The Theoretical Network

OSI Model• Networks are complicated structures

with many interrelated parts.

• To identify the various parts that makeup networks and how they work together , it is useful to have a network model.

• International standard organization (ISO) established a committee in 1977 to develop a reference model for understanding computer network communication.

• Open Systems Interconnection (OSI) reference model is the result of this effort.

• In 1984, the Open Systems Interconnection (OSI) reference model was approved as an international standard for communications architecture.

• Communication Architecture provides Strategy for connecting host computers and other communicating equipment and defines necessary elements for data communication between devices.

• OSI model is a layer architecture design and It is easy to debug network applications in a layered architecture network.

• Term “open” denotes the ability to connect any two systems which conform(obey the rules) to the reference model and associated standards.

• The OSI model describes how information or data makes its way from application programmes (such as spreadsheets) through a network medium (such as wire) to another application programme located on another network.

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

• This separation into smaller more manageable functions is known as layering.

7 Layers of OSI Reference Model

Application Layer

Presentation Layer

Data Link Layer

Network Layer

Physical Layer

Session Layer

Transport Layer

07

06

03

05

04

02

01

→ Provides services that directly support user applications

→ Translates data formats and adds encryption

→ Establishing and maintaining connections

→ Adds identifiers to processes and deals with error-handling information

→ Handles internetwork sequencing ,addressing and routing

→ Adds error checking information and organizes bits into frames

→ Transmits and receives bits over physical media

Physical Layer• Responsible for sending bits from one

computer to another.

• Deals with physical connection to the network and with transmission and reception of signals.

• Covers all - mechanical, electrical, functional and procedural - aspects for physical communication , Such as voltage levels, timing of voltage changes, maximum transmission distances, physical connectors, and other similar attributes are defined by physical layer specifications.

Data link Layer• Data link layer attempts to provide reliable

communication over the physical layer interface.

• Create and detect frame boundaries.• Handle errors by implementing an

acknowledgement and retransmission scheme.

• Implement flow control.• Supports points-to-point communication.• Supports simplex, half-duplex or full-duplex

communication.

Network Layer• Implements routing of frames (packets)

through the network.

• Defines the best possible path the packet should take from the source to the destination. Handles overcrowding in the network.

• Defines logical addressing so that any endpoint can be identified

• translates logical network addresses into physical machine addresses.

Transport Layer• Purpose of this layer is to provide a reliable

mechanism for the exchange of data between two processes in different computers.

• Ensures that the data units are delivered error free.

• Ensures that data units are delivered in sequence.

• Ensures that there is no loss or duplication of data units.

Session Layer• Session layer provides mechanism for

controlling the dialogue between the two end systems. It defines how to start, control and end conversations (called sessions) between applications.

• a logical connection to be established on an end-user’s request.

• Any necessary log-on or password validation is also handled by this layer.

• Session layer is also responsible for terminating the connection.

• also provide check-pointing mechanism • data can be retransmitted from the last

checkpoint.

Presentation Layer• Presentation layer defines the format in

which the data is to be exchanged between the two communicating entities.

• Also handles data compression and data encryption ,data translation, interpretation of graphics command.

Application Layer

• Application layer interacts with application programs and is the highest level of OSI model.

• such as file transfer, electronic mail, remote login etc.

• When a programmer writes an application program that uses network services ,application program will access this layer.

Communication between Layers

• Moving from the top, down - messages get larger and larger ! !   A message is passed down, and the lower layer adds a header to it.  This is called encapsulation.

• Moving from the bottom, up - messages get smaller and smaller ! !  A message is first stripped of it's header, and then the inner contents (the "data" portion) is passed up.  This is "decapsulation".

Although there is officially no layer 0 or 8 conceptually they exist as the physical link (the cable) and the User, as shown below.  Note how the packets get larger as each successive layer adds it's own header to the packet that was handed down to it from the layer above.

IEEE 802 categories• The institute of IEEE in February 1980

formed a project called project 802 to help define certain LAN standards, though “802” is sometimes associated with the date the first meeting was held — February 1980.

• IEEE 802 refers to a family of IEEE standards dealing with local area networks and metropolitan area networks.

• IEEE 802 splits the OSI Data Link Layer into two sub-layers named Logical Link Control (LLC) and Media Access Control (MAC), so that the layers can be listed like this:

1. Data link layer i. LLC Sub layerii. MAC Sub layer

2. Physical layer

• The IEEE 802 family of standards is maintained by the IEEE 802 LAN/MAN Standards Committee (LMSC). The most widely used standards are for the Ethernet family, Token Ring, Wireless LAN, Bridging and Virtual Bridged LANs. An individual Working Group provides the focus for each area.

