Computer Networks: Understanding Network Concepts and Protocols

This document explores fundamental concepts and protocols in computer networks. It delves into the layered architecture of TCP/IP, examining data encapsulation and decapsulation processes. The document also investigates the challenges of network loops and the role of Spanning Tree Protocol (STP) in preventing broadcast storms. Additionally, it examines the importance of Point-to-Point Protocol (PPP) in wide area networks (WANs) and its sub-protocols for data exchange. Finally, the document explores digital signal transmission in cellular telephony and the concept of collision domains in network topologies.

SIT202
Computer Networks
1

Contents
Question-1................................................................................................................................................3
Question 2................................................................................................................................................4
Question 3................................................................................................................................................6
Question 4................................................................................................................................................8
Question 5................................................................................................................................................9
Question 6..............................................................................................................................................11
2

Question-1
Consider two processes communicating over a TCP/IP network using the TCP protocol on an
Ethernet network. As data from a sending process moves through the protocol stack each layer will
encapsulate the payload and generate a Protocol Data Unit (PDU) which is then passed down to the
next lower layer. Prepare a diagram illustrating the layers, direction of data flow, peer layer
communication, and identify the PDU name and the structure of components (structure) of the PDU
and explain encapsulation steps for each layer.
At the receiving end, data is reconstructed form the digital signal received at the physical layer then
progresses up through the protocol stack to reach the process receiving the application layer protocol.
This whole process is known as de-capsulation. Explain the steps of de-capsulation at the receiving
end and show de-capsulation steps jointly with the encapsulation diagram of above.
1) In many networks, redundant links and devices are often added to ensure the network remains
available in the event of network failures. If one link stops working, the other link provides the
connection paths. If this approach was used without a loop avoidance scheme, however,
broadcasts would loop and flood endlessly in the network, potentially disabling the network.
In the network illustrated below, host F in LAN2 sends a frame to host A in LAN1. Explain
how the MAC address table in the switches would be updated and suffer thrashing, and why
the frame would be forwarded endlessly.
2) Recreate the necessary diagrams using the following diagram required for your explanation to
show the broadcasts. In your diagram show the MAC table updates for switch-1 and switch-2,
the location of frames in both LANs.
3

Question 2
An L2 loop happens inside the system when there are one or more than 1, Layer 2 path between the
endpoints of the system. A loop generates broadcast storms as broadcast and it multicasts, which are
carried forward by the switches out at every port, the switches will constantly regenerate the broadcast
message overflowing the network. Layer 2 header does not have TTL, if a frame is sent into a looped
kind of topology, it can loop forever. To resolve the issue we came up with STP protocol.
So when F send the frame it has no TTL value. So when Hub 2 receives it, it will flood it to Switch 1
and 2.
Switch 1 and 2 both are going to learn the MAC of F on respective port and flood on all other active
ports.
Switch 1 and 2 both are going to send it to Hub 1 which is further broadcasted to Switch 2 and Switch
1 respectively and all other active port.
Now Switch 1 and 2 both are learning MAC F on both end ports.
Similarly, same will be repeated by Hub and Switches and the L2 loop is formed and the switch will
become unstable.
4

Numbering over arrow shows traffic/Frame path travelling around the network in steps.
5

Question 3
When Alice is sending data 1st time to Bob it will prepare the Frame where it does not have MAC
address of Bob.
S-MAC:- L6, D-MAC:- ?,S-IP:- N6 , D-IP :- N5
When Alice send data to Bob it will perform AND operation between D-IP and Own Subnet and will
identify if it belongs to the same network.
As it belongs to a different network, it will try to resolve ARP for the Default gateway.
S-MAC:- L6, D-MAC :- FFFF:FFFF:FFFF, Sender IP:- N6,Target IP:- N2,Sender MAC:-
L6,Target MAC:- 0000:0000:0000
When R1 receives the Arp request, it will open L2 header and checks Target address. As its is of its
own, it will respond with its own MAC.
Now Frame is completed S-MAC:- L6, D-MAC :- L2,S-IP:- N6 , D-IP :- N5 and forwarded to R1.
When R1 receives Frame it will check D-Mac of its own and will open it and checks-IP if its is
available in Routing table. If not available, it is dropped. If available it is routed to R2.
Similarly AND operation is performed and ARP is resolved for R2 and packet formed is S-MAC:-
L3, D-MAC :- L4,S-IP:- N6 , D-IP :- N5.
6

