Introduction to the OSI Layer
The OSI model stands for Open System Interconnection. When Network first came into being, Computer could typically communicate with computer from same manufacturer. So the basic purpose of OSI is to break though this barrier.
Purpose of OSI Model –
The OSI model purpose is to meant to help vendors to create inter-operable network device and software in the form of protocols so that different vendor network could work in peaceable accord with each other.
OSI model is the primary architectural model for networks. It describes how data and network information are communicated from an application on computer through the network media to an application on another computer.
OSI Layered Approach –
The OSI reference model are breaks this approach in different layers. These layers are divided in to two groups
Two Groups are –
1. The Upper layers – It defines how the applications within the end station will communicate with each other as well as with users.
2. The Lower Layers – It defines how data is transmitted end to end.
The Network devices operate at all seven layers of OSI model are –
- Network Management System
- Network hosts
Descriptions of OSI Layers –
Layer 7 – Application
The Application layer is the main layer of the OSI model, and it provides the interface between the network protocol and the software running on the computer. The Application layer provides the necessary services that support applications. It provides the interface for e-mail, Telnet and File Transfer Protocol (FTP) applications and files transfers. This is the location where applications interrelate with the network
The common application protocols include
- File Transfer Protocol (FTP)
- Simple Mail Transfer Protocol (SMTP)
- Internet Message Access Protocol (IMAP),
- Post Office Protocol (POP)
- Hypertext Transfer Protocol (HTTP)
- Simple Network Management Protocol (SNMP).
- Network News Transfer Protocol (NNTP)
Layer 6 – Presentation
The primary responsibility of Presentation Layer is to define the syntax that network hosts use to communicate. Compression and encryption fall in the functions of this layer. It is sometimes referred to as the “syntax” layer and responsible for transforming information or data into format(s), which the application layer can use.
The functions performed at the Presentation layer of the OSI are:
- Protocol conversion
- Data translation.
- Data encryption and decryption
- Data compression
- Character set conversion
- Interpretation of graphics commands.
Data is translated at the Presentation layer when it is transmitted from the sender to the receiver. The application of the sender moves the data to the Presentation layer. The Presentation layer translates the data to a common format, which can be read by both computers. When the data is received, the Presentation layer translates the data into a format which the application can read.
Gateway services also function at the Presentation layer. A gateway can be defined as a connection point between networks, which run different systems and applications. Gateways are typically deployed through software. An example is Gateway Services for NetWare (GSNW).
Common gateways include:
- Gateways which cross platforms and file systems
- Systems Network Architecture (SNA) gateways enable PCs to communicate with mainframe computers.
- E-mail gateways enable data to be transmitted between different e-mail applications running the same protocol.
Layer 5 – Session
The Session Layer establishes process to process communications between two or more networked hosts. Under OSI, this layer is responsible for gracefully closing sessions (a property of TCP) and for session check pointing and recovery (not used in IP). It is used in applications that make use of remote procedure calls. The Session layer utilizes the virtual circuits created by the Transport layer to establish communication sessions.
The important functions performed at Session layer to establish, maintain and terminate communication sessions are summarized below:
- Establishes, terminates, and monitors communication sessions between applications.
- Name lookup and security functions.
- Placement the header information in a packet which determines the point where a message starts and the point where a message ends.
- Data synchronization, the layer performs synchronization between the Session layer of the data sender and the Session layer of the receiver of the data.
- Controls whether the communication or messages exchanged in a session are transmitted as full duplex messages or half duplex messages.
- Full duplex: Information is transmitted simultaneously, and in both directions.
- Half duplex: Information is transmitted in both directions, and flows in one direction at a time.
Layer 4 – Transport
The Transport Layer is responsible for the delivery of messages between two or more networked hosts. It handles fragmentation and reassembly of messages and controls the reliability of a given link.
The important functions performed at the Transport layer to enable network communication are listed below:
- Guaranteed data delivery
- Name resolution
- Flow control
- Error detection
- Error recovery
The Transport layer at each computer verifies that the application transmitting the data actually allowed to access the network. It also verifies that each end of the network connection can start the data transfer process. The transport protocols running on each host partaking in communication monitors the data transfer process, and monitors for errors as well.
The common Transport protocols utilized at this layer are:
-Transmission Control Protocol (TCP): TCP is a connection-orientation protocol that offers greater reliability when it comes to transporting data than what UDP, the other TCP/IP protocol, which works at this layer provides. With TCP, the application which sends the data receives acknowledgement or verification that the data was actually received.
