- The OSI model is a seven layer reference tool which is used for understanding data communication between different hosts .
- Each layer of the OSI model performs specific task to provide the service to the layers above it and uses services from the layers below it.
Advantages of Layering
- We can apply divide and conquer strategy by dividing problem into subproblems and then solving them one by one.
- With the concept of layering, encapsulation is possible.
- Data abstraction can also be done at different layers.
- Layering makes testing easy.
- The physical layer manages all the functions required to transmit a bit stream over a physical medium.
- It deals with the mechanical, electrical, functional, and procedural specifications of transmission medium and interface.
- Ethernet network interface card performs functions at both the physical layer and the data link layer.
Functions of Physical Layer:
- The physical layer defines characteristics of the interface between the transmission medium and physical devices.
- Physical layer defines the type of transmission medium.
- It deals with the transmission rate, i.e. the number of bits sent each second.
- It is the responsibiliyt of physical layer to perform clock synchronization of sender and receiver.
- It is concerned with the type connection established between the devices and the transmission medium.
- Point-to-point configuration: In point-to-point connection two devices are connected with each other through a dedicated link.
- Multipoint configuration: In multipoint connection a link is shared between several hosts.
- Physical layer is also concern with the physical topology.
- Different types of topologies:
- Bus Topology
- Star Topology
- Ring Topology
- Mesh Topology
- Hybrid Topology
- Different types of topologies:
- It deals with the direction of transmission also known as transmission mode, i.e. simplex, half-duplex or duplex.
- Physical layer works with bits and establishes connection by transferring bits over network.
- This layer is completely hardware biased and deals with hardware devices, some hardware used are Repeater and Hub.
- Data Unit: Bitstream
Data Link Layer:
- DLL transforms the bits from physical layer (a raw transmission data) to a reliable link.
- DLL is responsible for Node-to-Node delivery of data.
- It makes the physical layer appear error-free to the Network layer.
Functions of the Data Link Layer:
- Data Framing: Framing is dividing the stream of bits received from the network layer into manageable data units called frames and sending it to physical layer for further transmission. Segmentation of upper layer datagrams (packets) into frames.
- Flow Control: Flow control is needed to manage communication between a high-speed transmitting sender with the low-speed receiver.
- Error Control: Error control provides a mechanism of detecting and retransmit ting damaged or lost frames and also prevent duplication of frames. To achieve error control in DLL, a trailer is added at the end of each frame.
- Access Control: Gives mechanism to determine which device has control over the link at any given time, if two or more devices are connected to the same link.
- Physical Addressing: It's the responsibility of DLL to add a header to the frame to define the physical address of the sender (source address) and/or physical address of receiver (destination address) of the frame.
- DLL uses smart devices which have software and hardware as well, devices used: Bridges and switches.
- Data Unit: Frames
- Protocol Used at DLL: Simplex protocol, stop and wait protocol, sliding window, HDLC (High-Level Data Link Control), SDLC, NDP, ISDN, ARP, PSL, OSPF, NDP.
- The network layer is responsible for source to destination delivery of a packet possibly across multiple networks (links).
- It deals with the host-to-host delivery.
- If the two systems are connected with the same link, then there is no need of a network layer.
- If the two systems are connected by different networks then there is often a need of the network layer to accomplish source to destination delivery.
Functions of the Network Layer:
- Logical Addressing: If the packet transmitted between different networks, then we need a logical addressing system to identify the correct source and destination systems.
- Routing: Routing is a mechanism of travelling a data packet from source to destination through different routes. Independent networks or links are connected together with the help of routers or gateways which helps in traversing data packet. Routers route the packets to their final destination. It is Network layer's responsibility to provide a routing mechanism.
- Hardware Used: Routers
- Data Units: Packets
- Protocols Used in Network layer: IP (Internet, Protocol), NAT (Network Address Translation), ARP (Address Resolution Protocol), ICMP (Internet control Message Protocol), BGP (Border Gateway Protocol), RARP (Reverse Address Resolution Protocol), DHCP (Dynamic Host Configuration Protocol), BOOTP, OSPF.
- The transport layer is responsible for source to destination delivery of the entire message.
- It deals with end-to-end or port-to-port delivery of data.
- The network layer does not recognize any relationship between the packets delivered, Network layer recognize each packet as an independent entity, it doesn't concern with the fact that the different packets are of same message or not. But Transport layer takes the whole message intact and keep it in order.
Functions of Transport Layer
- Service Point Addressing: The transport layer header must contain a service point address (or port address).
- Segmentation and Reassembly: A message is divided into transmittable segments, each segment containing a sequence number.
- Flow Control: Flow control at transport layer is done end to end instead of performing it across a single link.
- Error Control: This layer deals with end to end delivery of data, so in this layer error control is done by ensuring that the entire message at the receiving transport layer is received without error (damage, loss or duplication). Error correction is usually done through retransmission of messages.
- Connection Control: Connection-oriented or connectionless approach is used by transport layer for delivering the segments.
- Hardware Used: Transport Gateway
- Data Unit: Segments
- Protocol Used: TCP (Transmission Control Protocol) for connection-oriented approach and UDP (User Datagram Protocol) for connectionless approach.
- The session layer is the network dialog controller in the OSI model.
- It's main responsibility is to establish, maintain and synchronize the interaction between different communicating systems.
- Session Layer plays an important role in keeping applications data separately.
Functions of Session Layer:
- Dialog Control: Session layer is fifth layer in OSI model that allows to establish the communication between two processes either in half-duplex or full-duplex. It allows applications functioning on devices to establish, manage and terminate a dialog through a network.
- Synchronization: The session layer allows a process to add checkpoints or synchronization points into a data stream.
- It's responsibility is to format the data and take care how data is sent out into the network.
- It's duty is to allow or deny any application or site to read and understand the message.
Functions of Presentation Layer:
- Translation: Various device uses different encoding system, so it is the functionality of presentation layer to provide interoperability between different encoding methods. At the sender end it changes the information from sender dependent format into a common format and at the receiver end it changes the common format into receiver's dependent format.
- Encryption and Decryption: Encryption and Decryption of data is the major responsibility of presentation layer. It is a mechanism to assure privacy to carry sensitive information. Encryption of data means transforming the original information of sender into another form(unreadable) and decryption mechanism works at the receiver end, it reverses the new form(unreadable) data into its original form.
- Compression: Presentation layer also uses a compression mechanism to reduce the number of bits to be transmitted. Data compression technique is important while transmission of multimedia such as text, audio and video.
- This layer enables the end user to access the network.
- Application layer provides UI (user interfaces) and support for services such as electronic mail (e-mail), remote file access and other types of distributed information services.
- Examples: Telnet, FTP, etc.
Functions of Application Layer:
- Network Virtual Terminal: NVT is a software version of a physical terminal and it allows a user to log on to a remote host. For this, the application layer creates a software emulation of a terminal at the remote device (host).
- File Transfer, Access and Management: It is duty of application layer to allow or deny a user from accessing files, retrieving files, managing files or controlling files on a remote computer.
- Mail Services: It provides Electronic messaging services(e-mail storage and forwarding).
- Directory Services: It also provides distributed database sources and access to global information about various objects and services.
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