Clearly explain the TCP/IP protocol model and the ISO-OSI model.
Clearly explain the TCP/IP protocol model and the ISO-OSI model.
The OSI model is a layered framework for the design of network systems that allows communication between all types of computer systems. It consists of seven separate but related layers, each of which defines a part of the process of moving information. The OSI model is composed of seven ordered layers:
- physical (layer 1)
- data link (layer 2)
- network (layer 3)
- transport (layer 4)
- session (layer 5)
- presentation (layer 6)
- application (layer 7)
In above we can clearly see the process of transferring the message between two devices(A and B).
At the physical layer, communication is direct: In Figure device A sends a stream of bits to device B (through intermediate nodes). At the higher layers, however, communication must move down through the layers on device A, over to device B, and then back up through the layers. Each layer in the sending device adds its own information to the message it receives from the layer just above it and passes the whole package to the layer just below it. At layer 1 the entire package is converted to a form that can be transmitted to the receiving device. At the receiving machine, the message is unwrapped layer by layer, with each process receiving and removing the data meant for it. For example, layer 2 removes the data meant for it, then passes the rest to layer 3. Layer 3 then removes the data meant for it and passes the rest to layer 4, and so on.. The passing of the data and network information down through the layers of the sending device and back up through the layers of the receiving device is made possible by an interface between each pair of adjacent layers. Each interface defines the information and services a layer must provide for the layer above it.
Now lets describe each layer process in details.
Physical Layer--The physical layer is responsible for movement of individual bits from one node to next node. It coordinates the functions required to carry a bit stream over a physical medium. It deals with the mechanical and electrical specifications of the interface and transmission medium. It also defines the procedures and functions that physical devices and interfaces have to perform for transmission to Occur.
The physical layer is also concerned with the following: Physical characteristics of interfaces and medium, data rate, physical topology , transmission mode etc.
Data Link Layer-- The data link layer transforms the physical layer, a raw transmission facility, to a reliable link. It makes the physical layer appear error-free to the upper layer (network layer). data link layer concerns with the following:
1.Framing. The data link layer divides the stream of bits received from the network layer into manageable data units called frames.
2.Physical addressing. If frames are to be distributed to different systems on the network, the data link layer adds a header to the frame to define the sender and/or receiver of the frame. If the frame is intended for a system outside the sender's network, the receiver address is the address of the device that connects the network to the next one.
3.Flow control. If the rate at which the data are absorbed by the receiver is less than the rate at which data are produced in the sender, the data link layer imposes a flow control mechanism to avoid overwhelming the receiver.
4.Error control. The data link layer adds reliability to the physical layer by adding mechanisms to detect and retransmit damaged or lost frames. It also uses a mechanism to recognize duplicate frames. Error control is normally achieved through a trailer added to the end of the frame.
5.Access control. When two or more devices are connected to the same link, data link layer protocols are necessary to determine which device has control over the link at any given time.
Network Layer--The network layer is responsible for the source-to-destination delivery of a packet, possibly across multiple networks (links). Whereas the data link layer oversees the delivery of the packet between two systems on the same network (links), the network layer ensures that each packet gets from its point of origin to its final destination.
If two systems are connected to the same link, there is usually no need for a network layer. However, if the two systems are attached to different networks (links) with connecting devices between the networks (links), there is often a need for the network layer to accomplish source-to-destination delivery. Network layer concerns with the following--
1.Logical addressing. The physical addressing implemented by the data link layer handles the addressing problem locally. If a packet passes the network boundary, we need another addressing system to help distinguish the source and destination systems. The network layer adds a header to the packet coming from the upper layer that, among other things, includes the logical addresses of the sender and receiver. We discuss logical addresses later in this chapter.
2.Routing. When independent networks or links are connected to create network of networks or a large network, the connecting devices (called routers or switches) route or switch the packets to their final destination.
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