Network Physical Structure — Topologies; Network Devices

Network Physical Structure — Topologies

What is Network Topology?

Network topology refers to how devices are connected in a network — the geometric pattern of the network.

Two types:

• Physical topology — actual physical layout of cables and devices

Logical topology — how data flows through the network

Topology Types

11.1 Bus Topology

All devices connected to a single shared cable (backbone).

Device1 -- Device2 -- Device3 -- Device4 -- Device5
|___________________ Bus _____________________|

Characteristics:

Feature	Description
Connections	All nodes on a single cable
Data flow	Broadcast — all devices receive all data
Failure	If backbone fails, entire network fails
Terminator	Required at both ends (prevents signal reflection)
Use	Old Ethernet (10Base2, 10Base5)

Advantages: Simple, low cost, easy to implement Disadvantages: Single point of failure, performance degrades with more nodes, hard to troubleshoot

11.2 Star Topology

All devices connected to a central hub or switch.

        Device1
           |
Device5 --Hub/-- Device2
        Switch
           |
        Device3
           |
        Device4

Characteristics:

Feature	Description
Central device	Hub or Switch
Failure	If central device fails, entire network fails
Node failure	Only affected node is isolated
Cables	Separate cable per device
Use	Modern Ethernet (10Base-T, 100Base-T)

Advantages: Easy to add/remove devices, fault isolation, high performance Disadvantages: Central device is single point of failure, more cabling than bus

11.3 Ring Topology

Devices connected in a closed loop; data travels around the ring.

Device1 → Device2 → Device3 → Device4 → Device5 → (back to Device1)

Characteristics:

Feature	Description
Data flow	Unidirectional (or bidirectional in dual ring)
Token	Token Ring — device must have token to transmit
Failure	Single break stops the ring
Dual ring	FDDI — second ring provides fault tolerance
Use	SONET/SDH, IBM Token Ring (historical)

11.4 Mesh Topology

Every device has a direct connection to every other device.

D1 -- D2
| \/ / |
| /\ / |
D3 -- D4

Full Mesh: n(n-1)/2 connections for n devices.

Characteristics:

Feature	Description
Connections	Every device to every other device
Reliability	Highest — multiple paths exist
Fault tolerance	Excellent
Cost	Very high (lots of cables)
Use	Internet backbone, military networks

Partial Mesh: Only some devices have redundant connections (cost-effective compromise).

11.5 Tree Topology (Hierarchical)

Combination of star and bus; devices arranged in a hierarchy.

         Root Switch
             |
    +--------+--------+
    |                 |
  Switch1          Switch2
  /    \           /    \
D1     D2        D3     D4

Use: Large enterprise networks, campus networks

11.6 Hybrid Topology

Combination of two or more topologies.

Example: Star-Bus, Star-Ring Use: Most real-world networks are hybrid

Network Devices

12.1 Hub

A hub is a basic networking device that connects multiple computers in a LAN.

• Operates at Layer 1 (Physical Layer) of OSI model
• Broadcasts data to all connected ports — does not filter
• All devices share the same collision domain
• Old technology — largely replaced by switches

[Hub] receives data on one port → sends it out ALL other ports
Even if data is meant for only one device

Types:

• Active Hub — regenerates and amplifies signal (powered)
• Passive Hub — just connects cables, no amplification

12.2 Repeater

A repeater extends the network by amplifying and regenerating signals.

• Operates at Layer 1 (Physical Layer)
• Extends the range of a single network segment

Does not filter data — forwards everything

• Used when cable length exceeds maximum

[Segment1: up to 100m] → [Repeater] → [Segment2: up to 100m]
                         (regenerates)          = 200m total

12.3 Bridge

A bridge connects two network segments and filters traffic using MAC addresses.

• Operates at Layer 2 (Data Link Layer)
• Filters by MAC address — only forwards necessary traffic
• Reduces collision domain
• Has a MAC address table (learns which MACs are on which segment)

Segment A ← [Bridge] → Segment B
   |           |           |
   | MAC table: |           |
   | MAC1 → A  |           |
   | MAC2 → B  |           |

Modern equivalent: Network switches (multiport bridges)

12.4 Switch

A switch connects multiple devices in a LAN and forwards data only to the intended recipient (by MAC address).

• Operates at Layer 2 (Data Link Layer) (Layer 3 switches also exist)
• Each port is its own collision domain
• Maintains a MAC address table
• Smarter and faster than hub

Switch receives frame → checks destination MAC → forwards ONLY to correct port
(Not broadcast like hub)

Types:

Type	Description
Unmanaged	Plug-and-play; no configuration needed
Managed	Configurable (VLANs, QoS, port mirroring)
Layer 2	Forwards based on MAC address
Layer 3	Forwards based on IP address (routing)
PoE Switch	Powers connected devices (IP cameras, phones)

12.5 Router

A router connects different networks and routes data between them using IP addresses.

• Operates at Layer 3 (Network Layer)
• Makes intelligent forwarding decisions based on IP address
• Maintains a routing table
• Connects LAN to WAN (internet)

Home Network (192.168.1.x) ← [Router] → Internet (public IP)

Router vs Switch:

Feature	Switch	Router
OSI Layer	Layer 2	Layer 3
Address used	MAC address	IP address
Scope	Within LAN	Between networks
Routing table	MAC table	IP routing table
Broadcast domain	Shared	Separate per interface

12.6 Gateway

A gateway connects networks using different protocols.

• Operates at all layers (particularly upper layers)
• Translates between different protocol stacks
• Example: Email gateway (SMTP to X.400)

12.7 Access Point (AP)

A wireless access point provides wireless connectivity to a wired network.

• Provides Wi-Fi access to wireless devices
• Connected to wired network via Ethernet

Multiple APs can cover large areas (enterprise Wi-Fi)

Device Summary by OSI Layer

Device	OSI Layer	Forwarding Based On
Hub	Layer 1 (Physical)	None — broadcasts
Repeater	Layer 1 (Physical)	None — regenerates
Bridge	Layer 2 (Data Link)	MAC address
Switch	Layer 2 (Data Link)	MAC address
Router	Layer 3 (Network)	IP address
Gateway	All layers	Protocol