Network OSI

8/8/2019 Network OSI

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Computer Networks

overview

Network Components

Overview of Network Management



Communication Architecture A communication architecture integrates

many protocols and the software that

implements them. These parts can be developed

independently of one another as long as the

developers adhere to the standard

interfaces between them.



OSI Reference Model



OSI Reference Model The model does not specify how to

implement the function in each layer.

The functions in each layer are defined byprotocol, which are implemented in software.

The software in each layer provides servicesto the layer above it.

Layering concept simplifies the developmentprocess and allows improvements of software in one layer without affecting other layers.



Physical Layer

This layer defines the electrical, mechanical,

functional specifications for hardware that

connects a device to the network. Physical layer standards would specify

Connector¶s size and shape

Exact number of pins The signals that can be used to those

pins



Data Link Layer

This layer specifies the procedures in order

to achieve reliable point-to-point transfer of

information between two devices. Data link layer can be divided into two

sublayers.

Logical Link Control

Medium Access Control



Data Link Layer (cont¶d)

CSMA/CD is implemented in Ethernet.

CSMA means that a computer waits until

there is no signal from any other computer on LAN before transmitting a message.

CD reduces the probability that a collision

will occur by backoff its transmission.

Bridge is a typical device working on data

link layer.



Network Layer This layer provides the mechanisms for

transporting a packet from the source

networks to the destination network. Packets are messages constructed by the

network layer and higher layers.

A router is called a network layer device

because it is controlled by protocols up to

and including the network layer.



Transport Layer Transport layer provides reliable and

unreliable transport protocols.

The reliable protocol, which is called theTransport Control Protocol (TCP), provides

the mechanisms that enable end-to-end

reliability.

TCP accomplishes this by the use of

automated acknowledgement and

automated retransmissions.



Transport Layer (cont¶d) The unreliable transport protocol, which is

called the User Datagram Protocol (UDP),

places no constrains on the network. Best-effort delivery from the source device

to the destination device is all that is

required.

UDP is used when speed is of the essence

and the effort spent on ensuring reliability is

not cost-effective.



Session Layer The Session layer provides the mechanisms

necessary to open and close multiple logical

connections between processes on differentPCs.

These logical connections are called

session.

Many logical connections can use the same

physical connection.



Presentation Layer

This layer provides a commonrepresentation of data transferred between

peer application layer entities on differentstations.

Abstract syntax notation.one (ASN.1) formatis useful for application layer.

However, the layers below the presentationlayer use a formant call transfer syntax.

The presentation layer translates between ASN.1 and transfer syntax.



Application Layer Common Application Service Elements

provide service generally useful to a variety

of application programs provided by vendors. Specific Application Service Elements

service the additional needs of particular

application programs.



Frames

Frame construction

Application program makes a request.

The request is passed to the appropriateapplication-layer-entity.

The application-layer-entity constructs an Application layer protocol data unit (PDU).

The Application layer PDU is then passedto the appropriate presentation-layer-entity. This entity constructs aPresentation layer PDU.



NetworkD

evice Workstations

Servers

Repeaters

Hubs

Bridges

Switches Routers

Probes



Workstations Workstations typical make use of the

services of all layers of a protocol stack

because they process the requests of application programs for information on

servers.

Server is like workstation making use of all

layers of the protocol stack.



Repeater Repeater amplifies the signal between

segments and extends the length of a LAN.

Repeaters are used on 10BASE2 and10BASE5 LANs.



Hubs A hub is a repeater with many input and

output ports.

Every transmission received by the hub onone port is amplified and retransmitted outof all other ports.

Hub do not examine information in any field

of the frame. Repeaters and hubs are consider a Physical

layer device.



Bridges Transparent bridge

divides a LAN into segments that use the

same media access control specificationand it controls what traffic is passedbetween segments.

uses bridge table to learn which devices

are on which segments. Translating bridges connects networks

using different MAC layer specifications.



Bridges (cont¶d) Errors and collisions that take place on one

segment are not propagated to devices on

the other segment. The use of switches rather than bridges is

commonplace today.



Switches A switch is a multiport bridge.

Switches are typically configured to forward

incoming frames to outgoing ports based ondestination MAC addresses associated with

those ports in the switch table.

Switches also can be configured to drop

frames based on MAC destination or to

redirect frames to other MAC address.



Switches (cont¶d) Higher layers of the protocol stack are now

being implemented in switches.

Layer 3 switch also can function like router.



Routers Router reads the Network layer header in

packet to find the destination network

address and then looks up the address inrouting table.

The routing table is a list of mappings of

network addresses to ports that will get the

packet to the destination networks by thebest means.



Routers (cont¶d)

Routing table



Probes

Probes are tools for learning about the

details of the communication between

devices. Probes can be placed on a network and the

information they gather transferred to the

Management Station fro analysis of the

traffic.



Outline

Network Components

Overview of Network Management



Network Management

Jobs of network management in early day

Attaching PCs and server to a LAN using

Network Interface Cards (NICs) Installing protocol stacks

Configuring NIC I/O addresses and

interrupts so as not to conflict with other NIC selections.

Using Ping application to ensure that all

devices on the network could

communicate with one another



Network Management (cont¶d)

Access control

The manager might write a script for the

server to control access to information onit.

The script would provide access to drives,

folders or files that use or group of users

needed.

Install a print server to manage the print

jobs from each of the PCs.



Network Management (cont¶d)

Network configuration

If the networks were to be divided into

subnets, then a bridge would be used to

connect them.

The manager would configure a routing

table to enable connectivity according to

network address, subnet address andsubnet mask.

Finally, installing user applications on

PCs is necessary for interfacing correctly

with OS.



Maintenance

After configuring primary network

ingredients, here comes the maintenance.

Network performance is not optimized, therewill be software and hardware failures and

network resources will change.

The manager is required to make estimates

of the network ca pacity .

There was a need for minimal security .



Maintenance (cont¶d)

New work management has divided intospecialties nowadays.

Administration Backing up servers

Maintaining security

Installing OS and applications

Network engineer

Be responsible for the health of thenetwork and its availability to user.



Network-Based Management

As network has grown larger and morecomplicated, network manager needed to findmore efficient way to determine and control the

state of network. Some protocols could be used by network

manager to remotely control devices.

Simple Network Management Protocol

(SNMP

) Management Information Base (MIBs)

Network Management Systems (NMS)



Network Management Protocol

Stack

Management Station and Management

Agent.



Network Management

Example

Step 1

NMS calls for the service of the managementprocess

Step 2

the management process call the program theimplements network management protocol.

Step 3

NMP constructs a request packet to the programon the management agent.

Step 4

NMP on the management agent cause therequest packet to be passed to the agent process.



Network Management Example

(cont¶d)

Step 5

The agent process accesses the value of therequested variable and passes it to NMP.

Step 6 NMP constructs the response packet which is

sent back to the management station.

Step 7

At the management station the program that

implements the NMP receives the responsepacket.

Step 8

The management process passes the requestedvalue to the application program.

Network OSI

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