OSI Model and Networking Fundamentals
OSI Reference Model: Overview and Organization
The OSI (Open Systems Interconnection) Reference Model is a universal standard for networking that was formulated to standardize communication between different network types such as LANs, MANs, and WANs. It was developed around the 1970s and has since set the standards for network communications between hardware and software across diverse systems. The model also standardizes the quality of network-related hardware devices and software, providing a framework for interoperability and design.
Governing Organizations behind the OSI Model
The OSI reference model has been founded and continuously governed by a group of key international organizations. These include the Institute of Electrical and Electronics Engineers (IEEE), the American National Standards Institute (ANSI), the International Telecommunication Union (ITU), and the International Organization for Standardization (ISO). Each organization contributes to the development, standardization, and promotion of networking protocols, methodologies, and technologies that underpin the OSI framework.
ISO, IEEE, ANSI, ITU: Brief Introductions
The ISO is described as a non-governmental worldwide federation of national standards bodies. Its goal in relation to computer networking is to ensure communication and proper networking standards are met, contributing significantly to the development of network protocol standards. IEEE is portrayed as an international organization of professionals focused on advancing technology that benefits human society. ANSI is a private, non-profit organization that monitors standards for US enterprises, government agencies, and international groups to ensure safe and high-quality products or services, conducting quality checks across a range of products including network equipment. ITU is a United Nations agency dedicated to ensuring safety and quality of information and communications technology products and services, setting standards for computer hardware and software such as routers, e-mail standards, wired and wireless media, and more.
OSI Model Features and Applications
The OSI model features help users assess several capabilities: how to choose appropriate network equipment for needs, how to design effective and proper networks, how to ensure equipment interoperability with other networks, and how to perform network-related troubleshooting. These features guide engineers in making informed architectural and operational decisions.
OSI Layer Stack and Nomenclature
The OSI layers are arranged from bottom to top as follows: Physical, Data Link, Network, Transport, Session, Presentation, and Application. Each layer is referred to by its actual layer name or by its numerical position in the reference model, i.e., L1 through L7. A common classroom example in the source material labels layers by both name and number, emphasizing that they can be identified either way.
Layer 7: Application Layer (as described in the transcript)
In the provided material, Layer 7 or the Application Layer is described as the topmost layer that interacts with software applications and data presentation. The bottom portion of the OSI stack is described in the source as being responsible for constructing frames and packet transmission and for handling physical network equipment such as cables and network devices. The transcript also notes that the middle layers coordinate communication between nodes and that the top layers handle direct communication with software applications and data presentation, including encryption, data management, and operating system functions. Together, these statements identify a Stack concept but with a labeling that appears inconsistent with standard OSI definitions. A faithful study note should recognize this discrepancy and align with the standard model where the Application Layer is Layer 7 at the top, responsible for user-facing services and applications.
Layer Details: Physical Layer (Layer 1)
The Physical Layer is located at the bottom of the OSI model. The hardware within this stack is responsible for initiating, transmitting, and detecting voltage signals that are crucial for data transfer and receiving. Network signals can be analog or digital. The Physical Layer encompasses several responsibilities:
1) It manages data transmission media, including wired and wireless channels such as coaxial cables, UTP cables, radio waves, and microwaves.
2) Network connectors are part of the Physical Layer.
3) It determines how signals transition from analog to digital form and detects signal errors.
4) Network Topology falls under the Physical Layer.
5) It includes network devices such as routers, hubs, and devices whose primary purposes are to send and receive signals with data.
There are two types of network signals: Digital and Analog. An Analog signal is continuous and varies in wavelength, measured by varying positive and negative voltage levels. Examples include natural human voice, light waves, radio, and telephone signals. In the transcript, an Analog signal example shows voltages such as 3.3 V for a positive level and 0 V as a reference.
Layer Details: Data Link Layer (Layer 2)
The Data Link Layer is the second layer and sits at the bottom part of the OSI model. It handles data transmission errors and ensures a steady flow of data to and from the Physical Layer. It is commonly implemented in network adapters and devices like routers. The Data Link Layer checks for duplication or incorrect or partially received data; if an error occurs, it requests retransmission. A Cyclic Redundancy Check (CRC) is a common error-detection method used here. The two important sublayers of this layer are:
Logical Link Control (LLC): monitors and manages flow control, error control, frame synchronization, and avoidance of network traffic.
