OSI 3

International & Access Foundation Programmes

• Computer Science Module, Semester 2, Part 4
• Focuses on networks: OSI Model and Ethernet

The OSI Model

• Developed by the International Organization for Standardization (ISO)
• Aims to standardize communication functions using abstraction layers.
• Groups similar communication functions into logical layers.
• Each layer serves the layer above it and is served by the layer below.
• It is crucial to network communication, especially today's protocols.

OSI Model Layers

• Layer 7: Application: Application issues, typically software-based. Handles communication between end-users.
• Layer 6: Presentation: Data conversion, encryption, and decryption.
• Layer 5: Session: Interhost communication. Maintains session order (starts and stops).
• Layer 4: Transport: Ensures delivery of entire messages. Handles end-to-end connections and reliability of communication.
• Layer 3: Network: Path determination and logical addressing.
• Layer 2: Data Link: MAC and LLC for physical addressing.
• Layer 1: Physical: Media, Signal, and binary transmission.

Layers of the OSI

• Simplify network design.
• Allow easy debugging of applications.
• Support easier network management.
• Layers follow rules called protocols (controls data format and timing of interactions).

The Upper Layers

• Handle application issues and typically implemented in software.
• Closest to the end user.
• Communication between users starts here.

The Lower Layers

• Handles data transport
• Physical layer and Data Link layer are implemented in hardware and software.

OSI in Networks

• Modern network communication protocols largely follow the OSI model.
• Ethernet and IP protocols are part of the OSI model.
• Included examples of Modern GSM Mobile Networks.

Networking

• Networking enables computers to exchange information.
• The Internet is a prominent networking example that interconnects millions of computers globally.
• Numerous smaller networks also play vital roles in daily life (e.g., libraries).
• Networking allows multiple devices to communicate and access information.

LAN's (Local Area Networks) & WAN's (Wide Area Networks)

• Local Area Networks (LANs) connect devices in a confined area, such as a building.
• Wide Area Networks (WANs) connect devices over large geographical areas, often using leased lines from telephone companies.
• Fiber optic connections have improved speeds and reliability in LANs and WANs.

Ethernet (IEEE 802.x)

• Original Ethernet was developed in the 1970s using coaxial cables.
• Initial data rate was 3 Mbps, with a CSMA/CD protocol.
• 10 Mbps Ethernet specification (IEEE 802.3) followed, significantly expanding the ability to share information from different nodes.

Ethernet (Advantages)

• Most popular and widely deployed network technology.
• New technologies are integrated with mechanics of operation from the original design.
• Devices communicate with each other over a shared cable.

Ethernet (Disadvantages)

• Practical network size limits due to physical cable limitations.
• Signal attenuation (weakening as it travels).
• Electrical interference from surrounding devices.

Terminology

• Protocols in networking are sets of rules controlling communication.
• Protocols act like languages, enabling different devices to communicate effectively.
• Medium (cable): Path for electrical signals.
• Segment: Shared medium within a network.
• Node: Device connected to the network segment.
• Frame: Short message used for network data communication.

Motivation for Local Area Networking

• Private ownership and limited coverage, and free selection of technology and services.
• Technical, economic, and organizational drivers drive demand: cost savings, sharable resources, and interaction between departments.

LAN Designs

• Peer-to-Peer: Simple way to share resources among connected devices.
• Client/Server: Centralized server controls access to resources and handles tasks such as backup and security management.

Servers

• HP Superdome 2: High-performance, mission-critical server platform.

Traditional Ethernet (Topology)

• Uses a topology that connects devices along a common cable.
• Potential for collisions if multiple devices request transmission at once.

Ethernet (CSMA/CD)

• Device listening for transmission before sending data to avoid collisions.
• If collision happens, a device resends data later.

Ethernet Disadvantages

• Practical size limitations due to cable length and signal weakening/attenuation over long distances.
• Susceptible to electrical interference from surrounding equipment.

