Application Layer An application layer is the topmost layer within the TCP/IP model. When one application layer protocol needs to communicate with another application layer, it forwards its information to the transport layer. Transport Layer It is responsible for the reliability, flow control, and ...
OSI stands for Open Systems Interconnection. It has been developed by ISO – ‘International
Organization for Standardization‘, in the year 1984. It is a 7 layer architecture with each layer
having specific functionality to perform. All these 7 layers work collaboratively to transmit the
data from one person to another across the globe.
1. Physical Layer (Layer 1) :
The lowest layer of the OSI reference model is the physical layer. It is responsible for the actual
physical connection between the devices. The physical layer contains information in the form of
bits. It is responsible for transmitting individual bits from one node to the next. When receiving
data, this layer will get the signal received and convert it into 0s and 1s and send them to the
Data Link layer, which will put the frame back together.
The functions of the physical layer are as follows:
, Bit synchronization: The physical layer provides the synchronization of the bits by providing a
clock. This clock controls both sender and receiver thus providing synchronization at bit
level.
Bit rate control: The Physical layer also defines the transmission rate i.e. the number of bits
sent per second.
Physical topologies: Physical layer specifies the way in which the different, devices/nodes are
arranged in a network i.e. bus, star, or mesh topology.
Transmission mode: Physical layer also defines the way in which the data flows between the
two connected devices. The various transmission modes possible are Simplex,
half-duplex and full-duplex.
* Hub, Repeater, Modem, Cables are Physical Layer devices.
** Network Layer, Data Link Layer, and Physical Layer are also known as Lower Layers or
Hardware Layers.
2. Data Link Layer (DLL) (Layer 2) :
The data link layer is responsible for the node-to-node delivery of the message. The main
function of this layer is to make sure data transfer is error-free from one node to another, over
the physical layer. When a packet arrives in a network, it is the responsibility of DLL to transmit it
to the Host using its MAC address.
Data Link Layer is divided into two sublayers:
Logical Link Control (LLC)
Media Access Control (MAC)
The packet received from the Network layer is further divided into frames depending on the
frame size of NIC(Network Interface Card). DLL also encapsulates Sender and Receiver’s MAC
address in the header.
, The Receiver’s MAC address is obtained by placing an ARP(Address Resolution Protocol) request
onto the wire asking “Who has that IP address?” and the destination host will reply with its MAC
address.
The functions of the Data Link layer are :
Framing: Framing is a function of the data link layer. It provides a way for a sender to
transmit a set of bits that are meaningful to the receiver. This can be accomplished by
attaching special bit patterns to the beginning and end of the frame.
Physical addressing: After creating frames, the Data link layer adds physical addresses (MAC
address) of the sender and/or receiver in the header of each frame.
Error control: Data link layer provides the mechanism of error control in which it detects and
retransmits damaged or lost frames.
Flow Control: The data rate must be constant on both sides else the data may get corrupted
thus, flow control coordinates the amount of data that can be sent before receiving
acknowledgement.
Access control: When a single communication channel is shared by multiple devices, the
MAC sub-layer of the data link layer helps to determine which device has control over
the channel at a given time.
* Packet in Data Link layer is referred to as Frame.
** Data Link layer is handled by the NIC (Network Interface Card) and device drivers of host
machines.
*** Switch & Bridge are Data Link Layer devices.
3. Network Layer (Layer 3) :
The network layer works for the transmission of data from one host to the other located in
different networks. It also takes care of packet routing i.e. selection of the shortest path to
transmit the packet, from the number of routes available. The sender & receiver’s IP addresses
are placed in the header by the network layer.
The functions of the Network layer are :
Routing: The network layer protocols determine which route is suitable from source to
destination. This function of the network layer is known as routing.
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