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OSI Reference Model
 Hierarchical Software Layers
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The ISO/OSI Reference Model
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Developed by the International Standards Organization
(ISO) to facilitate the international standardization of
communications protocols
For U.S.: ANSI (American National Standard Institute) www.ansi.org
OSI is ISO's Basic Reference Model for Open Systems
Interconnect (hence: ISO/OSI)
The Reference Model itself is not a Network Architecture
(does not specify any protocols or services)
ISO also developed Network Architecture standards
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The ISO/OSI Reference Model
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The model describes computer communications protocols
in a general sense to facilitate discussion
No assumptions are made regarding:
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Programming language bindings
Operating system bindings
Applications programming interfaces
Development of the model started in the mid-1970’s
Biggest Problems
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Very long time to complete the model and protocol standards
Very hard to understand the detailed standards
Difficult (expensive) to get the standards documents
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ISO/OSI 7-Layer Reference Model
7 Application
Application
6 Presentation
Presentation
5
Session
Session
4
Transport
Transport
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Network
Network
Network
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Data Link
Data Link
Data Link
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Physical
Physical
Physical
Physical Communications Media
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ISO/OSI Reference Model - Why 7 Layers?
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One layer for each level of abstraction
Each layer performs (ideally) a limited, well defined
function
Functions for each layer are selected with International
Standardization as a goal
Layer boundaries are chosen to minimize information
crossing the interface
Want to keep the model manageable (5 would have been
nice) but not have to jumble together distinct functions
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OSI Layer 1 - Physical layer
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Primary function is transmitting raw bits over a physical
communications channel
Primary design issues include: mechanical, electrical,
coding, physical characteristics
 How many pins in the connector
 what voltage represents a “1” versus a “0”
 etc.
By “raw bits” we mean there is no interpretation of the bits
- stream of bits in and bits out
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OSI Layer 2 - Data Link layer
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Primary function is to make Layer 1 into what appears to be a
channel free of undetected errors
Deals with data in chunks (typically 100s-1000s of bytes)
generally called Frames
This layer must create/recognize frame boundaries
 remember - Physical layer does not care
 often requires special bit patterns to signal boundaries
 may have to deal with possibility of pattern appearing in
data
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OSI Layer 2 - Data Link layer
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Among the key issues dealt with are:
 Error handling (e.g. corrupted frame)
 Flow control
 Providing various qualities of service
For Broadcast networks, a key issue is controlling access
to the channel:
 Use a sub-layer called the Media Access Control
(MAC) sub-layer
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OSI Layer 3 - Network layer
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Primary function is to control the operation of the layers
below
Among the key issues dealt with are:
 Routing packets from source to destination through the
network (or multiple networks) using static or dynamic
routing algorithms
 Controlling congestion in the network
 Accounting functions (for billing)
 Translating between protocols across heterogeneous
networks
 Concerned with Addressing
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OSI Layer 4 - Transport layer
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First end-to-end layer
Uses the network to (most often) provide higher layers
with a connection oriented, reliable, error free channel that
delivers messages (or byte stream) in order
May provide other types of services
Often performs multiplexing of multiple transport
connections over one or more network connections
Generally requires Address (or naming)
May also perform flow control
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OSI Layer 5 - Session layer
 Sort
of an unwanted layer, this layer is usually
very thin and little more than a pass through for
most protocols
 Manages dialog control (e.g. may manage who’s
turn it is to talk in a high-level half-duplex
protocol)
 Manages synchronization of transactions which
may need to be able to roll back in case of a crash
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OSI Layer 6 - Presentation layer
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than being concerned with moving
information, the Presentation layer is concerned
with the interpretation of information
representation
 Ensures that the syntax and meaning is the same
for each participant in a communication
 Provides for standard representation and may
provide capabilities for conversion of data
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OSI Layer 7 - Application layer
 The
layer where end-user applications live
 This is the highest level of abstraction and the
level which is of primary importance (for most
users)
 All the rest of the layers exist to support these
applications
 Layering exists so we can move these around to
different machines, and so they can communicate
across any platforms - Open Systems Interconnect
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Review - Functions of the OSI Layers
Layer
1 (physical): Transmission of bits
Layer 2 (data link): Transmission of
frames on one given link
Layer 3 (network): Routing of packets
through the network
Layer 4 (transport): End-to-end delivery
of messages
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Review - Functions of the OSI Layers
Layer
5 (session): Setup and
management of end-to-end
conversation, synchronization
Layer 6 (presentation): Formatting,
encryption, and compression of data
Layer 7 (application): user applications
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TCP/IP Protocol Suite
Advanced Research Project Agency
(ARPA) of DoD sponsored the development
of ARPANET in 1970s.
 TCP/IP has been adopted as the ARPANET
protocol suite
 TCP/IP became popular by the inclusion of
this protocol in BSD Unix system (a version
of Unix developed by University of
California @ Berkley)
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TCP/IP (cont.)
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Transport Layer-TCP (Transmission
Control Protocol)
• Provides fully reliable, connection-oriented
service
• Byte-stream transmission
 Network
Layer- IP (Internet Protocol)
• IP provides datagram service (used in packet
switching)
• It is connectionless unreliable service
• IP handles routing
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TCP/IP suite and OSI 7 Layer Model
Application
Presentation
Application
Process
Session
Transport
TCP
Network
IP
Data Link
Communication
Network
Most people define
TCP/IP as a 5 layer
protocol dividing the
Communication layer
into 2 separate layers
(Network & Physical)
Physical
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Network Structure
UDP: User Datagram Protocol:
ICMP: Internet Control
Message Protocol
IP: Internet Protocol
ARP: Address Resolution
Protocol
RARP: Reverse ARP
ICMP
user
processor
user
processor
TCP
UDP
IP
ARP
hardware
interface
OSI Layer 5-7
OSI Layer 4
RARP
OSI Layer 3
OSI Layer 1-2
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Port Number
user
user
user
user
user
user
user
proc.A proc.B proc.C proc.D proc.E proc.F proc.G
UDP
TCP/IP
protocol
suite
TCP
PEX
IP
Ethernet
interface
Ethernet cable 2
SPP
IDP
Ethernet
interface
Ethernet
interface
XNS
protocol
suite
Ethernet cable 1
Ethernet
interface
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Hierarchical Addressing Scheme
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UDP = User Datagram
Protocol
Connection defines the communication
link between two processes
16-bit UDP source port #
16-bit UDP dest. port #
protocol = UDP
internet 32-bit source addr
internet 32-bit dest. addr
frame type = IP
Ethernet 48-bit source addr
Ethernet 48-bit dest. addr
Ethernet
header
data
UDP
header
data
IP
header
UDP
header
data
IP
header
UDP
header
data
Ethernet
trailer
Ethernet frame
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