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.

 

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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:

1. Communication standards — Enable devices from different manufacturers to communicate reliably

2. 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:

Ethernet Standards

• Defined: IEEE 802.3 standard (established 1983)

• IEEE = Institute of Electrical and Electronics Engineers

 

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Copper Ethernet Cables (UTP)

Ethernet Standards by Speed

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

 

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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:

Full-Duplex Transmission

The pin arrangement enables full-duplex communication—devices can transmit and receive simultaneously because they use different pin pairs.

 

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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

 

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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:

1. Core (glass or plastic)

2. Cladding (surrounds and protects core)

3. Buffer (protective layer)

4. 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

 

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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)

 

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Quick Reference: When to Use What