OSI Layers

The bottom four layers of the OSI model are often referred to as the lower

layers:

·        Layer-4 – Transport layer

·        Layer-3 – Network layer

·        Layer-2 – Data-Link layer

·        Layer-1 – Physical layer

Protocols that operate at these layers control the end-to-end transport of data between devices, and are implemented in both software and hardware.

The Transport Layer :- 

The Transport layer (Layer-4) does not actually send data, despite its

name. Instead, this layer is responsible for the reliable transfer of data, by

ensuring that data arrives at its destination error-free and in order.

Transport layer communication falls under two categories:

Connection-oriented :– requires that a connection with specific agreed-upon parameters be established before data is sent.

Connectionless – requires no connection before data is sent. Connection-oriented protocols provide several important services.

Segmentation and sequencing – data is segmented into smaller pieces for transport. Each segment is assigned a sequence number, so that the receiving device can reassemble the data on arrival.

Acknowledgments – receipt of data is confirmed through the use of acknowledgments. Otherwise, data is retransmitted, guaranteeing delivery.

Flow control (or windowing) – data transfer rate is negotiated to prevent congestion.

Connection establishment – connections are established, maintained, and ultimately terminated between devices.

The TCP/IP protocol suite incorporates two Transport layer protocols:

Transmission Control Protocol (TCP) – connection-oriented

User Datagram Protocol (UDP) - connectionless

The Network Layer :- 

The Network layer (Layer-3) controls internetwork communication, and has two key responsibilities:-

Logical addressing – provides a unique address that identifies both the host, and the network that host exists on.

Routing – determines the best path to a particular destination network, and then routes data accordingly. 

Two of the most common Network layer protocols are:

The Data-Link Layer :-

While the Network layer is concerned with transporting data between  networks, the Data-Link layer (Layer-2) is responsible for transporting  data within a network.

The Data-Link layer consists of two sublayers:

·        Logical Link Control sublayer

·        Media Access Control  sublayer

The LLC sublayer serves as the intermediary between the physical link and all higher layer protocols. It ensures that protocols like IP can function regardless of what type of physical technology is being used.

Additionally, the LLC sublayer can perform flow-control and error checking, though such functions are often provided by Transport layer protocols, such as TCP.

The MAC sublayer controls access to the physical medium, serving as mediator if multiple devices are competing for the same physical link. Datalink layer technologies have various methods of accomplishing this -

Ethernet uses Carrier Sense Multiple Access with Collision Detection (CSMA/CD), and Token Ring utilizes a token.

Ethernet is covered in great detail in another guide. 

The Data-Link Layer :-

The Data-link layer packages the higher-layer data into frames, so that the

data can be put onto the physical wire. This packaging process is referred to as framing or Encapsulation.

·        FDDI (Fiber Distributed Data Interface)

·        802.11 Wireless

·        Frame-Relay

ATM (Asynchronous Transfer Mode) :-

The data-link frame contains the source and destination hardware (or physical) address. Hardware addresses uniquely identify a host within a network, and are often hardcoded onto physical network interfaces.

However, hardware addresses contain no mechanism for differentiating one network from another, and can only identify a host within a network.

Here I point on one line that The most common hardware address is the Ethernet MAC address.

The Physical Layer:-

The Physical layer (Layer-1) controls the signaling and transferring of raw bits onto the physical medium. The Physical layer is closely related to the Data-link layer, as many technologies (such as Ethernet) contain both datalink and physical functions.

The Physical layer provides specifications for a variety of hardware:

·        Cabling

·        Connectors and transceivers

·        Network interface cards (NICs)

·        Wireless radios

·        Hubs

Physical-layer devices and topologies are covered extensively in other guides.