Network Topology and Wireless Networking

Network topology describes how devices are arranged and connected.

Physical star topology

Every device connects directly to a central switch or hub with its own dedicated cable.

Advantages:

• Failure of one link affects only that device — others remain connected
• Centralised management at the switch
• Easy to add or remove devices

Disadvantage: if the central switch fails, the entire network goes down.

Logical bus topology

All devices share a single communication channel. Data transmitted by one device is visible to all others; each device checks whether the data is addressed to it.

Key distinction: physical and logical topologies can differ. A physical star network using a switch can operate as a logical bus — the switch receives data on one port and broadcasts or forwards it, so the logical channel is shared. This is how most wired Ethernet LANs work: physical star wiring with logical bus behaviour via the switch.

Client-server

• Dedicated servers provide services (web, email, file, database)
• Clients request services; servers respond
• Servers have more powerful hardware and dedicated roles
• Centralised control — easy to manage permissions, security, backups

Examples: web browsing (browser=client, web server), email (email client + mail server), corporate file sharing.

Peer-to-peer (P2P)

• All nodes are equal — each can act as both client and server
• No dedicated server — resources shared directly between peers
• Decentralised — no single point of failure for the network
• Harder to manage security and access control

Examples: BitTorrent file sharing, some VoIP systems (Skype originally), blockchain networks.

	Client-server	Peer-to-peer
Structure	Dedicated servers + clients	All nodes equal
Management	Centralised, easier	Decentralised, harder
Security	Easier to enforce	Harder to control
Scalability	Add more servers	Scales naturally with nodes
Single point of failure	Server is a risk	No single point of failure

WiFi is a family of wireless LAN standards based on IEEE 802.11.

Components:

• Wireless network adapter (client): hardware in a device (laptop, phone) that transmits and receives radio signals
• Wireless access point (WAP): connects the wireless network to the wired network (and Internet); manages wireless communication

Devices connect to a WAP by discovering and joining an available network identified by its SSID.

SSID (Service Set Identifier): the human-readable name of a wireless network (e.g. "HomeNetwork", "CoffeeShop_WiFi"). Devices scan for SSID broadcasts to discover available networks.

Wireless signals travel through the air — any device in range can attempt to receive them. Security measures are therefore critical.

Measure	How it works	What it protects against
WPA/WPA2	Encrypts all wireless traffic using a shared key (WPA-PSK) or enterprise authentication	Eavesdropping; prevents reading intercepted traffic
SSID broadcast disabled	AP stops advertising its network name	Hides the network from casual discovery — not strong security alone
MAC address allow lists	AP only allows connections from devices whose MAC addresses are pre-registered	Restricts which hardware can connect

WPA2 (Wi-Fi Protected Access 2) uses AES encryption and is the current standard for home/office networks.

Wireless networks cannot use collision detection (CSMA/CD used in wired Ethernet) because a transmitting device cannot simultaneously listen to the channel. Instead, WiFi uses CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance).

CSMA/CA process

1. Device listens to the channel before transmitting (carrier sense)
2. If the channel is busy, wait a random backoff period before retrying
3. If the channel is free, transmit

The random backoff reduces the chance that two devices, both waiting, transmit at exactly the same time.

RTS/CTS (Request to Send / Clear to Send) handshake

An optional enhancement to reduce hidden node collisions (two devices out of range of each other but both in range of the AP):

1. Device sends a short RTS (Request to Send) frame to the AP
2. AP replies with CTS (Clear to Send), heard by all devices in range
3. All other devices defer transmission during the reserved time window
4. Transmitting device sends data

RTS/CTS adds overhead but significantly reduces collisions in congested or complex wireless environments.

1. Confusing physical and logical topology

Physical topology = actual cable/radio connections. Logical topology = how data flows. A physical star (all cables to a switch) is often a logical bus (switch broadcasts, so all nodes share a channel). You must state both if asked to describe a network.

2. Claiming disabling SSID broadcast makes a network secure

SSID hiding is a minor inconvenience, not real security. The SSID is still visible in probe packets from connecting devices, and network scanning tools reveal hidden SSIDs easily. WPA2 encryption is the actual security mechanism.

3. Confusing CSMA/CA with CSMA/CD

CSMA/CD (Collision Detection) is used in wired Ethernet: a device detects a collision while transmitting and stops. CSMA/CA (Collision Avoidance) is used in WiFi: collisions are avoided by listening first and using random backoff, because radio transmission makes detection during transmission impossible.

4. Omitting RTS/CTS details when asked to explain CSMA/CA fully

If asked to explain how collisions are avoided in WiFi, describe CSMA/CA and then mention RTS/CTS as the optional handshake mechanism. Exam questions worth 4+ marks expect both.