Q1 An application on a device, which is connected to the Internet via a 100Mbit/s
Ethernet connection, generates a 1,200-byte block of data. The service provided by
the transport layer is reliable. How long does it take to transmit the Ethernet frame?
Explain clearly any assumptions made.
Q2 Interleaving is used in a VoIP application to conceal burst loss. Consider that 24
packets (numbered 1 to 24) of speech are to be transmitted and the interleaving
depth used is 8. All the packets are 225 bytes and the transmit rate is 10 Mbit/s.
i) If transmission starts at t = 0 seconds, at what time is transmission of packet
number 4 complete?
ii) Supporting your answer using the information given, explain a disadvantage of
the interleaving process.
Q3 An application is running on top of UDP at the transport layer and IP at the network
layer.
i) What type of service do UDP and IP provide and what are the characteristics of
such a service?
ii) What is the disadvantage of using such a service at the network layer?
iii) What is the advantage of using such a service at the network layer?
iv) Giving reasons, explain what needs to be done at the data link layer and the
application layer to ensure that the sending application’s data is replicated at
the receiver.
v) Why is there a need for UDP if it provides the same service as IP?
Q4 A company is implementing VoIP to carry voice calls between sites. WAN
connections between sites will carry voice and data. G.711 CODECs are used ,
generating 8,000, 8-bit samples per second. The speech sample size is 20ms and
the additional overhead required per packet is 25%.
i) How many bytes are there per packet?
ii) What is the total bandwidth required for 50 concurrent calls?
iii) The network administrator decides to change to a G.728 codec which operates
at 16kbps and has the same sampling period as the G.711 codec. Explain one
advantage and one disadvantage of this change.
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Q5 Two nodes, A and B, are attached to opposite ends of a 2km cable. Node A has one
frame of 9,744 bits (including all header and trailer bits) to send to node B. The
signal propagation speed on the cable is 0.5x10-8 s/m and the transmission rate is
100 Mbps. There are five routers between A and B, each inserting a 20-bit delay. If A
starts transmitting the frame at time t = 0s, and assuming the same link layer
protocol operates across all links, at what time does the last bit of A’s frame arrive at
B?
Q7. Using a frame sequence diagram, give an example of an exchange of frames
between X and Y that involves the three different HDLC frame types. Give a detailed
description of what is happening in the exchange. Include, where appropriate, frame
sequence numbers in the form [N(S), N(R)].
Q8 Packets of four different data classes D1, D2, D3 and D4 are queued for
transmission at a router. All packets are 500 bytes in size and the transmit rate is
10Mbps. At t = 0 seconds all queues are full, each containing 10 packets.
i) Assuming priority queueing, where D1 is the highest priority and D4 the
lowest priority, at what times will transmission of the last D1 and D4 packets
be complete?
ii) Assuming round-robin queueing, with the same assigned priorities as in (i), at
what times will transmission of the last D1 and D4 packets be complete?
iii) If weighted fair queueing is used and the weights assigned to D1, D2, D3 and
D4 are 0.2, 0.3, 0.1 and 0.4 respectively, at what times will transmission of
the last D1 and D4 packets be complete?
Q9 Two stations are communicating across a data link using the HDLC protocol and
both stations are busy generating frames. The data rate is 400Mbit/s, the end-to-end
delay is 20μs and the fixed frame size is 250 bytes of which 8 are overhead. A
window size of 4 is used and the ARQ mechanism is go-back-N. The protocol is set
up in such a way that for information frames all acknowledgements are piggybacked.
Assume that there are no frame errors.
i) Calculate the efficiency of the given data link as described above.
ii) What impact will increasing the window size to 12 have on the efficiency of
the link?
Q10 Using a diagram to help illustrate your answer, compare the role and scope of
operation of layer 2 source/destination addresses with that of layer 3 and layer 4
source/destination addresses in the exchange of messages between two end-points
in the Internet.
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