Sheet #1
Question one: Host A wants to send an e-mail of 2.5 MB to host B through your provider
network
R6
E3
T1
OC-1
Host A
H1
S1
R1
OC-3
R2
ISDN
R3
E1
R4
H2
Host B
T3
R5
53
H1..: Hub, S1…: Switch, R1…: Router
Perform the following activities:
a. If the administrator of this network will choose a path that minimizes the sending time.
List the path and hence calculate this time (using the given table)
b. Show at each device using the following illustrative figures
• The OSI model
• The change of the PDU's at each layer
How each of the network devices encapsulates data as it travels through the OSI layers
representing these devices
Question two: Consider two hosts, A and B, connected by a single link of rate R bps. Suppose
that the two hosts are separated by m meters, and suppose the propagation speed along the link is
s meters/sec. Host A is to send a packet of size L bits to Host B.
a.
Express the propagation delay, dprop, in terms of m and s.
b. Determine the transmission time of the packet, dtrans, in terms of L and R.
c. Ignoring processing and queuing delays, obtain an expression for the end to end delay.
d. Suppose Host A begins to transmit the packet at time t = 0. At time t = dtrans, where is the last
bit of the packet?
e. Suppose dprop is greater than dtrans. At time t = dtrans, where is the first bit of the packet?
f. Suppose dprop is less than dtrans. At time t = dtrans, where is the first bit of the packet?
g. Suppose s = 2.5 · 108, L = 120 bits, and R = 56 kbps. Find the distance m so that dprop equals
dtrans.
Question three: Consider an application that transmits data at a steady rate (for example, the
sender generates an N-bit unit of data every k time units, where k is small and fixed). Also, when
such an application starts, it will continue running for a relatively long period of time. Answer
the following questions, briefly justifying your answer:
a. Would a packet-switched network or a circuit-switched network be more appropriate for this
application? Why?
b. Suppose that a packet-switched network is used and the only traffic in this network comes
from such applications as described above. Furthermore, assume that the sum of the application
data rates is less than the capacities of each and every link. Is some form of congestion control
needed? Why?
Question Four: An international company has 4 floors in a building. The following table
shows the number of employees in each floor:
Floor
No
1
2
3
4
Distribution of Employees
Present
Number
60
220
50
40
Future
Increase
5
40
10
5
You are asked to design a network to connect the PC's of these employees. Draw the schematic
diagram for the suggested network, taking care of:
1. Use 24 port edge switches to connect PC's
2. Use a backbone switch to connect more than one edge switch
3. Use a router (if needed) with one Ethernet interface
Perform the following activities:
a. Draw a schematic diagram of the network
b. Show the throughput at each link assuming 10/100 Mbps NIC for each user. Hence
choose the kind of the cable from well-known types
N.B: Use standard Cisco networking symbols in your drawing. You can also make use of the
given table of LAN Ethernet cabling standards.
Question Five: An international company has 4 floors in a building. The following table
shows the number of employees in each floor:
Floor
No
1
2
3
4
Distribution of Employees
Maximum inter-distance
Employees Number
in meters
60
175
123
500
35
300
14
600
You are asked to design a network to connect the PC's of these employees. Draw the schematic
diagram for the suggested network, taking care of:
1. Use 24 port edge switches to connect PC's
2. Use a backbone switch to connect more than one edge switch
3.
4.
5.
6.
Use a router (if needed) with one Ethernet interface
Others devices if needed
The maximum inter distance is the maximum distance between any 2 employees.
Also assume that you are using UTP and that you design for the maximum interdistance for each floor.
Perform the following activities:
a. Draw a schematic diagram of the network showing the kind of cables used
b. Show the throughput at each link assuming 100 Mbps NIC for each user. Hence choose
the kind of the cable from well-known types
N.B: Use standard Cisco networking symbols in your drawing. You can also make use of the
given table of LAN Ethernet cabling standards
Question Sex: Host A wants to send the following message through a connection to host B.
Elsalamo Alykom, my phone is 02/234666.
The message has a header of the following fields:
1. Source ID
3 bytes
2. Destination ID
3 bytes
3. Message sequence
1 bytes
If the hosts A and B have ID’s of 101 and 123, and also this is the first message
Perform the following activities:
a. Draw the message showing its contents.
b. What is the values of the 5th, 7th and 16th bytes, hence draw them.
Question Seven: In this problem, we consider sending real-time voice from Host A to Host B
over a packet switched network (VoIP). Host A converts analog voice to a digital 64 kbps bit
stream on the fly. Host A then groups the bits into 56-byte packets. There is one link between
Hosts A and B; its transmission rate is 2 Mbps and its propagation delay is 10 msec. As soon as
Host A gathers a packet, it sends it to Host B. As soon as Host B receives an entire packet, it
converts the packet’s bits to an analog signal. How much time elapses from the time a bit is
created (from the original analog signal at Host A) until the bit is decoded (as part of the analog
signal at Host B)?
Question Eight:
Suppose N packets arrive simultaneously to a link at which no packets are currently being
transmitted or queued. Each packet is of length L and the link has transmission rate R. What is
the average queuing delay for the N packets?