Bandwidth vs. Speed


Railroad Track Analogy bandwidth vs. speed

To better see the difference between bandwidth and speed, consider a railroad where

  • All trains are 1 mile long.

  • The maximum speed of all trains is 100 mph.

  • All tracks are 1-way.

  • We will consider the speed of trains across the track system to be the number of trains per hour (let’s use tph) that cross a given section of the track system.

Then the bandwidth of 1 track is 100 tph (train per hour). The bandwidth of 4 parallel tracks is 400 tph. Let’s say we have a switching location (node in the network) with 3 parallel tracks on the left and 2 on the right. The bandwidth on the left set of tracks is 300 tph. The bandwidth on the right set of tracks is 200 tph.

Assuming the tracks are all 1-way from left to right, trains on the left could approach at a speed of 300 tph. However, since only 200 can leave to the right in an hour, once back to back trains on all 3 tracks on the left reach the switching node, the speed on both sides of the node will be reduced to 200 tph.

Assuming the tracks are all 1-way from right to left, the maximum speed on both sides of the switching node is 200 tph since only 200 tph can reach the higher 300 tph bandwidth on the left side.

The situation is similar in circuit connections. With a 1 Gbps circuit into your building:

  • If the device that is using that circuit is limited to a 100 Mbps bandwidth, 100 Mbps is the highest speed you’ll get across the 1 Gbps connection

  • If 3 devices are attempting to use the 1 G connection each at a speed of 750 Mbps, it won’t happen since that’s a total of over 2 Gbps attempting to go over a 1 G circuit. They will be limited to a total of 1 G. If their priorities are equal, each will be able to use 333 Mbps.

  • If a device on the 1 Gbps circuit is attempting to download a file from a web site at 1 Gbps but the web site server is only able to send data at 200 Mbps, that’s the maximum that will be sent to the 1 Gbps circuit.

  • Even if there is a 1 Gbps device communicating across the internet with a 1 Gbps server, there could be areas of the internet that will limit that speed cause of conflicting traffic.

Hopefully, the above analogy gives you a better grasp of the difference between speed and bandwidth.