Bandwidth (Throughput) is not everything about network performance

A look along the Coconut Palm Avenue (椰林大道).
National Taiwan University.

We like to compare the "bandwidth" numbers of different network systems. In my experience, bandwidth is the most asked property when I am introducing new technologies to other network workers. Bandwidth is very easy for people to understand, to do the calculations, and to compare which one is better than the others.

However, bandwidth is just not everything. Network with larger bandwidth is simply not a faster network at all.

Here I crafted an imaginary example, to show you why bandwidth is not as important as you might think of.

This imaginary example is called: "Trans-Pacific High Bandwidth Pipe".


I live in Taiwan. Naturally, I will start from Taiwan. Suppose I want to build a data network pipe from Taiwan to United States, across the Pacific, to link up two of my data centers.

Edited from this picture on Wikipedia.

The imaginary system works like this: whenever I want to transmit data from Taiwan to United States, I write all my data onto 4-Terabyte hard disk drives. Then, I pack all such hard disk drives into one standard 40-foot container, and ship this container to United States.

The travel time for a container ship to go trans-pacific should be far less than 100 days. Let me just assume it is 100 days, including the packing and unpacking time.

Of course, when I want to transmit data in the reverse way, I just repeat the same process: write data onto 4-Terabyte HDDs, pack them into one container, and ship it back to Taiwan. And let me just assume that it also takes 100 days.

One hundred days! For round-trip time it is two hundred days! I guess nobody would ever want this "slow" solution at all.


However, let's analyze the bandwidth of this imaginary pipe. The result is very interesting.


[Dimension for 40-foot Container]

40 feet container dimension in millimeters: 12,032 x 2,352 x 2,385
http://en.wikipedia.org/wiki/Intermodal_container#Specifications


[Dimension for 3.5-inch Hard Disk Drive]

4-Terabyte 3.5-inch Hard Disk Drive dimension in millimeters: 26.11 x 101.6 x 146.99
http://m.seagate.com/gb/en/support/internal-hard-drives/desktop-hard-drives/desktop-hdd/#specs

Calculation of "Gbps" of this imaginary pipe

After some calculation, a single container can be packed with over 160,000 thousand such HDDs. Total bits transmitted per trip is over 5 billion Gigabits. After divided by around 8 million seconds (that is 100 days), the bandwidth is around 621 Gbps.

This imaginary pipe is indeed 621 Gbps. Not bad, isn't it!

(You can open the following Google Spreadsheet to verify the calculation yourself.
https://docs.google.com/spreadsheets/d/1E0x6D2cxb6Kf-Ow2AzlZxQKhocZibkt7oxVR6Sah4O0/edit?usp=sharing)


Conclusion

Bandwidth is a metric of how much data a network can serve concurrently. The larger the bandwidth is the larger the concurrency. That's all. Nothing else.

Larger bandwidth is not faster at all! We can see it easily from this imaginary example.


Ok, the Last Words

By the way, the word "bandwidth" is very misleading. I prefer to use the word "throughput" or "bit rate" when talking about data networks.

In radio communication, a band is a range of frequencies on the whole spectrum. Given the same type of radio encoding technology, the wider the band (thus the larger bandwidth), the more channels we can put in the same band. Since every channel can carry the same amount of data, we can transmit more data bits with larger bandwidth.

That is how we have the notion that larger bandwidth means higher bit rate. Somehow this notion is erroneously brought into data networks.

So, I will leave the name "bandwidth " only for radio communication.