Megabits And Gigabytes
If you currently use hosting, or you've been looking into it, you know that one of the important considerations is how much 'bandwidth' or 'traffic' you get with a given plan - in other words, how busy your site can get. Nearly all hosting plans will specify a fixed amount of bandwidth, usually on a monthly basis, with their plans - the more expensive plans typically getting more bandwidth included. If your site(s) become busy enough to exceed this traffic limit in any given month, you will be assessed an overage, and pay for the difference.
Overwhelmingly, Managed Hosting providers will express these bandwidth limits in units called 'Gigabytes', abbreviated as 'GB'. But if you are evaluating Colocation plans, you will most commonly find bandwidth being expressed in units called 'Megabits', which is common shorthand for Megabits-per-second, abbreviated as 'Mbps'. Some Colocation providers do price bandwidth in GB, but it is less common.
If you are used to thinking in terms of Gigabytes, this can be confusing. Why the difference? What exactly does Megabits-per-second mean, and how does it compare to Gigabytes? Well, Megabits and Gigabytes are both measure traffic, but they do it in different ways, as follows:
Megabits-per-second it a measurement of the rate of traffic across a certain point (your uplink connection in a colocation data center, for example). Given the time constant of one second, a greater number of bits accross the interface indicates an increase in traffic. A Megabit is just over one million bits, and is equivalent to 128 kilobytes (eight bits equal one byte). So, to give you something of a concrete picture, if you had a website with an average webpage size of about 50 kilobytes, and it was constantly serving 2 to 3 webpages per second around the clock, that website would be doing approximately 1 Mbps of traffic.
Since the switches and routers that provide you with a connection to the Internet have built-in counters to capture rate data, measuring it and logging it is a straightforward task. Your actual traffic usage in Mbps is typically evaluated over a month's time by your Colocation provider, and most providers will use the 95th percentile method to arrive at the final number (more about that later). Megabits-per-second can be charted on an X-Y graph, with the vertical access representing the data rate, and the horizontal access representing the passage of time, like this:
As you can see, the above graph shows the varying traffic rates over a 24-hour period, based on samples taken every five minutes and also gives, peak, average and current values for data in both directions.
In contrast, Gigabytes is simply a cumulative total of the bytes of data in and out of a certain point. Theoretically, the number is computed by directly tallying all bytes as they go in and out. In practice, usually one month's worth of traffic will be tallied to arrive at the final number - and to see if it exceeds the Gigabytes per month that was included with your hosting plan.
One Gigabyte is equal to just over one million Kilobytes - so as you can imagine, computing traffic in this manner for a busy website can be cumbersome! In practice, hosting companies will usually use a variety of techniques, including scanning of log files, or coverting rate data (Mbps) to Gigabytes by way of a formula. The end result, while not entirely inaccurate, must often be considered an approximation.
So, to sum up, Mbps epresses a data rate, GB expresses a total. Is one method better than the other? Not necessarily. A bit of adjustment in thinking is needed when switching between the two. 1 Mbps sustained over a 30-day period is equivalent to just over 320 GB, so one can use that as a basis for comparing bandwidth pricing. It is important to keep in mind that traffic rates will ebb and flow constantly over time, so that yield is purely hypothetical!
If you are not yet thuroughly confused, there is one more topic to consider with regards to bandwidth: 95th Percentile Monitoring.