ov 5 - 1 copyright © 2011 element k content llc. all rights reserved. networking models the osi...

OV 5 - 1Copyright © 2011 Element K Content LLC. All rights reserved.

Networking Models

The OSI Model The TCP/IP Model


The OSI Model

Application

Presentation

Session

Transport

Network

Data Link

Physical


Layer 1: The Physical Layer

The Physical layer of the OSI model: Provides the means for transmitting data packets over a physical medium. Specifies the electrical and mechanical characteristics of the network. Receives fully formatted data packets from the Data Link layer. Includes technologies such as: Ethernet, Fast Ethernet, ATM, token ring,

and FDDI.


Layer 2: The Data Link Layer

The Data Link layer of the OSI model: Is responsible for the error-free transfer of data packets on a network. Is responsible for grouping data packets into frames and attaching the

address of the receiving node to each frame – thus forming the data packet. Adds error correction and detection codes to frames. Includes two sublayers: MAC and LLC.

Devices and protocols: Switches operate at this layer. PPP and SLIP are the protocols that operate at this layer.


Layer 2: The Data Link Layer (Cont.)

Data Link Sub-Layer

Description

LLC

Identifies the Network-layer protocols and manages frames between the

layers. Controls the the Physical layer device and also performs error control. Checks the CRC of the frame, and either ACKs or NACKs the data. Controls data flow to maintain the bandwidth of the network medium. Tells the Data Link layer how to handle the frame it receives.

MAC

Defines how packets are placed on the media. Is responsible for the carrier sense to detect collision in a contention-

based network. Is responsible for the token in a token passing network. Controls elements of addressing such as error notification, the frame

delivery sequence, and flow control in an Ethernet network which uses

contention-based media access.


Layer 3: The Network Layer

The Network layer of the OSI model: Is responsible for addressing and routing the data packets from a source to

a destination through a network. Ensures delivery of packets across a network. Is responsible for controlling congestion on the network by taking proper

routing decisions. Defines protocols for interconnecting two or similar networks such as IP,

ARP, ICMP, DHCP, RIP, OSPF, BGP, and IGMP.


Layer 4: The Transport Layer

The Transport layer of the OSI model: Accepts data from upper layers and breaks it into smaller units known as

segments. Passes the segments to the lower layers and ensures that all segments

arrive correctly at the receiving end. Adds a sequence number to reconstruct original sequence of segments.

This is useful in case of out of order sequencing. Is responsible for error correction and sending acknowledgments at the

network level. Gateways operate at this layer. TCP, UDP, IPSec, PPTP, RDP and L2TP are the protocols that operate this

layer.


Network- and Transport-Layer Protocols

Protocol Family

Function

Reliability protocols

Provides a method of ensuring reliable data transfer. For example, a header or trailer might contain a Checksum value or request that you need to acknowledge received data by sending an acknowledgement message back to the sender.

Connection protocols

Establish and maintain a connectionless or connection-oriented service for the upper layers. In a connection-oriented service, the sending and receiving nodes maintain constant communication to mediate the transfer of data. Sequencing, flow control, and reliability are monitored at both ends. In a connectionless service, the message is packaged, delivered, and sent. The message is transferred only if communication exists between the two nodes.

Routing protocols

Provides a method of ensuring data transfer to the correct destination. In an unswitched network, routing is virtually unnecessary because the nodes are directly connected. In a switched network, however, the routing protocol determines the path a packet will take to reach its destination. This function is particularly important and complex in a packet-switched network, because there can be many possible paths to a destination and many intermediary devices such as routers along the path. Routing protocols determine the strategies used to transmit data through the network.


Layer 5: The Session Layer

The Session layer of the OSI model: Establishes and connection between network devices and applications,

maintains the connection, and then terminates or reestablishes it when required.

Controls how, when, and for how long a device can transmit or receive data. Specifies procedure for the connection, termination, and reestablishment of

sessions. Specifies the procedures for synchronizing data transfer between two

devices with different data transmission rates. Sockets and session establishment in TCP function at this layer.


Layer 6: The Presentation Layer

The Presentation layer of the OSI model: Is responsible for encoding data into a standard network-compatible format,

which enable devices with different representation techniques to communicate with each other.

