The OSI Model: Understanding the Seven Layers of Computer Networks Part - 02

 The OSI Model: Understanding the Seven Layers of Computer Networks

Layer 1 – The Physical Layer

The physical layer of the OSI model defines connector and interface specifications, as well as the medium (cable) requirements. Electrical, mechanical, functional, and procedural specifications are provided for sending a bit stream on a computer network.

Components of the physical layer include:

• Cabling system components

• Adapters that connect media to physical interfaces

• Connector design and pin assignments

• Hub, repeater, and patch panel specifications

• Wireless system components

• Parallel SCSI (Small Computer System Interface)

• Network Interface Card (NIC)

Note: The Physical Layer of the OSI model is only part of a LAN (Local Area Network).

Layer 2 – The Data Link Layer

Layer 2 of the OSI model provides the following functions:

• Allows a device to access the network to send and receive messages

• Offers a physical address so a device’s data can be sent on the network

• Works with a device’s networking software when sending and receiving messages

• Provides error-detection capability

Common networking components that function at layer 2 include:

• Network interface cards

• Ethernet and Token Ring switches

• Bridges

Layer 3 – The Network Layer

Layer 3, the network layer of the OSI model, provides an end-to-end logical addressing system so that a packet

of data can be routed across several layer 2 networks (Ethernet,Token Ring, Frame Relay, etc.). Note that net-

work layer addresses can also be referred to as logical addresses.

Initially, software manufacturers, such as Novell, developed proprietary layer 3 addressing. However, the net-

working industry has evolved to the point that it requires a common layer 3 addressing system.The Internet

Protocol (IP) addresses make networks easier to both set up and connect with one another.The Internet uses

IP addressing to provide connectivity to millions of networks around the world.     

Layer 4 – The Transport Layer

Layer 4, the transport layer of the OSI model, offers end-to-end communication between end devices through a

network. Depending on the application, the transport layer either offers reliable, connection-oriented or con-

nectionless, best-effort communications.

Some of the functions offered by the transport layer include:

• Application identification

• Client-side entity identification

• Confirmation that the entire message arrived intact

• Segmentation of data for network transport

• Control of data flow to prevent memory overruns

• Establishment and maintenance of both ends of virtual circuits

• Transmission-error detection

• Realignment of segmented data in the correct order on the receiving side

• Multiplexing or sharing of multiple sessions over a single physical link

The most common transport layer protocols are the connection-oriented TCP Transmission Control Protocol

(TCP) and the connectionless UDP User Datagram Protocol (UDP).

Layer 5 – The Session Layer

Layer 5, the session layer, provides various services, including tracking the number of bytes that each end of

the session has acknowledged receiving from the other end of the session.This session layer allows applica-

tions functioning on devices to establish, manage, and terminate a dialog through a network. Session layer

functionality includes:

• Virtual connection between application entities

• Synchronization of data flow

• Creation of dialog units

• Connection parameter negotiations

• Partitioning of services into functional groups

• Acknowledgements of data received during a session

• Retransmission of data if it is not received by a device

Layer 6 – The Presentation Layer

Layer 6, the presentation layer, is responsible for how an application formats the data to be sent out onto the

network.The presentation layer basically allows an application to read (or understand) the message.

Examples of presentation layer functionality include:

• Encryption and decryption of a message for security

• Compression and expansion of a message so that it travels efficiently

• Graphics formatting

• Content translation

• System-specific translation

Layer 7 – The Application Layer

Layer 7, the application layer, provides an interface for the end user operating a device connected to a net-

work.This layer is what the user sees, in terms of loading an application (such as Web browser or e-mail); that

is, this application layer is the data the user views while using these applications.

Examples of application layer functionality include:

• Support for file transfers

• Ability to print on a network

• Electronic mail

• Electronic messaging                        

• Browsing the World Wide Web

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