Internet Speed Issue – Bandwidth VS. Throughput

Most of Internet users (I met) are a bit confused about meaning and definition of Bandwidth and Throughput . If you are non-computer person or just happy with Internet connectivity, probably those terms, bandwidth and throughput, are in out of your sight. However some people who are living with e-commers field, Stock trader, most of IT professionals are really sensitive for downloading Internet speed.  Size of Internet and ISP’s backbone speed is quickly upgraded day by day. I heard one of tier1 ISP, Verizon, provides 40Mbps download and 20Mbps upload Fios service for business. Furthermore, price is really competitive.

Anyway, if you have a high speed broadband or serial links (DS3, OCs and Metro Ethernet) for Internet, do you believe maximum download speed is same as your subscribed Internet speed? In another word, if you have DS3 45Mbps (bandwidth) link to ISP, do you think you will get 45Mbps downloading speed? or TCP throughput, when you download files or transferring data to other location? Most of people might say, of course that’s what I am paying for. However answer is "Yes" and "NO". Let’s figure why that is!

• Internet Speed Issue – Bandwidth VS. Throughput
• Internet Speed Issue – How to troubleshoot(FTP)
• Internet Speed Issue – How to troubleshoot(IPERF)
  
• Internet Speed Issue – Why TCP window size is matter
• Internet Speed Issue – How to use TCP window scaling option
• Internet Speed Issue – Optimal MTU size on VPN or others

 

 Definition

First, let’s see what is definition of Bandwidth and Throughput. From the Wikipedia,

Definition of Bandwidth is
"A data rate measured in bits per second"

Definition of Throughput is
"Average rate of successful message delivery over a communication channel. These data may be delivered over a physical or logical link, or pass through a certain network node."

 

Bandwidth

In general, both Bandwidth and Throughput are usually measured in bit per second.
Meaning of Bandwidth is not too difficult to understand. Also, scope of bandwidth is normally considered between ISP edge device (router) and CPE.
See below diagram shown DS3 (45Mbps) VS. T1 (1.5Mbps). Simply says DS3 is 28 times bigger than T1. Easy~

Pic 1 – Bandwidth VS. Throughput

Throughput

Understanding of Throughput is sort of trick and you might need to use your brain a bit. See below picture shown simple testing environment involved a server and client devices are located at US and other client device is located far away at Asia (China). All connection to ISPs is 45Mbps (high-speed Internet).

Let’s assume below conditions

• All devices have same TCP window size 64K bytes(65535bytes, standard size for most of window OS, but not NT and older)
• Testing devices are only machines are connected to ISP on each locations.
• Testing devices are only machines are generated traffic between locations.
• No congestion between nodes.
• No packet lost betwween nodes.
• RTT(Round Trip Time) is Server(New York) to Client (Chicago) = 30ms
• RTT(Round Trip Time) is Server(New York) to Client (Japan) = 200ms

** Above RTT is average of realistic value between nodes.

Pic 2 – Bandwidth VS. Throughput

 

Pic 3 – Bandwidth VS. Throughput

Based on TCP Throughput calculation formula, throughput of upload / download between each node are below.

 

TCP Throughput NY to Chicago = 512000 (64000 bytes x 8) bits / 0.03 second = 17066666 bps = 17Mbps 

 

 

TCP Throughput NY to Japan = 512000 (64000 bytes x 8) bits / 0.2 second = 2560000 bps = 2.560Mbps

 

 

 

Here is a theoretical TCP throughput under the assumption; both clients at Chicago and Japan won’t reach 45Mbps bandwidth which is subscribed bandwidth.

So, while the client at Chicago was downloading a file or transferring data from the server at New York, both end of DS3 links will be utilized only 37% of total bandwidth. Also, in other case, both side of DS3 will be utilized only 5.6% of total bandwidth.

So, we can imagine that simply 3 clients can transfer data at the same time without overutilization in the first case (Pic 2), and on the second case (Pic 3); about 17.5 clients can transfer data.

Now, you might say "It is not true, I get more than 35Mbps throughput at similar environment!" Okay Okay, hold on~. Yes you can get more throughputs. I didn’t say you can get better throughput. We have reviewed in general circumstance. We will talk about how we can increase throughput on other article within Internet Speed Issue series. – "Internet Speed Issue – How to use TCP window scaling option".

I hope you got some idea about what are Bandwidth VS. Throughput.

"Do not screaming that Throughput is not improved after you have upgraded Bandwidth."