Comparison of latency and bandwidth

This article is a comparison of latency and bandwidth in telecommunications. A common misunderstanding of communication is that having more bandwidth means a "faster" (lower-latency) connection. But, in many cases, the reverse is true, depending on context and needs.

Table of contents

1 Definitions 2 How latency and bandwidth interplay 3 The postal service is "faster" than the Internet 4 Conclusion

Definitions

Bandwidth : how much information can be transferred over a connection in a given period of time. It's usually measured in bits per second or bytes per second. EG: 1.5 Mbps, (1.5 million bits per second) or 150 MBps. (150 million bytes per second)

Latency: how much time it takes for a response to return from a request. Usually this is measured in a simple time value. On the Internet, this is typically in "ms". 1,000 ms is 1 second in time.

How latency and bandwidth interplay

Latency and bandwidth together determine the "speed" of a connection, and the "speed" of a connection can vary widely depending on your needs.

To view a web-page over a 56 Kbps modem (56,000 bits per second) from a server 3,000 miles away is done very effectively over the Internet. Latency is low (typically far less than a second) and the size of an average web page (around 30-100 kilobytes) will transfer in 5-10 seconds or so.

However, to transfer the contents of a DVD over a modem could take a week or more at this rate. Simply packing the DVD into an envelope and mailing would be faster!

Using a T1 line with similar latencies, you could download that web page in under a second, which is a significant improvement. To download a 5 GB DVD over a 1.5 Mbps connection would take at least 55 hours, or about 2.5 days.

At this rate, the 1.5 MB T1 line is roughly comparable to shipping the DVD disk in the mail! Which is really faster?

The postal service is "faster" than the Internet

The postal service has a latency of about 3 days in most cases, but the bandwidth that can be put into a box (in the form of DVD disks) is incredible.

Assume that you ship 500 DVDs in a medium sized-box from LA to New York. To match this amount of bandwidth, you'd have to transfer 9,645,061 bytes of information *every second*. This is roughly 65 T1 lines worth of bandwidth! At a typical monthly cost of $500 per line per month, to transfer DVD disks at a rate comparable to your mid-sized package, you'd be investing $32,500 per month.

It's been reported recently that http://netflix.com transferred more information in an average day than the entire Internet. From the above, you can see this is very likely true.

Conclusion

This relationship between bandwidth and latency would also explain why satellite Internet has not been very popular. Although the bandwidth of a satellite connection can be very high and very economical, the latency added by the round trip through the satellite makes low-latency Internet connectivity (AKA ping time) such as network gaming a very bad experience.

Similarly, video is a high-bandwidth, high-latency application. If all television in use were to be transferred over the Internet, the network would buckle immediately as current infrastructure is insufficient by several orders of magnitude to handle the amount of information required. However, the high latency inherent in driving to your local video store and renting a movie to watch is perfectly acceptable.

Understanding this key difference in "speed" can greatly help one understand the implications of speed.