The OSI Model

	Created in the late 1970's, the OSI (Open Systems Interconnection) model was created by the International Organization for Standardization
	OSI Model was designed to help vendors create interoperable network devices.
	Is the primary architectural model for networks
	Describes how data and network information are communicated from applications on one computer, through the network media, to an application on another computer.
	Breaks the approach for this communication into layers

The Cisco three-layer model

	Created by Cisco
	Used to help design, implement, and maintain any size network
	Core to understanding Cisco internetworking
	Used to effectively design and purchase the correct Cisco equipment to meet needs

The Layered Approach

A reference model is a conceptual blueprint of how communications should take place. It addresses all the processes required for effective communications and divides these processes into logical groups called layers. When a communication system is designed in this manner, it's known as layered architecture.

Software developers can use a reference model to understand computer communication processes and to see what types of functions need to be accomplished on any one layer. If they are developing a protocol for a certain layer, all they need to concenr themselves with is the specific layer's functions, not those of any other layer. Another layer and protocol will handle the other functions. The technical term for this idea is binding. The communication processes that are related to each other are bound, or grouped together, in a particular layer.

Advantages of Reference Models (Exam Topic 8)

The OSI Model, like the Cisco three-layer model, is hierarchial, and the same benefits and advantages can apply to any layered model. The goal of this model is to allow vendors to interoperate. The benefits of the OSI and Cisco model include:

	Dividing the complex network operations into more manageable layers
	Changine one layer without having to change all layers. This allows application developers to specialize in design and development.
	Defining the standard interface for the "Plug-and-play" multivendor integration.

	The OSI reference model was created to help facilitate data transfer between network nodes. One of the greatest functions of the OSi specifications is to asssit in data transfer between disparate hosts, such as between a Unix host and a PC.
	It provides the framework for creating and implementing networking standards, devices, and internetworking schemes.
	Contains seven different layers, divided into two groups
	Top three layers (application, presentation, & session) define how applications within end stations will communicate with each other, and between users
	User interfaces with the computer at the application layer, and the upper layers are responsible for applications communicating between hosts.
	Top three layers are not aware of anything to do with networking or network addresses. That is the responsibility of the bottom four layers.
	The bottom four layers (transport, network, data link, & physical) define how data is transmitted end-to-end, transmitted through a physical wire, through switches and routers, and how to rebuild a data stream from a transmitting host to a destination host's application.

The OSI Layers explained

The Application Layer

The Application Layer of the OSI model is where users communicate to the computer. It is responsible for identifying and establishing the availability of the intended communication partner and determining if sufficient resources for the intended communication exist. Although computer applications sometimes require only desktop resources, applications may unite communicating components from more than one network application, file transfers, e-email, network management activities are examples.

Examples of Application layers programs and protocols are:

	World Wide Web
	Email Gateways - using SMTP protocol or X.400 standard
	Electronic Data Interchange (EDI) - a composite of specialized standards and processes that facilitates the flow of tasks between businesses.
	Special Interest Bulletin boards: mIRC, Chat rooms, BBS's, software sharing
	Internet Navigation utilities
	Financial transaction services

The Presentation Layer

The presentation layer gets it's name from it's purpose, it presents data to the application layer. It is essentially a translator and provides coding and conversion functions. By providing translation services, the presentation layer ensures that data transferred from the Application lyaer of one system can be read by the application layer of another host.

The OSI has protocol standards that define how standard data should be formatted. Tasks like data compression, decompression, encryption, and decryption are associated with this layer. Graphic and visual image presentation protocols, and movie and sound standards,  are managed via the presentation layer, for example.

	PICT- The picture format used by Macintosh or PowerPC programs
	TIFF - Tagged Image File Format
	JPEG - Joint Photographic Experts Group photo standards
	MIDI - Musical Instrument Digital Interface
	MPEG - Moving Picture Experts Group standard for compression and coding of motion video. Provides for digital storage and bit rates up to 1.5 Mbps
	QuickTime

The Session Layer

The Session Layer is responsible for setting up, managing, and then tearing down sessions between Presentation layer entries. It also provides dialog control between devices, or nodes. It coordinates communication between systems and serves to organize their communication by offering three different modes:   simplex, half-duplex, and full-duplex. It keeps different applications data separate from otherr applications data, such as is implemented in the messaging functions of the Windows API.

Examples of Session-layer protocols and interfaces (as per Cisco):

	NFS - Network File System. Developed by Sun, used with TCP/IP and Unix workstations to allow transparent access to remote resources
	SQL - Structured Query Language. Developed by IB< to provide a simpler way to define information requirements on both local and remote systems
	RPC - Remote Procedure Call. A broad client/server redirection tool used for disparate service environments. It's procedures are created on clients and performed on servers. The basis for client/server applications
	X Window - USed by intelligent terminals for communicating with remote Unix computers, allowing them to operate as locally attached monitors.
	ASP - Appletalk Session protocol. A client/server mechanism for AppleTalk networks
	DNA SCP - Digital Network Architechture Session Control Protocol. DECnet Session-layer protocol

The Transport Layer

The transport layer includes an assembly of protocols that are responsible for initiating the communication between devices. It breaks down outgoing transmissions and reassembles incoming transmissions, and multiplexes host to host communications to handle simultaneous application communications. It can provide or not provide error recovery of transmission streams, and can re-assemble data packets that arrive out of order. It also provides for logical connections between hosts on an internetwork. These communications can be handled by reliable protocols (such as TCP), or simple, and therefore unreliable protocols (such as UDP). The transport layer utilizes Flow Control and Connection Oriented Communication to maintain reliable data passing. This layer is the 'front -end' for the communication & networking layers (layers 1-4), and, as such, hides the inner workings of such communications from the upper layers (layers 5-7) that handle how applications and programs work locally on a host device.

Flow Control

Flow control manages the rate at which data is sent between hosts. It can be employed by the sender, the receiver, any mediaries in between, or any combination of the previous. This is the mechanism in which the transport layer assures data integrity. It maintains that a sender cannot overflow the buffers of a receiving end while utilizing a connection-oriented communication between the two ends of the data stream. It does this by assuring that the segments delivered are acknowledged back to the sender upon thier reception, retransmitting any segments that were not acknowledged, sequencing segments back into their proper order upon arrival, and that the data flow is managed to avoid congestion, overloading, and data loss.

There are three methods (required knowledge for the CCNA exam) for instituting flow control

	Buffering - The use of memory to hold incoming data in storage until it can be processed
	Congestion avoidance - The notification of the sender that the receiver cannot handle the incoming data stream, possibly because it's buffers are nearing capacity. The sender then holds transmission until the receiver sends another message that it is ready to again receive and process data. This process is commonly employed by Synchronous Data Link Control (SDLC) and Link Access Procedure, Balanced (LAPB) serial data-link protocols. This method can also be used to 'slow-down' data transmission, such as employed by the ICMP source quench message, which slows the sender down until the quench message is no longer received.
	Windowing - A window is the maximum amount of data that a sender can send without getting an acknowledgement from the receiver.