Comparison of latency and bandwidth

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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.

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, e.g.: 1.5 Mb/s, (1.5 million bits per second) or 150 MB/s. (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 milliseconds. 1 000 ms is equal to 1 second.

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 kb/s modem (56 000 bits per second) from a server 3 000 miles away is done very effectively over the Internet. Latency is fairly low (typically about a quarter of a second) and the size of an average web page (around 30-100 kilobytes) will transfer in less than 10 seconds.

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!

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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 this 1.5 Mb/s connection would take at least (***A 1.5Mb/s connection can transfer 5GB MUCH faster than 55 hours. This figure needs to be fixed, and possibly alters the relevance of the next paragraph(s).***) 55 hours, or 2.3 days.

At this rate, the 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 amount of information that can be put into a box (e.g. many DVD discs) 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 9.6 MB of information every second. This is roughly 65 T1 lines worth of bandwidth! At a typical monthly cost of $500 per line, you'd be investing $32 500 per month to transfer DVD disks at a comparable rate.

It has been reported recently that Netflix 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 (1-2 seconds) makes low-latency Internet tasks such as network gaming a very bad experience.

Similarly, video is currently a high-bandwidth, high-latency application. If all television broadcasts in use were to be transferred over the Internet, the network would buckle immediately as the 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.