Physical Layer - Layer 1 of OSI model

- The physical layer is level one in the seven level OSI model. Actually, it is last layer that receives and processes the data from the sending device while it is first layer to receive the data at the destination end.
- It performs services requested by the data link layer.
- The Physical Layer defines the Mechanical, Electrical, Procedural and Functional specifications for activating, maintaining and deactivating the physical link between communication network systems.
- The basic objective of this layer is to transform the data in the form that is needed to be carried through the transmission media over the network. The transmission media either bounded or unbounded , carries the data in the form of electromagnetic waves or radio waves.
- The Physical Layer is responsible for bit-level transmission between network nodes.
The main functions of physical layer are :

- Definition of Hardware Specifications: The details of operation of cables, connectors, wireless radio transceivers, network interface cards and other hardware devices are generally a function of the physical layer. Devices used in the Physical Layer are
* Network Interface Cards (NIC)
* Transceivers
* Repeaters
* Hubs
* Multi Station Access Units (MAU’s)
- Encoding and Signaling: The physical layer is responsible for various encoding and signaling functions that transform the data from bits that reside within a computer or other device into signals that can be sent over the network.
- Data Transmission and Reception: After encoding the data appropriately, the physical layer actually transmits the data, and of course, receives it.
- Topology and Physical Network Design: The physical layer is also considered the domain of many hardware-related network design issues, such as LAN and WAN topology. There are four possible kinds of topologies:
* Bus
* Star
* Ring
* Mesh
In general, then, physical layer technologies are ones that are at the very lowest level and deal with the actual ones and zeroes that are sent over the network.

The Transport Layer - Layer 4 of OSI model

The Transport Layer of the OSI model is responsible for delivering messages between networked hosts. The Transport Layer should be responsible for fragmentation and reassembly.

- This layer converts the data received from the upper layers into segments and prepares them for transport.
- The Transport layer is responsible for end-to-end (source-to-destination) delivery of entire messages.
- It allows data to be transferred reliably and uses sequencing to make sure that the order of packets is maintained.
- It also provides services such as error checking and flow control.
- In case IP, lost packets arriving out of order must be reordered.
- The size and complexity of a transport protocol depends on the type of service it can get from the network layer.
- The transport layer can accept relatively large messages, but there are strict message size limits imposed by the network (or lower) layer.
- Two transport protocols, Transmission Control Protocol (TCP) and User Datagram Protocol (UDP), sits at the transport layer.
- TCP establishes connections between two hosts on the network through 'sockets' which are determined by the IP address and port number. It keeps a track of the packet delivery order and the packets that must be resent.
- UDP provides a low overhead transmission service, but with less error checking.
- The Transport layer protocols are either connectionless or connection-oriented.
- Connection-oriented means that a connection (a virtual link) must be established before any actual data can be exchanged. e.g. TCP.
- In Connectionless, the sender does not establish a connection before it sends data, it just sends it without guaranteeing delivery. e.g. UDP.

Data Segmentation

Data segmentation is the process by which the transport layer uniquely handles all data passed to and from different upper-level applications. For example, if a user is browsing the web and checking email at the same time, each program would be passing data and waiting for a reply on a unique port number. The Transport layer ensures that data is passed to the correct application.