OSI Reference Model

The O.S.I. model (O.S.I. - Open System Interconnection) is a way of sub-dividing a System into smaller parts (called layers) from the point of view of communications. A layer is a collection of conceptually similar functions that provide services to the layer above it and receives services from the layer below it. On each layer an instance [ Process] provides services to the instances at the layer above and requests service from the layer below. For example, a layer that provides error-free communications, across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that make up the contents of the path. Conceptually two instances at one layer are connected by a horizontal protocol connection on that layer.

HISTORY:

In 1978, work on a layered model of network architecture was started and the International Organization for Standardization (ISO) began to develop its OSI framework architecture. OSI has two major components: an abstract model of networking, called the Basic Reference Model or seven-layer model, and a set of specific protocols.


The concept of a 7 layer model was provided by the work of Charles Bachman, then of Honeywell. Various aspects of OSI design evolved from experiences with the ARPANET, the fledgling Internet, NPLNET, EIN, CYCLADES network and the work in IFIP WG6.1. The new design was documented in ISO 7498 and its various addenda. In this model, a networking system is divided into layers. Within each layer, one or more entities implement its functionality. Each entity interacts directly only with the layer immediately beneath it, and provides facilities for use by the layer above it.

Protocols enable an entity in one host to interact with a corresponding entity at the same layer in another host. Service definitions abstractly describe the functionality provided to an (N)-layer by an (N-1) layer, where N is one of the seven layers of protocols operating in the local host.

Description of OSI layers

 

 

OSI / REFERENCE  MODEL

The OSI reference model is an arbitrary hierarchical stratification (layering) of computer networking functions.  The stratification consists of  seven  layers.

There are various ways of implementing the protocols at any given layer. OSI is a CONCEPTUAL model.

 Protocols are prescriptive methods which delineate the communication that is to take place at a given layer.

Several other well known and widely used reference models have NOT been recognized as standards, e.g., IBM’s SNA (also a layered approach),  and the DoD developed protocols (which include TCP/IP – Transmission Control Protocol/Internet Protocol, SMTP – Simple Mail Transfer Protocol, FTP – File Transfer Protocol) are often used in UNIX and other environments.

The OSI Reference Model

The OSI reference model consists of seven layers, not including layer 8, the end user’s application, and layer 0 the physical transmission media

8.  O/S or User Application

       7.    Application Layer

6.   Presentation Layer

5.   Session Layer

4.   Transport Layer

3.   Network Layer

2.   Data Link Layer

1.   Physical Layer

Physical Transmission Media   

The OSI Reference Model

LAYER 7 – The APPLICATION Layer

                The top layer of the OSI model

                Provides a set of interfaces for sending and receiving applications to  gain access to and use network services, such as:  networked file transfer, message handling and database query processing

LAYER 6 – The PRESENTATION Layer

                Manages data-format information for networked communications (the network’s translator)

                For outgoing messages, it converts data into a generic format for network transmission; for incoming messages, it converts data from the generic network format to a format that the receiving application can understand

                This layer is also responsible for certain protocol conversions, data encryption/decryption, or data compression/decompression

                A special software facility called a “redirector” operates at this layer to determine if a request is network related on not and forward network-related requests to an appropriate network resource

LAYER 5 – The SESSION Layer

                Enables two networked resources to hold ongoing communications (called a session)  across a network

                Applications on either end of the session are able to ex hange data for the duration of the session

                This layer is:

                Responsible for initiating, maintaining and terminating sessions

                Responsible for security and access control to session information (via session participant identification)

                Responsible for synchronization services, and for checkpoint services

LAYER 4 – The TRANSPORT  Layer

                Manages the transmission of data across a network

                Manages the flow of data between parties by segmenting long data streams into smaller data chunks (based on allowed “packet” size for a given transmission medium)

                Reassembles chunks into their original sequence at the receiving end

                Provides acknowledgements of successful transmissions and requests resends for packets which arrive with errors

LAYER 3 – The NETWORK Layer

                Handles addressing messages for delivery, as well as translating logical network addresses and names into their physical counterparts

                Responsible for deciding how to route transmissions between computers

                This layer also handles the decisions needed to get data from one point to the next point along a network path

                This layer also handles packet switching and network congestion control

LAYER 2 – The DATA LINK Layer

                Handles special data frames (packets) between the Network layer and the Physical layer

                At the receiving end, this layer packages raw data from the physical layer into data frames for delivery to the Network layer

                At the sending end this layer handles conversion of data into raw formats that can be handled by the Physical Layer

LAYER 1 – The PHYSICAL Layer

                Converts bits into electronic signals for outgoing messages

                Converts electronic signals into bits for incoming messages

                This layer manages the interface between the the computer and the network medium (coax, twisted pair, etc.)

                This layer tells the driver software for the MAU (media attachment unit, ex. network interface cards (NICs, modems, etc.)) what needs to be sent across the medium

                The bottom layer of the OSI model

The OSI Reference Model

     

TCP/IP

Transmission Control Protocol/Internet Protocol  (TCP/IP) is a DoD developed, widely accepted and used communications protocol.  TCP/IP has only four layers, which roughly correspond to groups of the OSI model.  The Internet, many internal business networks and some home networks used TCP/IP.

The four layers in TCI/IP are:

                Application Layer

                Transport Layer

                Internet Layer

                Network Interface Layer

TCP (Transmission Control Protocol) is the portion of TCP/IP that is responsible for reliable delivery of data

TCP is a TRANSPORT protocol in the OSI and TCP/IP models

IP (Internet Protocol) is the portion of TCP/IP that provides addressing and routing information

IP is a NETWORK protocol in the OSI model and an INTERNET protocol in the TCP/IP model

                     Tcp                      vs              OSI

Application                                                        7 Application

Transport                                                            6 Presentation

Internet                                                               5 Session

Network Interface                                          4 Transport

7 Application                                                     3 Network

6 Presentation                                                  2 Data Link

                                                                                1 Physical

More information on the OSI layers are to be posted in future posts...