CSCI 245
Computer Networks
Beloit College
Computer Science
Eyad Haj Said
hajsaide@beloit.edu
Meeting hours:
TR 8:00 AM-9:45 AM, Beloit College, Science Building – 116
,
Office Hours
Monday 1:00 pm – 2:30 pm,
Tuesday 12:00 pm -1:30 pm, or
Wednesday 2:00 pm – 3:30 pm, or
by appointment in person or Zoom
CSCI 245
Computer Networks
 Pre-requisite
• CSCI 175 Computer Architecture
 Textbook
• Data Communications and Networking with TCP/IP Protocol Suite, Forouzan, McGraw Hill.
 Reference book
• Computer Networks, Tanenbaum, 5th Edition, Pearson.
.
Course Objectives

Identify the network topologies and network types.

Understand the layered communication architectures (OSI and TCP/IP).

Understand the fundamentals of data transmission principles

Identify the characteristics of the various transmission media.

Understand the concepts of error detection techniques.

Understand the functionality of data link layer and services related to it.

Become familiar with Ethernet and IEEE standards

Identify the principles of routing and switching

Describe IP protocol specification and operation

Recognize transport layer services, protocols and performance.

Understand the client/server model and key application layer protocols

Recognize growth and control of the Internet and its social implications

Understand the basics of data and network security
Topics
 Introduction to
data communication and networking
 Network Models
 Physical
layer and data transmission
 Transmission media
 Data
link layer and error detection and correction.
 Wired
LAN and Ethernet.
 Network layer and
 Transport layer,
IP
UDP ,and TCP
 Application layer and standard
 Introduction
client-server protocols.
to data and network security.
Assessment Overview
Assessment Task
Weighting
Midterm Exam 3/7/2024
20%
Quizzes
25%
Homework Assignments
30%
Final Exam 5/7/2002 at 9 am
25%
Policies – Homework Assignments
 Four homework assignments
will be assigned throughout the course
period.
 Students
are responsible for submitting their homework by submitting all
related files.
 Some of
the homework assignments are based on teamwork homework
assignments. However, each student in the team should submit the work.
 Penalty
for late submission - 15% of the maximum mark specified for the
assessment will be deducted for each day.
 Submitting
the homework more than two days after the due date will not
be accepted, and the homework will be marked as zero.
Policies – Quizzes
 Quizzes will be
given throughout the semester.
 The
quizzes will be taken in the first 10 to 15 minutes or the last 10 to 15 minutes
of the class without any announcement.
 At least

one quiz will be taken in each week.
Understanding topics covered during the class and the previous lectures are
essential to obtain a high score in the quiz.
 No makeup
quizzes without any medical/emergency excuse.
Policies – Attendance

Attendance is very important and expected for all class meetings. If a conflict
arises with the scheduled class meeting time, please contact me before the class
meeting (or as soon afterwards as possible in the case of an emergency).
Students with Disabilities

A If you have a disability and need accommodations, contact Learning Enrichment
and Disability Services (LEADS) located on 2nd floor Pearsons (north side), 608363-2572, learning@beloit.edu or through olesena@beloit.edu . For
accommodations in my class, you are to bring me an Access Letter from the
Director of LEADS and then we will discuss how to implement the
accommodations. Contact that office promptly; accommodations are not
retroactive.
Policy on Inclusivity
 Inclusivity is
a demonstration of equity and social justice through awareness,
understanding, and respect for the differences in identity, culture, background,
experience, and socialization, and the ways in which these forms of difference
impact how we live and learn. Inclusivity requires equitable, institution-wide
representation and access to resources. In practice, this manifests itself by each
individual being aware of, committed to, and responsible for the well-being and
care of all students, staff, and faculty.
 The
land on which we gather is a sacred indigenous place located within the
ancestral territories of the sovereign Sac and Fox, Ho-Chunk, and Potawatomi
Nations, as recognized by treaties between those nations and the United States of
America. Recognizing and honoring Native occupancy is new at Beloit College, but
it is fitting in view of our commitment to inclusion and anti-racism
Getting to Know you

