[AusNOG] Why not Symmetric ingress and egress?

[Mark Smith]

On Thu, 17 Jun 2010 09:29:45 +0930
John Lindsay <JLindsay at internode.com.au> wrote:

> It's mostly a way of differentiating business services from domestic services.
> 
> Access seekers will be paying by the megabit up and down.
> 
> When I say differentiating what I mean of course is "enabling charging more for".
> 
> Based on ISP experience asymmetric service keeps traffic in better balance.
> 
> There are also some limits in the FTTH network architecture that mean there is less total bandwidth in the back channel.
> 
> It is easy to prioritise the traffic from one head end to hundreds of end users.  It is much harder to control those end points when they each want to transmit over one shared path.  The network uses much the same protocol as WiFi for controlling this.
> 

I think it would actually be more economic if symmetric was used.

Lets say you have 100 customers on 100 Down/8 Mbps Up. If, at your
peak traffic time, 50% of them were fully utilising their 100Mbps down,
to avoid congestion, your backhaul will need to be 5 Gbps towards
the customers. As you end up buy symmetric backhaul, you'll also be
buying 5Gbps upstream bandwidth. Even with all 100 customers utilising
their 8Mbps up, you'll only ever utilise 800Mbps of the upstream 5Gbps.
What a waste of 4.2 Gbps upstream backhaul!

If, OTOH, you can convince those customers to accept 50 Mbps/50 Mbps,
you only have to buy 2.5 Gbps symmetric bandwidth for backhaul, and will
possibly be able to fully utilise the upstream backhaul you've paid
for. So you're buying less backhaul bandwidth in total, and gaining
better efficiency out of what you've bought. This efficiency carries
right through your network to your expensive transit links.

How hard would it be to convince people to accept 50/50? It seems 66%
of the (debatably) technically savvy Whirlpooleans would be fine
with it -

http://forums.whirlpool.net.au/forum-replies.cfm?t=1462456 

It'd also avoid the performance problems that TCP has with asymmetry -

RFC3449 - "TCP Performance Implications of Network Path Asymmetry"

http://tools.ietf.org/html/rfc3449

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.

Regards,
Mark.