Media Access Control (MAC): checks the logical address of a network device. A MAC address is a unique hexadecimal identifier, for example, 0004AC8428DE, typically located on the network interface hardware. It can be formatted as 00-04-AC-84-28-DE or 00:04:AC:84:28:DE. MAC addresses follow a pattern: the first half identifies the vendor (who manufactured the device) and the second half is unique to the interface. The MAC layer’s purpose is to transform bits and format them into frames; a frame is the unit of data on a network.
Layer Details: Network Layer (Layer 3)
The Network Layer is the third layer responsible for regulating and monitoring the passage of packets along routes on the network. Its responsibilities include managing physical routes (cables and wireless paths) and logical routes (software paths). The Network Layer acts as a traffic director, setting itineraries for packets and seeking efficient paths using a process called discovery, which uses metrics to gather information about the location of different networks and nodes. A packet is defined as a discrete unit of data formatted for transmission over a network.
Layer Details: Transport Layer (Layer 4)
The Transport Layer is the fourth layer and is responsible for delivering data from the sending node to its destination node, ensuring reliable data delivery. Its responsibilities include fragmenting messages into smaller units to enable reliable end-to-end transmission.
Layer Details: Session Layer (Layer 5)
The Session Layer builds and maintains proper communication between nodes, identifying the duration of transmission and recovery from transmission errors. It can establish which node transmits first and governs how long a node can transmit, as well as how to recover from transmission errors if a lower layer breaks a session. The Session Layer defines two modes of data transfer:
Two-way Alternate Mode (TWA), also known as half-duplex, where there is a single channel for sending and receiving, so sending and receiving cannot occur simultaneously (analogy: a Walkie Talkie).
Two-way Simultaneous (TWS), also known as full duplex, where nodes can transmit and receive data simultaneously, making it more efficient (used in full-duplex communications).
Layer Details: Presentation Layer (Layer 6)
The Presentation Layer is responsible for data formatting, syntax checking, and data presentation logic. It governs how data is formatted for the receiving system and supports character encoding schemes. The material discusses multiple encoding schemes:
EBCDIC (Extended Binary Coded Decimal Interchange Code): an 8-bit coding method for a 256-character set used by IBM and older computers.
ASCII (American Standard Code for Information Interchange): an 8-bit character coding method that supports 128 characters and is commonly used by UNIX/Linux, Windows, and macOS systems. Older IBM systems used EBCDIC; newer systems typically use ASCII.
Layer Details: Application Layer (Layer 7)
The Application Layer is the topmost layer and manages the user’s direct access to applications and network services. It handles tasks such as remote access control of files and printers, email, file transfer, and overall message handling for network services.
How Communication Between Stacks Happens
A simplified analogy shows two computers on a LAN communicating under the OSI model. The OSI model regulates standards for LAN communication and inter-networking (e.g., WANs and LANs). It constructs a message at the Application layer, and this data traverses downward through the layers to the Physical Layer, with each layer adding its own header (or processing) before transmission. At the receiving end, the data travels up through the layers, being interpreted at each corresponding layer.
Sample OSI Communication Scenario
A sample scenario describes a local server in an MIS office being accessed by a workstation in another department. A redirector at the Application layer locates a shared drive; the Presentation layer ensures the data format is ASCII; the Session layer establishes and maintains the link; the Transport layer monitors transmission and reception errors; the Network layer routes the packet along the shortest path; the Data Link layer formats frames and verifies addresses; and the Physical layer converts data to an electrical signal. This illustrates how each layer contributes to end-to-end communication.
OSI Layer-to-Hardware and Software Mapping
The transcript provides a mapping between OSI layers and corresponding network hardware or software components. Examples include:
Application layer: API, Sockets, WinSock, and user-oriented programs like web browsers and email clients.
Presentation layer: data translation, encryption, graphics formatting (e.g., .gif, .jpg), and gateways.
Session layer: network software drivers, half- and full-duplex capabilities, remote procedure calls, and gateways.
