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 or a little less than a mile to allow for spacing between the trains.
Each train can go a maximum of 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 (trains 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.
First, assume the tracks are all 1-way from left to right. Then 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 average speed on both sides of the node will be reduced to 200 tph.
Next, assume 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. So you see that, for this network, the maximum speed is 200 tph in either direction.
The situation is similar in data 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 website at 1 Gbps but the website server sends data at a maximum speed of 200 Mbps, 200 Mbps is 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. If the concept is still a little fuzzy to you, read the next section.
Simpler Railroad Track Analogy bandwidth vs. speed
Let’s again consider the same railroad with the same characteristics as above except that we will look at an area where there is a single one-way track. Recall the bandwidth of the track is 100 tph which means there can be a maximum of 100 tph across this track. The maximum speed is 100 tph. Now let’s say we have alternating freight trains and passenger trains all 1 mile long as above. In other words, every other train is either a freight train or a passenger train. Then the maximum speed of the freight trains is 50 trains per hour, and the maximum speed of the passenger trains is 50 trains per hour. The bandwidth of the track is still 100 tph.
Now let’s consider another example. Instead of only having equal numbers of freight trains and passenger trains, let’s say that 25% of the trains are military transports, 35% are freight trains, and 40% are passenger trains. With the track bandwidth remaining at 100 tph, if all trains have the same priority, you will have maximum speeds of 25 military transport trains per hour, 35 freight trains per hour, and 40 passenger trains per hour.
