01 Computer Network Introduction
Table of Contents
Overview
A computer network is a collection of interconnected computing devices that exchange data and share resources. Networks form the foundation of modern computing infrastructure including the internet, enterprise systems, cloud computing, and IoT ecosystems.
Key Characteristics
Connectivity
Physical (Ethernet, fiber) and logical (VPNs, virtual networks) connections
Resource Sharing
Hardware, software, and data sharing across devices
Scalability
Support for network growth from PANs to global networks
Reliability
Fault tolerance through redundancy and error correction
Primary Functions
Data Communication
Transfer information between devices
TCP/IP, UDP protocols
Resource Sharing
Shared access to resources
File/print servers
Distributed Processing
Divide tasks across nodes
Cloud computing, distributed systems
Reliability
Maintain operations despite failures
Redundancy, failover mechanisms
Network Architecture
Network architecture defines the structure and design of a network, including physical and logical components.
Network Edges and Core
┌─────────────────────────────────────────────────┐
│ NETWORK EDGE (End Devices + Access Networks) │
│ [PC] [Laptop] [Phone] [Server] │
│ ↓ ↓ ↓ ↓ │
│ [Ethernet] [WiFi AP] [DSL/Cable Modem] │
└───────────────────┬─────────────────────────────┘
↓
┌─────────────────────────────────────────────────┐
│ NETWORK CORE (ISP Infrastructure) │
│ [Router]────[Router]────[Router] │
│ │ │ │ │
│ [Router]────[Router]────[Router] │
│ • High-speed backbone links │
│ • Internet Exchange Points (IXPs) │
│ • Tier 1/2/3 ISPs │
└─────────────────────────────────────────────────┘Network Edge Components:
End Devices: Clients, servers, IoT devices, edge computing nodes
Access Networks: DSL, cable, FTTH, Ethernet, Wi-Fi, cellular (4G/5G)
Access Points: Switches, wireless APs, modems, SOHO routers
Network Core Components:
Core Routers: High-capacity packet switches (Tbps throughput)
Backbone Networks: Fiber optic links (10G, 40G, 100G, 400G Ethernet)
ISP Hierarchy:
Tier 1: Global reach, peer with other Tier 1, no transit payments
Tier 2: Regional/national, purchase transit from Tier 1
Tier 3: Local access, purchase transit from Tier 2
IXPs: Direct peering infrastructure between ISPs
Network Services: DNS, DHCP, CDNs, firewalls
Network Types and Topologies
Classification by Scope
PAN
0-10m
Personal devices, low power
Bluetooth, USB, NFC
LAN
10m-1km
Single building/campus, high speed
Ethernet, Wi-Fi
MAN
1-50km
City-wide, fiber backbone
Metro Ethernet
WAN
50km+
Long distance, interconnects LANs
Internet, MPLS
Common Topologies
Bus Topology:
[Node A]──[Node B]──[Node C]──[Node D]
Shared BusPros: Simple, low cost
Cons: Single point of failure, limited scalability
Star Topology:
[Central Switch]
/ | \
[A] [B] [C]Pros: Easy management, fault isolation
Cons: Central device is single point of failure
Ring Topology:
[A]→[B]→[C]→[D]→[A]Pros: Predictable performance
Cons: Break disrupts entire network
Mesh Topology:
[A]─┬─[B]
│ ×│× │
[D]─┴─[C]Pros: High reliability, multiple paths
Cons: Complex, expensive
Data Switching Methods
Circuit Switching
Establishes a dedicated communication path for the entire connection duration.
Setup → Data Transfer (Dedicated Path) → TeardownCharacteristics:
Connection-oriented with call setup
Reserved bandwidth for entire duration
Constant delay, guaranteed QoS
Inefficient resource utilization (idle periods waste capacity)
Examples: PSTN, ISDN
Packet Switching
Data divided into packets, independently routed through the network.
Message: "HELLO" → Packets: [HE][LL][O]
Each packet routed independently
May arrive out of order → Reassemble at destinationCharacteristics:
Connectionless (datagram) or connection-oriented (virtual circuit)
Statistical multiplexing (shared resources)
Store-and-forward at routers
Variable delay, efficient resource use
Packet Structure:
┌──────────────┬─────────────────┐
│ Header │ Payload │
├──────────────┼─────────────────┤
│ • Src Addr │ │
│ • Dst Addr │ Application │
│ • Seq # │ Data │
│ • Protocol │ │
│ • Checksum │ │
└──────────────┴─────────────────┘Examples: Internet (TCP/IP), Ethernet
Comparison
Connection
Dedicated
Shared
Resource Allocation
Fixed (entire duration)
Dynamic (on-demand)
Efficiency
Low (idle resources wasted)
High (statistical multiplexing)
Delay
Constant, predictable
Variable (congestion-dependent)
Setup
Required
Minimal/none
Best For
Voice, continuous data
Bursty data (web, email)
Failure Impact
Entire call disrupted
Packets reroute
Network Performance Metrics
Latency
Total time for data to travel from source to destination.
