Show understanding of the characteristics of a LAN (local area network) and a WAN (wide area network)

2.1 Networks – The Internet

Objective

Show understanding of the characteristics of a LAN (Local Area Network) and a WAN (Wide Area Network) and the related concepts required by the Cambridge AS & A‑Level Computer Science syllabus – topologies, network devices, client‑server & peer‑to‑peer models, cloud‑computing models, wired vs wireless media, Ethernet/CSMA‑CD, switching methods, the OSI/TCP‑IP models, IP addressing, performance metrics and security measures.

Key Definitions

• LAN (Local Area Network): A network that interconnects devices within a limited geographical area (e.g., a single building, floor or campus).
• WAN (Wide Area Network): A network that spans a large geographical area, linking multiple LANs, cities, or countries.

Network Topologies

• Star – all devices connect to a central switch or hub (most common in modern LANs).
• Bus – devices share a single communication line (legacy Ethernet).
• Ring – each device connects to two neighbours forming a closed loop (e.g., Token Ring).
• Mesh – multiple redundant paths between devices (typical for WAN backbones).
• Hybrid – combination of two or more basic topologies (e.g., star‑bus in a campus network).

Diagram suggestion: a small figure showing each topology side‑by‑side with labels.

Typical Network Devices

• Switches – forward Ethernet frames within a LAN using MAC addresses (full‑duplex, no CSMA‑CD).
• Routers – forward IP packets between different networks (LAN ↔ WAN).
• Wireless Access Points (WAPs) – provide Wi‑Fi connectivity.
• Firewalls – filter traffic based on IP, ports, protocols and state; can be hardware or software.
• VPN Gateways – terminate encrypted IPSec or SSL/TLS tunnels over a WAN.
• Core / Edge Routers – high‑capacity devices used in WAN backbones.
• Modems, Satellite Dishes, WAN Optimisers – specialised equipment for long‑distance links.

Network Models

Client‑Server vs Peer‑to‑Peer (P2P)

• Client‑Server – dedicated server(s) provide resources/services to many clients.

  • Example (exam‑style): a school’s web‑mail service where all students (clients) connect to a central mail server.

• Peer‑to‑Peer – each node can act as both client and server, sharing resources directly.

  • Example: a group of students sharing a large video file directly between their laptops using a P2P application.

Cloud‑Computing Models and Network Implications

• Public cloud – services delivered over the Internet by third‑party providers (e.g., AWS). Requires reliable WAN links and often a VPN or TLS for secure access.
• Private cloud – cloud infrastructure operated solely for one organisation, usually hosted on its own WAN or data‑centre network.
• Hybrid cloud – combination of public and private clouds; typically linked by a secure VPN tunnel or dedicated MPLS circuit.

Wired vs Wireless Media

• Wired media

  • Copper twisted‑pair: Cat 5e (100 MHz, up to 1 Gbps), Cat 6 (250 MHz, up to 10 Gbps), Cat 6a (500 MHz, up to 10 Gbps).
  • Coaxial cable – used for legacy cable TV and some broadband services.
  • Fibre‑optic: multimode (850 nm/1300 nm, up to 10 Gbps) and single‑mode (1310 nm/1550 nm, up to 100 Gbps+).
  • Advantages: high bandwidth, low attenuation, predictable latency.
  • Key parameters: signal‑to‑noise ratio (SNR) and attenuation (dB/km).

• Wireless media

  • Wi‑Fi (IEEE 802.11a/b/g/n/ac/ax) – typical indoor range 30‑100 m, bandwidth up to 9.6 Gbps (Wi‑Fi 6E).
  • Bluetooth – short‑range, low‑power, up to 2 Mbps.
  • Cellular (LTE/5G) – wide‑area coverage, bandwidth from 10 Mbps to >1 Gbps, higher latency.
  • Satellite – global coverage, high latency (≈ 500 ms) and limited bandwidth.
  • Disadvantages: susceptibility to interference, variable latency, lower data rates compared with wired links.

Ethernet & CSMA‑CD

Ethernet is the dominant LAN technology.

• Half‑duplex Ethernet uses Carrier Sense Multiple Access with Collision Detection (CSMA‑CD) – a device listens, transmits if idle, aborts and retries after a collision.
• Full‑duplex Ethernet (most modern LANs) eliminates collisions; devices can send and receive simultaneously, so CSMA‑CD is not used.
• Switch‑based LANs replace hubs, providing dedicated full‑duplex links to each port.

Switching Methods

• Packet switching – data is divided into packets that travel independently via the most efficient route. Used by the Internet and most WAN technologies.

