hop 9 of 32 · 5 min

Hubs, switches and how a frame finds its port

Connecting several machines needs a device in the middle, and which device you use changes the network's behaviour completely.

Hub, switch, router

Hub: repeats every incoming signal to every other port. One shared collision domain, everybody sees everybody's traffic. Obsolete, and useful to understand because a switch is defined against it.

Switch: learns which device is on which port and forwards only there. Each port is its own collision domain, so there are effectively no collisions.

Router: forwards between different networks, using IP addresses rather than MAC addresses.

	Hub	Switch	Router
Layer	physical	data link	network
Decides by	nothing	MAC address	IP address
Collision domains	one	one per port	one per port
Broadcast domains	one	one	one per interface

How a switch learns

Nothing is configured. It builds the table from traffic it sees:

code

1. a frame arrives on port 3 from aa:bb:cc:...
   -> record that address as reachable via port 3
2. a frame arrives for dd:ee:ff:...
   -> known? forward to that port only
   -> unknown? flood to every port except the one it came from
3. the reply teaches the switch where dd:ee:ff is

Entries expire after a few minutes so a device that moves is relearned. This is why the very first frame to a new device is flooded and everything after is not.

Circuit and packet switching

Two fundamentally different ways to move data, and the distinction is a standard exam question.

Circuit switching, as in the traditional telephone network: a dedicated path is reserved for the whole conversation. Guaranteed bandwidth and constant latency, and the capacity is wasted during silence.

Packet switching, as in the internet: data is split into packets, each routed independently, sharing links with everyone else. Efficient because idle users consume nothing, at the cost of variable delay and possible loss.

code

circuit:  reserve  ->  use  ->  release     predictable, wasteful
packet:   send each packet independently    efficient, variable

The internet chose packet switching, which is why a video call degrades under congestion rather than being refused at the start. A phone call was refused up front and then guaranteed, which is a genuinely different trade rather than a worse one.

Hubs, switches and how a frame finds its port