Open System Interconnection (OSI) Model

INTRODUCTION

OSI model has become the primary architecture for the computer network. It was originated in 1982 and has become the most efficient ISO standard network model. The OSI model describes how information moves through a network medium. There are seven layers of OSI model. Each model is specific in its task.

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As the message travels up the OSI Model on the other side, the headers and footers are stripped away, finally revealing the message at the top. To differentiate the evolution of the message as it moves along the model, different names are used to identify the message. The following names are commonly used.

Physical Layer

The physical layer handles low-level rules for transmitting bits. This layer encodes or decodes bits and sends or receives the stream of data. This is the pavement of the information superhighway. The physical layer defines:

• Electrical properties
• Transmission media
• Transmission devices
• Physical topology
• Data signaling
• Data synchronization
• Data bandwidth

Electrical properties define the rules how the message travels over transmission media, which then create bound or unbound pathways for transmission of bits. Again, these media compose the concrete of the information superhighway. Transmission devices provide midpoints and functionality to the transmission media. They are the raw implementation tools.

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Without translation devices, bits would scatter aimlessly through space. Physical topologies provide the form and function. They are defined by connection types and geographical arrangements of networking nodes. They often determine which medium is best certain circumstances.

An example includes bus, star, ring and cellular topologies. Data signaling encompasses coding and timing rules for digital and analog communications. These rules govern how the bits are received at the other side. Data synchronization defines one of two states for the electronic transmission message: asynchronous or synchronous.

Finally, data bandwidth is the capacity of transmission media, which determines the communications methodology and transmission speed. Most LANs are baseband communications, whereas WANs operate in the broadband arena.

Data Link Layer

The Data Link Layer organizes physical bits into logical groups called frames. These are contiguous series of bits grouped together as a unit of data. The Data Link Layer also detects and sometimes corrects errors. It also controls data flow and identifies computers on the network through physical addressing.

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To accomplish all this, the Data Link Layer is organized into two sub-layers:

• Media Access Control (MAC)
• Logical Link Layer (LLC)

Media Access Control (MAC):

These protocols are the rules of the road. Your data signals must use one of the three different protocols to gain access to the media. They are:

• Contention
• Token passing and
• Polling

Physical addressing distinguishes one location from another. The data frames need address so that they know where to go and what to do. Addressing works on the hierarchy structure. The data frames need three addresses. They are:

• Physical address
• Logical address
• Service address

Logical Link Control (LLC):

This layer consists of the Functions:

Frame synchronization: It forces senders and receivers to agree on the boundaries of data frames. This takes place as follows:

1. Asynchronous – sends each frame separately
2. Synchronous – sends on the basis of synchronized clocks
3. Isochronous – incorporates advantages of above both

Flow Control:They are typically made up of devices with different transmission speeds like storage and processing capabilities. In diverse world, we need to develop rules that protect slower devices but don’t hinder faster ones. These rules are called flow control. Data Link flow control regulates how much data can be sent over the transmission media in a specified period of time. That is achieved by:

• Window flow control
• Guaranteed rate flow control

Error Checking: It uses a complex calculation called the Cyclical Redundancy Check (CRC). CRC ensures the integrity of the packet by calculating a 16-32 bit number based on the contents of the packet.

Network Layer

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Network Layer is concerned primarily with moving data from point A to point B. In general, network layer has four functions:

• Logical Addressing
• Switching
• Routing
• Network Control

Logical Addressing

Logical addressing helps to determine a logical path during internetwork communications. The network relies on three types of addresses:

1. Physical address
2. Logical address
3. Service address

Switching

Most large internetworks consist of multiple physical paths between the sender and receiver. Intermediate routers are responsible for moving the packets across these diverse links. Switching makes this possible.

• Circuit Switching:It connects the sender and receiver by a single path during the conversation. Telephone switching equipment, for example, uses address number (in the form of the country area, office, truck codes, etc) to establish a path that connects the sender’s telephone to the receiver’s telephone.

