Skip to main content

Networking Models

Updated Jan 16, 2018 ·

Overview​

Networking models and standards define how hardware and software systems communicate and share information. Key goals include:

  1. Reliable Communication - Ensure data is sent and received correctly.
  2. Layered Functions - Separate responsibilities into layers for easier management.
  3. Packet-Based Transmission - Use packets as the basic unit of communication.
  4. Standardized Protocols - Consistent routing, addressing, and control mechanisms.
  5. Layer Extensibility - Allow additional functionality without disrupting core layers.
  6. Vendor-Neutral and Scalable - Work across different vendors and scale efficiently.

Layers​

Most network models have at least two layers:

  • Upper Layer (Host/Application Layer)

    • Handles connection integrity and session management.
    • Transforms data into a universal format.
    • Facilitates application-level communication.

  • Lower Layer (Media/Transport Layer)

    • Receives bits from the physical medium
    • Converts the received bits into frames.
    • Adds routing information to create packets.
    • Prepares data for higher layers to process.

OSI Model​

The Open Systems Interconnection (OSI) Model is a conceptual framework for describing the communication structure of interconnected computer systems, comprising seven layers.

  • Application, Presentation, Session (Layers 5-7)

    • Handle data formatting and session management
    • Enable applications to talk across different systems
    • Example: SNMP (Layer 7)

  • Transport Layer (4)

    • Uses TCP/UDP protocols for reliable/connectionless delivery
    • Ensures data is delivered in order and without errors

  • Network Layer (3)

    • Handles routing and packet transmission
    • Determines logical addressing and path selection
    • Example protocols: IP, ICMP, IGMP

  • Data Link Layer (2)

    • Manages frames and error detection/correction
    • Controls access to physical medium and devices like switches

  • Physical Layer (1)

    • Converts data into electrical, optical, or radio signals
    • Hardware standards like cables, connectors, and signaling

Encapsulation and De-encapsulation​

When data is transmitted over a network, it goes through a process of encapsulation and de-encapsulation:

  • Encapsulation

    • Data is wrapped with protocol information as it moves down the layers
    • Each layer adds its own header (and sometimes footer)
    • Headers/footers are used for routing, error checking, and delivery
    • Prepares the data for transmission over the physical medium

  • De-encapsulation

    • Data is unpacked as it moves up the layers
    • Each layer removes its corresponding header/footer
    • Information is processed after header/footer removal
    • Original data is correctly delivered to receiving application

TCP/IP​

Transmission Control Protocol/Internet Protocol (TCP/IP) is platform-independent but resource-intensive. It is designed for ease of use rather than security. It predates the OSI model.

  • Application Layer

    • Defines transport layer protocol
    • Example: Telnet, FTP, SMTP, DNS

  • Transport Layer

    • TCP: Connection-oriented, reliable, full-duplex.
    • UDP: Connectionless, fast, supports broadcast/multicast.

  • Internet Layer

    • Handles packets and routing.

    • ICMP: Ping and network health checks.

  • Network Interface Layer

    • Manages data flow on physical networks
    • Handles framing and error detection for data
    • Controls access to physical network (e.g., Ethernet, Wi-Fi)

Transmission Types​

Networks can send data in several ways depending on the target:

  • Unicast: One-to-one communication between a sender and a single receiver.
  • Broadcast: One-to-many communication to all devices on a network segment.
  • Multicast: One-to-many communication to a specific group of devices.
  • Anycast: One-to-one-of-many communication, delivered to the nearest or best receiver.

TCP Handshake​

The SYN, SYN-ACK, and ACK handshake is a process used in the TCP (Transmission Control Protocol) to establish a connection between two devices on a network.

  1. SYN (Synchronize)

    • Initiates the connection request.
    • The sender indicates its intention to establish a connection.
    • The sender picks an initial sequence number.

  2. SYN-ACK (Synchronize-Acknowledge)

    • Acknowledges the receipt of the SYN packet.
    • Indicates acceptance of the connection request.
    • The receiver also selects an initial sequence number.

  3. ACK (Acknowledge)

    • Confirms the acknowledgment of the SYN packet.
    • Establishes the connection.
    • Data transfer can begin after the ACK is received.

This three-step handshake ensures that both the sender and receiver are ready to exchange data and have agreed upon initial sequence numbers for reliable communication.

UDP​

User Datagram Protocol (UDP) is a connectionless protocol that enables fast data transmission without establishing a connection, which makes it suitable for low-latency applications like gaming and streaming.

  • Connectionless, reducing overhead but less reliable
  • No error checking or data recovery, leaving this to the application
  • Lightweight, fast, and efficient for small data transmissions
  • Supports broadcasting and multicasting
  • Ideal for live broadcasts and conferencing

DNP3​

Distributed Network Protocol (DNP3) is a multilayer communication protocol primarily used in utility and industrial automation systems to enable reliable and efficient data exchange between control equipment.