After I call your name can you tell everyone
•how long you have been at Beloit College or
•where you are from if transfer/exchange
•something interesting about yourself
•If you are visiting for the semester or don’t yet have access to the
Advanced CS lab then let me know so I can get that done
I
want us to be interactive • Ask questions or give thoughts at
any time.
Software
 Wireshark
 Packet Tracer
CSCI 245
Computer Networks
Chapter one
Introduction To Data
Communication and
Computer Networks
Slides are based on Data Communications and Networking book
Data Communication
 Data
communications are the exchange of data between two devices via some
form of transmission media.
 Data
communication system is combination of software and hardware and its
effectiveness depends on
• Delivery
• Accuracy
• Timeliness
• Jitter
Data Communication System components
Data Representation
 Text
 Numbers
 Images
 Audio
 Video
Data Flow
Networks
A
network is the interconnection of a set of devices capable of communication.
 Device can be
a host (end system)
• Large computer, desktop, laptop, workstation, cellular phone, or security system.
 Device can also
be a connecting device
• such as a router, a switch, a modem that changes the form of data, and so on.
 Devices are
connected via transmission media
• Such as air, cable
A
network must be able to meet a certain number of criteria.
• Performance:
– Transit time: Amount of time required for a message to travel from one device to another
– Response time: elapsed time between the inquiry and response.
• Reliability, and
• Security
Networks – Physical Structure – Types of Connection
 Link is
a communication pathway that transfers data from one device to another.
Networks – Physical Structure – Physical Topology
 Physical
topology refers to way in which a network is laid out physically.
• Geometric representation of the relationship of all the links and the linking devices to one
another.
– Linking devices are called nodes
Networks – Physical Structure – Physical Topology – Mesh Topology
Networks – Physical Structure – Physical Topology – Star Topology
Networks – Physical Structure – Physical Topology – Bus Topology
Tap is a connector
Drop line is a connection
running between the
device and the main cable
Long cable acts as backbone to
link all devices in a network
Networks – Physical Structure – Physical Topology – Ring Topology
Networks Types
 The
criteria of distinguishing one type of network from another is difficult and
sometimes confusing.
 Few criteria
such as size, geographical coverage, and ownership to make this
distinction
• Local Area Network (LAN)
• Wide Area Network (WAN)
• Internetwork
• Internet
Networks Types – LAN
 LAN
is usually privately owned and connects some hosts in a single office, building,
or campus.
• can be as simple as two PCs and a printer in someone’s home office, or
• it can extend throughout a company and include audio and video devices.
 Each
host in a LAN has an identifier, an address, that uniquely defines the host in
the LAN.

A packet sent by a host to another host carries both the source host’s and the
destination host’s addresses.
Networks Types – LAN
Networks Types – WAN
 WAN is
an interconnection of devices capable of communication.
• LAN is normally limited in size; a WAN has a wider geographical span, spanning a town, a
state, a country, or even the world.
• LAN interconnects hosts; a WAN interconnects connecting devices such as switches, routers,
or modems.
• LAN is normally privately owned by the organization that uses it; a WAN is normally created
and run by communication companies and leased by an organization that uses it.
Networks Types – Point to Point WAN
Networks Types – Switched WAN
Networks Types – internetwork made of two LANs and one WAN
Internetwork or internet (note the lowercase i) is two or more
networks that can communicate with each other.
Networks Types – heterogeneous network made of three LANs and four WANs
Networks Types – Internet
 The
most notable internet is
called the Internet (uppercase I ),
and is composed of thousands
of interconnected networks.
Networks Types – Internet
 Backbones
are large networks owned by companies such as Version and AT&T.
• Connected via complex switching systems (Peering Point)

