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Can someone explain how latency, bandwidth, and capacity are related please?

lorisarvendu
Grafter
Posts: 328
Registered: 26-08-2007

Can someone explain how latency, bandwidth, and capacity are related please?

I'm trying to get my head round latency on a broadband connection.  I kind of get that it is delay due to time taken for a packet to get somewhere (so latency to parts of the US should be higher than latency to places in the UK).  For example I do a traceroute to www.google.com (which a lookup tells me is located in Calfornia) and I get a time of 31ms. 
The Thinkbroadband Ping Graph tells me my minimum latency is 20ms, so I can never get a ping or trace outside of my router than will be less than 20ms.

But how come a traceroute to www.bbc.co.uk takes longer (33ms) than a traceroute to Google in California?  And a traceroute to www.plus.net takes even longer (38ms).
Also why do I get large yellow spikes of latency on a TBB ping graph, generally between 10am and midnight?  What causes latency to go up at those times?  If I did a trace to the above addresses at this time, would they take longer?
I've Googled and there are some sites that explain latency, but they are quite old, mention modems, and also treat it as a static concept - your network's latency is what it is, and you can't increase it or decrease it.  But that doesn't explain why latency does seem to go up and down on my connection.
I also kind of get that when people say "speed" they actually mean "capacity"...or do they mean "bandwidth"?
Any explanation (or link to a good explanation) would be appreciated!

-Dave (Confused of Derbyshire)
A tortoise? What's that?
You know what a turtle is? Same thing.
6 REPLIES
Community Veteran
Posts: 19,099
Thanks: 434
Fixes: 21
Registered: 31-08-2007

Re: Can someone explain how latency, bandwidth, and capacity are related please?

Firstly, Google have peer points in this country, so pings won't necessarily go to the USA.
Secondly ping times will depend on how quickly the 'pinged' device responds. That will depend on not only how fast the equipment at that end is, but how busy it is at the time. Some sites will be on slower responding servers than others.
Plusnet's Portal is not on the same servers as it's "network" - the part you use to connect to the Internet, and as you've read the latency on that is what it is, except it could take a bit longer when things get quite congested or busy, as with any device.
Speed, capacity, bandwidth. IMHO these terms are frequently misused and some of them get interchanged which leads to confusion.
For example, your Thomson Modem/Router will refer to the "Bandwidth" of your Downstream and Upstream connection when it's referring to the sync speed of your connection. Whilst it can be technically argued that it is bandwidth in the sense that  it an expression of the maximum amount of bits/sec that flow in each direction over the connection, it's poor use IMHO. Others will refer to it as Rate, Connection Speed, Sync speed for example.
Speed is what it says,  but it's generally the data rate at a given instant which may include overheads. The "given instant" could be more continuous depending on the context, eg. when referring to your sync speed.
A lot of people will be interested in their  "Download speed" which is the rate at which data actually arrives at you computer. This will be limited by the speed of your connection as well as anything else in between and at the other end.
Capacity is really the maximum bandwidth that any connection is capable of handling,  but it isn't necessarily handling that rate most of, or for all of the time.
I hope that gives a better understanding in fairly simple terms, without making the explanation too technical. It's quite possible a few pedants may come along quoting more precise definitions  Roll eyes
lorisarvendu
Grafter
Posts: 328
Registered: 26-08-2007

Re: Can someone explain how latency, bandwidth, and capacity are related please?

Thanks for that.  You could have made it more technical if you wanted to, I'm an IT technician at a university! 
Also thankfully I have a Netgear router, not a Thomson.
A tortoise? What's that?
You know what a turtle is? Same thing.
Community Veteran
Posts: 19,099
Thanks: 434
Fixes: 21
Registered: 31-08-2007

Re: Can someone explain how latency, bandwidth, and capacity are related please?

The advantage of not getting too technical is that any other non-techie readers have a chance of understanding most of it Smiley
xreyuk
Grafter
Posts: 231
Registered: 23-05-2014

Re: Can someone explain how latency, bandwidth, and capacity are related please?

To expand, latency depends on so many factors.
Latency could be variable due to how many routes you have to take to a destination address (routes change dynamically as they get congested, go down etc). Depending on the way everything is plumbed together, you could get to various points a lot quicker than others.
An example of the above would a Plusnet gateway. I usually connect to the gateway ptw-bng01, as it gives me the lowest latency. However, if I get disconnected and reconnect to that gateway, I see ping times vary by ~4ms depending on the routing taken to that gateway. So usually it's 21ms, I reconnect and it's now 25ms, it only drops back down to 21ms if I keep trying to reconnect and get the original routing.
Add that to how long equipment takes to process packets (every piece of equipment adds delay), how old that equipment is (older equipment tends to be slower), there are a lot of variables that can affect response time.
My response time to the BBC is lower than my ping graph from think broadband.
Community Veteran
Posts: 19,099
Thanks: 434
Fixes: 21
Registered: 31-08-2007

Re: Can someone explain how latency, bandwidth, and capacity are related please?

I think your use of the word "dynamically" there could cause confusion.
Once connected to a Gateway, the "route" to it does not change unless the connection goes down.
xreyuk
Grafter
Posts: 231
Registered: 23-05-2014

Re: Can someone explain how latency, bandwidth, and capacity are related please?

Indeed, the route to the gateway doesn't change, but the route of the packets after they leave the gateway can.
Smiley