Help,

I am trying to work out how many gig transfer i get a month from a fixed connection. I have used 256k as a guide but is there an easy way to finds out or to calculate how this works for different speeds of connection?

Any help much appreciated. :D

If you use a constant 256kbps, that will add up to 80GB over a month (depending upon which month it is). After subtracting off overheads, that would really mean about 65GB of files.

However, if you have a capped line, it is impossible to use all the "available" bandwidth, both due to technical issues (TCP tries very hard to avoid filling a pipe) and demographic issues (more traffic during the day than at night). If your bandwidth is capped at 256kbps you'll probably only manage to use an average of around 128kbps, maybe a bit lower.

As a rough estimate, a capped line can run at around 50% capacity, while a "95th percentile" line can run at around 66% of rated capacity; either way, each 1kbps translates into around 0.32 GB/month of packets, or 0.26 GB/month of files.

Originally posted by cperciva
If you use a constant 256kbps, that will add up to 80GB over a month (depending upon which month it is). After subtracting off overheads, that would really mean about 65GB of files.

However, if you have a capped line, it is impossible to use all the "available" bandwidth, both due to technical issues (TCP tries very hard to avoid filling a pipe) and demographic issues (more traffic during the day than at night). If your bandwidth is capped at 256kbps you'll probably only manage to use an average of around 128kbps, maybe a bit lower.

As a rough estimate, a capped line can run at around 50% capacity, while a "95th percentile" line can run at around 66% of rated capacity; either way, each 1kbps translates into around 0.32 GB/month of packets, or 0.26 GB/month of files.

I know that TCP overhead can be up to 10-15% of the traffic on a pipe depending on various parameters, but I have seen one of our lines (512Kbps) capped at 100% of capacity for about 8 hours continuously (outgoing), so real life observation contradicts the above. We use a Cisco 3620 and MRTG which queries the router using SNMP every 5 minutes. Also at times I login to the router itself and observe 100% utilization on the circuit.

Maybe what cperciva says is true if BGP is used.

Originally posted by brav0
I have seen one of our lines (512Kbps) capped at 100% of capacity for about 8 hours continuously (outgoing),

I wrote "TCP tries very hard" ;)

Sometimes even TCP can't avoid maxing out a line, but when that does happen it means that each individual connection is running *very* slowly -- and it also means that you're going to have relatively high packet loss, which cuts down on the "useful" bandwidth used.

1kbps translates into around 0.32 GB/month of packets, or 0.26 GB/month of files.

Thanks for the help guys and allowing for the ups n downs of tcp and the different suppliers setups etc, if i was looking for a rule of thumb, how do you caluclate that above,

How do you calculate out that 1kbps = 0.32GB/month and whats the difference between the packets and the files ?

Originally posted by cptkoi
1kbps translates into around 0.32 GB/month of packets, or 0.26 GB/month of files.
...
How do you calculate out that 1kbps = 0.32GB/month and whats the difference between the packets and the files ?

(1000 bits/sec) * (60 sec/min) * (60 min/hr) * (24 hr/day) * (30 days/month) = 2,592,000,000 bits/month

Since there are 8 bits per byte, dividing the above with 8 gives you 324,000,000 bytes per month, or 324 MB, or 0,324 GB.

As for packets and files, try this: when you request a file it does not travel over the internet in one continuous piece. The router chops the data that comprise the file into smaller pieces called packets. It also adds some information to each packet, called header, which contains information about the recipient of the packet and other fun bits and bytes. It also waits for confirmation from the recipient that the packet was received. If it doesn't get receipt confirmation, it resends the packet. So you can see that the TCP protocol is actually very resilient and in most cases, as long as there is a live connection it manages to get the file to the requestor.

The difference between 0.32GB in packets and 0.26 GB in actual data is the extra information that the router sends (header, etc) which is used only for the transfer of the data and is then discarded. Typically that amounts to 10 to 15% of the useful data (ie file size) transferred. In other words, in order for the router to transmit a file with a size of say 100MB, it will actually send 115MB of data over the line.

Hope the above helps

so if i understand these figures I am not even getting 1GB a month in transfer on a 256kbs leased line ?:confused:

You may find this link useful and interesting http://www.speedguide.net/Cable_modems/bandwidth.shtml

Thanks for that link and all the help guys :D