-User Datagram Protocol (UDP): UDP is a connectionless protocol that does not provide reliable data transport, No acknowledgements are transmitted.
Layer 3 – Network
The Network Layer is primarily responsible for establishing the paths used for transfer of data packets between devices in the network. Network routers operate at this layer, which is commonly be divided into three sub-layers: Sub network access, Sub network-dependent convergence, and Sub network-independent convergence.
One of the main functions performed at the Network layer is routing. Routing enables packets to be moved among computers, which are more than one link from one another.
The functions performed at the Network layer of the OSI model are listed below:
- Traffic direction to the end destination
- Addressing; logical network addresses and services addresses
- Routing functions; route discovery and route selection
- Packet switching
- Packet sequence control
- End-to-end error detection, from the data sender to the receiver of data.
- Congestion control
- Network layer flow control and Network layer error control
- Gateway services
Layer 2 – Data Link
The Data Link Layer is primarily responsible for communications between adjacent network nodes. Network switches and hubs operate at this layer which may also correct errors generated in the Physical Layer.
The Data-link layer of the OSI model enables the movement of data over a link from one device to another, by defining the interface between the network medium and the software on the computer. The Data-link layer maintains the data link between two computers to enable communications.
The responsibilities of the Data-link layer include:
- Packet addressing
- Media access control
- Format the frame used to encapsulate data
- Error notification on the Physical layer
- Managing of error messaging specific to the delivery of packets.
It ensures that frames are transmitted from one computer to another computer with no errors. It also establishes error-free connections between two devices.
Layer 2 manages the ordering of bits, packets, and from data segments. The ensuing result is called frames. Frames contain data that is already arranged in an orderly manner. The Data-link layer receives packets from the Network layer and structures these packets into frames. The frames are then moved to the Physical layer for sending. A cyclic redundancy check (CRC) is added to the data frame. The CRC detects damaged frames. The computer at the receiving end can request the cyclic redundancy check (CRC) so that it can verify that the frame is not damaged. The Data-link layer can determine when a frame is lost. It also requests any lost frames to be re-transmitted. By performing these tasks, the Data-link layer makes it possible for data bits to be transmitted in an organized manner.
The Data-link layer is divided into the following two sublayers:
1.Logical Link Control (LLC) sublayer: The LLC sublayer provides and maintains the logical links used for communication between the devices.The functions at the LLC sublayer of the Data-link layer include the following:
- Error checking
- Frame synchronization
- Flow control
2.Media Access Control (MAC) sublayer: The MAC sublayer of the Data-link layer controls the transmission of packets from one network interface card (NIC) to another over a shared media channel. A NIC has a unique MAC address, or physical address. This address identifies the particular NIC on the network. To ensure that these addresses are unique, the MAC addresses are usually permanently burned in the memory of the NIC. The MAC sublayer handles media access control, which essentially prevents data collisions. It provides for the allocation of network access to computers, and more importantly, it prevents computers from transmitting data simultaneously.The common media access control methods are listed below.
- Token Passing; utilized in Token Ring and FDDI networks
- Carrier Sense Multiple Access/Collision Detection (CSMA/CD); utilized in Ethernet networks.
- Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA); utilized in AppleTalk networks.
Layer 1 – Physical
The Physical Layer handles the bit level transmission between two or more network nodes.
The first layer in the OSI model is the Physical layer which transmits raw bit streams over a physical medium. The Physical layer deals with establishing a physical connection between computers to enable communication. The physical layer is hardware specific and deals with the actual physical connection between the computer and the network medium. All devices that function at the Physical layer handle signalling. Data handled at the layer are in bits (1s and 0s). The 1s and 0s are in represented by pulses of light or electricity. Components in this layer include connectors, cable types, pin-outs, and voltages which are defined by the applicable standards organization.
The details on the actual physical connection defined at this layer include:
- Physical topologies of the network.
- Network connection types and how cable is attached to the Network Interface Card (NIC).
- Data encoding: This relates to the analog and digital signaling methods utilized to encode data in the signals.
- Bit synchronization
The specifications of the Physical layer include:
- Physical layout of the network
- Voltage changes and the timing of voltage changes.
- Data rates
- Maximum transmission distances
- Physical connectors to transmission mediums
The issues normally clarified at the Physical Layer include:
- Whether data is transmitted synchronously or asynchronously
- Whether the analog or digital signaling method is used
- Whether baseband or broadband signalling is used.