Why not max speed on ADSL when line capability > connection speed?

[MickeyD]

@Logo - I'm aware of it (transmission networks 101). I added the "less overheads" to indicate that he will never get full line speed, whether on capped or uncapped accounts.

 

[Bryn]

Capped and uncapped has nothing to do with the overhead, the overhead doesn't change on which account you are as they both use the same network. Overhead is the result of the routing, header and encryption bits that form part of your data packets you send out over the network. Without this you won't have a network. It also has nothing to do with contention ratios as you state later. Contention ratio is the ISP selling 100mb capactity 200 people at 1mbps again nothing to do with network overhead.

To the OP and to MickeyD you can read up on network overhead and the maximum possible throughput at this link http://pflog.net/dsl_overhead/

In short OP the overhead forms part of the data and is not stripped out of the packets when being transmitted between source and destination therefor you will never get full line speed. The downloaders you use are right at the end of the line and only receive the actual data packet and they only count this in your data throughput speeds while the ISPs count the headers in to your throughput speed. on ADSL you are looking at overhead of about 15 to 20 percent. Some ISPs allow for this in that they allow the line to "burst" over the threshold for a short period of time, this is to aid with small downloads such as normal browsing and email downloads, but as soon as a large file is being downloaded you will see it spike in the beginning and then settle down to whatever speed your account is set to.

Perfect answer to my question. Very informative. Will hit that link now. Thanks!

 

[qinglung]

The link that Logo posted explains is nicely.

I like to think of TCP overhead as the packaging when you want to post something, let's say a box of DVDs

The packaging has the destination address on it, and you put in some bubble wrap to make sure the DVDs arrive safely. When posting it, you pay for the weight of the DVDs and the packaging, but in the end you just throw away the packaging. All you really want are the DVDs.

Now, when downloading your DVDs (legally, of course) the overhead in the packets include the destination of where the packets should go, as well as checksums to do error-checking to make sure your data arrives safely (and some other stuff). Again, you're only interested in the DVD data so the overhead gets thrown away, but those bits of overhead still needs to be downloaded.

 

[Icarium]

I'm in the same situation as the OP and in my case I do often get around 500kb/s download speeds. Then again speedtest reckons I'm on a 5mb connection even though my ISP account is 4mb.