- Overview
- Network Architecture
- Network Types and Topologies
- Data Switching Methods
- Network Performance Metrics
- Protocol Layers
- Summary
- References
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.
| Characteristic | Description |
|---|---|
| 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 |
| Function | Description | Implementation |
|---|---|---|
| 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 defines the structure and design of a network, including physical and logical components.
┌─────────────────────────────────────────────────┐
│ 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
| Type | Range | Characteristics | Examples |
|---|---|---|---|
| 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 |
Bus Topology:
[Node A]──[Node B]──[Node C]──[Node D]
Shared Bus
- Pros: 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
Establishes a dedicated communication path for the entire connection duration.
Setup → Data Transfer (Dedicated Path) → Teardown
Characteristics:
- Connection-oriented with call setup
- Reserved bandwidth for entire duration
- Constant delay, guaranteed QoS
- Inefficient resource utilization (idle periods waste capacity)
Examples: PSTN, ISDN
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 destination
Characteristics:
- 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
| Aspect | Circuit Switching | Packet Switching |
|---|---|---|
| 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 |
Total time for data to travel from source to destination.
Components:
Total Latency = Transmission Delay + Propagation Delay +
Processing Delay + Queueing Delay
1. 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 speed
Propagation 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
Actual data transfer rate achieved.
Throughput = Data Transferred / Time
Bottleneck 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 Mbps
Factors affecting throughput:
- Bandwidth, latency, packet loss
- Protocol overhead
- Network congestion
- TCP window size
Maximum data transfer capacity of a link.
Bandwidth-Delay Product (BDP):
BDP = Bandwidth × RTT
Represents amount of data "in flight" on the network.
Example: 100 Mbps × 50 ms = 625 KB in flight
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)
┌────────────────────────────────────────────────┐
│ 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 │
└────────────────────────────────────────────────┘
┌────────────────────────────────────────────────┐
│ 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) │
└────────────────────────────────────────────────┘
| Aspect | OSI Model | TCP/IP Model |
|---|---|---|
| Layers | 7 | 4 |
| Origin | Theoretical (ISO) | Practical (Internet) |
| Adoption | Less common | Dominant |
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).
Security Objectives:
- Confidentiality: Encryption (SSL/TLS, IPsec)
- Integrity: Checksums, digital signatures
- Authentication: Certificates, passwords
- Availability: DDoS protection, redundancy
- Non-repudiation: Digital signatures
Common Threats:
| Threat | Description | Mitigation |
|---|---|---|
| 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
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
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