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Average Delay - Network Design - Exam Paper, Exams of Computer Science

Aliah UniversityComputer Science

These are the Exam Paper of Network Design which includes Transmission Link, Mean Number, Arrival Rate, Mean Number, Mean Delay, Suitable Queuing, Queuing Model, Packets Experience, Wireless Access etc. Key important points are: Average Delay, College Lan, Mail Downloads, Transmission Time, Exponentially Distributed, Wireless Broadband, Broadband Subscriber, Subscriber Module, Transmission Buffer, Data Rate

Typology: Exams

2012/2013

Uploaded on 03/25/2013

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Cork Institute of Technology
Bachelor of Science (Honours) in Software Development and
Computer Networking – Award
(NFQ– Level 8)
Network Design
January 2007
(Time: 2 Hours)
Answer any four questions for full marks Examiners: Dr. J. Buckley
Dr. A. Kinsella
Dr Dirk Pesch
Dr Susan Rea
Q1.
(a) Consider a college LAN with a large number of PCs and a single e-mail server.
The server receives 200 requests for e-mail downloads per minute during normal
operation. Requests arrive according to a Poisson process. The server requires 0.4
seconds on average to process a request and the average transmission time on the
LAN is 0.25 seconds. Both processing time and transmission time are
exponentially distributed.
1. Calculate the total time each user has to wait until she receives her e-mails from
the server.
2. By what factor can the e-mail request rate be increased if during peak times,
users are willing to wait three times as long for a reply compared to the normal
case of part i) of the question?
[10 marks]
(b) N = 20 PCs are connected via a switch to a wireless broadband subscriber module
connecting an office LAN to a wireless internet service provider. Each computer
generates fixed size IP packets of 8192bits according to a Poisson process with rate
λ = 90/sec. Due to transmission impairments the data rate on the wireless link
varies over time, which results in effectively 5 data rates Ri, i = 1, …, 5, depending
on the error rate on the link. The rates are R1 = 512kbit/s for 10% of the time, R2 =
1024kbit/s, 15% of the time, R3 = 1536kbit/s, 30% of the time, R4 = 2048kbit/s,
25% of the time, and R5 = 2550kbit/s, 20% of the time.
NOTE: You can neglect the delay on the office LAN
1. Calculate the average delay data packets experience in the wireless subscriber
module before transmission.
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Download Average Delay - Network Design - Exam Paper and more Exams Computer Science in PDF only on Docsity!

Cork Institute of Technology

Bachelor of Science (Honours) in Software Development and

Computer Networking – Award

(NFQ– Level 8)

Network Design

January 2007

(Time: 2 Hours)

Answer any four questions for full marks Examiners: Dr. J. Buckley

Dr. A. Kinsella Dr Dirk Pesch Dr Susan Rea

Q1.

(a) Consider a college LAN with a large number of PCs and a single e-mail server. The server receives 200 requests for e-mail downloads per minute during normal operation. Requests arrive according to a Poisson process. The server requires 0. seconds on average to process a request and the average transmission time on the LAN is 0.25 seconds. Both processing time and transmission time are exponentially distributed.

  1. Calculate the total time each user has to wait until she receives her e-mails from the server.
  2. By what factor can the e-mail request rate be increased if during peak times, users are willing to wait three times as long for a reply compared to the normal case of part i) of the question? [10 marks]

(b) N = 20 PCs are connected via a switch to a wireless broadband subscriber module connecting an office LAN to a wireless internet service provider. Each computer generates fixed size IP packets of 8192bits according to a Poisson process with rate λ = 90/sec. Due to transmission impairments the data rate on the wireless link varies over time, which results in effectively 5 data rates Ri , i = 1, …, 5, depending on the error rate on the link. The rates are R 1 = 512kbit/s for 10% of the time, R 2 = 1024kbit/s, 15% of the time, R 3 = 1536kbit/s, 30% of the time, R 4 = 2048kbit/s, 25% of the time, and R 5 = 2550kbit/s, 20% of the time. NOTE: You can neglect the delay on the office LAN

  1. Calculate the average delay data packets experience in the wireless subscriber module before transmission.
  1. Calculate the memory size of the transmission buffer that the subscriber modules requires based on average demand so that no packets are lost.
  2. What single data rate would be required for the wireless link in order to maintain the same average delay if the packet size was exponentially distributed with mean 8192bits? [15 marks] [Total: 25 marks]
Q2.

(a) A small Private Automatic Branch Exchange (PABX) telephone system provides 40 internal telephone connections and has 6 external lines. Call generation rate per internal telephone connection is 2 per hour with 40% of all telephone calls having outside destinations. The average call holding time is 3 minutes.

  1. Calculate the external call blocking probability for the PABX.
  2. How many external lines (rounded to the nearest integer) would be required to maintain an external call blocking probability of less than 5%. [15 marks]

(b) The input to a router is a data packet stream modelled by a Poisson process with arrival rate λ = 50 packets/sec. The length of the data packets is exponentially distributed with mean L = 1024bytes.

  1. Assume that the router has m = 4 output lines of data rate R = 128kbit/s, which transmit packets from the head of the queue. Calculate the probability that arriving packets have to queue before transmission.
  2. Assume that the 4 output lines are aggregated into a single output line of data rate R = 512kb/s. Calculate the probability that an arriving packet has to queue for this case. What do you observe and discuss your observations in the context of efficient design of multiplexers and routers. [10 marks]
Q4.

(a) Using the Bellman-Ford’s algorithm, develop the least-cost routing table for source node s = 7 for the network of 8 nodes shown in Figure 2. The link costs are valid in both directions. In your answer also provide the least cost with each route between the source node and any other node. [10 marks]

3

3

1

6

4

7

4

8

2 5

1

5

3

1 4

2

2 2

Figure 2

(b) Using the Ford-Fulkerson algorithm, calculate the maximum flow f between source node s = 1, and destination node d = 4 for the network of 7 nodes shown in Figure 3. Link capacities and flow direction are shown with each link.

20

10 5

7

5

(^3 )

(^15 )

20

7

Figure 3 [15 marks]

Q5.

(a) Briefly describe the differences between stochastic and deterministic simulation and summarise the steps required to build a credible computer simulation model. [10 marks]

(b) Briefly describe the structure of a stochastic discrete-event computer simulation program for an M/M/m/m queuing system. You may use the functionality of a particular simulation system or programming language to help you in your description. [15 marks]