Data and Computers Exam One

1. Data and Computer Exam 1
Question 1
If computer B is connected to computer A and C by two point-to-point links, then it is possible to
send messages from A to C by sending them first from A to B, then from B to C. This is called
store-and-forward transmission. A store-and-forward transmission from A to C via B is more
efficient if the transmission from B to C can start before that from A to B is completed. Use this
concept to answer the following questions:
a. Suppose A wants to send a long message to C and that the transmission of that message on a
direct link takes one minute. How long would it take for the message to be fully transmitted
from A to C?
b. Now suppose that the long message in (a) is decomposed into 60 packets that take one second
each to be transmitted on a direct link. How long would it take for the message to be fully
transmitted (as packets) from A to C?
Answer for blank # 1: 2 minutes
Answer for blank # 2: 61 seconds
Question 2
A 5-story office building has the floor plan of the following figure for each floor. A local
network is to be installed that will allow attachment of a device from each office on each floor.
Attachment is to take place along the outside wall at the baseboard. Cable or wire can be run
vertically through the indicated closet and horizontally along the baseboards. The height of each
story is 8 feet. What is the minimum total length of cable or wire required for star topology?
Which floor’s closet should house the center of star? Please use the floor plan given at
homework 3 except the height of each story is 8 feet now.
a. Length on each floor ______ feet
b. Total length of cable _______ feet for the star topology?
c. Which floor’s closet should house the center of star? _____
Answer for blank # 1: 1330 feet
Answer for blank # 2: 7332 feet
Answer for blank # 3: 3
Question 3
Define the following parameters for a switching network:

2. N = number of hops between two given end systems
L = message length, in bits
B = data rate, in bits per second (bps), on all links
P = packet size
H = overhead (header) bits per packet
S = call setup time (circuit switching or virtual circuit), in seconds
D = propagation delay per hop, in seconds
For N = 5, L = 4800, B = 9600, P = 1024, H = 24, S = 0.1, D = 0.01, compute the end-to-end
delay for circuit switching? Assume that there are no acknowledgments
Answer:
Circuit Switching: = N X D + L/B = 5 X 0.01 + 4800/9600 =0.55 T=
C1 + C2 = 0.1 + 0.55= 0.65seconds
Question 4
Define the following parameters for a switching network:
N = number of hops between two given end systems
L = message length, in bits
B = data rate, in bits per second (bps), on all links
P = packet size
H = overhead (header) bits per packet
S = call setup time (circuit switching or virtual circuit), in seconds
D = propagation delay per hop, in seconds
For N = 5, L = 4800, B = 9600, P = 1024, H = 24, S = 0.1, D = 0.01, compute the end-to-end
delay for datagram packet switching. Assume that there are no acknowledgments.
Answer: 1.011 seconds
Question 5
The ...............layer is the layer closest to the transmission medium.
Physical
data link
network
transport
Question 6
As the data packet moves from the lower to the upper layers (OSI Model), headers are ………..
Added
Removed
Rearranged

3. Modified
Question 7
Mail services and directory services are available to network users through the ………… layer.
Data link
Session
Transport
Application
Question 8
Ten out of fourteen stations on a LAN accept the same transmission. The destination address is
probably a …………… address.
……………..refers to the way a network is laid out.
Topology
Access method
Addressing
Formatting
Question 10
The medium interface on a …………….topology is a passive device that is either internal or
external to the station.
Bus
Ring
Star
2 and 3
Unicast
Multicast
Broadcast
None of the above
Question 9

4. Question 11
In a ring topology data is transmitted in……………direction(s).
one
two
three
four
Question 12
Briefly comment on the following ethical question raised by communications networks: Should
unauthorized access to a computer account be considered illegal or does the responsibility lie
with the owner of the account?
It should be considered illegal for unauthorized access to a computer account.
Question 13
Light travels between point A and point B in 0.01 seconds. What is the distance between A and
B?
3 km
30 km
300 km
3,000 km

5. Question 14
A bit rate of 10 Kbps means that ..............bits are sent in 10 seconds.
100,000
20,000
10,000
1000
Question 15
Differential Manchester encoding has...........signal levels.
1
2
3
4