Ring Topology

• Stations are connected in a cycle.
• Data moves sequentially around the ring.
• Token controls access to the ring.

Star Topology

• A central switch connects devices.
• A switched Ethernet employs digital switches.

Mesh Topology

• Each node serves as a relay for other nodes.
• Provides redundancy and reliable data propagation.
• Common in wireless settings.

Wireless Network Topology

• Allows replacing fixed-wire connections.
• Includes ad-hoc and portable to fixed networks (e.g., using Wi-Fi)

Linking Networks (Bridges)

• Bridges connect separate Ethernet segments (increasing network spans, without increasing total collision domain).
• They help regulate network traffic.

Repeaters

• Improve signal integrity and extend the transmission distance.
• Important for wired and wireless Ethernet (or, Wi-Fi) connections.

Switched Ethernet

• Modern implementation using dedicated segments.
• Eliminates shared-medium collisions and supports high data rates (especially in full-duplex implementations).
• Allows creating dedicated paths between stations.

Ethernet Switches

• Hardware devices facilitating connections for multiple devices.

Transmission Mediums

• Physical pathways for signal transmission in a network.
• Examples: twisted pair, coaxial cable, fiber optic cable, microwave, infrared, and satellite.

Bound Media

• Physical mediums involving physical cable connections. Examples include twisted pair, coaxial, and fiber optics.

Unbound Media

• Transmission via electromagnetic waves (e.g., radio, microwave, infrared, satellite).

The Electromagnetic Spectrum

• Unbound mediums utilize portions of the electromagnetic spectrum.
• Different frequencies are suited for various purposes.

Microwave Point-to-Point

• High-frequency transmission.
• Primarily line-of-sight transmission.

Infrared (IR) Point-to-Point

• High-frequency, short-range communication.
• Line-of-sight. Often used for short range (e.g., remote controls).

Satellite

• Geosynchronous orbit for long-distance communication.
• Used for communication between countries and continents.

Comparisons of Transmission Mediums

• Comprehensive factors for choosing transmission mediums (cost, bandwidth, performance, and attenuation characteristics).

Evaluation Factors

• Costs, bandwidth needs, latency requirements (real-time data), ease of installation, maintenance requirements, and projected lifespan.

Modulation

• Converting data to a signal suitable for transmission. Includes techniques such as amplitude modulation (AM) and frequency modulation (FM) methods for efficient transmission of data.

Digital Modulation

• Converting digital data (binary) into a modulation signal format (analog) for transmission.

Radio Frequencies

• Frequencies used for wireless communication, spanning a broad range.
• Example types include radio (AM/FM), TV broadcasts, shortwave radio, short-range networks.

Twisted Pair

• Two copper wiring, twisted around each other, used for cabling systems
• Offers advantages in both cost and ease of installation.

Co-axial Cable

• Copper wire with insulation and a protective copper mesh, which can handle higher data rates compared to twisted pair cable
• Typical applications are cable television and other data transmission needs between two or more locations.

Fibre Optic Cables

• Light pulses transmitted through glass fiber.
• High-bandwidth, long-distance transmission. High data rates.
• Components include the core, cladding, and buffer to facilitate greater signal carrying capacity (and, minimizing signal attenuation).

Fibre Optic Modes

• Multimode: multiple light paths, modal dispersion common
• Single mode: single light path, less modal dispersion, greater bandwidth

Fibre Optic Cable Design

• Core and cladding act as an optical waveguide.
• Core diameter and material properties affect performance.

Fibre Optic Transmission

• Light rays travel through the fibre core by total internal reflection.

Fibre Optic Construction

• Use of glass (silica) for high bandwidth and signal propagation.
• Attenuation is minimized for long-distance networking.

Disadvantages of Fiber Optics

• Termination is more complex than other mediums (potentially demanding special tools).
• Repeaters needed for longer distances.
• Can be more fragile than other mediums.