Provider networks use the service of
the backbones for fee.
 Customer
networks are networks
at the edge of the Internet.
• Use the services provided
by Interent.
Protocol Layering
 A protocol defines
the rules that both the sender and receiver and all intermediate
devices need to follow to be able to communicate effectively.
• When communication is simple, simple protocol is needed
• When the communication is complex, we need a protocol at each layer, or protocol
layering.
Protocol Layering
 A protocol defines
the rules that both the sender and receiver and all intermediate
devices need to follow to be able to communicate effectively.
• When communication is simple, simple protocol is needed
• When the communication is complex, we need a protocol at each layer, or protocol
layering.
Postal carrier facility
Protocol Layering
 The
Protocol Layering is important. Why?
 Layer
can be considered as black box with inputs and outputs.
• Separate services from implementation.
• Some intermediate systems do not need to deal with all layers.
Postal carrier facility
Protocol Layering Principles
 bidirectional
communication, we need to make each layer so that it is able to
perform two opposite tasks, one in each direction.
 the
two objects under each layer at both sites should be identical.
 logical
connection between each layer
• layer-to-layer communication
TCP/IP PROTOCOL SUITE
 The
Transmission Control Protocol / Internet Protocol (TCP/IP) reference model is
a set of protocols that allow communication across multiple networks.
 It
is a hierarchical protocol made up of interactive modules.
 The
term hierarchical means that each upper level protocol is supported by the
services provided by one or more lower level protocols.
 TCP/IP is
thought of as a five-layer model
TCP/IP PROTOCOL SUITE
TCP/IP PROTOCOL SUITE – Layered Architecture
TCP/IP PROTOCOL SUITE – Layered Architecture
Logical connections
TCP/IP PROTOCOL SUITE – Layered Architecture
Identical objects (messages)
Identical objects (segment or user datagram)
Identical objects (datagram)
Identical objects (datagram)
Identical objects (frame)
Identical objects (frame)
Identical objects (bits)
Identical objects (bits)
TCP/IP PROTOCOL SUITE – Physical Layer
 Responsible
for carrying bits in a frame across the link.
 Transmission
medium carries the electrical or optical signals
• Bits coming from transport layers are transformed and sent through the transmission medium
• We consider the logical unit between two physical layers in two devices is a bit
TCP/IP PROTOCOL SUITE – Data Link Layer
 Data
link layer is responsible for taking the datagram and moving it across the
link
• The next link to travel is determined by router
• The datagram coming from the network layer is encapsulated into packets called frames
 When
the datagram travel from host to destination, several overlapping sets of
links are used
 Routers
 Many
are responsible for choosing the best route for each packet.
protocols can be used in data link layers in TCP/IP suit protocol
• Each protocol provide different services
– Error detection
– Complete error detection and correction
TCP/IP PROTOCOL SUITE – Network Layer
 The
network layer is responsible for creating a connection between the source
computer and the destination computer.

The communication at the network layer is host-to-host.
 The
network layer in the Internet includes the main protocol, Internet Protocol
(IP), that defines the format of the packet, called a datagram at the network
layer
TCP/IP PROTOCOL SUITE – Transport Layer
 The
connection between the two transport layers is end-to-end connection.
 The
message coming from the application layer encapsulated it in a transport
layer sends it, through the logical connection, to the transport layer at the
destination host.
• The packet called a segment or a user datagram in different protocols
 TCP
(Transmission Control Protocol)
• Connection-oriented protocol that first establishes a logical connection between transport
layers at two hosts before transferring data.
• It creates a logical pipe between two TCPs for transferring a stream of bytes.
• TCP provides flow control, error control, and congestion control to reduce the loss of
segments due to congestion in the network.
• Flow control: matching the sending data rate of the source host with the receiving data rate of
the destination host to prevent overwhelming the destination.
• Error control: to guarantee that the segments arrive at the destination without error and
resending the corrupted ones
TCP/IP PROTOCOL SUITE – Transport Layer

