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Understanding Loopback Adapter Through Its Structure

#beginners #learning #networking #tutorial

Introduction

While studying for CompTIA Network+, I couldn't grasp what a Loopback Plug actually does. I understood that it was used for testing, but had no idea how it worked in practice. As a result, I kept getting questions about it wrong. Once I understood the structure behind it, everything clicked. So I decided to write it down.

What is NIC

NIC stands for Network Interface Card.
A NIC is a component inside a computer that enables it to connect to a network.

Familiar example 

Computer = Human beings
NIC      = Ears and mouth

Without ears, we cannot hear voice.
Without mouth, we cannot convey voice.

Similar to these example, computers cannot receive and send signal without NIC.
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Physical position

NIC is built-in computers.
The visible part is the port where you plug in a LAN cable - the RJ-45 port.

PC's back or sides
┌───────────────────────────────┐
│  [USB] [USB] [RJ-45] [HDMI]   │
└───────────────────────────────┘
                  ↑
              NIC port
     The hole to insert LAN cable
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The role for NIC 

1. Electric signal ↔ Digital data
　→ It transfer electric signal in Network to the data that computers can read

2. MAC address management
　→ Each NIC has a unique identifier called a MAC address
　→ The only number in the world

3. Control for sending and receiving
　→ It controls the timing for sending and receiving data
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Relationship with OSI reference model 

Layer 2  Data Link ← Management for MAC address
Layer 1  Physical  ← Sending and receiving electric signal
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NIC involves in both Physical and Data Link layers.

What is Lookback Plug

A loopback Plug is a dedicated testing tool used to verify whether a NIC is functioning correctly.

Normally, other devices is needed to connect to Network, but lookback plug enables us to test for NIC without cables or other devices.

Normal connection：
PC → cable → switch → the other PC

Loopback Plug：
PC → signal loops back to itself → received by itself
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Kinds

There are two types in Lookback Plug.

1. RJ-45 Loopback Plug（ for copper wire cables）
2. Fiber Loopback（ for optical fiber）
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The structure of RJ-45 Lookback Plug

Inside the RJ-45 connector, the sending pins (TX) and receiving pins (RX) are directly connected.

【RJ-45 Loopback Plug （front）】

┌───────────────────────────────────────────────┐
│  1     2     3     4     5     6     7     8  │
│  TX   TX    RX  　  　         RX      　  　  │
└───────────────────────────────────────────────┘
     │     │ │                   │
     │     └ │ ──────────────────┘    
     └───────┘
    （Connect TX to RX）
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The sent signal loops back inside the plug and returns to the receiving pin.

The structure of Fiber Lookback

The Fiber Loopback connects the TX and RX ports using an external cable.

【Fiber Loopback structure】

        ┌────────────────┐
        │                │
   TX ──┤ >>>>>>>>>>>>>> ├── RX
        │ Electric fiber │
        └────────────────┘

   Sent electric signal turn around to the receiving port
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【Connector （front）】

  送信側          受信側
┌──────┐   ┌──────┐
│ [TX] │~~~│ [RX] │
│ port │   │ port │
└──────┘   └──────┘
   └──────────┘
Connect with loop cable
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What is the test purpose with those adapters 

Insert Loopback Plug to NIC port
　↓
Test tools send a signal
　↓
TX （sending pin） sends a signal
　↓
The signal turn around to RX (Receiving pin) within Loopback Plug 
　↓
The same signal comes back
　↓
✓ If it returns → NIC・ port is normal
✗ If it doesn't return → NIC・ port has problems
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What specifically can we learn 

✓ What we can know：
・Whether NIC can send and receive signal
・Whether physical port breaks
・Whether NIC itself is normal without cables

✗ What we cannot know：
・Cable issues
・Switch issues
・IP address settings issues
・Internet connection issues
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How to use in troubleshooting

When PC cannot connect to Network, lookback plug is used to isolate the problems.

Step 1：Test with Loopback Plug
　→ failed → NIC・ port is the cause of problem
　→ success → NIC・ port is not the cause of problem

Step 2：Test with cable
　→ failed → cable is the cause of problem
　→ success → cable is not the cause of problem

Step 3：Test with switch
　→ failed → switch is the cause of problem
　→ success → network settings is the cause of problem
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** Isolating the root cause step by step** is the fundamental approach to troubleshooting.

Relationship with OSI reference model

Lookback plug is used for test in *Physical layer (Layer 1) *.

Layer 7  Application    │
Layer 6  Presentation │  Not subjecting to testing
Layer 5  Session           │
Layer 4  Transport       │
Layer 3  Network         │
Layer 2  Data Link        │
─────────────────────────────
Layer 1  Physical     ← Subjecting to testing with Loopback Plug
─────────────────────────────
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The standard approach is to confirm Physical Layer first, then work upward through each layer.

Summary 

NIC = Computer's components to connect to network
　→ NIC port for LAN cable inserting
　→ Involves in Physical and Data Link layers

Loopback Plug = Plug for testing whether NIC port is normal
　→ Sent signal turn around itself
　→ Kinds：RJ-45・Fiber
　→ What it can test： Physical normalcy in NIC・ port
　→ What it cannot test： cable・ switch・ settings problems
　→ OSI層：Only Physical Layer （Layer 1）
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Conclusion

I used to struggle with Loopback Plug because I didn't even know what a NIC was in the first place.
Not knowing what NIC was made it impossible to understand what the plug was actually testing.
Once I understood NIC first and then the structure of the Loopback Plug, everything fell into place.

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