Transport layer: Layer 4 switching, flow control software, and related capabilities.
Network layer: gateways, routers, routing protocols, and Layer 3 switches.
Data Link layer: network interface cards, intelligent hubs and bridges, Layer 2 switches, and gateways.
Physical layer: networking cabling, connectors, multiplexers, transmitters, receivers, transceivers, hubs, repeaters, and gateways.
Additional Notes and Key Takeaways
The seven-layer OSI model is described as the foundation of LAN and WAN communications. Bottom layers handle data connectivity, frame formation, encoding, signal transmission, and the conversion between digital and analog signals. Middle layers establish and maintain sessions (network traffic control). Upper layers handle data presentation and encryption, including user-facing services. Information is transported over LANs via LAN transmission or access methods.
Signal Types: Analog and Digital (and Interference)
There are two main signal types: Analog and Digital. Analog signals are continuous and vary in wavelength, measured by positive and negative voltage levels. Examples include natural human voice, light waves, radio, and telephone signals. In the Analog example, voltages such as 3.3 V (positive) and 0 V (reference) are used. Digital signals use discrete voltage levels to represent binary 0s and 1s; they are the preferred method for most LANs and high-speed WANs. The Digital signal section describes wiring and encoding concepts, including differences between electrical signals and optical representations. Two types of signal interference are discussed: Electromagnetic Interference (EMI) and Radio Frequency Interference (RFI). EMI occurs when magnetic fields produced by devices (fans, motors, heaters, air conditioning, etc.) disturb signals. RFI occurs due to disturbances from radio-emitting devices such as radios, televisions, substandard computers, or TV equipment.
Data Link Layer: CRC, LLC, and MAC Details
A key feature of the Data Link Layer is error detection using CRC. The LLC sublayer handles flow control, error control, frame synchronization, and avoidance of network traffic. The MAC sublayer checks the logical address of a network device using MAC addresses like 0004AC8428DE, which can be formatted as 00-04-AC-84-28-DE or 00:04:AC:84:28:DE. MAC addresses are not random; the first half identifies the vendor, and the second half is unique to the device interface. The Data Link Layer’s purpose is to transform bits into frames, with a frame being the unit of data on a network.
Network Layer: Packets, Routes, and Discovery
The Network Layer regulates the passage of packets along both physical and logical routes. It acts as the network’s traffic director and uses metrics and discovery to determine efficient paths to different networks and nodes. A packet is a discrete data unit formatted for transmission.
Transport Layer: Reliability and Segmentation
The Transport Layer provides reliable end-to-end delivery, fragmenting messages into smaller units to enable manageable transfer and error control.
Session Layer: Two Modes of Transmission
The Session Layer introduces two modes: TWA (Two-way Alternate mode) or half-duplex, with a single data lane that cannot support simultaneous send/receive; and TWS (Two-way Simultaneous) or full duplex, where sending and receiving can occur at the same time, increasing efficiency.
Presentation Layer: Encoding Standards and Character Sets
The Presentation Layer handles data formatting and encoding, including ASCII and EBCDIC. ASCII supports 128 characters and is widely used in modern systems, whereas EBCDIC is an older 8-bit code used by IBM and some legacy systems. The layer also covers data translation, compression, and encryption as part of data presentation.
Application Layer Functions
The Application Layer manages user access to network resources and services, enabling remote access to files and printers, and supporting email, file transfer, and management tasks.
Tables, Mappings, and Hardware/Software Correspondence
The material provides a direct mapping of OSI layers to specific hardware and software components, highlighting how different technology interfaces with each layer. This mapping helps clarify where certain protocols and tools operate within the stack, reinforcing the practical understanding of OSI in real networks.
Summary of Key Concepts
The OSI model provides a layered approach to networking, with seven layers spanning from physical transmission to application-level services. Each layer has distinct responsibilities, interfaces with adjacent layers, and serves as a building block for reliable, scalable, and interoperable networks. Understanding the roles of each layer, the interactions between layers, and how data moves from an application to the physical medium (and back) is essential for diagnosing issues, designing networks, and evaluating equipment and software in real-world scenarios.