Assignment 1: CMPT 371 

Assignment 1: CMPT 371 
Please be aware that you must show your work for each problem to receive full points. At a
minimum you should provide an equation or verbal explanation expressing what you are doing.
 For example an equation might be
 velocity = distance / elapsed time = 40km/5hr = 8km/hr.
 For example a verbal explanation might be
 Velocity is calculated by dividing the distance by the time it took to travel the
 distance so 40km/5hr=8km/hr
LESS than half points will be given for providing the correct numerical answer only.
1) A system uses statistical time division multiplexing, there are 56 channels (56 different
signals) being combined into one multiplexed signal. Each channel carries frames with a
total length of 1200 bytes. Each frame has a header of 60 bytes. Each channel supports a
maximum bit rate of 9.6Mb per second (1KB = 1000 bytes, 1MB = 1000 KB, 1GB = 1000 MB,
1Kb = 1000 bits, 1Mb = 1000 Kb, 1Gb = 1000 Mb). Assume that the average frame rate
passing through each channel is 555 frames/s. Assume that the line carrying the
multiplexed signals can carry signals at a maximum bit rate of 550 Mb/s.
a) [3 points] What is the maximum number of frames per second supported by each
channel?
b) [3 points] What the average percentage of the time each channel is used?
c) [3 points] What fraction of the information being sent is overhead?
d) [3 points] What are the average bit rate and the average frame rate summed over all
signals?
e) [4 points] What is the average data rate summed over all signals? (data only)
f) [3 points] What percentage of the line’s capacity will be utilized?

2) Consider a transmission line with a transmission rate of 360 Mibps (1Mibps = 220 bits per
second).
a) [4 points] How many users would the line support? Make the following assumptions:
i. Each user has a 15 Mibps connection
ii. The line is shared using synchronous TDM
iii. Assume that 85% of the capacity of the line may be used for user connections.
b) [4 points] How many users can the line support? Why? Make the following assumptions
i. Each user has a 15 Mibps connection
ii. Each user’s connection is used an average of 34% of the time
iii. The line is shared using synchronous TDM
iv. Assume that 85% of the capacity of the line may be used for user connections.
c) [4 points] How many users can the line support? Make the following assumptions
i. Each user has a 15 Mibps connection
ii. Each user’s connection is used an average of 34% of the time
iii. The line is shared using statistical TDM
iv. Assume that 85% of the capacity of the line may be used for user connections.
d) [8 points] Suppose there are 66 users, using 10 Mibps connections at the same time. Each
user uses their connection 38% of the time. Find the probability that at any given time,
exactly 22 users are transmitting simultaneously. Derive an equation then calculate the
requested value (Hint: use the binomial distribution).

3) [20 points] Consider a local network with one router, that router is connected to the
internet through its first network card (interface). The router is connected to the local
network through a second interface. All hosts on the local network including the router
are on the same physical network segment. A proposal has been made to add a HTTP
proxy server to a host in the local network. All HTTP traffic will be processed by the
proxy server before being sent onto the Internet. HTTP queries would be sent to the
internet only if they could not be satisfied from the HTTP cache. The purpose of adding
the proxy server is to reduce traffic on the Internet connection.
 The local network supports a bit rate of 900 Mbs.
 The internet connection supports a bit rate of 250 Mbs
 The average traffic on the local network, excluding HTTP traffic, is 320 Mbs.
 The average traffic on the Internet connection, excluding HTTP traffic, is 85 Mbs.
 The average number of internet queries from the network is 135 queries per second
 Each query has an average size of 1.18 Mb.
 If a HTTP proxy server is added the number of queries satisfied by the cache will be
62%
Determine each of the following:
a) HTTP traffic intensity for the local network
b) HTTP traffic intensity for the internet without the proxy server.
c) The total traffic intensity for the local network without the proxy server.
d) The total traffic intensity for the internet connection without the proxy server
e) The total traffic intensity for the internet connection with the proxy server
Is a proxy server needed?
Why?
Will the addition of the proxy server solve any problems with traffic intensity your
identified? Why?
HINT: traffic intensity = traffic(Mbs/sec)/data rate(Mbs)


