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TAKUMI SUGATA
TAKUMI SUGATA

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Crosstalk is Just Like Music Leaking from Someone's Earphones on a Crowded Train.

#network #comptia #crosstalk #beginners

Introduction

While studying for CompTIA Network+, I couldn't visualize what Crosstalk actually meant. I could memorize that it refers to interference from adjacent copper cable. But, I didn't understand why the interference occurs and why the finer twist is faster.

Once I understood from the structure, everything fell into place.

What is Crosstalk

To receive electric interference from copper cable.

An electromagnetic field occurs around a copper cable when electric current flows 
　↓
The electromagnetic field affects the other copper cables 
　↓
Unintended signals ride on the copper cables
　→ This is Crosstalk
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Familiar example 

In a crowded train, you may have experienced music leaking from someone else's earphones. 
That leaked sound mixes with the music you are listening.
　→ This is Crosstalk image
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The problems Crosstalk cause 

・Decline in communication speed
・Data corruption
・Unstable connection
・Rise in error rate
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2 kinds of Crosstalk

There are 2 kinds of Crosstalk depend on occur place

1. Internal Crosstalk (Inside cable) 

8 Copper cables interfere each other inside the same LAN cable 

┌─────────────────┐
│ ○ ○ ○ ○ ○ ○ ○ ○ │ ← 8 cables inside LAN cable 
└─────────────────┘
　　↑
This copper cable interfere each other in electromagnetic field
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Feature：
・The most common Crosstalk
・Cannot be physically separated since the interference occurs inside the cable itself. 
・Twisted pair and shielding are the solusions
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2. External Crosstalk / Alien Crosstalk (Between cables) 

Other cables adjacent to a cable interfere each other 

Cable A ──────────────
Cable B ──────────────  ← Run side by side
　↑
These two cables interfere mutually
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Feature：
・It tends to occur when cables are bundled or run side by side
・The interference will be greater when it run next to a power cable
・Physically separating the cables is the solution
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Why more twisting reduce interference

This is the core concept of this article

The mechanism of Twisted pair 

When two copper cables are twisted ：

Copper cable A → Generates a clockwise electromagnetic field 
Copper cable B → Generates a counterclockwise electromagnetic field
    ↓
Each electromagnetic field cancels each other out
　↓
Leakage to outside decreases
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The relationships between rough twisting and interference 

Coarse twisting：
　→ Cancellation is weak 
　→ More leakage
　→ Huge interference（Frequent Crosstalk）

Fine twisting：
　→ Cancellation is strong
　→ Less leakage
　→ Minimal interfeernce（Less Crosstalk）
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More Twists → Less Leakage → High Speed 

More Twists
　↓
Cancellation in electromagnetic field is stronger
　↓
Less Leakage
　↓
Signals are stable
　↓
High Speed
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** This is the reason why higher CAT standard enable faster communication speeds **

Solution for Crosstalk

Solutions differ from each kind

Solution for Internal Crosstalk

** 1. Twisted pair **

Every 2 copper cables are twisted together
　→ Cancellation in electromagnetic field
　→ Decline in interference

The more twisting, the greater the effect
　→ Higher CAT standard is more twisting
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*2. Shield *

Over CAT6a：Whole cables are covered with shield
CAT7    ：Each pair are covered with individual shield

It can prevent leakage from inside electromagnetic field
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Solution for External Crosstalk

** 1. Physical separation **

Do not bundle each LAN cable
Do not run next to power cable 
　→ Physical separation can reduce interference
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*2 . Shield *

The cable covered with shield are used
　→ It blocks interference from outside sources
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Solution summary table

Solution Internal Crosstalk External Crosstalk
Twisted pair ✓ Valid △ Less valid
Shield ✓ Valid ✓ Valid
Physical separation ✗ Invalid ✓ Valid

Relationship with CAT standard 

CAT5  → Coarse twisting
　→ More leakage・ More interference → Until 100Mbps

CAT5e → More twisting
　→ Less leakage・ Less interference → 1Gbps

CAT6  → More twisting + spine added to the center
　→ 10Gbps（Until 55m）

CAT6a → Add whole shield
　→ Prevent from interference from outside  → 10Gbps（100m）

CAT7  → Add individual shield to each pair
　→ No Internal Crosstalk → 10Gbps（100m）
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Summary 

Crosstalk ：The phenomenon of receiving electrical interference from copper cables

2 kinds of Crosstalk：
Internal Crosstalk
　→ Interference caused by copper cable in the same cable 
　→ Cannot separate physically
　→ Twisted pair・ Shield are the solution

External Crosstalk （Alien Crosstalk）
　→ Interference caused by adjacent cables
　→ Can separate physically
　→ Shield is also the solution

Core：
More Twists → Less Leakage → Less Crosstalk → High Speed
　→ This is the reason why higher CAT standards rise in speed
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Conclusion

I used to struggle with memorizing what Crosstalk was.
Once I understood the flow below, everything clicked naturally.

Internal Crosstalk  = Interference inside cable
　→ Solution : Twisted pair・ Shield

External Crosstalk  = Interference between cables
　→ Solution : Physical separation・ Shield

More Twists → Less Leakage → High Speed
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