Computer Networks Exam questions PDF

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COMPUTER NETWORKS (14ELC004) June 2015

2 Hours

Answer 3 out of 4 questions. Each question carries 20 marks. Any University approved calculator is permitted.

1.

Answer the following questions: a)

b)

When the local DNS server cannot resolve the IP address for a DNS enquiry, where will the local DNS server forward the enquiry to?

[2 marks]

If an application is based on UDP protocol, does the application provide reliable transmission?

[2 marks]

c)

How many “handshakes” are required to establish one TCP connection?

[2 marks]

d)

Which layer applies the “congestion control”?

[2 marks]

e)

Can the CSMA/CA protocol solve the “hidden station problem” in the wireless network?

[2 marks]

In the WiFi network applying the CSMA/CA protocol, when a mobile station receives a packet, it waits for a certain duration before it sends back the acknowledgement “ACK”. Why does the mobile station do not immediately send back the “ACK” after it receives the packet?

[2 marks]

The fibre network is often called high-speed network. Is it because the fibre cable has small transmission delay or small propagation delay?

[2 marks]

Which technique is used to achieve transparent transmission in the data link layer?

[2 marks]

f)

g)

h)

i)

Can error detection alone achieve reliable transmission?

[2 marks]

j)

Why is the “port” introduced in the transport layer?

[2 marks]

1 of 4

/continued

2.

This question contains 2 parts a)

The routing table of the router “A” is shown as following Destination network N1 N2 N3 N5

Distance to destination 4 2 3 5

Next hop B C F G

Suppose the router “A” receives the updating information from another router “C” as Destination network N1 N2 N3 N4

Distance to destination 2 3 2 7

Find the new routing table of “A”.

b)

[10 marks]

The routing table of a router is shown as following Destination network 128.96.39.0 128.96.39.128 192.4.153.128 192.4.153.0 128.96.40.0 128.96.40.10

Subnet mask

Next hop

255.255.255.128 255.255.255.128 255.255.255.192 255.255.255.192 255.255.255.128 255.255.255.128

Port m0 Port m1 R2 R3 R4 R5

Suppose the router receives a packet. The destination IP address of the received packet is “128.96.40.12”. Determine the next hop for this packet. [10 marks]

2 of 4

/continued

3.

In an Ethernet network, there are 3 stations which are always ready to transmit. The maximum delay to transmit one packet between any 2 stations is 1 second. The probability of transmission for every station during a contention slot is p=0.6.

Answer the following questions: a)

b)

In order to handle the collision, which protocol is applied in the Ethernet? What is the duration of one contention slot? What is the probability that, during one contention slot, the Ether (or the bus) is occupied for data transmission? Note, during a contention slot, if only one station transmits and the other 2 stations do not transmit, the Ether is occupied by the transmitting station. In all other cases, the collision occurs and the Ether cannot be used for data transmission.

c)

[4 marks]

What is the probability that the contention interval has j slots? Note that the contention interval has j slots implies that the Ether is not occupied during the first (j-1) contention slots and occupied at the jth slot.

d)

[4 marks]

[4 marks]

From c), calculate the average number of slots for the contention intervals. Further from the duration for one contention slot obtained in a), obtain the average duration of the contention intervals.

Note, for any 0...