  • Example: a web‑page request from a laptop to a remote server is broken into IP packets that each follow the best available path.

• Circuit switching – a dedicated communication path is established for the duration of a session. Used by traditional telephone networks.

  • Example: a voice call over the public switched telephone network reserves a fixed channel for the whole call.

OSI and TCP/IP Models

IP Addressing

• IPv4 – 32‑bit address written as four octets (e.g., 192.168.1.10). Subnet mask determines the network portion.

  • Example: /24 = 255.255.255.0 → 256 addresses, 254 usable hosts.

• IPv6 – 128‑bit address written in hexadecimal groups (e.g., 2001:0db8:85a3::8a2e:0370:7334).
• Public vs Private IPv4 ranges

  • 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16.

• Static vs Dynamic addressing – static is manually configured; dynamic is assigned by DHCP.
• Domain Name System (DNS) – translates human‑readable domain names to IP addresses.
• CIDR notation – compact representation of network prefix (e.g., 192.168.10.0/23).

Performance Metrics

• Bandwidth – maximum data‑transfer capacity (bits per second, bps).
• Latency – time for a packet to travel from source to destination (milliseconds, ms).
• Jitter – variation in latency; important for real‑time services.
• Packet loss – percentage of packets that never reach the destination.

Example Calculation

File size = 500 MB (≈ 4 000 Mbit)

Transfer time = File size ÷ Bandwidth

• LAN (1 Gbps): 4 000 Mbit ÷ 1 000 Mbps = 4 s
• WAN (100 Mbps): 4 000 Mbit ÷ 100 Mbps = 40 s

Network Security Measures

• VPN (Virtual Private Network) – encrypted tunnel (IPSec or SSL/TLS) over a public WAN.
• Firewalls – packet‑filtering (stateless) or stateful inspection; can also be application‑aware.
• Intrusion‑Detection/Prevention Systems (IDS/IPS) – monitor traffic for malicious patterns.
• Access‑Control Lists (ACLs) – filter traffic on routers/switches based on IP, protocol, or port.
• Encryption – TLS/SSL for web traffic, IPSec for VPNs, WPA3 for Wi‑Fi.

Characteristic Comparison: LAN vs WAN

Real‑World Case Study: University Campus Network

Scenario: A university has three campuses (A, B, C). Each campus has its own LAN and all campuses must share student records, access a central cloud‑based learning management system (LMS), and provide high‑speed research links to a national research WAN.

• LAN design (each campus)

  • Star topology centred on a 10 Gbps core switch.
  • Wired Ethernet (Cat 6a) for desktops/labs; Wi‑Fi 6 (802.11ax) for mobile devices.
  • VLANs separate student, staff and research traffic.
  • DHCP assigns private IPv4 addresses (10.0.x.0/16); internal DNS resolves hostnames.
  • Perimeter firewall on the campus edge router.

• WAN design (inter‑campus)

  • Dedicated MPLS leased lines (2 Gbps) between campuses.
  • Edge routers run IPSec VPN to the national research WAN (packet‑switched).
  • Public IPv4 block (203.0.113.0/24) advertised via BGP.
  • Measured latency ≈ 15 ms; jitter < 5 ms – sufficient for live video lectures.

• Cloud integration

  • Hybrid cloud: private OpenStack for sensitive research data; public AWS for the LMS.
  • Secure VPN tunnel from the campus WAN to the AWS VPC.

• Security measures

  • Perimeter firewalls + ACLs on edge routers.
  • TLS for all web services; WPA3 for Wi‑Fi.
  • Regular vulnerability scanning and IDS/IPS monitoring.

Typical Use‑Cases

• LAN: office file sharing, local printing, internal email servers, LAN gaming, laboratory instrumentation.
• WAN: connecting branch offices, accessing cloud services, inter‑university research collaborations, global e‑commerce platforms, remote backup to a data centre.

Summary

A LAN is characterised by a limited geographic scope, high bandwidth, low latency, and ownership by a single organisation. It typically uses Ethernet (full‑duplex), star or hybrid topologies, and provides services such as file sharing and local printing. A WAN covers much larger distances, often relies on third‑party infrastructure, exhibits higher latency and variable bandwidth, and incurs greater cost. WANs employ a variety of media (leased fibre, satellite, MPLS), use packet‑switching, and require additional security mechanisms such as VPNs, firewalls, IDS/IPS and ACLs. Mastery of these differences, together with IP addressing, protocol layers, performance metrics and security controls, enables students to design efficient, secure networks that meet the needs of both local users and distributed organisations.