Faculty of Information Engineering & Technology

The Networks Department

Course: Communication Networks [NETW501] Dr. –Ing. Maggie Mashaly

Eng. Mariham Wasfy

Eng. Yomna Atef

Assignment Deadline Sunday 24/05/2020 at 11:59 PM

(Please submit a PDF file including your clear answers)

Name : ___________________________________________________________________

ID : ___________________________________________________________________

Group : ___________________________________________________________________

Part 1: Answer the Following Questions:

1. The performance of a client-server system is influenced by two network factors: the

bandwidth of the network (how many bits/sec it can transport) and the latency (how many

seconds it takes for the first bit to get from the client to the server). Give an example of a

network that exhibits high bandwidth and high latency. Then give an example of one with

low bandwidth and low latency.

2. A factor in the delay of a store-and-forward packet-switching system is how long it takes to

store and forward a packet through a switch. If switching time is 10 μsec, is this likely to be

a major factor in the response of a client-server system where the client is in New York and

the server is in California? Assume the propagation speed in copper and fiber to be 2/3 the

speed of light in vacuum.

3. If the unit exchanged at the data link level is called a frame and the unit exchanged at the

network level is called a packet, do frames encapsulate packets or do packets encapsulate

frames? Explain your answer.

4. A bit string, 0111101111101111110, needs to be transmitted at the data link layer. What is

the string actually transmitted after bit stuffing?

5. Consider the delay of pure ALOHA versus slotted ALOHA at low load. Which one is less?

Explain your answer.

6. Two networks each provide reliable connection-oriented service. One of them offers a reliable

byte stream and the other offers a reliable message stream. Are these identical? If so, why is

the distinction made? If not, give an example of how they differ.

7. An image is 1024 x 768 pixels with 3 bytes/pixel. Assume the image is uncompressed. How

long does it take to transmit it over a 56-kbps modem channel? Over a 1-Mbps cable modem?

Over a 10-Mbps Ethernet? Over 100-Mbps Ethernet?

8. Data link protocols almost always put the CRC in a trailer rather than in a header. Why?

9. Consider the interconnected LANs shown in figure below. Assume that hosts a and b are on

LAN 1, c is on LAN 2, and d is on LAN 8. Initially, hash tables in all bridges are empty and

the spanning tree shown in Figure (b) is used. Show how the hash tables of different bridges

change after each of the following events happen in sequence, first (a) then (b) and so on.

i. (a) a sends to d.

ii. (b) c sends to a.

iii. (c) d sends to c.

iv. (d) d moves to LAN 6.

v. (e) d sends to a.

10. One consequence of using a spanning tree to forward frames in an extended LAN is that some

bridges may not participate at all in forwarding frames. Identify three such bridges in the

previous question. Is there any reason for keeping these bridges, even though they are not used

for forwarding?

11. In some networks, the data link layer handles transmission errors by requesting damaged

frames to be retransmitted. If the probability of a frame’s being damaged is p, what is the mean

number of transmissions required to send a frame? Assume that acknowledgements are never

lost.

12. Suppose that x bits of user data are to be transmitted over a k-hop path in a packet-switched

network as a series of packets, each containing p data bits and h header bits, with x >>p + h.

The bit rate of the lines is b bps and the propagation delay is negligible. What value of p

minimizes the total delay?

13. Compare the delay in sending an x-bit message over a k-hop path in a circuit-switched network

and in a (lightly loaded) packet-switched network. The circuit setup time is s sec, the

propagation delay is d sec per hop, the packet size is p bits, and the data rate is b bps. Under

what conditions does the packet network have a lower delay?

14. Two CSMA/CD stations are each trying to transmit long (multiframe) files. After each frame

is sent, they contend for the channel, using the binary exponential backoff algorithm. What is

the probability that the contention ends on round k, and what is the mean number of rounds per

contention period?

15. Consider the subnet of figure below. Distance vector routing is used, and the following vectors

have just come in to router C: from B: (5, 0, 8, 12, 6, 2); from D: (16, 12, 6,0, 9, 10); and from

E: (7, 6, 3, 9, 0, 4). The measured delays to B, D, and E, are 6, 3,and 5, respectively. What is

C's new routing table? Give both the outgoing line to use and the expected delay.

