The Seven layer's Of OSI Model.

The International Standards Organization (ISO) developed the Open Systems Interconnection (OSI) Reference Model to define functional communications standards. This reference model is widely used by equipment manufacturers to assure their products will interoperate with products from other vendors. The OSI model describes the functions that are performed in data communications. The model architecture is seven layers those are listed below with each layer defining specific functions.

Layer 7 Application
The Application layer provides the interface to the user. Any user application that requires network communication accesses the communication resources through this layer. This layer also is responsible for finding and determining the availability of communication partners. Typical applications in the TCP/IP protocols are Simple Mail Transfer Protocol (SMTP), Telnet, and File Transfer Protocol (FTP).

Layer 6 Presentation
The Presentation layer is responsible for encoding and decoding data that is passed from the application layer to another station on the internetwork. This layer is responsible for encoding data in a format that the receiving station can interpret and for decoding data received from other stations. Data compression and encryption are accomplished at this layer. Typical coding schemes include ASCII, EBCDIC, MPEG, GIF, and JPEG.

Layer 5 Session
The session layer is responsible for creating, managing and termination sessions that are used by entities at the presentation layer. The session layer is responsible for coordinating the service requests and responses generated and received by a station when it is communication with other entities on the internetwork.

Layer 4 Transport
The Transport layer implements reliable internetwork data transport services that are transparent to upper-layer protocols. The services include flow control, multiplexing, and error checking and recovery. If virtual circuits are needed for the communication to be accomplished, they are built and maintained at this layer. Flow control is responsible for making sure that a sending station does not transmit data faster than the receiving station can process it. Multiplexing allows multiple applications to share a common network interface. Error checking is implemented to discover errors on transmission and to provide a recovery mechanism when errors are found. Typical error recovery includes retransmission of the data.

Layer 3 Network
The Network layer defines routing services that allow multiple data links to be combined into an internetwork. The Network layer defines network-addressing schemes that logically identify network devices. The logical network addresses are different from the physical addresses defined at the MAC layer, and are used by routing protocols running at this level to transfer packets from one network to another. The most common network addressing protocols are IP, IPX, and AppleTalk. Typical routing protocols that run at this level are RIP, OSPF, IGRP, and NLSP.

Layer 2 Data Link
The Data Link layer provides reliable transit of data across a physical network link. The Data Link layer also defines the physical network-addressing scheme, such as the MAC Address on network interface cards in a workstation connected to a LAN. The Data Link layer also defines the topology of the network (bus, star, dual ring, and so on). Flow control at the Data Link layer is defined to ensure receiving stations are not overrun with data before they can process data already received. The Institute of Electrical and Electronics Engineers (IEEE) has redefined the Data Link layer into two sublayers. The sublayers are the Logical Link Control (LLC) layer and the Media Access Control (MAC) layer. The LLC and MAC sublayers are defined in the IEEE 802.2 standards. The LLC manages communications between devices over a single link of a network. The MAC sublayer manages access to the physical medium from multiple upper-level protocols. The MAC layer also defines the MAC address, which uniquely identifies devices at the Data Link layer.

Layer 1 Physical
The Physical layer defines the parameters necessary to build, maintain, and break the physical link connections. It defines the characteristics of the connectors, data transmission rates and distances, and the interface voltages.

OSI (Open System Interconnection) reference model and it's seven layers explained.

OSI is a standard description or "reference model" for how messages should be transmitted between any two points in a telecommunication network. This model is designed to interconnect computers, but is now applied in several other areas, like wireless.

” A reference model is a conceptual blueprint of how communications should take place. it addresses all the processes required for effective communication and divides these processes into logical grouping called layers. ”

The seven Layers of the OSI Model :-
Layer 7: Application Layer
Layer 6: Presentation Layer
Layer 5: Session Layer
Layer 4: Transport Layer
Layer 3: Network Layer
Layer 2: Data-Link Layer
Layer 1: Physical Layer

Application Layer

In this layer we have the User Interfaces, which are created by the data itself (email, file transfer, etc). This is where the data is sent and received by users. Such requests are made by applications according to the protocols used.
This layer is probably that you are more used to. You interact directly with it for example when using a program to read or send email, or communicate through instant messaging.