Now routing table is checked and the packet is routed to R2.
R2 is going to open L2 header and checks L3 header and found destination IP is directly connected to
it.
As again, if this is first-time communication then ARP is resolved for Bob MAC address.
S-MAC:- L3, D-MAC :- L5,S-IP:- N1 , D-IP :- N5.
Bob is going to receive the frame and checks if destination MAC is of its own. If yes then opens the
L2 header and check destination IP. If it is of its own then it will encapsulate and accept data and send
back the response.
7

Question 4
1) Explain the importance of PPP with respect to wide area networks (WANs)? (1)
>> Point-to-Point Protocol connections are utilized for connecting LANs to service provider WANs
on which traffic can be sent in an encrypted form which increases the security.
Point-to-Point Protocol enables the real-time use of multiple Network layer protocol. Some of the
more familiar NPCs (network layer protocols) are:
1. Internet protocol control protocol (IPC)
2. AppleTalk Control Protocol (ACP)
3. Novell IPX Control Protocol (NIP)
4. Cisco Systems Control Protocol (CSC)
2) Explain how PPP enables users to exchange data in WAN by using sub-protocols such as LCP,
PAP, CHAP, NCP, IPCP.
Point-to-Point Protocol is the second Layer protocol of the WAN. It has 2 components:
1. LCP negotiates the connection. LCP is utilized in the start, arrange, and examine the data link
connections.
2. NCP encapsulates traffic. Network Control Protocol (NCP) is used for creating and organizing
different Network layer protocol.
3. Password Authentication Protocol directs everything in plain texts.
4. CHAP uses an MD5 hash to encrypt the data.
5. IPCP is used for arranging, allowing, and disabling the Internet Protocol at both ends of the
PPP link.
8

Question 5
1) Explain why modulation of a digital signal is required for transmission in digital cellular
telephony?
 Digital modulation techniques are used to enhance the communication quality.
 A digital signal, consist of 0s and 1s. As it is made of the square signal with a very small rise
time and fall time. To communicate such a signal, there is a need of channel with limitless
bandwidth, which does not exist. All microwave devices have finite bandwidth again and can't
handle these rectangle-type pules. So still if we want to signals we have to modulate it to send
it at right bit-rate.
 Variation in the strength, tone, or pitch of one's voice is done with “excellent voice
modulation" .
2) Briefly explain digital signal transmission on a bandpass channel.
 There are two different ways of transmitting of digital signal: baseband and broadband
transmission.
 In baseband pulse transmission (BPT), a data stream is represented in the form of a distinct
PAM (pulse-amplitude modulated) signal which is transferred over low-pass channels.
 In digital pass band transmission (DPT), the incoming stream of data is modulated on a carrier
wave with definite frequency and then it is transmitted via a band-pass channel.
 Baseband transmission is transmitting a digital signal on a channel without altering the digital
signals into analogue signals and it requires modulations of the signals.
Digital to analogue and analogue to digital conversion: digital-to-analogue conversion is done
when the digital data signal is converted into an analog signal. Whereas in analogue to analog
conversion, low pass analog signals are changed into band pass analog signal. The data transfer from
one computer to other computer requires conversion of data into analog signal. Analog signals are
then rectified to show digital signal.When it is received at receiving end it is decoded from analog to
digital at receiving end.
9

10

Question 6
Total 9 collision domain found marked in a circle with numbering.
Switches: Switch has a single collision domain for each interface. So if we have 3 links connected
then it has 3 collision domains.
Hub: It’s a L1 device which is 1 collision domain.
Bridges are similar to switches which also have 1 collision domain per interface.
Router: The router itself doesn't have a collision domain. A router only creates multiple broadcast
domains.
 Switch 1 = 2 collision Domain
11

 Switch2 = 3 Collision Domain
 Switch3 = 2 Collision Domain
 Hub1 = 1 Collision Domain
 Hub2= 1 Collision Domain
 Hub3= 1 Collision Domain
 Hub4= 1 Collision Domain
 Hub5= 1 Collision Domain
 Bridges= 3 Collision Domain
In respect to Topology 9 Collision Domains exist as marked in topology.
12