When considering hosting providers, you'll often notice that bandwidth commitments vary greatly. There's a reason for that; you don't need as much “bandwidth” as you're often lead to believe. You'll find many providers offering plans not limited by bandwidth, but by throughput (the actual amount of data transferred). To add to this confusion, they'll often still call this measurement “bandwidth”.
This posting is intended to help you establish realistic expectations, based on our examples of real world situations.
With many hosting providers, what they typically call “bandwidth” is really two specific metrics:
Unless you have a truly unmetered connection, understanding the above differences is extremely critical.
Let's clarify one critical point before proceeding. Consider the following internet connection options:
The types of connections that you'll find inside a datacenter are commonly ethernet (copper or fiber-based). These are nearly always “Full Duplex”, wheras your dial-up, DSL and broadband style connections are designed to run at half-duplex. Full-duplex network connections mean that data can be sent AND received at the listed speed simultaneously. So your datacenter's 10Mbit ethernet connection can really do roughly 20Mbps, while your 10Mbit home broadband connection can only do roughly 10Mbps.
Marketing often gets a bad rap when selling you “MegaBits”, many techies would argue its an attempt to obscure what you're really purchasing…I'll leave that debate for another posting. The reality is, data transfer speeds are typically measured in bits, while data quantity is measured in bytes. The difference is significant; it takes eight bits to make one byte.
Understanding Bits vs Bytes:
Put another way, if you have a 100MB file, and you have a 10Mbps ethernet connection, it will take you not 10 seconds to transfer this file, it will take you roughly 80 seconds.
Our typical datacenter/colocation plan is a 10Mbps connection, with a 1TB/month transfer limitation. This means you're able to transfer data at speeds of up to 20Mbps, but not continuously for that entire month. Why? Consider that a 20Mbps connection is really:
6.5TB/month (6,480GB) is most-certainly more than a 1TB/month allotment.
Whenever considering the network resources required to transfer a certian file, the inherent protocol overheads are often overlooked. TCP alone introduces an approximate 8% overhead, just for its headers and other necessary meta-data, add HTTP overheads and even MIME/BASE64 encoding (transparently used when transferring files via email), you'll see anywhere from 15-30% “hidden” network consumption. Pad those estimates!
An unmetered connection means a connection of a certain speed, of which you can use all or none of the available capacity.
A burstable connection means being able to send data at high (burst) speeds, but if you don't use that high speed all the time, you don't have to pay for it.
If you transfer more data than you were allotted, many providers will charge an over-usage fee. This can vary from between two to ten+ times your normal rates; such fees protect providers from customers using more resources than originally requested, without shutting-down legitimate customer services. You should monitor your usage throughout the month in order to prevent possible overage charges.
We recommend you utilize vnstat for monitoring your usage; it's an excellent command-line based tool designed to log and trend up to a year's worth of network activity.
Providers use three methods to measure and charge for monthly usage.
Committed Information Rate (CIR) billing is often where you're assigned a certain allotment, such as 1Mbit/sec. Then enabled to utilize up to 100% of that, without worry of any overage fees or other limiting factors.
Total Bytes Per Month is the allotment of a certain amount of data per month, with fees established to cover any overage. For example, you purchase a 1TB/month plan, with a $1 per GB, per month, overage fee. The benefits here are a significant cost-savings, with minimal performance compromises; setup on a 10Mbit circuit, and assigned a generous 1TB/month allotment, you're enabled to burst up to 10Mbit speeds, but are only paying for the data you use. Although you must already have an excellent understanding of your bandwidth needs.
The 95th percentile method more closely reflects needed capacities. The 95th percentile enables a short (less than 36 hours, given a monthly billing period) burst in traffic without overage charges. The 95th percentile says that 95% of the time, the usage is at or below this amount. Conversely, 5% of the samples may be bursting above this rate.
95th percentile billing is similar to the CIR billing plan, but without digging into specifics, you can pay significantly less, or more, depending on your site's typical traffic patterns.
These numbers will fluctuate monthly, and a good portion of sites often end up using little or no bandwidth. Keep in mind this only considers web-based traffic. Other data, such as backups, file transfers, ssh connections, mail, etc. can easily add 10-25% to your monthly usage. Streaming music, VoIP services, and other bandwidth-intensive processes must also be watched closely.
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