A

time 0 t/2 t 3t/2 2t 5t/2 3t 7t/2 4t 9t/2 5t 11t/2 6t

B

C

D

Central Station

Part 2: Answer the Following Problems:

Topic 1: Static and Dynamic MAC a) Consider the network shown below where terminals A, B, C, and D are communicating over a bus

with a central station. The transmission time for all packets is t units.

A B C D

t t t

Central Station 1.5t

i. Provided that terminals A, B, C, and D start their transmissions at tA = 2.25t, tB = 0.5t, t c=

2t and tD = 0 respectively. Assuming that A, C implement 1- persistent CSMA/CD and B, D

implement Non-persistent CSMA/CD. Show the transmission and reception activity for the

four stations and the central station reception activity in the plot below until 6t. (Assume the

timeout is 3t and backoff interval is equal to 1.5t for all terminals. Assume a collision if overlap interval is

greater than zero).

A

time 0 t/2 t 3t/2 2t 5t/2 3t 7t/2 4t 9t/2 5t 11t/2 6t

B

C

H

ii. Indicate which of the four packets will be received correctly and which will suffer from

collision?

iii. What is the throughput of the network in the 6t period?

b) Consider the network shown below where terminals A, B, C, and H are communicating over a

bus. The distance between stations A and B is 2d meters. The distance between stations A

and H is d meters. The distance between stations H and C is 3d meters. The propagation

delay between stations A and C is t sec. The transmission time for all packets is t units.

Frames arrive at the four stations A, B, C and H at times tA = 0, tB = 3t/4, tC = 5t/2 and tH = 11t/4

respectively. If we assume that the stations A, C, H apply 1-persistent CSMA/CD while station

B applies Non-Persistent CSMA. Show the transmission activity for the four stations in the plot

below until 6t. (Assume that if collision is detected or occurs for terminals A & C the back-off window size is 2

slots while if collision is detected or occurs for terminal B & H the back-off window size is 1 slot).

A C B

H

d

2d

3d

c) The configuration below shows an extended LAN with nodes A, C, D, G. B1 is a bridge and H1 is

a hub. Frames arrive at the four stations A, C, D and G at times tA = 0.5t destined to D, tC = t

destined to D, tD = 1.75t destined to G, tG = 3.75t destined to A respectively. All Frames

require transmission time of t. If we assume that the stations A, C, D, G, B1 apply 1-Persistent

CSMA/CD. (Assume that if collision is detected or occurs for nodes A, C, D, G and bridge B1, the backoff

window size is greater than 6t. For a collision to occur two frames MUST overlap for a time that is GREATER

THAN ZERO).

Show the transmission activity for the four stations and B1 until 6t.

H1

B1

LAN 1 LAN 2

LAN 3 LAN 4

A

C

D G

t sec

t/2 sec 2T sec

t sec t/2 sec

t sec

t/2 sec 2t sec

t sec

A

time 0 t/2 t 3t/2 2t 5t/2 3t 7t/2 4t 9t/2 5t 11t/2 6t

C

D

G

B1

d) Assume round robin polling applied for 4 users A, B, C and D. Each user is allowed to transmit

only 2 frames per poll except for C which is allowed to send 3 frames per poll. Assume at t =

0 the polling order starts with station A followed by B, C, and D. If stations A, B, C and D need

to transmit 9, 7, 5 and 6 frames respectively. Assume frame size is 1500 BYTES. Assume the

time used to send a polling message is negligible as well as the propagation time. Calculate

the minimum transmission rate required for station B to be able to complete transmission of

all its frames before 312 ms?

Topic 2: ARQ Protocols a) In the figure shown below, frames are generated at node A and sent to node C through node B.

Determine the time required to transfer a message of size 11700 bytes from A to C based on the

following: • The data rate between A and B is 80 kbps

• The data rate between B and C is 100 kbps

• The propagation delay is 10 µsec/mile for both lines

• All data frames are 1000 bits long; ACK frames are separate frames of length 64 bits.

• Between A, B and C Stop-and-Wait ARQ is used.