Components:
Total Latency = Transmission Delay + Propagation Delay +
Processing Delay + Queueing Delay1. Transmission Delay: Time to push bits onto link
T_trans = L / R
where L = packet size (bits), R = bandwidth (bps)Example: 1500 bytes, 100 Mbps → 0.12 ms
2. Propagation Delay: Time for signal to travel through medium
T_prop = D / S
where D = distance, S = propagation speedPropagation speeds:
Copper/Fiber: ~2 × 10⁸ m/s (2/3 speed of light)
Wireless: ~3 × 10⁸ m/s (speed of light)
Example: 3000 km fiber → 15 ms
3. Processing Delay: Router processing time (μs to ms)
4. Queueing Delay: Wait time in router queues (most variable)
Round-Trip Time (RTT): Time for packet + acknowledgment return
Throughput
Actual data transfer rate achieved.
Throughput = Data Transferred / TimeBottleneck Link: Slowest link limits end-to-end throughput
Link 1: 10 Mbps → Link 2: 100 Mbps → Link 3: 1 Mbps → Link 4: 10 Mbps
Bottleneck ↑
End-to-end throughput ≈ 1 MbpsFactors affecting throughput:
Bandwidth, latency, packet loss
Protocol overhead
Network congestion
TCP window size
Bandwidth
Maximum data transfer capacity of a link.
Bandwidth-Delay Product (BDP):
BDP = Bandwidth × RTTRepresents amount of data "in flight" on the network.
Example: 100 Mbps × 50 ms = 625 KB in flight
Packet Loss and Jitter
Packet Loss: Percentage of packets that fail to reach destination
Loss Rate = (Packets Lost / Total Sent) × 100%Causes: Buffer overflow, transmission errors, routing failures
Jitter: Variation in packet delay over time
Important for real-time applications (VoIP, video conferencing)
Protocol Layers
OSI Model (7 Layers)
┌────────────────────────────────────────────────┐
│ 7. APPLICATION │ HTTP, FTP, SMTP, DNS │
├────────────────────────────────────────────────┤
│ 6. PRESENTATION │ SSL/TLS, JPEG, encryption │
├────────────────────────────────────────────────┤
│ 5. SESSION │ NetBIOS, RPC │
├────────────────────────────────────────────────┤
│ 4. TRANSPORT │ TCP, UDP │
├────────────────────────────────────────────────┤
│ 3. NETWORK │ IP, ICMP, routing │
├────────────────────────────────────────────────┤
│ 2. DATA LINK │ Ethernet, Wi-Fi, MAC │
├────────────────────────────────────────────────┤
│ 1. PHYSICAL │ Cables, fiber, radio │
└────────────────────────────────────────────────┘TCP/IP Model (4 Layers)
┌────────────────────────────────────────────────┐
│ 4. APPLICATION │ HTTP, FTP, SMTP, DNS │
│ │ (Combines OSI 5, 6, 7) │
├────────────────────────────────────────────────┤
│ 3. TRANSPORT │ TCP, UDP │
├────────────────────────────────────────────────┤
│ 2. INTERNET │ IP, ICMP, routing │
├────────────────────────────────────────────────┤
│ 1. LINK │ Ethernet, Wi-Fi │
│ │ (Combines OSI 1, 2) │
└────────────────────────────────────────────────┘Model Comparison
Layers
7
4
Origin
Theoretical (ISO)
Practical (Internet)
Adoption
Less common
Dominant
Encapsulation
Data gains headers as it moves down the protocol stack:
Application: [Data]
Transport: [TCP][Data]
Network: [IP][TCP][Data]
Data Link: [Eth][IP][TCP][Data][FCS]
Physical: 01001011010...Each layer adds its header; receiving end reverses the process (decapsulation).
Network Security Fundamentals
Security Objectives:
Confidentiality: Encryption (SSL/TLS, IPsec)
Integrity: Checksums, digital signatures
Authentication: Certificates, passwords
Availability: DDoS protection, redundancy
Non-repudiation: Digital signatures
Common Threats:
Eavesdropping
Traffic interception
Encryption
Man-in-the-Middle
Intercept and alter
Authentication + encryption
Denial of Service
Resource exhaustion
Rate limiting, filtering
IP Spoofing
Forge source address
Ingress filtering
Security Mechanisms:
Firewalls: Packet filtering, stateful inspection
VPNs: Encrypted tunnels over public networks
IDS/IPS: Intrusion detection/prevention
Encryption: SSL/TLS, IPsec, WPA2/WPA3
Summary
Computer networks enable communication and resource sharing between distributed devices. Key concepts:
Architecture:
Network edge: end devices and access networks
Network core: routers, ISPs, backbone links
Hierarchical ISP structure (Tier 1/2/3)
Switching:
Circuit switching: dedicated path, constant delay, inefficient
Packet switching: shared resources, variable delay, efficient
Performance:
Latency: sum of transmission, propagation, processing, queueing delays
Throughput: limited by bottleneck link
Bandwidth: maximum link capacity
Packet loss and jitter affect quality
Protocol Layers:
OSI: 7-layer theoretical model
TCP/IP: 4-layer practical model
Encapsulation: adding headers at each layer
Security:
Objectives: confidentiality, integrity, authentication, availability
Threats: eavesdropping, MITM, DoS, spoofing
Mechanisms: firewalls, VPNs, encryption, IDS/IPS
References
Course Materials:
CSEE 4119: An Introduction to Computer Networks - Columbia University
Textbooks:
Kurose, James F., and Keith W. Ross. Computer Networking: A Top-Down Approach. 8th Edition, Pearson, 2021.
Standards:
RFC 791: Internet Protocol
RFC 793: Transmission Control Protocol
RFC 768: User Datagram Protocol
ISO/IEC 7498-1: OSI Reference Model
Last updated