CCNA 1 Chapter 2 OSI model Objectives General

CCNA 1 – Chapter 2 OSI - model

Objectives • General model of communication • The OSI reference model • Comparison between OSI and TCP/IP reference model

Overview • In the early 1980 s, companies began proprietary to implement their (private) network. • Every company developed its own networking devices & software. • So , each network implemented its own specifications.

incompatible • The result was networks (because of the absence of Standards. • Subsequently, the communication between networks was very difficult. • There was a need to develop networking standards to ensure the Interoperability between Networks.

• ISO created OSI model in 1984. In this chapter we will study: • The importance of OSI model • the basic functions of each OSI layer. • Data flow over networks.

Example of layering model Has his own Director general responsibilities (functions) secretary Fax Has her own responsibilities (functions) Provides fax services (functions) • Each layer performs a group of functions (layer N).

(layer N) (layer N-1) (layer N-2) Has his own Director general responsibilities (functions) secretary Fax Has her own responsibilities (functions) Provides fax services (functions) • Each layer provides services to the layer above it.

• English director general is going to send a fax to French director general. (layer N) (layer N-1) (layer N-2) Director general (English) (French) layer N Secretary layer (English/ French) N-1 Fax layer N-2

Director general (layer N) (English) Writes the text Peer-to-peer (French) Translated text Passes the text (with instructions) (layer N-1) (layer N-2) Secretary Translates the text (English/ French) Peer-to-peer (English/ French) Secretary Sent the fax picks up the text Fax Physical communication

(layer N) (layer N-1) (layer N-2) Director general (English) (French) Secretary (English/ French) Fax • Change of one layer (director general, secretary or fax) will not require a change of any other layer.

OSI reference model

Networking concepts. . . • Networking is the interconnection of workstations, peripherals and other devices. • All devices must speak the same language Protocol: is the rules that governs the data format and data exchange.

• Layer N in one computer communicates with layer N on another computer. Peer –to –peer communication Using layer N protocol

Source , destination, and data packets: • Data is sent in bits, 1 s, and 0 s. • Data stream is broken into smaller units, called Packets. Why ? Øevery computer can take its turn in transmitting data. ØIf packet is lost, only a small amount of data is retransmitted.

ØPackets can take different paths. • Each packet must include the identity of the sending device Source address and the identity of the receiving device Destination address

Different Media Types AIR carries light, radio, microwave Media—material through which data packets travel

Why a layered Network model? • It divides a network communication process into smaller parts: ØEasier to develop network components. ØEasier to design networks. ØEasy to troubleshoot network problems. ØEasier to manage networks.

• It standarizes network devices and software: ØEnsures compatibility (interoperability) between devices from different vendors. • It prevents changes in one layer from affecting the other layers: ØLayers are developed quickly. ØAccelerates the evolution of networks. • Simplifies the learning process.

The seven layers of OSI model • Data flow is divided into seven smaller and manageable steps. • What devices operate at each layer. • As a result, you will understand how to trouble shoot network problems.

Functions of each layer

The application layer: • Provides services to user’s applications. • Ensures the availability of intended communication partner. • Ensures the existence of sufficient network resources (ex. NIC). • Synchronizes and establishes agreement on error recovery. • Data integrity.

The application layer: • services to user’s applications. • Data format. • Insures data is readable by the receiving system. • Translates between different data formats. • Compression /decompression • Encryption/ decryption.

The session layer: • services to user’s applications. • Data format. • Establishes, manages and terminates commun. between presentation layer entities. • Dialog management (half/full duplex). • Synchronization (checkpoints).

• Provisions for efficient data transmission. • Reporting of session, presentation and application layer errors (errors above transport layer).

The transport layer: • services to user’s applications. • Data format. • Dialog and conversations. • End - to - end services. • Segmentation / reassembly : • Reliability : üerror recovery üacknowledgment.

• Flow control : üSliding windows. • Establishes , manages and terminates virtual circuits (not physical circuits). • Shields the upper layers from transport details.

The network layer: • services to user’s applications. • Data format. • Dialog and conversations. • Reliability, flow control, error recovery. • IP addressing. • Path selection. • Packet routing.

The data link layer: • services to user’s applications. • Data format. • Dialog and conversations. • Reliability, flow control, error recovery. • Addressing / path selection. • Point –to- point services. • Reliability over the physical link.

• Media access control. • Network topology. • Error detection. • Flow control. • Physical addressing. • Framing.

The physical layer: • services to user’s applications. • Data format. • Dialog and conversations. • Reliability, flow control, error recovery. • Addressing / path selection. • Framing, access control. • Binary transmission.

• Electrical characteristics of physical medium. • Pin assignment. • Timing of bits. • Voltage levels • Data rate. • Distances. • Encoding schemes.

encapsulation

Encapsulation wraps data with necessary protocol information before transit. Data stream Data header Data trail (peer-to-peer communication- PDUs) 0011110101001101011100011001

TCP / IP model

• The U. S. Department of Defense (Do. D) created the TCP/IP reference model because it wanted a network that could survive any conditions, even a nuclear war. • The main purpose was to move packets from any point to any other point regardless of the condition of any particular node.

The application layer: • Session and Presentation layers are thin. • TCP/IP model combines the functions of the upper three layers into one layer.

The transport layer: §Deals with Qo. S issues (TCP protocol): • reliability: üError recovery. üAcknowledgment. • flow control: üSliding windows. §TCP – connection oriented protocol.

The internet layer: • path determination. • Packet switching. • IP addressing. • Connectionless services (unreliable).

The network-access layer: • Host-network layer. • Combines the functions of physical and data link layers. • It includes the WAN and LAN technology.

TCP / IP protocol graph:

Similarities: • both have layers • both have application layers. • both have comparable transport and network layers • packet-switched (not circuit-switched) technology is assumed • networking professionals need to know both

Differences • TCP/IP combines the presentation and session layer issues into its application layer • TCP/IP combines the OSI data link and physical layers into one layer • TCP/IP appears simpler because it has fewer layers • TCP/IP protocols are the standards around which the Internet developed, so the TCP/IP model gains credibility just because of its protocols. In contrast, typically networks aren't built on the OSI protocol, even though the OSI model is used as a guide.

OSI # OSI Layer Name 7 application TCP/IP Layer Encapsul. / IP# Name Units. 4 application data 6 presentation 4 application data 5 session 4 application data 4 transport 3 transport segment 3 network 2 internet packet 2 Data link 1 Netw. access frame 1 physical 1 Netw. access bits

Good Luck