Question

1- Definition and introduction of its layer
2- Explain the network functions
3- Explain the data transfer method in the network
4- Mention the protocols if there is and explain them
5- Network positives and negatives effect

for the session layer

and transport layer

Answer 1

Introduction

OSI (Open Systems Interconnection model) Model: It can be described as a conceptual model used to define the processes involved during the network communication. In consists of 7 layers which are listed below.

• The Physical Layer
• The Data Link Layer
• The Network Layer
• The Transport Layer
• The Session Layer
• The Presentation Layer
• The Application Layer

Session Layer:

It is the 5^th layer in OSI network model. The main purpose of this layer is to track the sessions during a network communication, which starts from establishing, controlling and finally ending the session (between local and remote application/user).

Functions

· Dialog control: Allows two systems/users to interact with each other in full duplex or half duplex mechanism.

· Synchronization: It adds checkpoints after regular intervals to the stream of data that gets transferred. This ensures that there is no data loss occurred during the transferring.

(For instance, if a file is send with 100 characters, checkpoints will be added after 10 characters, which can be validated at the destination side)

· Token Management: This prevents deadlock situations, i.e., it prevents two systems or users attempting same operation at the same time.

In session layer, the data transfer occurs in either full duplex or half duplex method.

Protocols

· Remote procedure call protocol (RPC)

Remote Procedure Call (RPC) is a protocol that one program can use to request a service from a program located in another computer on a network without having to understand the network's details

· Point-to-Point Tunneling Protocol (PPTP)

Point-to-point tunneling protocol (PPTP) is a set of communication rules that govern the secure implementation of virtual private networks (VPN), which allow organizations a method of extending their own private networks over the public Internet via "tunnels."

· Session Control Protocol (SCP)

Session control protocol (SCP) is a method of creating multiple light-duty connections from a single TCP (Transmission Control Protocol) connection

· Session Description Protocol (SDP)

The Session Description Protocol (SDP) is a format for describing multimedia communication sessions for the purposes of session announcement and session invitation

Transport layer

It is the 4^th layer in OSI network model. Transport layer offers peer-to-peer and end-to-end connection between two processes on remote hosts. Transport layer takes data from upper layer (i.e. Application layer) and then breaks it into smaller size segments, numbers each byte, and hands over to lower layer (Network Layer) for delivery.

Functions

· Service Point Addressing:

Transport Layer header includes service point address which is port address. This layer gets the message to the correct process on the computer unlike Network Layer, which gets each packet to the correct computer.

· Segmentation and Reassembling:

A message is divided into segments; each segment contains sequence number, which enables this layer in reassembling the message. Message is reassembled correctly upon arrival at the destination and replaces packets which were lost in transmission.

· Connection Control:

It includes 2 types:

Connectionless Transport Layer: Each segment is considered as an independent packet and delivered to the transport layer at the destination machine.

Connection Oriented Transport Layer: Before delivering packets, connection is made with transport layer at the destination machine.

· Flow Control:

In this layer, flow control is performed end to end.

· Error Control:

Error Control is performed end to end in this layer to ensure that the complete message arrives at the receiving transport layer without any error. Error Correction is done through retransmission.

Protocols

There are two types of protocols in Transport layer.

TCP

Transmission Control Protocol (TCP) – a connection-oriented communications protocol that facilitates the exchange of messages between computing devices in a network. It is the most common protocol in networks that use the Internet Protocol (IP); together they are sometimes referred to as TCP/IP.

UDP

UDP divides messages into packets, called datagrams, which can then be forwarded by the devices in the network – switches, routers, security gateways – to the destination application/server. While UDP does not number or reassemble the datagrams, it does include port numbers in the datagram header that help distinguish different user requests and an optional checksum capability that can help verify the integrity of the data transferred.

Positives and negatives

Pros

· It is a truly generic model. And it is considered as a standard model in computer networking.

· Layers in OSI model architectures are distinguished according to the services, interfaces, and protocols.

· Since the protocols are hidden, any protocols can be implemented in this model. So I call it as a generic model. It has all flexibility to adapt to many protocols.

· It supports connection-oriented as well as connectionless services. So we can use the connection-oriented model when we need reliability and connection-less services when we need a faster data transmission over the internet.

Cons

· It may find sometimes difficult to fit a new protocol in this model. This is because this model was created before the invention of any of these protocols.

· The session layer is used for session management. Presentation layer deals with user interaction. Though they are useful, not as much as other layers in the OSI model.

· There is some duplication of services at various layers. Such as, both transport and data link layer have error control mechanisms.

· There is also interdependence among the layers. These layers cannot work in parallel. They have to be in wait to receive data from its predecessor.