[clug] upload speeds on transact

Andrew Smith andrew at coolchilli.com
Wed Oct 22 09:08:20 GMT 2008

80-85% is a reasonable average for most xDSL products and PPPoE 
framing.  The declared speeds are the ATM rate, so you have to take out 
ATM headers (5/53 bytes or ~10%), then 8 bytes of PPPoE framing (for 
every 64-1500 byte frame), then TCP/IP overhead (16 bytes from memory 
for every 64-1500byte frame).  You're left with speed at the Application 

xDSL is a killer on the TCP ACK side of things when downloading and 
using PPPoE.  RFC's require 1 ack per 2 packets, each ack is a minimum 
Ethernet packet in size (about 64bytes with padding), which is larger 
than 1 ATM packet, so it goes over 2.  I've seen quite a few ADSL links 
congest the upstream well in advance of the downstream.

If your routing allows it, a switch to PPPoA framing should see a bit of 
an increase.


Paul wrote:
> I've just upgraded to the transact rapid (8Mb/450k) and from using the 
> http://speedtest.velocity.net.au/ I can't get Upload speeds to get 
> anywhere near 450kb the closest I've got is 330kb .
> Is there a problem with upload bandwidth on transact? or is it my 
> location etc...?
> Thanks

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