4) Suppose two hosts A and B. Both hosts are part of a packet switched network. A user has a 3
Gb (3*230 bits) file to transfer across the connection. Assume that each link in the packet
switched network has a capacity of 900MBps, the propagation delay on each link is 0.0001s,
the processing delay at each host is 0.003s, there are no queuing delays, and the packet is
transmitted by A and by 9 additional hosts as it travels through the packet switched network
to B.
a) [5 points] Consider sending the file through the packet switched network as a single
packet. Assume that each of the intermediate hosts are store and forward nodes. How
long does it take to send the file from A to B?
b) [6 points] Assume that the file is segmented into packets containing 12000 bits each.
Each packet has a header of 200 bits. If a packet is partially full of data the remainder of
the data field is filled with zeros before the resulting full size packet is transmitted. How
long does it take for the file to be transmitted through the network (assume no queuing
delays).
c) [10 points] What is the optimal packet size to transmit this file through this network? To
find the optimum express the equation as a function of the amount of data in each packet,
Ipacket. Then take a derivative with respect to Ipacket to get the general expression. Finally
evaluate the expression to find the optimal packet size for the

5) Consider the HTTP protocol. HTTP is the protocol the protocol used for sending the contents
of web pages between hosts, from a web server to an agent (client like Firefox or explorer).
HTTP is also used by agents to make requests to web servers for particular web pages. HTTP
communications, both requests and replies travel through TCP connections. The packets sent
back and forth between an agent requesting web pages and the web server sending the web
page were captured using the packet sniffer Wireshark. All the provided files provide a list of
packets that were transmitted when web pages were requested using the chrome browser
on a windows 10 machine.
You can download Wireshark and open the pcapng data files supplied with this problem.
These files contain a great deal of information about each packet. I will discuss some of the
things you can do with Wireshark in the video that will be supplied soon after this assignment
is posted. If you try capturing your own packets, please be aware that many license
agreements specify you will not capture packets from the application, and using Wireshark
may flag you to network admins of the networks you work on as a potential hacker. You do
not need to capture any packets to complete this problem. Please use the supplied Wireshark
files.
Summary data was captured and is provided for you in the files HTTP2020summary.pcapng
and HTTP2020conversation.pcapng. Consider only the HTTP packets in the summary file
(ignore the packets labeled TCP). The conversation file contains both the HTTP packets and
the TCP packets used to transmit the data sent by the server in response the HTTP requests.
Both files include both the initial request for one or more web pages and the responses to
those requests.

Based on the information in these files answer each of the following questions. In each case
explain how the contents of the files supports the answer you have given.
For parts a), c} support for your answer should include reference to a particular frame or
group of frames in the file HTTP2020summary.pcapng, pointing out the useful evidence
within the packet or packets.
 For parts d), e} support for your answer should include reference to a particular frame or
group of frames in the file HTTP2020conversation.pcapng, pointing out the useful evidence
within the packet or packets.
a. [6 points] What is the difference between a basic HTTP GET request and a conditional
HTTP get request? After answering this question provide the following information to
illustrate your answer:
i. Identify by frame number an example packet from the file
HTTP2020summary.pcapng for a conditional Get request. Show the line in the file
you have selected in your solution. (The line from the list of Wireshark packets)
ii. Identify by frame number an example packet from the file
HTTP2020summary.pcapng for a Get request that is not conditional. Show the
line from the file you have selected in your solution and if possible one for a Get
that is not conditional.
iii. Explain briefly how you found the packets and the evidence within the packets tell
you if the packet is executing a GET or a conditional GET? Show the evidence you
site in your answer
b. [4 points] What is different about the responses to a basic HTTP GET command and to a
conditional HTTP GET request? What information does each type of response return?
Would you expect a different response to the conditional get if the web page had been
modified between the two requests? No evidence from packets is required for this
c. [4 points] What was the IP address of the computer running the browser (the agent)?
What was the IP address of the web server it queried? Explain why the evidence you used
from the file HTTP2020summary.pcapng file to demonstrate the addresses you chose
were the addresses of the agent and the server?
d. [2 points] For one of the HTTP GET responses shown in file
HTTP20202conversation.pcapng how many packets were used to carry the HTTP GET
response from the server to the client? How did you determine your answer?
e. [4 points] Were persistent or non-persistent connections used to download the webpage
information from the server? Explain why you think so? Were persistent or nonpersistent connections used to download the webpage information from the server?