UDP (User Datagram Protocol)
• It is a connectionless protocol that transmits user datagrams without first creating a logical
connection.
• In UDP, each user datagram is an independent entity without being related to the previous or
the next one (the meaning of the term connectionless).
• Does not provide flow, error, or congestion control.
• Attractive to an application program that needs to send short messages and cannot afford the
retransmission of the packets involved in TCP, when a packet is corrupted or lost.
TCP/IP PROTOCOL SUITE – Application Layer
 The
connection between the two application layers is end-to-end connection.
 The
application layers is process-to-process communication.

To communicate, a process sends a request to the other process and receives a
response
 The
Hypertext Transfer Protocol (HTTP) is a vehicle for accessing the World
Wide Web (WWW).
 The
Simple Mail Transfer Protocol (SMTP) is the main protocol used in
electronic mail (e-mail) service.
 The
File Transfer Protocol (FTP) is used for transferring files from one host to
another.
Encapsulation and De-capsulation
Encapsulation

At the application layer, the data to be exchanged is referred to as a message.
 The
message is passed to the transport layer.
 The
result is the transport-layer packet, which is called the segment (in TCP)
and the user datagram (in UDP).
 The
transport layer then passes the packet to the network layer.
 The
network layer takes the transport-layer packet as data and adds its own
header contains the addresses of the source and destination hosts.
 The
packets are called a datagram passes the packet to the data-link layer.
 The
data-link layer takes the network-layer packet as data and adds its own
header, which contains the link-layer addresses of the host or the next hop (the
router).
 The
result is the link-layer packet, which is called a frame passed to the physical
layer for transmission.
Decapsulation at Destination Host
 After
the set of bits are delivered to the data-link layer, this layer decapsulates the
datagram from the frame and passes it to the network layer.
 The
network layer decapsulates the user datagram from the datagram and
passes it to the transport layer.
 The
transport layer decapsulates the message from the user datagram and passes
it to the application layer.
Encapsulation and Decapsulation at Router
 After
the set of bits are delivered to the data-link layer, this layer decapsulates the
datagram from the frame and passes it to the network layer.
 The
network layer only inspects the source and destination addresses in the
datagram header and to find the next hop to which the datagram is to be
delivered.
 The
data-link layer of the next link encapsulates the datagram in a frame and
passes it to the physical layer for transmission.
Example
 Link layer addresses may called
• Link address
• Physical address
• MAC address
N is IP address, DNS is
used to determine the
destination IP address
L is link address
Address Resolution
Protocol is used to
determine the destination
link address
3 links and 2 routers
Addressing
Addressing

Any communication that involves two parties needs two addresses: source address and destination
address.

We need four pairs of addresses, one pair per layer

The physical layer does not need addresses

Each address is related to a specific layer in the TCP/IP architecture.

At the application layer, we use
• names to determine the sites www.somesite.com
• Email address somemail@Beloit.edu

At the transport layer, addresses are called Port numbers, and these define the application-layer
programs at the source and destination.
• Port numbers are local addresses that distinguish between several programs running at the same time.

At the network-layer, IP address the addresses are global, with the whole Internet as the scope.
• A network-layer address uniquely defines the connection of a device to the Internet.

The link-layer addresses, sometimes called MAC addresses, are locally defined addresses, each of
which defines a specific host or router in a network (LAN or WAN).
Multiplexing and Demultiplexing

Multiplexing in this case means that a protocol at a layer can encapsulate a packet from several
next-higher layer protocols (one at a time)

Demultiplexing means that a protocol can decapsulate and deliver a packet to several next-higher
layer protocols (one at a time).

The TCP/IP protocol suite uses several protocols at some layers, we have multiplexing at the source
and demultiplexing at the destination.
OSI Model
OSI Model vs. TCP/IP
Scenario
Scenario