What is this 95th Percentile (or, the difference between throughput and transfer)?
Many, if not most hosting companies sell and bill bandwidth based on a method called the 95th percentile. Many, if not most customers, don’t have a clue what the 95th percentile really is. In this article, I’ll try to shed some light on what 95th percentile is.
In order to explain, we must first understand the difference between the two types of bandwidth billing methods. Those two types are TRANSFER (95th percentile billing) and THROUGHPUT (per-gig billing). Let’s look at them individually….
Throughput is the actual total SIZE of the combined files that are sent by the server. Throughput is sold in Gigabytes (GB) and is an aggregate monthly total. So, for example, let’s say you have a web page called THISPAGE.HTML and the actual page is 25k, on this page you have 3 graphic images that are 25k each which is a total of 100k. If 100,000 people downloaded that page over the course of a month then your Throughput would be calculated as 100kB X 100,000 = 10,000,000kB or 10GB. So for that month your THROUGHPUT would be 10GB. This does not take into account if all 100,000 people hit the server at the same time or were evenly spread out over the course of the month; it is still 10GB of THROUGHPUT for the month.
Now, on to TRANSFER, but before we begin let me state that in *NO* circumstances can you mix Throughput and Transfer. It is physically impossible (it’s like trying to add up gallons and nickles). They are two different things.
TRANSFER is measured in Megabits Per Second (Mbps) and measures how much information is traveling through the Internet “pipe” at any given time. I like to compare TRANSFER to water in a series of water pipes. Imagine that your home PC has a water hose connected to it instead of an Internet connection. The water hose is 1/2″ and is connected to the side of your house where it meets a 2″ pipe and your house is connected to the Water Main, which is a 12″ Pipe. In this example your Â½” water hose is your home Internet connection and your 2″ pipe to your house is your ISP and the 12″ water main is the backbone of the Internet. It does not matter how hard you try you are only going to get 1/2″ of water into your PC at any given time because the “pipe” is only a 1/2″ water hose.
Now if I were going to sell you water BY THE GALLON, that would be called Throughput (see above), or I can sell you a PIPE and just charge you for the amount of water that you push through the pipe at any given time…this is called TRANSFER. For example, if I take a measurement right now and you are pushing 1″ of water through the pipe and I look again in five minutes and you are pushing 1″ still and I look again in five more minutes and you are pushing 1/2″ and I look again in five more minutes and you are pushing 2″ then how big of a pipe do you need to accommodate your traffic flow without any water being backed up like a funnel??? You would need a 2″ pipe, but you are not using 2″ all the time, so why do you have to pay for a 2″ pipe all the time?? This is where the 95% comes in.
The 95th percentile (which is an industry standard) simply means that the hosting company will look at your pipe every five minutes and take a reading and add that reading to a long list that they keep for 30 days. At the end of the month that list will contain 8640 readings (there are 12 five minute intervals in an hour, 24 hours a day for 30 days). They will then take that list and sort it from the biggest number to the smallest number so that your largest five minute reading is on the top, the second largest is next, the third largest is next and so on. The top 432 entries (the top 5%) are discarded and the 433rd is considered your “95th Precentile” and that is the number that you pay for. The 95th percentile was designed to help chop off wild peaks and only bill you for what you are sustaining on a regular basis. This is a rolling 30 day number that is constantly changing. In other words, once you get the 8640 data points, every time a new data point is added and the list is sorted, the oldest data point is dropped off.
As for what is more advantageous, it depends on the traffic patterns of your site. THROUGHPUT (95%) is good for almost all sites with very few exceptions. TRANSFER is recommended for sites that have extremely high spikes or very inconsistent traffic. For example, if you have very high traffic every Monday but the rest of the week is very low traffic, then being billed on THROUGHPUT may be the best for you. In this case, you would have lots of big numbers due to that high traffic on Monday, which would create an inflated 95th percentile. However, very few sites have this type of traffic pattern.
With TRANSFER host should provide 95th percentile graphs (usually MRTG graphs which is the industry standard) and you can see your transfer yourself. You should check these graphs every day as they can indicate problems as well as let you know your traffic patterns. You should see highs and lows each day and these patterns of highs and lows should follow the sun. If you see a flat line across the top of the graphs then you know that your hosting company doesn’t have enough bandwidth to handle your needs (and this is much more common than one would think). ***IF YOU ARE BEING BILLED ON 95TH PERCENTILE MAKE SURE YOUR HOSTING COMPANY PROVIDES YOU WITH THOSE GRAPHS*** If they refuse, they obviously have something to hide.
Hopefully this helps you understand what 95th percentile is.