Protocols: BitTorrent, BGP ,BOOTP , DNS, DHCP , FTP , HTTP, HTTPS, IMAP, LDAP , MIME , NetBIOS , NNTP, NTP, POP3 , RADIUS , RDP, RTP, RTSP , SSH , SIP , SMTP, SNMP, SMB, Telnet, TFTP,

Presentation Layer

At the Presentation layer (Layer 6) data is first converted into a form that can be sent over a network. At this layer data is compressed and decompressed and encrypted or decrypted, depending on which direction it's traveling.

Protocols: SSL, TLS.

Devices: Gateways (translating protocols between different networks).

Session Layer:

This layer deals with communication between two devices. For example: when the user goes to a website, the user’s computer must open a session between itself and server hosting the website, thus allowing the user to receive the website in the first place. The same goes for any sort of communication, ie VOIP etc.

Function: start, manage and terminate sessions for the presentation layer, eg TCP sessions.

Transport Layer:

This layer manages the end-to-end control (for example, determining whether all packets have arrived) and error-checking. It ensures complete data transfer.
This layer is responsible for resending any packets that do not receive an acknowledgment from the destination address. It's also responsible for any problems that are associated with fragmentation of packets.

Network Layer :

This layer handles the routing of the data (sending it in the right direction to the right destination on outgoing transmissions and receiving incoming transmissions at the packet level). The network layer does routing and forwarding.

Data Link Layer:

The Data Link layer (Layer 2) is responsible for sending data to the Physical layer so that it can be put onto the "wire" or network media. The Data Link layer is subdivided into two other layers: the Logical Link Control (LLC) and the Media Access Control (MAC) layers. The LLC connects the Data Link layer to the higher-level protocols such as IP at the Network layer. The MAC layer connects the Data Link layer to the physical connection and provides the MAC address. The Data Link layer also defines the technology that is used for the network. This layer can also perform checksums, which are calculations that the system uses to make sure that packets are not damaged in transit.

Physical Layer:

The Physical layer (Layer 1) defines the physical characteristics of the network such as the type of cable that must be used as well as the voltage that will be used to transmit data through the network. Since the Physical layer defines these characteristics, it also establishes the topology of the network. Many standards are defined at this layer, such as the IEEE 802.3 standard for Ethernet as well as the IEEE 802.5 standard for Token Ring networks.

The Cisco Three-Layer Hierarchical Model.

Cisco layer model consist of the following three layers.

The Core layer

The Distribution layer

The Access layer

Each layer plays a role and specific responsibilities are assigned to these three logical layers. These layers are same like the network layers of OSI reference model.

Core Layer

Core layer is know as core of network as it is on the top of the network and it is responsible to transfer heavy amount of traffic in reliable and quick manner. Objective of core layer is to speed up the network traffic as much as possible. Traffic at core layer is common for most of users and user data is transported to distribution layers which forwards requests if it is required. If core layer is affected by a failure, every user is affected on network. Fault tolerance is main thing to consider on this layer.
The main responsibility of core layer is to see heavy traffic, so speed and traffic issues are concerned at this layer. What are functions of core layer not do the following things.

Protect the network from slowing down the traffic, use of access lists, routing between different Virtual Local Area Networks (VLANs) and Packet Filtering.

Protect network from workgroup access support.

Don't expand core as network grows. Try to overcome performance issue by adding routers and prefer to upgrade devices over expansion.

However while designing core, we want to do the following things.

Design the core by using data link technologies which provides speed and redundancy like FDDI, Fast Ethernet and ATM.

Prefer to select protocols with low convergence time and fast redundant link connectivity.

Distribution Layer

It is also known as workgroup layer and it is called communication point between access and core layer. Basic function of distribution layer is routing, filtering and WAN access and find out the method by which packets can access the core. This layer must find out the fastest mechanism to handle network operations like how to handling and forwarding a file to server on request. After finding best path, distribution layer forward request towards core layer and then to the right service. Policy implementation is done on distribution layer and you can exercise flexibility defining network operations.

Here are the functions which should be done at distribution layer.

Implementations of access lists for filtering interesting traffic and blocking uninteresting traffic.

Security and network policy implementation containing address translation and firewalls.

Static routing redistribution

Enabling routing between all VLANs

Defining broadcast and multicast domains

The Access Layer

User and workgroup access to network and resources is defined at access layer and this layer is also known as desktop layer.

Here are some functions of access layers.

Manage access control and policy

Create separate collision domains

Connectivity of workgroup through distribution layer

DDR (Double Data Rate) and Ethernet switching technology are mainly used in access layer with Static routing.