Some Working groups Name Description

IEEE 802.1 Bridging (networking) and Network Management

IEEE 802.2 LLC

IEEE 802.3 Ethernet

IEEE 802.4 Token bus

IEEE 802.5 Defines the MAC layer for a Token Ring

IEEE 802.6 MANs(DQDB )

IEEE 802.7 Broadband LAN using Coaxial Cable

IEEE 802.8 Fiber Optic TAG

Protocol Stacks• As network management is easier due to

the layered architecture.• Each layer has its own a set of rules and

requirements called protocol.• The protocol defines the format of the

data being exchanged, and the control and timing for the handshake between layers.

• Protocols that work together to provide a layer or layers of OSI model are protocol stacks or suit.

• The protocol stack is an implementation of a computer networking protocol suite. The terms are often used interchangeably. Strictly notice, the suite is the definition of the protocols, and the stack is the software implementation of them.

• Protocol stacks are often divided into three major sections: media, transport, and applications.

• A particular operating system or platform will often have two well-defined software interfaces: one between the media and transport layers, and one between the transport layers and applications.

a) The media-to-transport interface defines how transport protocol software makes use of particular media and hardware types ("card drivers"). For example, this interface level would define how TCP/IP transport software would talk to Ethernet hardware.

b) The application-to-transport interface defines how application programs make use of the transport layers. For example, this interface level would define how a web browser program would talk to TCP/IP transport software.

Internet protocol suite TCP/IP MODEL

• The Internet protocol suite is the networking model and a set of communications protocols used for the Internet and similar networks. It is commonly known as TCP/IP.

• The Internet protocol suite and the layered protocol stack design were in use before the OSI model was established.

• It is one of the commonly used networks today.

The layers of the protocol suite near the top are logically closer to the user application, while those near the bottom are logically closer to the physical transmission of the data.

•  It was developed by Department of defend’s Advanced projects Research Agency DARPA, an agency of the United States Department of Defense under its project on network interconnection , started in 1969.

• TCP/IP provides end-to-end connectivity specifying how data should be formatted, addressed, transmitted, routed and received at the destination.

• It has four layers,

The link layer

The internet layer (IP)

The transport layer

The application layer

• Application layer (process-to-process):

This is the scope within which applications create user data and communicate this data to other processes or applications on another or the same host. This is where the higher level protocols such as HTTP (hypertext transfer protocol), etc. operate.

• Transport layer (host-to-host):

• This is where flow-control, error-correction, and connection protocols exist, such as TCP.

• This layer deals with opening and maintaining connections between Internet hosts.

Internet layer:

• The internet layer has the task of exchanging data grams across network boundaries.

• It defines and establishes the Internet. This layer defines the addressing and routing structures used for the TCP/IP protocol suite.

• Link / Network access layer:• Despite the different semantics of layering in

TCP/IP and OSI, the link layer is often described as a combination of the data link layer (layer 2) and the physical layer (layer

1) in the (OSI) protocol stack

• Management of reliable data delivery,

• access to physical networks

• Binary data transmission

Working

As data is being sent from one computer, it will pass from the top layer to the bottom. On the receiving end, the data will then be rebuilt from the bottom layer to the top.

Each layer a packet of information travels through adds what is called a header. Think of it in terms of a Russian doll.

• When the packet is being rebuilt on the receiving end, each header is unpackaged the same way.

• Since each layer of the TCP/IP model does a unique task separate of the other layers, we refer to the data package at each layer with different names. For instance, the data package at the Application Layer is called a message, while the same data package at the Internet Layer is called a datagram.

• Once the data packet leaves link layer it is converted into a bit stream of electrical pulses, commonly referred to as 1’s and 0’s.

• all transport layer in TCP implementations must choose whether or how to provide reliability. UDP(User Datagram Protocol) provides data integrity/errorless data via a checksum but does not guarantee delivery; TCP (Transmission Control Protocol) provides both data integrity and delivery guarantee by retransmitting until the receiver acknowledges the reception of the packet.

Difference

Connectionless vs. connection oriented protocols• These are the two ways in which

communications between computers can be arranged.

• Former assumes that all data will get through ,do not check delivery & sequencing .

• Faster working

• UDP/IP

• Connectionless Network Service is an OSI Network Layer service.

Connectionless vs. connection oriented protocols• Later presume that some data will be

lost or disordered .

• Assures data delivery in sequential order by retaining data and negotiating for retransmission .

• Reliable

• TCP/IP

Which is better? Historically, a few companies tried to create

protocol stacks based on ISO/OSI, mostly throughout the 1980s (Unger man-Bass, even the early versions of Microsoft NetBEUI).

However, no implementation ever seen was "complete" in that it covered all 7 layers. The ISO/OSI model is simply more theoretical than practical.

On the other hand, the TCP/IP model is easy to implement and there are numerous implementations of it.

Either way, during the mid 1980s, it became clear that the stack of TCP/IP protocols that was built according to the TCP/IP model were winning in the market. Whatever "OSI inspired" products there were, disappeared from the market.