Computer Networks - part V

prunelimitNetworking and Communications

Oct 23, 2013 (4 years and 20 days ago)

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Computer Networks
-

part V

THE OSI MODEL

LAYERED TASKS

Figure 1
-

Tasks involved in sending a letter


There are 3 different activities at the sender site and 3 at the receiver site.


Must be done in the order of the layers.


Each layer at the sending site uses the services of the layer right below it.

THE OSI MODEL

1.
Established

in

1947
,

the

International

Standards

Organization

(ISO)

is

a

multinational

body

dedicated

to

worldwide

agreement

on

international

standards
.


2.
An

ISO

standard

that

covers

all

aspects

of

network

communications

is

the

Open

Systems

Interconnection

(OSI)

model
.

It

was

first

introduced

in

the

late

1970
s
.


3.
ISO

is

the

organization
.

OSI

is

the

model
.

4.
Topics

covered
:

1.
Layered

Architecture

2.
Peer
-
to
-
Peer

Processes

3.
Encapsulation


It is now considered the primary Architectural model for
inter
-
computer communications.


The Open Systems Interconnection (OSI) reference model is
a descriptive network scheme. It ensures greater
compatibility and interoperability between various types of
network technologies.


It describes how information or data makes its way from
application programmes through a network medium to
another application programme located on another
network.


The OSI reference model divides the problem of moving
information between computers over a network medium
into SEVEN smaller and more manageable problems.

THE OSI MODEL

Figure 2
-

Seven layers of the OSI model

THE OSI MODEL


Why use a layered approach ?


Data communications requires complex procedures


Sender identifies data path/receiver


Systems negotiate preparedness


Applications negotiate preparedness


Translation of file formats


For all tasks to occur, a high level of cooperation is
required


Provide framework to implement multiple specific
protocols per layer

THE OSI MODEL


Advantages of Layering


Easier application development


Network can change without all programs being
modified


Breaks complex tasks into subtasks


Each layer handles a specific subset of tasks


Communication occurs


between different layers on the same node or stack
(INTERFACES)


vertical communications


between similar layers on different nodes or stacks
(PEER
-
TO
-
PEER PROCESSES)


horizontal communications


The OSI Reference Model is composed of seven layers, each
specifying particular network functions.


The process of breaking up the functions or tasks of
networking into layers reduces complexity.


Each layer provides a service to the layer above it in the
protocol specification.


The lower 4 layers (Layers 4, 3, 2, and 1) are concerned with
the flow of data from end to end through the network.


The upper three layers of the OSI model (Layers 7, 6 and 5)
are orientated more toward services to the applications.


Data is
Encapsulated with the necessary protocol information
as it moves down the layers before network transit.

THE OSI MODEL


The seven layers of the OSI
model ca be divided into
two categories:


Upper layers (Application
set


3 layers)


Lower layers (Transport set


4 layers)

THE OSI MODEL


Network support layers : Layers 1, 2, 3


Concerned with flow of data from end to end through
Network


Combination of HW & SW


Physical layer always implemented in Hardware


User support layer : Layer 5, 6, 7


Always implemented in Software.



It allows interoperability among unrelated software systems


Transport layer (Layer 4) : links the two subgroups


Layers glued together by interfaces


Each interface defines what info & services it must provide
for the above layer

THE OSI MODEL

(
A
)
ll

7. (
A
)
pplication

(
A
)way

(
P
)
eople

6. (
P
)
resentation

(
P
)
izza

(
S
)
eem

5. (
S
)
ession

(
S
)
ausage

(
T
)o

4. (
T
)
ransport

(
T
)
hrow

(
N
)
eed

3. (
N
)
etwork

(
N
)
ot

(
D
)
ata

2. (
D
)
ata

Link

(
D
)o

(
P
)
rocessing

1. (
P
)
hysical

(
P
)lease

Mnemonics

Figure 3
-

The interaction between layers in the OSI model

Network
support
layers

User
support
layers


Data exists at each layer contained within a unit called a
Protocol Data Unit (PDU).


Data Encapsulation is the process of adding a header to
wrap

the data that flows down the OSI model.



The 5 Steps of Data Encapsulation are:


1
.

The Application, Presentation and Session layers create
DATA from users' input.


2. The Transport layer converts the DATA to SEGMENTS


3. The NW layer converts the Segments to Packets
(datagram)


4. The Data Link layer converts the PACKETS to FRAMES


5. The Physical layer converts the FRAMES to BITS.


Data Encapsulation

Figure 4
-

An exchange using the OSI model

Layer

PDU Name

7. Application

Data

6. Presentation

Data

5. Session

Data

4. Transport

Segment

3. Network

Packet

2. Data Link

Frame

1. Physical

Bits

PDU’s and the OSI Model

LAYERS IN THE OSI MODEL

Figure 5
-

Physical layer

The physical layer is responsible for movements of individual bits from one hop
(node) to the next.


The interface and the type of the physical transmission medium


Raw bits
-
> signals


Bit duration


How the devices are connected to the media (point
-
to
-
point, or multipoint)


How devices are connected to each other(mesh, star, ring, bus, or hybrid)


The direction of transmission( simplex, half
-
duplex, or full
-
duplex).

Figure 6
-

Data link layer

The data link layer is responsible for moving frames from one hop
(node) to the next.


Makes the raw transmission facility (physical layer), reliable. Error
-
free to the upper layer
(network).


Divides the stream of bits into frames (data units)


Adds a header to define the send and/or receiver of the frame


Flow control mechanism to avoid overwhelming the receiver (receiver slower than
sender). Detect and retransmit damaged or lost frames. Recognize duplicate frames.

Figure 7
-

Hop
-
to
-
hop delivery


Communication at the data link layer occurs between two adjacent nodes.


For a to f, 3 partial deliveries are made. a to b, b to e, and e to f. Different headers.

Figure 8
-

Network layer

The network layer is responsible for the delivery of individual
packets from the source host to the destination host.


Ensures that each packet gets from origin to final destination.


If two systems are connected to the same link, there is usually no need for a
network layer. If different links, need.


Network layer adds logical addresses of the sender and receiver


Routing: routers/switchers route or switch the packets to their final destination.

Figure 9
-

Source
-
to
-
destination delivery

When packet gets B, B makes a decision based on the final F. B is a router, it uses
its routing table to find that the next hop is router E, so send to E.

Figure 10
-

Transport layer

The transport layer is responsible for the delivery

of a entire message from one process to another.


Ensures the whole message arrives intact and in order, overseeing both error
control and flow control at the source
-
to
-
destination level.


Service
-
point addressing: specific process (like email, msn, etc)


A message is divided into segments, containing a sequence number

Figure 11
-

Reliable process
-
to
-
process delivery of a message

Figure 12
-

Session layer


Establishes, maintains, and synchronized the interaction among communicating
systems.


Synchronization: allows a process to add checkpoints, or synchronization points, to a
stream of data.

•Responsible for enforcing the rules of dialog (e.g., Does a connection permit half
-
duplex or full
-
duplex communication?), synchronizing the flow of data, and
reestablishing a connection in the event a failure occurs.

Figure 13
-

Presentation layer

The presentation layer is responsible for
translation, compression, and encryption.

• Provides for data formats, translations, and code conversions.

• Concerned with syntax and semantics of data being transmitted.

• Encodes messages in a format that is suitable for electronic transmission.

• Data compression and encryption done at this layer.

• Receives message from application layer, formats it, and passes it to the session layer.

Figure 14
-

Application layer

The application layer is responsible for

providing services to the user.

Figure 15
-

Summary of layers