CAB222 - Lecture 1: Network Architecture OSI Model

LAN (Local Area Network)

Small-area network within a building or campus where devices are close and share local switching or Wi-Fi

WAN (Wide Area Network)

Network linking distant sites via a carrier across cities or countries

MAN (Metropolitan Area Network)

City-scale network connecting multiple LANs across a metro region

Internetwork

Multiple LANs joined by routers to act as one larger network

Internet

Public global internetwork using open standards such as TCP IP and HTTP

Intranet

Private internal network for an organisation’s staff

Extranet

Controlled access for trusted external partners into parts of an intranet

Packet (OSI Layer 3 Network)

Network-layer unit carrying IP source and destination with payload used for routing

Frame (OSI Layer 2 Data Link)

Link-layer unit that encapsulates a packet with MAC addresses plus an FCS for error detection used for switching

Bit (OSI Layer 1 Physical)

Smallest unit of data represented as a physical signal 0 or 1 on wire fiber or radio

Client

Application or device that requests data or services such as a web browser

Server

Application or device that provides data or services such as a web server

Protocol

Agreed rules for formatting and exchanging data such as HTTP TCP IP

NIC (Network Interface Card)

Hardware that places bits on the medium and receives bits from it

NIC Driver

Software that hands properly formatted frames to the NIC and accepts frames from it

Switch

Layer 2 device that connects hosts inside a LAN and forwards frames using MAC addresses

Router

Layer 3 device that connects networks and forwards packets using IP addresses and routes

Access Point (AP)

Wireless bridge that connects Wi-Fi devices to the wired LAN

Message flow 4 steps

App creates message then system formats then protocols package with IP TCP then driver and NIC transmit signals

Why small chunks

Improves fairness fits buffers and limits retransmission to only the errored chunk

Layering concept

Split functions into layers so components can change independently and troubleshooting is simpler

Network Architecture

Blueprint for how a network is built arranged governed and how data moves

Reference Models TCP IP and OSI

Conceptual maps that place functions in layers so different systems interoperate

Encapsulation

Each layer adds its header and sometimes trailer as data moves down and the receiver removes them on the way up

Application Layer 7

Provides user-facing network services such as web email and name resolution clients

Presentation Layer 6

Handles data representation including formatting compression and encryption

Session Layer 5

Starts manages and ends conversations including who talks when and checkpointing

Transport Layer 4

End to end delivery segmentation reliability ordering and flow control with TCP or UDP

Network Layer 3

Logical addressing and routing to find best path using IP and routing protocols

Data Link Layer 2

Local link delivery framing MAC addressing and error detection for switching

Physical Layer 1

Sends and receives bits as electrical optical or radio signals and defines media and connectors

Troubleshooting L3

Wrong IP or subnet and routing problems

Troubleshooting L2

MAC or frame errors collisions duplex or speed mismatch and bad drivers

Troubleshooting L1

Damaged cables interference loose connectors and faulty NIC or port

Scope memory aid

LAN building then MAN city then WAN country or world

Data path memory aid

Application then Protocols then Driver or NIC then Medium

Encapsulation memory aid

Headers added going down and removed going up

Hardware building blocks

NIC (interfaces host to network), network medium (cable/fiber/radio), interconnecting devices: router (between networks), switch (within LAN), access point (wireless to wired bridge)

Software components

Client (requests service), server (provides service), protocols (rules/format), NIC driver (hands frames to/from NIC)

Four steps of a send

1) App accesses resource; 2) Client formats message; 3) Protocols package (e.g., IP/TCP) and pass to driver; 4) Driver/NIC emit signals on medium

Layering concept (alt)

Each step is a “layer” with a specific task; layers work together so parts can change independently and be troubleshot separately

Encapsulation essentials

Sender adds headers (and sometimes trailer) at each layer; receiver removes them; think nested envelopes

Why reference models

Define layered functions (OSI/TCP-IP) so multi-vendor systems interoperate and implementations remain modular/manageable

OSI structure facts

Seven layers; each serves the one above; same-layer peers communicate logically across hosts

Transport layer highlights

Segmentation due to MTU; reliability via ACKs/retransmission; ordering/re-sequencing; flow control (e.g., TCP)

Network layer highlights

Logical addressing and best-path routing; routers operate here; key protocols include IP/ARP/ICMP

Data Link layer highlights

Frames = header + trailer (FCS/CRC); MAC addressing; switches/NICs operate here; common issues: FCS errors, collisions, duplex/speed mismatches

Physical layer highlights

Converts bits↔signals (voltage/light/radio); includes cables, connectors, repeaters/hubs; common issues: bad cables, interference, loose connectors, faulty NIC/port

Purposes of layering

Clear modules, easier changes without breaking others, simpler implementation and troubleshooti