Interaction Between OSI Model Layers

Like many ISO standards, much of its formal theory does not make it into the real world of actual implementation, but the powerful concepts that the OSI model present are a key element in most modern network system designs.  Anyone who has worked with data networking or security has likely heard the terms “layer three” or “layer two” or “application layer.”  This terminology stems directly from the ISO model and how it is applied to practical solutions. The model concepts are conventionally used to design and troubleshoot networks, and the seven-layer model is standard fare on any network engineering certification exam or interview.  Careful study of the model can show us
support for concepts we have learned from more conventional forms of information security theory, and understanding and applying the model to information security scenarios can also help us assess and address information security threats in a network environment, allowing us to organize efforts to make security assessments and perform forensic analysis of compromised systems and threats presented in theory and found in the wild.

A given layer in the OSI model generally communicates with three other OSI layers: the layer directly above it, the layer directly below it, and its peer layer in other networked computer systems. The data link layer in System A, for example, communicates with the network layer of System A, the physical layer of System A, and the data link layer in System B.

One OSI layer communicates with another layer to make use of the services provided by the second layer. The services provided by adjacent layers help a given OSI layer communicate with its peer layer in other computer systems. Three basic elements are involved in layer services: the service user, the service provider, and the service access point (SAP).

The service user is the OSI layer that requests services from an adjacent OSI layer. The service provider is the OSI layer that provides services to service users. OSI layers can provide services to multiple service users. The SAP is a conceptual location at which one OSI layer can request the services of another OSI layer.

Ethernet Technologies

The term Ethernet refers to the family of local-area network (LAN) products covered by the IEEE 802.3 standard that defines what is commonly known as the CSMA/CD protocol. Three data rates are currently defined for operation over optical fiber and twisted-pair cables:

• 10 Mbps—10Base-T Ethernet
• 100 Mbps—Fast Ethernet
• 1000 Mbps—Gigabit Ethernet

Other technologies and protocols have been touted as likely replacements, but the market has spoken. Ethernet has survived as the major LAN technology (it is currently used for approximately 85 percent of the world’s LAN-connected PCs and workstations) because its protocol has the following characteristics:

• Is easy to understand, implement, manage, and maintain
• Allows low-cost network implementations
• Provides extensive topological flexibility for network installation
• Guarantees successful interconnection and operation of standards-compliant products, regardless of manufacturer

The original Ethernet was developed as an experimental coaxial cable network in the 1970s by Xerox
Corporation to operate with a data rate of 3 Mbps using a carrier sense multiple access collision detect (CSMA/CD) protocol for LANs with sporadic but occasionally heavy traffic requirements. Success with that project attracted early attention and led to the 1980 joint development of the 10-Mbps Ethernet Version 1.0 specification by the three-company consortium: Digital Equipment Corporation, Intel Corporation, and Xerox Corporation.

OSI Reference Model

The Open Systems Interconnection (OSI) reference model describes how information from a software application in one computer moves through a network medium to a software application in another computer. The OSI reference model is a conceptual model composed of seven layers, each specifying particular network functions. The model was developed by the International Organization for Standardization. The ISO-OSI model consists of seven layer architecture. It defines seven layers or levels in a complete communication system. The OSI model divides the tasks involved with moving information between networked computers into seven smaller, more manageable task groups. A task or group of tasks is then assigned to each of the seven OSI layers. Each layer is reasonably self-contained so that the tasks assigned to each layer can be implemented independently.

A handy way to remember the seven layers is the sentence "All people seem to need data processing." The beginning letter of each word corresponds to a layer.

• All-Application layer- Layer 7
• People-Presentation layer- Layer 6
• Seem-Session layer- Layer 5
• To-Transport layer- Layer 4
• Need-Network layer- Layer 3
• Data-Data link layer- Layer 2
• Processing-Physical layer - Layer 1

OSI Layer illustrates the seven-layer OSI reference model.

Feature of OSI Model :

1. Big picture of network is understandable through this OSI model.
2. We see how hardware and software work together.
3. We can understand new technologies as they are developed.
4. Troubleshooting is easier by separate networks.
5. Can be used to compare basic functional relationships on different networks.

The seven layers of the OSI reference model can be divided into two categories: upper layers and lower layers.
The upper layers of the OSI model deal with application issues and generally are implemented only in software. The highest layer, the application layer, is closest to the end user. Both users and application layer processes interact with software applications that contain a communications component. The term upper layer is sometimes used to refer to any layer above another layer in the OSI model.
The lower layers of the OSI model handle data transport issues. The physical layer and the data link layer are implemented in hardware and software. The lowest layer, the physical layer, is closest to the physical network medium (the network cabling, for example) and is responsible for actually placing information on the medium.

Interconnected Internetwork

An internetwork is a collection of individual networks connected by intermediate networking devices, that functions as a single large network. Internetworking refers to the industry, products, and procedures that meet the challenge of creating and administering internetworks.

Figure below illustrates some different kinds of network technologies that can be interconnected by routers and other networking devices to create an internetwork.

The first networks were time-sharing networks that used mainframes and attached terminals. Such environments were implemented both IBM. Then Local-area networks (LANs) evolved around the PC revolution. LANs enabled multiple users in a relatively small geographical area to exchange files and messages, as well as access shared resources such as file servers and printers. 

Wide-area networks (WANs) create connectivity of LANs with geographically dispersed users. Some of the technologies used for connecting LANs include T1, T3, ATM, ISDN, ADSL, Frame Relay, radio links, and others.

Internetworking evolved as a solution to three key problems: isolated LANs, duplication of resources, and a lack of network management. Isolated LANs made electronic communication between different offices or departments impossible. Duplication of resources meant that the same hardware and software had to be supplied to each office or department, as did separate support staff. This lack of network management meant that no centralized method of managing and troubleshooting networks existed.

Bismillah

Bismillah...
Let's start in the name of God.
Fill our soul with the light from God.
Let our mind to think about God.
Let's start our activities in the name of God.