The OSI Model

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Chapter 2
The OSI Model
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Modern computer networks are designed in a
highly structured way. To reduce their design
complexity, most networks are organized as a
series of layers, each one built upon its
predecessor. You can use the concept of layers
to help you understand the action that occurs
during the flow of information from one computer
to another.
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The OSI Reference Model is based on a proposal
developed by the International Organization for
Standardization (ISO). The model is called the
OSI (Open Systems Interconnection) Reference
Model because it deals with connecting open
systems - that is, systems that are open for
communication with other systems.
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The OSI Reference Model has seven layers. The
principles that were applied to arrive at the
seven layers are as follows

• A layer should be created where a different level
  of abstraction is needed.
• Each layer should perform a well defined
  function.
• The function of each layer should be chosen with
  an eye toward defining internationally
  standardized protocols.
• The layer boundaries should be chosen to minimize
  the information flow across the interfaces.
• The number of layers should be large enough that
  distinct functions need not be thrown together in
  the same layer out of necessity, and small enough
  that the architecture does not become awkward or
  bulky.

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All communications on a network originate at a
source, then travel to a destination. The
information that travels on a network is referred
to as data, packet, or data packet. The source
address in a packet specifies the identity of the
computer (the host) that sends data and
information to other computers in a network. The
destination address specifies the identity of the
computer that finally receives the data and
information.
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During your study of networking, you will hear
references to the word "medium". (Note The
plural form of medium is media.) In networking, a
medium is a material through which data travel.

• telephone wires
• Category 5 UTP (used for 10Base-T Ethernet)
• coaxial cables (used for cable TV)
• optical fibers (thin glass fibers that carry
  light)

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A protocol is the set of rules that determines
the format and transmission of data.
Layer m on one computer communicates with layer m
on another computer. The rules and conventions
used in this communication are collectively known
as the layer m protocol."
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The OSI reference model is the primary model for
network communications. Although there are other
models in existence, most network vendors, today,
relate their products to the OSI reference model.

• It describes a method of how information or data
  moves from one computer to another computer.
• It is considered the best tool available for
  teaching people about sending and receiving data
  on a network.
• In the OSI reference model, there are seven
  numbered layers, each of which illustrates a
  particular network function.

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The Application Layer
Layer 7
The Presentation Layer
Layer 6
The Session Layer
Layer 5
OSI Model
The Transport Layer
Layer 4
The Network Layer
Layer 3
The Data Link Layer
Layer 2
The Physical Layer
Layer 1
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An easy way to help you memorize the names of the
layers might be to use a mnemonic device such as
ACRONYMS
ALL
AWAY
7
7
POINTLESS
PEOPLE
PIZZA
STUDENTS
SEEM
SAUSAGE
TEACH
TO
THROW
NOT
NEED
NOT
DO
DATA
DO
PLEASE
PROCESSING
PLEASE
1
1
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Each individual OSI layer has a predetermined set
of functions that it must perform in order for
communication to occur.
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Layer 7 The Application Layer

• The OSI layer that is closest to the user
• It provides network services to the users
  applications
• It differs from the other layers in that it does
  not provide services to any other OSI layer
• Examples of such application processes are
  spreadsheet programs, word processing programs,
  and banking terminal programs
• If you want to remember Layer 7 in as few words
  as possible, think of browsers.

The Application Layer
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Layer 6 The Presentation Layer

• It ensures that the information that the
  application layer of one system sends out is
  readable by the application layer of another
  system.
• If necessary, the presentation layer translates
  between multiple data representation formats
• It is concerned with data structures and
  negotiation data transfer syntax.
• If you want to think of Layer 6 in as few words
  as possible, think of encoding, representation of
  data, ASCII (data format).

The Application Layer
The Presentation Layer
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Layer 5 The Session Layer

• The session layer establishes, manages, and
  terminates sessions between two communicating
  host.
• The session layer provides its services to the
  presentation layer (it manages data exchange
  between presentation layer entities).
• If you want to remember Layer 5 in as few words
  as possible, think of dialogues and conversations.

The Application Layer
The Presentation Layer
The Session Layer
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Layer 4 The Transport Layer

• The transport layer segments data from the
  sending host's system and reassembles the data
  into a datastream on the receiving host's
  system.
• Issues such as how reliable transport over an
  internetwork is accomplished are the concern of
  the transport layer.
• The transport layer establishes, maintains, and
  properly terminates virtual circuits. In
  providing reliable service, transport error
  detection-and-recovery and information flow
  control are used.
• If you want to remember Layer 4 in as few words
  as possible, think of quality of service, and
  reliability.

The Application Layer
The Presentation Layer
The Session Layer
The Transport Layer
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Layer 3 The Network Layer

• The network layer is a complex layer that
  provides connectivity and path selection between
  two end systems that may be located on
  geographically diverse networks.
• The network layer is concerned with logical
  addressing (IP)
• If you want to remember Layer 3 in as few words
  as possible, think of path selection, switching,
  addressing, and routing.

The Application Layer
The Presentation Layer
The Session Layer
The Transport Layer
The Network Layer
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Layer 2 The Data Link Layer

• The data link layer provides reliable transit of
  data across a physical link.
• The data link layer is concerned with physical
  addressing (MAC), network topology, media access,
  error notification, ordered delivery of frames,
  and flow control.
• If you want to remember Layer 2 in as few words
  as possible, think of frames and media access
  control.