  • Widely used in SCADA systems for real-0time monitoring and controll
  • Robust communication, time-stamped data, and event logging
  • Ensure interoperability between different vendors' equipment

How It Works:

  1. Remote Terminal Unit (RTU)
  • The RTU gathers data from sensors or field devices
  • Data is sent to the SCADA master station
  1. Communication Links
  • Channels (wired or wireless) that facilitate the exchange of data
  • Data is exchanged between RTUs and the SCADA master station.
  1. SCADA Master Station
  • Central system that receives data from RTUs and processes it
  • Issues control commands back to the RTUs
  • Enables real-time monitoring and control of the entire network.

Diagram:

Network Types​

Local Area Network (LAN)​

A LAN is a network that connects devices within a small, localized area.

  • Covers a small area like a home, office, or campus
  • High-speed connectivity for local devices
  • uses Ethernet or Wi-Fi for fast and reliable communication
  • Typically owned and managed by a single organization

Metropolitan Area Network (MAN)​

A MAN connects multiple LANs across a city or metropolitan area.

  • Covers a city or metropolitan region
  • Larger than a LAN but smaller than a WAN
  • Used by service providers to connect businesses or campuses
  • Provides higher-speed connectivity than WANs for urban areas

Metropolitan area network architectures are commonly built upon the following layers:

  • Access

    • Connects customer devices to the provider’s network
    • May include routers, switches, or optical interfaces

  • Aggregation/Distribution

    • Collects and forwards traffic from the access layer
    • Optimizes traffic flow and performs load balancing

  • Metro

    • Intermediate layer, routes traffic across the metropolitan area
    • Provides redundancy and high-capacity backbone connections

  • Core

    • Routes traffic to destination aggregation network efficiently
    • Connects to WAN or other MANs for long-distance communication

Wide Area Network (WAN)​

A WAN connects networks across large geographic distances.

  • Connects networks over cities, countries, or even continents
  • Often uses leased lines, MPLS, VPNs, or satellite links
  • Enterprise connectivity, remote office access, and internet backbones
  • Managed by service providers rather than individual organizations

Circuit-Switched vs Packet-Switched​

Circuit-switched is a communication method where a dedicated path is established for the entire session.

  • Dedicated path for the duration of the session
  • Ensures consistent bandwidth and low delay
  • Used in traditional telephone networks

Packet-switched breaks data into packets and sent over shared network paths.

  • More efficient use of bandwidth, but has variable delay
  • Common in modern IP networks and the Internet

Multiplexing Techniques​

Multiplexing is a method of combining multiple signals to share a single communication channel efficiently.

  • Time-Division Multiplexing (TDM)

    • Multiple signals share a channel by taking turns in fixed time slots
    • Ensures predictable timing and low interference
    • Common in digital telephony

  • Statistical Time-Division Multiplexing (STDM)

    • Dynamically allocates time slots based on active demand
    • More efficient than fixed TDM when traffic is bursty
    • Reduces wasted bandwidth

  • Frequency-Division Multiplexing (FDM)

    • Each signal occupies a separate frequency band simultaneously
    • Common in analog radio and TV transmission
    • Requires filters to avoid overlap between bands

  • Wavelength-Division Multiplexing (WDM)

    • Optical fiber variant of FDM using multiple light wavelengths
    • Increases fiber capacity without laying more cables
    • Used in long-distance and high-speed networks

Bit Rate Categories​

Bit rate categories define how much data is transmitted per unit time and the guarantees provided by the network.

  • CBR (Constant Bit Rate)

    • Connection-oriented, fixed throughput
    • Suitable for voice/video applications that need consistent delivery
    • Predictable performance with low latency

  • UBR (Unspecified Bit Rate)

    • Connectionless, no guaranteed throughput
    • Best for non-critical applications like file transfers
    • No reserved bandwidth, may experience variable delay

  • VBR (Variable Bit Rate)

    • Connection-oriented, throughput varies depending on traffic
    • Balances efficiency and service quality
    • Good for applications tolerant to delays
    • Example uses: Email or web browsing

  • ABR (Available Bit Rate)

    • Connection-oriented
    • Dynamically adjusts throughput based on network load
    • Ensures fair bandwidth allocation among users
    • Useful in shared networks where traffic fluctuates

Session Layer Communication Modes​

The session layer defines how two applications communicate over a network.

  • Simplex

    • Communication flows in one direction only
    • Rare in practice; mostly used for broadcasting or sensors

  • Half-Duplex

    • Two-way communication, but only one side sends at a time
    • Example: walkie-talkies

  • Full-Duplex

    • Two-way communication simultaneously
    • Example: telephone conversations, Ethernet networks

SNMP​

Simple Network Management Protocol (SNMP) is used to monitor and manage network devices.

  • Tracks device status, performance, and alerts
  • Consists of a manager (central server) and agents (network devices)
  • Enables network administrators to maintain health, configure devices, and respond to issues