Network Model
Network is combination of hardware and software that sends data from one device to another device. The Hardware consists of the physical equipment that carries signals from one point location to another location.The software consists of instruction sets that make possible the services that we expect from a network. We use the concept of layers in networking to understand the concept of networking.
What topics covered here :
- What is OSI Model ?
- The OSI model is composed of seven ordered layers:
- OSI Model Purpose
- Peer To Peer Process
OSI Model : Networking Model
OSI Model is a Set of protocols that allows any two different systems to communicate regardless of their underlying architecture.
Networking Model OSI Model is Seven (7) layered model established in 1947.In OSI Model OSI stands for Open systems Interconnection Model. It was first introduced in 1970s. The OSI Model is not a Protocol It is a model for understanding and designing a network architecture that is flexible robust and interoperable.
The OSI model is composed of seven ordered layers:
- Physical layer
- Data link layer
- Network layer
- Transport layer
- Session layer
- Presentation layer
- Application layer
OSI Model : PURPOSE
peer-to-peer process :
Physical Layer :
The physical layer is also concerned with the following:
- Physical characteristics of interfaces and medium.
- Representation of bits :- The physical layer data consists of a stream of bits (sequence of Os or 1s) with no interpretation.
- Data rate :- The transmission rate-the number of bits sent each second-is also defined by the physical layer.
- Synchronization of bits :- The sender and receiver not only must use the same bit rate but also must be synchronized at the bit level.
- Line configuration :- The physical layer is concerned with the connection of devices to the media. In a point-to-point configuration, two devices are connected through a dedicated link. In a multipoint configuration, a link is shared among several devices.
- Physical topology :- The physical topology defines how devices are connected to make a network.
- Transmission mode :- The physical layer also defines the direction of transmission between two devices: simplex, half-duplex, or full-duplex.
Data Link Layer
Data link layer transforms the physical layer, a raw transmission facility, to a reliable link. It makes the physical layer appear error-free to the upper layer (network layer) relationship of the data link layer to the network and physical layers.
- Framing :- The data link layer divides the stream of bits received from the network layer into manageable data units called frames.
- Physical addressing :- If frames are to be distributed to different systems on the network, the data link layer adds a header to the frame to define the sender and/or receiver of the frame.
- Flow control :- If the rate at which the data are absorbed by the receiver is less than the rate at which data are produced in the sender, the data link layer imposes a flow control mechanism to avoid overwhelming the receiver.
- Error control :- The data link layer adds reliability to the physical layer by adding mechanisms to detect and retransmit damaged or lost frames. It also uses a mechanism to recognize duplicate frames. Error control is normally achieved through a trailer added to the end of the frame.
- Access control :- When two or more devices are connected to the same link, data link layer protocols are necessary to determine which device has control over the link at any given time.
Network Layer
- Logical addressing :- The physical addressing implemented by the data link layer handles the addressing problem locally.
- Routing :- When independent networks or links are connected to create intemetworks (network of networks) or a large network, the connecting devices (called routers or switches) route or switch the packets to their final destination. One of the functions of the network layer is to provide this mechanism.
Transport Layer
Transport layer is responsible for process-to-process delivery of the entire message. A process is an application program running on a host. Whereas the network layer oversees source-to-destination delivery of individual packets, it does not recognize any relationship between those packets. It treats each one independently, as though each piece belonged to a separate message, whether or not it does. The transport layer, on the other hand, ensures that the whole message arrives intact and in order, overseeing both error control and flow control at the source-to-destination level.
- Service-point addressing :- Computers often run several programs at the same time. For this reason, source-to-destination delivery means delivery not only from one computer to the next but also from a specific process (running program) on one computer to a specific process (running program) on the other. The transport layer header must therefore include a type of address called a service-point address (or port address). The network layer gets each packet to the correct computer; the transport layer gets the entire message to the correct process on that computer.
- Segmentation and reassembly :- A message is divided into transmittable segments, with each segment containing a sequence number. These numbers enable the transport layer to reassemble the message correctly upon arriving at the destination and to identify and replace packets that were lost in transmission.
- Connection control :- The transport layer can be either connectionless or connectionoriented. A connectionless transport layer treats each segment as an independent packet and delivers it to the transport layer at the destination machine. A connectionoriented transport layer makes a connection with the transport layer at the destination machine first before delivering the packets. After all the data are transferred, the connection is terminated.
- Flow control :- Like the data link layer, the transport layer is responsible for flow control. However, flow control at this layer is performed end to end rather than across a single link.
- Error control :- Like the data link layer, the transport layer is responsible for error control. However, error control at this layer is performed process-toprocess rather than across a single link. The sending transport layer makes sure that the entire message arrives at the receiving transport layer without error (damage, loss, or duplication). Error correction is usually achieved through retransmission.
Session Layer
The services provided by the first three layers (physical, data link, and network) are not sufficient for some processes. The session layer is the network dialog controller. It establishes, maintains, and synchronizes the interaction among communicating systems.
- Dialog control :- The session layer allows two systems to enter into a dialog. It allows the communication between two processes to take place in either halfduplex (one way at a time) or full-duplex (two ways at a time) mode.
- Synchronization :- The session layer allows a process to add checkpoints, or synChronization points, to a stream of data.
Presentation Layer
- Translation :- The processes (running programs) in two systems are usually exchanging information in the form of character strings, numbers, and so on. The infonnation must be changed to bit streams before being transmitted. Because different computers use different encoding systems, the presentation layer is responsible for interoperability between these different encoding methods. The presentation layer at the sender changes the information from its sender-dependent format into a common format. The presentation layer at the receiving machine changes the common format into its receiver-dependent format.
- Encryption :- To carry sensitive information, a system must be able to ensure privacy. Encryption means that the sender transforms the original information toanother form and sends the resulting message out over the network. Decryption reverses the original process to transform the message back to its original form.
- Compression :- Data compression reduces the number of bits contained in the information. Data compression becomes particularly important in the transmission of multimedia such as text, audio, and video.
Application Layer
The application layer enables the user, whether human or software, to access the network. It provides user interfaces and support for services such as electronic mail, remote file access and transfer, shared database management, and other types of distributed information services
- Network virtual terminal :- A network virtual terminal is a software version of a physical terminal, and it allows a user to log on to a remote host. To do so, the application creates a software emulation of a terminal at the remote host. The user's computer talks to the software terminal which, in turn, talks to the host, and vice versa. The remote host believes it is communicating with one of its own terminals and allows the user to log on.
- File transfer :- access, and management. This application allows a user to access files in a remote host (to make changes or read data), to retrieve files from a remote computer for use in the local computer, and to manage or control files in a remote computer locally.
- Mail services :- This application provides the basis for e-mail forwarding and storage.
- Directory services :- This application provides distributed database sources and access for global information about various objects and services.