The Application Layer
The Presentation Layer
The Session Layer
The Transport Layer
The Network Layer
The Data Link Layer
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Layer 1 The Physical Layer

• The physical layer defines the electrical,
  mechanical, procedural, and functional
  specifications for activating, maintaining, and
  deactivating the physical link between end
  systems (data transmission across the network
  media).
• Characteristics such as voltage levels, timing of
  voltage changes, physical data rates, maximum
  transmission distances, physical connectors, and
  other similar attributes are defined by physical
  layer specifications (various types of networking
  media).
• If you want to remember Layer 1 in as few words
  as possible, think of signals and media.

The Application Layer
The Presentation Layer
The Session Layer
The Transport Layer
The Network Layer
Data Flow (Host) Layers
The Data Link Layer
The Physical Layer
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OSI Model - Function of each Layer
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Data Encapsulation

• Encapsulation allows computers to communicate
  data.
• If one computer (host A) wants to send data to
  another computer (host B), the data must first be
  packaged by a process called encapsulation.
• As the data moves down through the layers of the
  OSI model, it receives headers, footers, and
  other information.
• NOTE The word "header" means that address
  information has been added. It is control
  information placed before data when encapsulating
  that data for network transmission.

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Data Encapsulation
DATA
DATA
DATA
FRAME HEADER
FRAME TRAILER
NETWORK HEADER

• DATA - alphanumeric characters are converted to
  data that can traverse the internetwork.

• SEGMENT - Package the data for end-to-end
  transport. By using segments, the transport
  function ensures that the message hosts at both
  ends of the e-mail system can reliably
  communicate.

• PACKET - The data is put into a packet or
  datagram that contains a network header with
  source and destination logical addresses (IP).

• FRAME - The frame allows connection to the next
  directly connected network device on the link.
  The frame header contains information (e.g.
  physical addresses) required to complete the data
  link functions.

• BITS - The frame must be converted into a pattern
  of ones and zeros (bits) for transmission on the
  medium (usually a wire).

010111010110000101111011010101100101101110101111
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Data Encapsulation
Encapsulation wraps data with the necessary
protocol information before network transit.
Therefore, as the data packet moves down through
the layers of the OSI model, it receives headers,
trailers, and other information.
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Data Encapsulation
Different layers in the OSI model have different
groupings for the data. Each layer uses its own
layer protocol to communicate with its peer layer
in the other system. Each layers protocol
exchanges information, called protocol data units
(PDU).

• The PDUs for the lower layers are very commonly
  used
• the transport layer deals with segments
• segments are encapsulated into packets
• packets can be fragmented and are encapsulated in
  frames
• frames become a bit stream on the physical media.

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Data packets on a network originate at a source
and are then sent to a destination.

• Each layer depends on the service function of the
  OSI layer below it.
• The lower layer uses encapsulation to put the PDU
  from the upper layer into its data field then it
  adds whatever headers and trailers the layer
  needs to perform its function.

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The TCP / IP Model
The U.S. Department of Defense (DoD) created the
TCP/IP model because it wanted a network that
could survive any conditions, even a nuclear war.
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The TCP / IP Model
The TCP/IP model has four layers the application
layer, the transport layer, the Internet layer,
and the network layer.
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The TCP / IP Model

• Application Layer The TCP/IP combines all
  application-related issues into one layer, and
  assumes this data is properly packaged for the
  next layer. It includes session and presentation
  layer details.

• Transport Layer deals with the
  quality-of-service issues of reliability, flow
  control, and error correction.

• Internet Layer the purpose of the Internet
  layer is to send source packets from any network
  on the internetwork and have them arrive at the
  destination independent of the path and networks
  they took to get there.

• Network Access Layer It is the layer that is
  concerned with all of the issues that an IP
  packet requires to actually make a physical link,
  and then to make another physical link.

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All TCP/IP applications use IP for network
services.
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Comparing TCP / IP with OSI
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Comparing TCP / IP with OSI

• Similarities
• both have layers
• both have application layers, though they include
  very different services
• both have comparable transport and network layers
  
• packet-switched (not circuit-switched) technology
  is assumed
• networking professionals need to know both

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Comparing TCP / IP with OSI

• Differences
• TCP/IP combines the presentation and session
  layer issues into its application layer
• TCP/IP combines the OSI data link and physical
  layers into one layer
• TCP/IP appears simpler because it has fewer
  layers
• TCP/IP protocols are the standards around which
  the Internet developed, so the TCP/IP model gains
  credibility just because of its protocols. In
  contrast, no networks are built around specific
  OSI-related protocols, even though everyone uses
  the OSI model to guide their thinking.

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Many networking professionals have different
opinions on which model to use. You should become
familiar with both.

• Remember that there is a difference between a
  model (i.e. layers, interfaces, and protocol
  specifications) and an actual protocol that is
  used in networking.
• You will use the OSI model as the microscope
  through which to analyze networks, but you will
  also use the TCP/IP protocols throughout the
  curriculum.
• You will focus on TCP as an OSI Layer 4 protocol,
  IP as an OSI Layer 3 protocol, and Ethernet as a
  Layer 2 and Layer 1 technology.