Hub
1. Definition
A hub is a Layer-1 (physical layer) network device — a multiport repeater. It connects several computers in a star arrangement but does no intelligent processing: whatever signal arrives on one port is regenerated and repeated out of every other port. It has no memory, no MAC address table and no concept of addresses.
2. Structure and Working
PC-1 ─────┐
PC-2 ─────┤ HUB
PC-3 ─────┤ ┌────────────────────────┐
PC-4 ─────┴───►│ port1 port2 port3 port4│
│ ONE COLLISION DOMAIN │
│ bandwidth SHARED │
└────────────────────────┘
Frame from PC-1 is repeated to PC-2, PC-3 and PC-4 (all of them!)
Only the machine whose MAC address matches keeps the frame.
| Property | Behaviour |
|---|---|
| OSI layer | Physical layer (Layer 1) — only bits, no frames |
| Forwarding decision | None — repeats to all other ports |
| Address table | None (no MAC learning) |
| Collision domain | One for the whole hub — all ports contend |
| Broadcast domain | One |
| Duplex | Half duplex (shared medium, CSMA/CD) |
| Bandwidth | Shared — a 10 Mbps hub gives 10 Mbps total, so 10 nodes each get ~1 Mbps at best |
| Latency | Very low (simple regeneration) |
3. Types of Hub
- Passive hub — just a connection point; does not amplify or regenerate the signal; unpowered.
- Active hub — powered; regenerates (amplifies and retimes) the signal, so it can extend the cable distance beyond normal limits.
- Intelligent / manageable hub — includes a management interface (SNMP, port statistics, port enable/disable), used for monitoring.
- Switching hub — the older name for a switch; a true hub and a switch are different devices, and the term is a common source of confusion in exams.
4. Advantages
- Cheapest and simplest multi-port device — no configuration, no software, plug and play.
- Extends distance — an active hub regenerates the signal so a segment can be longer than a single cable run.
- Star-style wiring — nodes are cabled individually to the hub, so cabling is simpler than a bus with T-connectors.
- One dead node does not kill the others (unlike a bus break) as long as its own link is intact.
- Useful where performance is not a concern — a small lab, a temporary setup, or a quick fault-isolation aid (a hub lets a monitoring PC see all traffic, which is handy for learning and sniffing in a lab).
5. Disadvantages
- No traffic filtering — everything is broadcast to all ports: poor security (any station can sniff others' traffic) and wasted bandwidth.
- One shared collision domain — every transmission competes with every other; as nodes increase, collisions and retransmissions rise and throughput collapses.
- Shared bandwidth — 10/100 Mbps total, divided among all active users, and half duplex besides.
- No VLANs, no QoS, no port security, no monitoring in the basic models.
- Single point of failure — if the hub fails, the whole network segment goes down.
- Obsolete — hubs have been completely replaced by switches, which cost about the same today.
6. Hub vs Switch vs Router
| Basis | Hub | Switch | Router |
|---|---|---|---|
| OSI layer | Layer 1 (physical) | Layer 2 (data link) | Layer 3 (network) |
| Unit forwarded | Bits / signals | Frames | Packets |
| Address used | None | MAC address | IP address |
| Forwarding | Floods to all ports | Forwards to the destination port | Forwards to the best next hop |
| Collision domain | One for all ports | One per port | One per port |
| Broadcast domain | One | One (unless VLANs) | One per interface (normally blocks broadcasts) |
| Bandwidth | Shared | Dedicated per port | Dedicated per port |
| Duplex | Half | Full duplex capable | Full duplex capable |
| Connects | Devices in one LAN | Devices inside a LAN | Different networks (LAN to WAN/Internet) |
| Cost/complexity | Lowest/simplest | Moderate, some configuration | Highest, substantial configuration |
7. Exam Points
- Define a hub and state the OSI layer it operates at — 3 marks.
- Explain the working of a hub with a diagram; why is it called a multiport repeater? — 5 marks.
- Differentiate a hub and a switch — 5 marks, almost certain.
- Explain collision domain and broadcast domain with a hub and a switch — 5 marks, frequent.
- List the advantages and disadvantages of a hub — 5 marks.
- Why have hubs become obsolete? — 3 marks.