Day 2: Interfaces and Cables

Interfaces and Cables
Overview
Switches provide multiple ports for device connectivity—typically 24 ports on a standard switch.
These ports are RJ-45 (Registered Jack) connectors, the standard connection point for network devices.
 
 
Ethernet: The Standard
What is Ethernet?
Ethernet is a collection of standardized network protocols and hardware specifications that define how devices communicate over wired networks.
Why Standards Matter
Network protocols and standards serve two critical purposes:
Communication standards — Enable devices from different manufacturers to communicate reliably
Hardware standards — Ensure physical compatibility between devices and cables
Speed and Measurement
Connections between devices operate at specific speeds, measured in bits per second (bps).
Bit = a single value: 0 or 1
Byte = 8 bits
Speed Conversions:
Unit
Bits
1 kilobit (Kb)
1,000
1 megabit (Mb)
1,000,000
1 gigabit (Gb)
1,000,000,000
1 terabit (Tb)
1,000,000,000,000
Ethernet Standards
Defined: IEEE 802.3 standard (established 1983)
IEEE = Institute of Electrical and Electronics Engineers
 
 
Copper Ethernet Cables (UTP)
Ethernet Standards by Speed
Speed
Common Name
Standard
Cable Type
Max Distance
10 Mbps
Ethernet
802.3i
10BASE-T
100m
100 Mbps
Fast Ethernet
802.3u
100BASE-T
100m
1 Gbps
Gigabit Ethernet
802.3ab
1000BASE-T
100m
10 Gbps
10 Gigabit Ethernet
802.3an
10GBASE-T
100m
Notation:
BASE = Baseband Signaling (digital signal, single information channel)
T = Twisted Pair (copper cable type)
UTP Cables (Unshielded Twisted Pair)
What it is:
Standard copper cabling for most Ethernet connections
No metallic shield around the wires
Contains twisted pairs of wires
Why twist the wires?
The twisting protects against EMI (Electromagnetic Interference) from external sources
Helps maintain signal integrity over distance
Wire Count by Speed:
10/100BASE-T: Uses 2 pairs (4 wires)
1000BASE-T / 10GBASE-T: Uses 4 pairs (8 wires)
All 4 pairs are bidirectional (can transmit and receive simultaneously)
This enables faster transmission than older standards
 
 
Cable Pinout and Device Communication
RJ-45 Connector Pin Layout
An RJ-45 plug has 8 pins numbered 1–8 at each end of the cable.
Devices use specific pins to transmit (TX) and receive (RX) data:
Device Type
TX Pins
RX Pins
Router
1, 2
3, 6
Firewall
1, 2
3, 6
PC
1, 2
3, 6
Switch
3, 6
1, 2
Full-Duplex Transmission
The pin arrangement enables full-duplex communication—devices can transmit and receive simultaneously because they use different pin pairs.
 
 
Cable Types: Straight-Through vs. Crossover
Straight-Through Cables
When to use:
Connecting a PC or Router to a Switch
Connecting a Firewall to a Switch
Any device with TX on pins 1-2 to a device with RX on pins 1-2
Why it works:
One end has pins arranged the same as the other end
TX pins from one device connect to RX pins on the other device
No pin swapping needed
Crossover Cables
When to use:
Connecting two similar devices directly (Router to Router, Switch to Switch, PC to PC)
These devices have the same TX/RX pin configuration, so they would conflict with a straight-through cable
How it works:
One end of the cable has pins swapped to create proper TX/RX alignment
Pins are crossed: 1↔3, 2↔6, 3↔1, 6↔2
Crossover Pin Mapping:
End A → End B
PIN 1 → PIN 3
PIN 2 → PIN 6
PIN 3 → PIN 1
PIN 6 → PIN 2
Auto MDI-X
Modern equipment simplifies this:
Most switches, routers, and PCs now include Auto MDI-X capability
Devices automatically detect the neighbor's transmit pins and adjust their receive pins accordingly
Works with either straight-through or crossover cables
Eliminates the need to manually choose cable type
 
 
Fiber-Optic Cables
When to Use Fiber
Fiber-optic cables are used for high-speed, long-distance connections between network devices.
Standard: IEEE 802.3ae (and 802.3z for 1 Gbps)
SFP Transceiver
SFP (Small Form-Factor Pluggable):
A module that allows fiber-optic cables to connect to switches and routers
Separate TX and RX cables (unlike copper which uses the same cable for both directions)
Removes the need for pin management—fiber uses dedicated paths for each direction
Fiber Cable Structure
A fiber-optic cable has 4 main components:
Core (glass or plastic)
Cladding (surrounds and protects core)
Buffer (protective layer)
Jacket (outer protective sheath)
Two Types of Fiber
Single-Mode Fiber
Characteristics:
Narrower core than multimode
Light enters at a single angle (mode) using a laser-based transmitter
Allows the longest cable distances of all Ethernet media
More expensive (laser-based SFP transmitters are costly)
Use case: Long-distance backbone connections between buildings or data centers
Multimode Fiber
Characteristics:
Wider core than single-mode
Light enters at multiple angles (modes) using an LED-based transmitter
Allows longer distances than UTP, but shorter than single-mode
Less expensive than single-mode (LED-based transmitters are cheaper)
Use case: Medium-distance connections within a data center or building
Fiber-Optic Standards
Standard
Speed
Mode
Max Distance
1000BASE-LX
1 Gbps
Multimode / Single
550m (MM) / 5km (SM)
10GBASE-SR
10 Gbps
Multimode
400m
10GBASE-LR
10 Gbps
Single
10km
10GBASE-ER
10 Gbps
Single
30km
 
 
UTP vs. Fiber-Optic: Comparison
UTP (Unshielded Twisted Pair)
Advantages:
Lower cost
RJ-45 ports are inexpensive
Simple to install and terminate
Disadvantages:
Maximum distance: ~100m
Vulnerable to EMI (Electromagnetic Interference)
Security risk: Cables emit a faint signal that can be intercepted or copied
Not suitable for long distances
Fiber-Optic
Advantages:
Longer maximum distances (hundreds of meters to tens of kilometers)
No vulnerability to EMI
More secure: Signals cannot leak from the cable; no risk of interception
Suitable for long-distance and high-security applications
Disadvantages:
Higher cost than UTP
SFP ports are more expensive than RJ-45 ports
Single-mode fiber is significantly more expensive than multimode
Requires specialized installation and termination tools
More fragile (fiber can break if bent too sharply)
 
 
Quick Reference: When to Use What
Situation
Media
Reason
PC to Switch (same building)
UTP
Cost-effective, sufficient distance
Switch to Switch (same building)
UTP or Fiber
UTP for short runs; Fiber for long runs or security
Router to Router (different buildings)
Fiber
Longer distances, EMI immunity
Data center backbone
Fiber
Long distances, high throughput, security
Home/office network
UTP
Cost, simplicity, Auto MDI-X