The OSI Model &

TCP/IP model
Out Line
• Introduction OSI
• OSI History
• OSI Layers
• Introduction TCP/IP
• TCP/IP Layers
• Layering Considered Harmful?

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Introduction OSI

• The Open System Interconnection Reference

Model (OSI Reference Model or OSI Model) is a
description for layered communications and computer
network protocol & transmission design.
• It divides data movement into seven layers which, from
top to bottom, are the Application, Presentation,
Session, Transport, Network, Data Link, and Physical
Layers. It is therefore often referred to as the OSI Seven
Layer Model.

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OSI History

• In 1978, the International Standards Organization (ISO) began

to develop its OSI framework architecture.

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OSI History
• The concept of a 7 layer model was provided by the work of
Charles Bachman, then of Honeywell.
•  Various aspects of OSI design evolved from experiences with
the Advanced Research Projects Agency Network (ARPANET)
and the fledgling Internet.

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OSI Layers
OSI Model
Data unit Layer Function
7. Application Network process to application
Data representation, encryption
Data 6. Presentation
Host and decryption
layers 5. Session Interhost communication
End-to-end connections and
Segments 4. Transport
reliability, Flow control
Path determination and
Packet 3. Network
logical addressing
Media
Frame 2. Data Link Physical addressing
layers
Media, signal and binary
Bit 1. Physical
transmission
Going from layer 1 to 7: Please Do Not Throw Sausage Pizza Away 6
Going from layer 7 to 1: All People Seem To Need Data Processing
Layer1: Physical Layer
• The Physical Layer defines the electrical and physical
specifications for devices. In particular, it defines the
relationship between a device and a physical medium.
• This includes the layout of pin, voltages, cable specification,
hubs, repeaters,  network adapters, host bus adapters, and
more. 0’s and 1’s.

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Layer1: Physical Layer
• The major functions and services performed
by the Physical Layer are:
• Establishment and termination of a connection to
a communication medium.
• Singles being sent and received 0’s and 1’s.

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Layer 2: Data Link Layer
• The Data Link Layer provides the functional
and procedural means to transfer data
between network entities and to detect and
possibly correct errors that may occur in the
Physical Layer.
• MAC Address is used at this layer.
• Switches function at this layer.

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Layer 3: Network Layer
• The Network Layer provides the functional and procedural
means of transferring data from a source to a destination via
one or more networks.

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Layer 3: Network Layer
• The Network Layer performs
• network routing functions,
• Routers operate at this layer—sending data throughout the
extended network and making the Internet possible.

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Layer 4: Transport Layer
• The Transport Layer provides transparent transfer of data
between end users, providing reliable data transfer services to
the upper layers.
• The Transport Layer controls the reliability of a given link
through flow control, segmentation/desegmentation, and
error control.

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Layer 5: Session Layer
• The Session Layer controls the dialogues (connections)
between computers.
• It establishes, manages and terminates the connections
between the local and remote application.

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Layer 6: Presentation Layer
• The Presentation Layer establishes a context
between Application Layer entities, in which the
higher-layer entities can use different syntax and
semantics, as long as the presentation service
understands both and the mapping between
them.
• This layer provides independence from differences
in data representation (e.g., encryption) by
translating from application to network format,
and vice versa.
• This layer formats and encrypts data to be sent
across a network, providing freedom from
compatibility problems. 14

• It is sometimes called the syntax layer.

Layer 7: Application Layer
•  The application layer is the OSI layer closest to the end user,
which means that both the OSI application layer and the user
interact directly with the software application.
• Application layer functions typically include:
• identifying communication partners,
• determining resource availability,

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Layer 7: Application Layer
•  Some examples of application layer implementations include
• Hypertext Transfer Protocol (HTTP)
• File Transfer Protocol (FTP)
• Simple Mail Transfer Protocol (SMTP)

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Introduction TCP/IP
• The Internet Protocol Suite (commonly
known as TCP/IP) is the set
of communications protocols used for
the Internet and other similar networks.
• It is named from two of the most important
protocols in it:
• the Transmission Control Protocol (TCP) and
• the Internet Protocol (IP), which were the first
two networking protocols defined in this 17
standard.
TCP/IP Layers
OSI TCP/IP
Application Layer
Application Layer 　
Presentation Layer TELNET, FTP, SMTP, POP3, SNMP,
NNTP, DNS,NIS, NFS, HTTP, ...
Session Layer

 Transport Layer Transport Layer 　

TCP , UDP , ...

Network Layer  Internet Layer 　

 IP , ICMP, ARP, RARP, ...

Data Link Layer

Link Layer 
 FDDI, Ethernet, ISDN, X.25,...
Physical Layer

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TCP/IP Some Protocol
Layer Protocol
DNS, TFTP, TLS/SSL, FTP, Gopher, HTTP, IMAP, IRC, NNTP, 
POP3, SIP, SMTP, SMPP, SNMP, SSH, Telnet, Echo, RTP, 
PNRP, rlogin, ENRP
Application
Routing protocols like BGP and RIP which run over TCP/UDP,
may also be considered part of the Internet Layer.

Transport TCP, UDP, DCCP, SCTP, IL, RUDP, RSVP
IP (IPv4, IPv6), ICMP, IGMP, and ICMPv6

Internet OSPF for IPv4 was initially considered IP layer protocol since it

runs per IP-subnet, but has been placed on the Link since 
RFC 2740.
Link ARP, RARP, OSPF (IPv4/IPv6), IS-IS, NDP
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