Bandwidth is a measurement of the amount of data that a server or network transmits/receives over a given amount of time. Bandwidth is measured and billed for in several different ways – each with its own advantages and disadvantages.

Bandwidth is measured from the switch port of each network connected device – usually using SNMP – a network protocol to transfer statistics to a monitoring server. These figures are then collated by the network statistics package which translates them into either an excel style spread sheet or more commonly an easy to digest network graph, such as the one below:

Bandwith Measurement Network Graph

Bandwith Measurement Network Graph

You can see in the above image many statistics about the network device – in/out bandwidth, average and maximum bits per second, along with the 95th percentile figure for this graph.

How is Bandwidth measured?

Bandwidth is measured in bits per second, as shown above on the left hand side of the graph – 8.0m at the largest amount. It is important to know that there is a very big difference between mega “bits” and mega “bytes” – the physical connection of your network hardware (i.e. switch, server or router) would be measured in mega “bits” always, but traffic could be measured in megabits or megabytes per second.

Since the megabytes figure will be larger than the megabits figure (equation to follow shortly) most industry service providers like to give a total transfer based on this figure – however most bandwidth providers use megabits.

To translate bits to bytes is quite easy – there are 8 bits in a byte. In order to convert Mb (bits) into MB (bytes) divide by 8. Conversely, to convert MB into Mb multiply by 8.

So for the example above, we see that the maximum outbound figure of 8.55 Mbs actually equates to a transfer rate of 8.55*8 = 68.4 MB per second – luckily the above server is on a giga bit connection – otherwise you would be thinking about upgrading your server network hardware.

The 95th percentile method of bandwidth measurement

Graph

Image Source – Luis Llerena

Another figure you will see on the above graph is the 95th percentile figure. This figure is used extensively by bandwidth providers to measure bandwidth – it is a method of calculating a realistic figure of bandwidth usage by artificially removing the top 5% of traffic – therefore taking some of the large spikes off the top – so using the above as an example – the 95th percentile figure is 7.19Mbits but the peak level is 8.55Mbits – many ISP’s use this method for calculating capacity management – in effect making sure they have enough bandwidth for at least 95% of the time.

And the alternative bandwidth measurement methods?

Sometimes bandwidth is measured by the total transfer method – this is a method of measuring how many megabytes, gigabytes, terabytes or petabytes a network device transfers in a given time. Usually this is calculated on a calendar month, and usually you don’t get bandwidth graphs, like the one above, as the numbers are always going up rather than fluctuating.

This total transfer method is only useful in a certain amount of scenarios, but not very easy to diagnose when your traffic levels fluctuate – again, this is dependent to the service provider and their stats packages.

Bandwidth and you

So which way should you go? As with many things in life, it is a case of “horses for courses” – you may find that the total transfer method is good for you so you can calculate how many times something has been downloaded. Or you may find that your bandwidth usage has spikes in it, which the 95th percentile method is useful for removing thereby reducing your bill. Whichever method you use though, it’s important to understand the underlying methods of bandwidth calculation.

Bandwidth lingo

Bandwidth has a lingo all of its own. Here’s a handy list:

56 kbit/s Modem / Dialup
1.5 Mbit/s ADSL Lite
1.544 Mbit/s T1/DS1
10 Mbit/s Ethernet
11 Mbit/s Wireless 802.11b
44.736 Mbit/s T3/DS3
54 Mbit/s Wireless 802.11g
100 Mbit/s Fast Ethernet
155 Mbit/s OC3
600 Mbit/s Wireless 802.11n
622 Mbit/s OC12
1 Gbit/s Gigabit Ethernet
2.5 Gbit/s OC48
9.6 Gbit/s OC192
10 Gbit/s 10 Gigabit Ethernet
100 Gbit/s 100 Gigabit Ethernet

The 56 kilobits at the top harks back to the days of the internet when it was in its infancy and dialup internet access was the norm. You then progress through current and future specifications including emerging 100 Gigabit Ethernet. The most common connections available are 100Mbit “fast Ethernet” which is prevalent in most home user PCs ; 1 Gigabit Ethernet – found in upper end office PCs and entry level servers; and 10 Gigabit Ethernet, usually found in networking equipment such as top of the rack switches and edge routers.

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