Hubs Extend Ethernet Physically | Generated by AI
Question: Explain in English why the correct answer to the question about Ethernet extension devices is A (Hub), including a detailed explanation of the distinction between physical layer extension and data link layer extension.
Answer:
The question is testing the classification of Ethernet extension methods based on the OSI model layers where the extending devices operate.
Key Concept: Ethernet Extension Methods
Ethernet networks can be extended in two main ways:
- Physical Layer Extension (Layer 1)
- Purpose: Extend the physical reach / cable distance or connect more devices at the electrical/signal level.
- These devices do not look at MAC addresses or frame structure.
- They simply amplify/regenerate electrical signals or electrically connect multiple cable segments.
- Main devices used:
- Repeater (Repeater): regenerates and amplifies the signal to extend cable length.
- Hub (Hub): multi-port repeater — electrically connects multiple devices into one collision domain. It broadcasts every received frame to all other ports.
- Data Link Layer Extension (Layer 2)
- Purpose: Connect network segments intelligently, reduce collisions, segment traffic, and filter frames.
- These devices read MAC addresses and make forwarding decisions based on the Ethernet frame.
- They create separate collision domains (and sometimes separate broadcast domains when using VLANs).
- Main devices used:
- Bridge (Bridge): connects two network segments, learns MAC addresses, and forwards frames only to the necessary segment.
- Switch (Switch): multi-port bridge — learns MAC addresses for each port and forwards frames only to the destination port (greatly reduces collisions).
Why the answer is A. Hub
-
The question states: “物理层扩展使用的设备主要有中继器（Repeater）和（）”
→ “The main devices used for physical layer extension are repeaters and ( ).” - Since repeater is explicitly named as a physical layer (Layer 1) device, the second device must also be a Layer 1 device.
- Among the options:
- A. Hub → physical layer (multi-port repeater) → correct
- B. Bridge → data link layer → incorrect for this blank
- C. Switch → data link layer → incorrect
- D. Router → network layer (Layer 3) → incorrect
Therefore, the only correct choice that fits “physical layer extension” together with repeater is A. Hub.
Quick Summary Table
| Device | OSI Layer | Main Function | Collision Domains | Used for Physical Extension? | Used for Data Link Extension? |
|---|---|---|---|---|---|
| Repeater | Layer 1 | Signal regeneration & distance extension | 1 (same domain) | Yes | No |
| Hub | Layer 1 | Multi-port repeater, broadcasts everything | 1 (same domain) | Yes | No |
| Bridge | Layer 2 | MAC-based forwarding between segments | 2+ | No | Yes |
| Switch | Layer 2 | MAC-based forwarding, one per port | 1 per port | No | Yes |
| Router | Layer 3 | IP routing between networks | Separate | No | No |
Conclusion: Hubs are classified as physical layer devices (just like repeaters), so they are used for physical layer extension of Ethernet networks. That is why the correct answer is A.
References:
- None (explanation based on standard networking knowledge and OSI model principles)