Layered Task, OSI Model, TCP/IP Model Chapter 2 Layered Task, OSI Model, TCP/IP Model

Protocol Layers In Chapter 1, we discussed that a protocol is required when two entities need to communicate. When communication is not simple, we may divide the complex task of communication into several layers. In this case, we may need several protocols, one for each layer. Let us use a scenario in communication in which the role of protocol layering may be better understood. We use two examples. In the first example, communication is so simple that it can occur in only one layer.

Layered TASK We use the concept of layers in our daily life. As an example, let us consider two friends who communicate through postal mail. The process of sending a letter to a friend would be complex if there were no services available from the post office. Topics discussed in this section: Sender, Receiver, and Carrier Hierarchy

Figure 2.1 Tasks involved in sending a letter

Why layered communication? To reduce complexity of communication task by splitting it into several layered small tasks Functionality of the layers can be changed as long as the service provided to the layer above stays unchanged makes easier maintenance & updating Each layer has its own task Each layer has its own protocol

Reference Models OSI reference model TCP/IP

OSI Reference model Open System Interconnection 7 layers Each layer performs a well define function Functions of the layers chosen taking internationally standardized protocols Number of layers – large enough to avoid complexity

Seven layers of the OSI model

The interaction between layers in the OSI model

OSI Layers

Physical layer Transporting bits from one end node to the next type of the transmission media (twisted-pair, coax, optical fiber, air) bit representation (voltage levels of logical values) data rate (speed) synchronization of bits (time synchronization)

Physical Layer Physical layer is responsible for electrical, mechanical and procedural checks. Data will be converted into Binary(i.e) 0’s & 1’s. Data will be in the form of electrical pulses if it is Coaxial or twisted pair cable and in the form of light if it is fiber optic cable. Devices working on physical layer are hubs, Repeaters, Cables, Modems etc.

Note The physical layer is responsible for movements of individual bits from one hop (node) to the next.

Data Link layer Transporting frames from one end node to the next one - framing - physical addressing - flow control - error control access control

Data Link Layer Data Link layer is divided into two sub layers: LLC- Logical Link Control It talks about WAN protocols E.G. PPP, HDLC, Frame-relay MAC- Media Access Control it talks about physical Address. It is a 48 bit address i.e. 12 digit hexadecimal Number. It is also responsible for Error Detection.

Data Link layer - example-

Note The data link layer is responsible for moving frames from one hop (node) to the next.

Figure 2.8 Network layer

Network Layer Network Layer is responsible for providing best path for data to reach the destination. Logical Addressing works on this layer. Router is a network device layer device. It is divided into two parts Routed Protocols e.g. IP, IPX, Apple Talk Routing Protocols e.g. RIP, IGRP, OSPF, EIGRP

the source host to the destination host. Note The network layer is responsible for the delivery of individual packets from the source host to the destination host.

Figure 2.9 Source-to-destination delivery

Figure 2.10 Transport layer

Note The transport layer is responsible for the delivery of a message from one process to another.

Transport Layer It is responsible for end to end conectivity. It is also known as the heart of OSI Layers. Following tasks are performed at transport layer: Identifing service Multiplexing & De-multiplexing Segmentation Sequencing & Reassembling Error Correction Flow Control