Have you ever wondered what speed your Internet Service Provider should be delivering to be compliant with the contract? Do you get confused regarding the nominal speed of your connection and how it translates into an effective downloading speed? If so this guide should suit you, I will try to clarify the most frequent doubts Internet users face regarding the speed of their connection.
Back to the basics
Remember that a bit (binary digit) is the single piece of information in digital systems, it is either a 0 or a 1. A byte, instead, is a group of 8 bits. When we talk about computer memories or data storage 1 kilobit refers to 1024 (2^10) bits, 1 megabit refers to 1024 kilobits (or 1024 x 1024 bits), 1 kilobyte refers to 1024 bytes and so on.
In telecommunications, however, transmission rates have traditionally been declared in bits per second (bps) and 1 kilobit refers to 1000 bits and not 1024 as in data storage, after all we are talking about discrete signal pulses that describe the bandwidth.
Telecommunications bit rates
Bps = 1 bit/s
Kbps = 1.000 bits/s
Mbps = 1.000 Kbits/s or 1.000.000 bits/s
Gbps = 1.000 Mbit/s or 1.000.000.000 bits/s
Tbps = 1.000 Gbit/s or 1.000.000.000.000 bits/s
Byte = 8 bits
Kilobyte = 1024 bytes or 8192 (8 x 1024) bits
Megabyte = 1024 Kilobytes or 1.048.576 (1024 x 1024) bytes or 8.388.608 bits
Gigabyte = 1024 Megabytes or 1.073.741.824 bytes or 8.589.934.592 bits
Terabyte = 1024 Gigabytes or 1.099.511.627.776 bytes or 8.796.093.022.208 bits
Sometimes hard disk or memory manufacturers take advantage of the confusion between bits and bytes. For example they usually label products with GB, say a 100 GB hard disk, but what they are really shipping is 100 billions of bytes and not 100 Gigabytes. 100 billions of bytes are equal to around 93 Gigabytes (100 / 1,024 ^ 3), meaning the clueless customer is losing almost 10% of what he thought he was buying.
How to calculate download speed?
OK you purchased that DSL service but are not sure how the speed declared by the Internet service provider translates into effective downloading speed? Here is what you need to do to find it out.
First of all notice how most software, including web browsers, measure the download rate in Kilobytes per second (as the picture of the Firefox download window shows). The Internet service providers, on the other hand, declare speed in bits per second. Suppose you have a DSL running at 512 kbps in downstream. The first thing to do is to divide that number by 8 so we transform kilobits (1000 bits) per second into 1000 bytes per second. After that we need to divide the obtained number by 1,024 because we are now talking about data storage, therefore 1 kilobyte must be equal to 1024 bytes and not 1000 bytes as we have assumed for the transmission rate.
So 512 kbps / 8 = 64.000 bytes
then 64.000 bytes / 1,024 = 62,5 Kilobytes/s or kB/s
Internet speed = Download speed
256 kbps = 31,3 KB/s
512 kbps = 62,5 KB/s
1 mbps = 122,1 KB/s
5 mbps = 610,3 KB/s
10 mbps = 1220,7 KB/s
Consider you want to download a file large 640 Megabytes. We know this is equal to 655360 kilobytes (640 x 1024), therefore if your Internet speed is 1 Mbps you will download at 122,1 KB/s, meaning it will take 89,5 minutes to download the file completely.
The principle of Oversubscription
It is important to understand how Internet Service Providers operate. They basically purchase connectivity (guaranteed bandwidth) from Tier 1 operators and resell it to the larger public. Statistically, however, not every customer will be using Internet at the same time, and even the ones who do will not require full bandwidth (this pattern is changing with the diffusion of video streaming services, but for the sake of simplicity we will not consider the effect).
This concept allow ISPs to oversubscribe their DSL lines, a process that was already used by telecom operators with old telephony lines. The ratio of oversubscription may vary from 1:1 to 50:1 or more depending on the quality of the service the ISP is planning to offer. This means that if the ISP purchased a 10 Megabit line they could offer 1 Megabit DSL subscriptions to 50 homes (5:1 ratio) with a high quality of service or they could offer the same 1 Megabut DSL to 300 homes (30:1 ratio) with a less reliable service.
Committed Information Rate
Now the one thing you need to check on your DSL or cable contract, preferably prior to signing it, is what is called Committed Information Rate or CIR. This is basically the minimum bandwidth your service provider guarantees, at any time and under any circumstance. Consider you decided to subscribe to the 1 Megabit DSL plan offered by the ISP we mentioned above, who bought a 10 Megabit line from a tier 1 carrier. Should they use a 5:1 ratio the plan will be offered to 50 households, meaning that they can offer a CIR of around 200 kbps.
Calculating the CIR is pretty complex, but we can get a good estimation dividing the bandwidth of the dedicated line the ISP is using by the number of subscribers. Therefore 10 Mbps, which is equal to 10.000 kbps, divided by 50 customers will result in a minimum bandwidth of 200 kbps per customer, which is pretty good for current standards.
- Before signing a broadband contract make sure you know what is the Committed Information Rate and compare that CIR from different providers to evaluate the best deal. Try also to ask what is the compensation you will receive should the ISP fail to meet that minimum bandwidth.
- If it is too late for point 1, meaning you already have a broadband contract going on, inquire your Internet service provide about the CIR anyway because it will tell you what is the minimum bandwidth you should expect.
- Now that you have both the declared speed and the CIR of your broadband connection you want to convert those into effective downloading speed. Suppose you have a 2 Mbps DSL connection with a CIR of 128 kbps. This means you should have a peak download speed around 244 KB/s (kilobytes per second) and a minimum download speed of 15,6 KB/s.
- Finally, monitor the download speed you are getting through out the day. Should it go below the CIR (something not uncommon during peak hours of the day) make sure you contact your ISP to fix it or to get compensated if you incurred any losses due to the poor quality of the service.