Adds services such as data compression and encryption. MME, SSL, TLS, GIF, JPEG, and TIFF are the protocols and technologies

that operate at this layer.


Layer 7: The Application Layer

Offers services and utilities that enable application access to the network and its resources.

HTTP, DNS, FTP, Gopher, NFS, NTP, SMTP, SNMP, and Telnet are the technologies, protocols, and services that function at this layer.


Application-, Presentation-, and Session-Layer Protocols

Protocol Family Functions

Terminal-emulation protocols

Allow the user access to a text terminal and all its applications. Enable computers to act as standard terminals so that they can

access hosts.

File access and file transfer protocols

File access protocols: Enable nodes to access files on the network. Provide a common means to access network files.

File transfer protocols:

Enable copying of files between network storage and other storage, such as a computer's local disk drive.

Email protocols Provide for email delivery and handling of messages.

Remote-action and multiple-session protocols

Remote-action protocols: Determine whether processes should be performed remotely on a client node or directly by a server. Required for setting up a client-server relationship.

Multiple-session protocols: Enable multiple network links to be established.


Application-, Presentation-, and Session-Layer Protocols (Cont.)

Protocol Family Functions

Network management protocols

Provide tools for setting up and maintaining the network.

Task-to-task protocols

Enable software processes to communicate over the network.

Codeset and data structure protocols

Define the representation of data. Translate data for nodes that use different coding schemes.


The OSI Data Communication Process

Application

Presentation

Session

Transport

Network

Data link

Physical

Sender Receiver

44

33

22

11

1. Data added to Application layer

2. Data forwarded to lower layers from sender

3. Data added to Physical layer

4. Data is forwarded to the Application layer


The TCP/IP Protocols

TCP/IP

Enables computers to communicate over all

types of networks

Enables computers to communicate over all

types of networks


The TCP/IP Network Model

Transport

Network Interface

Application

Internet


Layers in the TCP/IP Network Model

TCP/IP Layer Functions

Application

Provides definition of protocols for file, mail, and hypertext

transfer. Handles the encoding of data and controlling of the sessions. Defines socket services and other utilities over TCP/IP.

Transport

Provides connection establishment and communication services. Defines protocols for end-to-end transfer of data, along with error

and flow controls.

Internet

Provides addressing and routing services. Controls congestion on the network. Involves transferring data from a source to destination network

when multiple networks are connected together.

Network Interface

Provides services to send and receive data packets on the

network. Defines protocols for moving data frames between

adjacent nodes, and for accessing the medium by the devices. Defines the protocols for encoding and transmitting data over the

network media.


Comparison of the OSI and TCP/IP Models

Application

Presentation

Session

Transport

Network

Data Link

Physical

Transport

Network Interface

Application

Internet

TCP/IP model

OSI model


Comparison of the OSI and TCP/IP Models

Category Description

Similarities

Both the models have a similar architecture. Both the models have an Application, Transport, and Network layer. Both the models have their lowest layer connected to the actual

physical network.

Dissimilarities

OSI was developed to standardize networking. TCP/IP was specifically

developed to execute Internet-related tasks The OSI reference model consists of seven architectural layers

whereas the TCP/IP only has four layers. The TCP/IP model does not have a Session or a Presentation layer. The Application layer in TCP/IP handles the responsibilities of layers

5, 6, and 7 in the OSI reference model. The TCP/IP model combines the OSI Data Link and Physical layers

into the Network Interface layer. The OSI reference model was invented before protocols, therefore the

functionality allotted in each layer is not very optimized. But the

TCP/IP protocols are the standards around which the Internet was

developed and therefore, the functionality allotted at each layer is

perfectly optimized.


Data Encapsulation

Process of adding delivery information to the actual data transmitted on each layer.

Takes place in the transmission end as data is passed down the layers. At the receiving end, the reverse process of removing the added information

is done as data passes to the next higher layer in a process called de-encapsulation.

The added information is called a header if it is before the data, or a trailer if it is added after the data.


Protocol Bindings

Network interfaceNetwork interface

Protocols bound to the network interface

Protocols bound to the network interface


Reflective Questions

1. What are the Physical layer devices that you have come across in

your network?

2. What are the similarities and differences between the OSI model and

the TCP/IP model?

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