what is the maximum data
rate?
2. Consider building a CSMA/CD network running at 1 Gbps over a 1-km cable with no
repeaters. The signal speed in the cable is 200,000 km/sec. What is the minimum
frame size?
3. Consider the subnet in the following. Distance vector routing is used, and the
following vectors have just come in to router C: from B: (5, 0, 8, 12, 6, 2); from D:
(16, 12, 6, 0, 9, 10); and from E: (7, 6, 3, 9, 0, 4). The measured delays to B, D, and E,
are 6, 3, and 5, respectively.
B C
A
E F
D
2
8
Figure 1. Illustration of the original subnet topology.
(a) What is C’s original routing table? Give both the outgoing line (next-hop table) to
use and the expected delay.
(b) What is C’s new routing table?
4. According to Fig. 1, determine the routing table for node F using Dijkistra algorithm.
Show the steps.
2. 5. In Transport Layer, symmetric connection release can be achieved by using a three-
way handshake, as shown in Fig. 2.
Figure 2. Three-way handshake protocol for connection release.
(a) Assuming a timer is NOT applied in the protocol, explain the two-army problem
in this protocol.
(b) Assuming a timer is used in the protocol, explain how the connection is released
if the ACK in the third handshake is lost.
6. In Fig. 3, the maximum flow between two adjacent routers are set as 1.
Router 1 Router 2 Router 3
Router 4 Router 5 Router 6
A
B
C
A
B
C
Figure 3. Flow control.
(a) Determine flow control using Max-Min fairness.
(b) Explain why your flow control is optimal. (Note: you need to illustrate 3 possible
scenarios, which are increasing the flow for A/B/C respectively.)
7. Congestion prevention policies applied to Data Link layer and Transport layer are
similar.
(a) List the congestion prevention policies in Data Link layer and Transport layer.
(b) Is it acceptable to adopt the policies used in Data Link layer and apply them to
the Transport layer? Why or why not?
3. 8. The address IPv4 is 32-bit long, which is not enough for all devices on earth.
(a) Explain how IPv4 addresses are assigned to support current devices. (Hint: your
device might share the same IPv4 address as your neighbor’s. Why and how?)
(b) Suppose you send a packet from your smart phone (e.g., IP: 192.168.0.1 and a
port number according to your router) to your neighbor’s smart phone (with the
same IP address according to his/her router). Explain in words how network
layer protocols determine (and translate) source and destination IP addresses.
9. IPv6 is slowly taking over.
(a) Draw the IPv6 fixed header.
(b) How does IPv6 solve the problem of IP address shortage.
(c) Why is there no different classes of network in IPv6?