• Message Switching: It does not establish a dedicated path between two stations. Instead , a conversation is divided into messages. Each message is packaged with its own destination address and then transmitted from router to router through the internetwork.

• Packet Switching:It combines the advantage of both circuit and message switching. Packet switching breaks the datagrams into small parts called packets. Each packet is constructed with source and destination address that allow it to work its way through internetwork and find its destination.

Routing

Routing intermediate network devices are used to make intelligent decisions about how the message should travel from point A to point B. Routing involves two simple steps:

1. Route discovery and
2. Route selection

Network Control

Network control administers internetworking and routing. These tasks will ensure that we maintain a steady speed and rejoin with all the rest of the datagrams at the other end. Network layer administrative task include:

• Network flow control
• Network sequencing
• Network error checking

Transport Layer

It organizes packets into segments and delivers them reliably to upper- layer services. If packets are not delivered to the destination correctly by the network layer, transport functions come to rescue. This layer can initiate retransmissions and inform the upper layers that we are trying again.

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The OSI Transport Layer consists of the following components:

1. Service addressing
2. Segmentation
3. Transport control

Service addressing: It provides a doorway to upper-layer services. Remember, this is the last point of connectivity before we get to the actual network services. Each message is destined for a particular service. The transport layer uses connection Ids, ports and sockets to make sure that we find the right one.

Segmentation: It is the housekeeping task that package messages into acceptable sizes. Many upper-layer services need specified-sized messages and segmentation does the trick.

Transport control: It includes error checking and flows control. Error checking is the Transport Layer’s main function. At this point, we are concerned mostly with the sage arrival of the network messages. We don’t care what they look like. That’s the Data Link Layer’s job.

Session Layer

The Session Layer opens a dialogue between the sender and receiver. It also monitors conversations and makes sure that everyone is getting along.

The session Layer does all this by using three simple steps:

1. Connection establishment
2. Data transfer and
3. Connection release

Connection establishment: It initiates the dialogue between two systems. It uses networking protocols to find a dialogue path and communications media to send messages back and forth (data transfer).

Data Transfer:During data transfer,the session layer manages the dialogue and ensures reliable conversations through simplex, half duplex or full duplex transmissions. Finally, connection release ends the dialogue and closes the connection. Incidentally, connection release can be either planned or accidental.

Connection release: Finally, connection release ends the dialogue and closes the connection. Incidentally, connection release can be either planned or accidental.

Presentation Layer

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The Presentation Layer is the OSI translator. It transforms the upper-layer message into a mutually agreed-upon format. At this point, the dialogue is open and data transfer has begun, but before the Application Layer can read the message, the Presentation Layer must take care of two things.

1. Translation
2. Encryption

Translation: It is the Presentation Layer’s main task. It is necessary when two system speaking different language try to communicate most of the time. The translation can happen in the variety of ways like Bit order, byte order, the character code and file syntax.

Encryption:It is a secondary task for the Presentation Layer. It is necessary for sensitive data and operating system security, such as default authentication. This level of encryption and decryption supports both public and private keys.

Application Layer

The Application Layer sits at the very top of the OSI Model. It is the ultimate goal of networking i.e. service providing. The Application Layer does the services task as:

1. Service Advertisement and
2. Service Availability

The Application Layer uses special networking protocols to provide file, print, message, application and database services. Service advertisement lets other systems and users know what services are available. Providers use active or passive techniques to define the scope of their network services.

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Service Availability is the next step. Once a service has been advertised, it must be made available. Service availability can be accomplished in several ways including OS Call Interception, remote operation, and collaborative computing.

References:

Khanal, R.C. Khanal, R.C. Computer Concept for XII. Pashupatigriha Marga, Thapathali, Kathmandu, Nepal: Ekta Books Distributors Pvt. Ltd., 2010. -88-93.

Adhikari, Deepak Kumar.,et.al., Computer Science XII,Asia Publication Pvt.Ltd