[AusNOG] Why not Symmetric ingress and egress?

[Paul Brooks]

On 17/06/2010 8:09 PM, Mark Smith wrote:
>
> My guess is that ADSL was chosen as the broadband technology to use,
> rather than a symmetrical DSL technology (not in Australia, I'm
> talking by the broadband groups who standardise it i.e. the Annex M
> people), because it sounded right for the way people were using the
> Internet at the time (consumers rather than producers), rather than
> understanding that the Internet protocols have operated over symmetric
> links for most of their life and it is therefore an unstated design
> assumption. If that wasn't the case, I don't think the above RFC would
> exist and be a Best Current Practice RFC.
>    
Nice try Mark, but your guess would be wrong!. ADSL was developed as a 
technology for video delivery several years before Berners-Lee invented 
HTTP, back when 'broadband' meant using multiple RF carriers over a wide 
frequency range on coaxial cable, and many years before anyone 
considered a mass market for Internet connectivity existed, and it was 
being used for email, gopher, linking university library catalogues, and 
providing remote terminal access through TELNET protocol to mainframes. 
ADSL was developed by Bell Labs to allow them to deliver television 
(over ATM) over their copper wire network, in response to the cable TV 
network operators starting to do telephone calls, and so competing with 
the telco's main product.

ADSL is highly asymmetric because originally that works for TV - high 
bandwidth towards the viewer, and a bit of capacity for channel-change 
information back to the network - nothing to do with the Internet. It 
then started being used for Internet because it matched people's traffic 
patterns, and also had much longer range than the early HDSL symmetric 
systems which stopped working after about 2.3km (depending on wire guage).

I'd also challenge that TCP/IP has worked and was designed for symmetric 
links for most of its life. Most of its life, and particularly in the 
early days when it was being developed, TCP/IP has been carried over 
serial circuits and dial-up - in the early days 1200/75 bps (asymmetric, 
deliberately because it matched peoples use of online resources at the 
time - fast enough to carry keypresses at a moderate typing rate towards 
the timeshared mainframe, and faster towards the user to provide fast 
full-screen refresh on 80x25 character terminals and line printers), and 
later still over 56kbps dial-up, which was actually 56kbps down, 33 kbps 
upstream assymmetric- or thereabouts. Granted early high-bandwidth 
trunks between sites were often symmetric 56kbps leased-lines, and later 
up to 1500 kps T1s until fairly recently when SDH was developed ;-)

Still - lets not allow facts to get in the way of a good story! :-)

Regards,
     Paul.