Quiz 1 Solution
COSC650 Computer Networks
Fall 2009
Name ______________________
1. Which of the OSI layers handles each of following:
1) Collecting a stream of bits of raw bits into a larger aggregate called a frame.
_____B_______
2) Unit of data exchanged among nodes is called a packet rather than a frame.
_____C_________
3) The lowest layer for dealing with a process-to-process channel and its unit is a
message. _______D________
4) Encoding data. (e.g., representing character strings using ASCII and Unicode)
_____F______
5) Related to the properties of the interface to a transmission medium. ____A_____
6) File transfer, electronic mail, terminal access to remote computers are resided.
______G________
7) Managing dialog control and tie together different types of data streams.
_______E_______
a) Physical layer
d) Transport layer
g)Application layer
b) Data Link layer
e) Session layer
c) Network layer
f) Presentation layer
2. How long does it take to transmit x KB over a y-Mbps link?
3. Suppose a 128-Kbps point-to-point link is set up between Earth and a rover on Mars.
The distance from Earth to Mars is approximately 55 Gm, and data travels over the link
at the speed of light (3 x 108 m/sec.)
a) Calculate the minimum RTT fro the link.
(55 x 10 ^9)/(3 x 10^8) = 184 secs
RTT = 2 x 184 = 368
b) Calculate the delay x bandwidth product for the link.
184 x 128 x 10^3 = 2.81 MB
c) A camera on the rover takes pictures of its surroundings and sends these to
Earth. How quickly after a picture is taken can it reach Mission Control on Earth?
(Assume that each image is 5 MB in size).
Transmission Delay for 5 MB = 41,943,040 / (128 x 10^ 3) = 328 secs.
Thus total time = 328 + 184 = 512 secs
4. Hosts A and B are each connected to a switch S via 10-Mbps. The propagation delay
on each link is 20µs. S is a store-and-forward device; it begins transmitting a received
packet 35 µs after it has finished receiving it. Calculate the total time required to
transmit 10,000 bits from A to B.
A
S
B
a) As a single packet.
b) Consider two 5,000-bit packets sent one right after the other.
At the given table of times, fill them up.
Time
T=0
T= 500
T= 520
T= 555
T= 1000
T= 1055
T = 1075
T = 1575
Event
start
A finishes sending packet 1, starts packet 2
Packet 1 finishes arriving at S
packet 1 departs for B
A finishes sending packet 2______
packet 2 departs for B
bit 1 of packet 2 arrives at B.
last bit of packet 2 arrives at B
5. Explain the effective throughput briefly.
The actual amount of data we can transmit for a total transfer time. Transferring a larger
amount of data will improve the effective throughput.
Infinitely large transfer size will make effective throughput to be network bandwidth.