• There are NO errors

2000 miles

A B C

1000 miles

Sender

Receiver

Time 0 1 2 3 4 5 6 7 8 9 10 11 12

b) Consider a link that uses GO Back 3 with a sending window size of WS = 3. Suppose that frames

use a time-out value of 3 units. Also assume that ACK transmission time and the processing

time are negligible. The figure below shows the operation of the ARQ protocol during the

sending of M frames.

i. What is the minimum number of bits that should be used for sequencing?

ii. Insert the appropriate messages in the white boxes in the below figure.

iii. Show the sending window state at time instants shown in the below table. (Note: use to represent frame sequences that are allowed to be sent. Use to represent outstanding

window frames. t = k+ means the instant right after t = k)

t=0+ t=1+ t=2+ t=3+ t=4+ t=5+ t=6+ t=7+ t=8+ t=9+ t=10+ t=11+ t=12+

Fr_0

Fr_1

Fr_2

Fr_3

iv. How many frames are successfully forwarded to the upper layer?

Fr0

c) Consider a link that uses Selective Repeat ARQ with a sending window size of WS = 4 and

receiving window size of WR = 2. Suppose that frames use a time-out value of 4 units. Also

assume that ACK transmission time and the processing time are negligible. The figure below

shows the operation of the ARQ protocol during the sending of M frames.

i. What is the minimum number of bits that should be used for sequencing?

ii. Insert the appropriate messages in the white boxes in the figure below.

iii. Show the sending window state at time instants shown in the table below. (Note: use to

represent frame sequences that are allowed to be sent. Use to represent outstanding frames. t = k+

means the instant right after t = k).

t=0+ t=1+ t=2+ t=3+ t=4+ t=5+ t=6+ t=7+ t=8+ t=9+ t=10+

Fr 0

Fr 1

Fr 2

Fr 3

iv. Show the receiving window state at time instants shown in the table below. (Note: use to

represent frame sequences that are allowed to be sent. Use to represent outstanding frames. t = k+

means the instant right after t = k).

t=2+ t=3+ t=4+ t=5+ t=6+ t=7+ t=8+ t=9+ t=10+ t=11+ t=12+

v. How many frames are forwarded successfully to the upper layer?

LAN 1

B2

B5

LAN 2

LAN 3

(1)

(2)

(3)

(2)

LAN 4

LAN 5

LAN 6

B4

B6

B1

B3

(1)

(2)

(1)

(1)

(2)

(2)

(1)

(3)

(1)

(2)

Topic 3: Switching and Routing a) Six stations (S1-S6) are connected to an extended LAN through transparent bridges (B1 and B2),

as shown in the figure below. Initially the forwarding tables are empty. Fill in the below tables

with appropriate entries, after the frames listed below have been completely transmitted.

S2 transmits to S1,

S5 transmits to S4,

S3 transmits to S5,

S1 transmits to S2,

S4 transmits to S3.

b) Generate the spanning tree for the network shown in the figure below. Assume the cost is the

number of LANs in the path to the root bridge. Draw the resultant spanning tree by marking

on the figure below, the root bridge and in the table below insert letter R to indicate a root

port, letter D to indicate a designated port or insert letter X otherwise.

B1 B2 B3 B4 B5 B6

Port (1)

Port (2)

Port (3)

B1 B2

Station Port Station Port

S3

S2 B1

S1

(1)

(2)

B2 (2)

(1)

(3)

S5 S6 S4

c) Given the packet switching network shown below. Use the Dijkstra routing algorithm to

compute shortest path from node “E” to all nodes of the network.

Step

(Iteration) N A B C D F

Initial {E}

1

2

3

4

5

d) The number shown next to each link of the network shown below is the probability of the

link working in correctly. It is assumed that links fail independently of each other. Apply the

Bellman ford algorithm to find the best path to destination “C” where for each any given

destination the best path used is the one which the probability that all the links will stay

intact is MAXIMAL.

Iteration A B D E F

Initial (-1,∞) (-1,∞) (-1,∞) (-1,∞) (-1,∞)

1

2

3

4

5

6

7

Best of luck.

Stay home, Stay safe.