Summary of "Lec-12: Hub in Computer Networks | Physical layer devices"

Main ideas / concepts

Hub as a Physical Layer device
- A hub operates at the Physical Layer (Layer 1).
- It is pure hardware: no software/logic is involved in deciding where frames should go.
Hub vs. Repeater
- Both hub and repeater work at the Physical Layer.
- Key difference (port count):
  - A repeater is typically a 2-port device.
  - A hub is a multiport repeater (commonly 4-port, 8-port, 16-port, 26-port hubs are mentioned).
- Extra functionality in hubs:
  - Unlike repeaters, hubs can include small LED indicators to show conditions such as:
    - whether power is on/off
    - whether there is a problem
Forwarding behavior
- A hub performs forwarding:
  - If A sends data, when it reaches the hub, the hub forwards it ahead.
Broadcasting behavior (no filtering)
- A hub has no filtering capability (because it’s hardware-only and not making decisions based on MAC addresses).
- Therefore, if A sends data to B, the hub effectively broadcasts the signal to other connected devices too (e.g., C and D also receive it).
Impact on traffic
- Because the hub broadcasts to everyone (no filtering), unnecessary traffic increases, leading to high traffic.
Collision domain
- Collisions can occur inside the hub.
- Example described:
  - If A and B transmit at the same time, their signals reach the hub simultaneously and then collide among devices (including devices like C and D).
- In general, with N devices, collision count can increase with N, and overall collisions are higher as the number of devices grows.
Comparison with bridges/switches/routers
- Bridges, switches, and routers are described as hardware + software devices (more intelligent than hubs/repeaters).
- Therefore, hubs and repeaters are cheaper than bridges/switches/routers.