Lecture 1 Notes

mustardpruneΔίκτυα και Επικοινωνίες

23 Οκτ 2013 (πριν από 3 χρόνια και 5 μήνες)

61 εμφανίσεις

Lecture 1 Notes

Marking Scheme:

Tests: 2 @ 15% each

Assignments/Labs: 30%

Project: 10%

Final Examination: 30%

DCN230 is for all intents and purposes an introductory data communications course disguised under the
course title “Protocols: Standards and Mode
ls”. This title reflects the fact that virtually all data
courses introduce

concepts in relation to the two best known models

called the OSI
and the TCP/IP or Internet Model as it is often called.

Models are useful for a number of purposes,

including education and standardization. Standards are
required so that heterogeneous networks can communicate with one another. A layered approach to
the understanding of networks breaks down the vastly complex process of networking into manageable
. Note that the OSI layers describe the functionality of the various seven levels described, but not
the technologies that enable it. For example, if it is said that a network interface card is a Layer Two
device, we understand that it performs the Layer T
wo function of data linkage without any reference to
how this linkage is achieved technologically.

Why do we have two models? The OSI model was developed in 1970s by the ISO as the first systematic
attempt to describe networked communications. An open sys
tem is a set of protocols that allows any
two different systems to communicate regardless of their underlying architecture. We will define a
protocol as a rule, or set of rules that govern communication. For example, the protocol that governs

in this classroom involves the use of English, a certain volume or voice level, the custom
of raising one’s hand to ask or answer a question, etc.
The OSI Model is technically not a protocol, then.
It is a model for understanding and designing a network a
rchitecture that is flexible, robust, and

When the internet was launched it quickly became apparent that the protocol actually in use was that of
the TCP/IP model, which had existed prior to the OSI Model. The TCP/IP protocol suite is the

protocol family that governs internet communications. We might say, then, that the OSI model is a
theoretical reference model, while the TCP/IP or Internet Model is the practical implementation of the
layers described in the OSI model. We will e
xamine the OSI layers today and compare them with those
of the Internet Model, but after today, all references will be to the Internet Model.

Note that I will present the Internet Model as a four
layer model but that some accou
nts involve a five
layer mod
el. My account is
consistent with the Cisco curriculum that some of you may wish to pursue
certification in, so my account may prove to be more useful.

he benefits of using a layered model


Assists in protocol design

Fosters competition

Changes i
n one layer do not affect other layers

Provides a common language

The following figure demonstrates the parallels between the OSI and the Internet Model layers:

Note the numbering of the layers from highest to lowest

i.e. t
he “highest” layer or the t
opmost layer
has the highest number. Note the number and the ordering of the layers, because as the semester
progresses, we will often refer to layers by number rather than name.

Layer Functions

Note that the following list names, but does describe detai
ls of the function of each of the OSI layers.
See your text for detailed descriptions.

Physical Layer (1)

Physical characteristics of interfaces and medium

Representation of bits

Data rate

Synchronization of bits

Line configuration

( e.g. point
point or


Physical topology

Transmission mode

(simplex, half
duplex and full duplex)

Data Link layer (2)


Physical addressing


Flow control

(e.g. fall forward, fall back)

Error control


Access control


ed Layer 2 Protocol
PPP, ATM, Frame Relay

Network Layer (3)

Logical addressing


Associated Layer 3 Protocols: IPv4, IPv6, Novell IPX, AppleTalk, Connectionless Network Service (CLNS)

Transport Layer (4)

Process to process delivery

Source to dest
ination delivery

Service point addressing

Segmentation and reassembly

Connection control

Flow control

Error control

Associated Layer 4 Protocols: TCP, UDP

Session Layer (5)

Dialog control


Presentation Layer (6)




Application Layer (7)

Network virtual terminal

File transfer, access and management

Mail services

Directory services

Associated Layer 7 Protocols: DNS, HTTP, SMTP, POP, DHCP, FTP

Brief summary of TCP/IP Model Layer Functions (Note: inadequate

for test purposes)

Encapsulation and Protocol Data Units (PDUs)

The term encapsulation refers to the transformation of data as it makes its way down through the OSI
layers. Headers and trailers are added as the data moves from one layer

to the next. These are stripped
off at the receiving end as the data moves up through the layers until it is presented to the user in its
original format at the application layer. We will be using the network simulation software called Packet
Tracer this
semester. This software enables us to examine PDUs and their contents in a simulated
network environment and should help to bring life to these concepts.

Next Topic: Network Categories


A network serving a home, building or campus is c
onsidered a Local Area Network (LAN)

LANs separated by geographic distance are connected by a network known as a Wide Area Network

The internet is defined as a global mesh of interconnected networks

Metropolitan area network:

ks that serve an area of 1 to 30 miles, approximately the size of a
typical city
. (MAN)

Personal area network:

a network of a few meters, between wireless devices such as PDAs, laptops,
and similar devices


Final Topic: Standards Organizations

ernational Organization for Standardization (ISO)

This is a multinational body whose membership is drawn from the standards creation committees of
various governments throughout the world. The ISO is active in developing cooperation in the realms of
ific, technological and economic activity.

International Telecommunication Union

Telecommunication Standards Se
ctor (ITU

The United Nations formed a committee devoted to the research and establishment of standards for
telecommunications in general a
nd for phone and data systems in particular in response to a lack of
coordinated international effort until this time.

American National Standards Institute (ANSI)

This is a private, nonprofit corporation with no affiliation with the U.S. federal governme
nt. ANSI
activities are undertaken for the welfare and benefit of U.S. citizens

Institute of Electrical and Electronics Engineers (IEEE).

This is the largest professional engineering society in the world. It aims to advance theory

and product

quality I the fields of electrical engineering, electronics and radio

as well as in all related
branches of engineering. The IEEE oversees the development and adoption of international standards
for computing and communications

Electronic Industries Asso
ciation (EIA)

This is another non
profit organization devoted to the promotion of electronics manufacturing concerns.
Its activities include public awareness education and lobbying efforts in addition to standards
development. In the IT field, the EIA has
made significant contributions by defining physical connection
interfaces and electronic signaling specification for data communication.

these materials were extracted directly from Chapter 1 of your textbook