[AusNOG] Discussion point: Why aren't the NBNCo tails symmetric in speed?

[Paul Brooks]

On 9/04/2014 9:39 PM, Skeeve Stevens wrote:
> Hey Chris,
>
> I wasn't asked 'when' they were doing it, but more 'why' they aren't doing it from
> day dot on the speeds they've chosen, which from what I understand, the GPON being
> rolled out is fully capable of.

Because GPON has an underlying asymmetry ratio of 2:1, so it makes sense for the
services provisioned on the GPON to also approach a 2:1 ratio, to avoid one direction
becoming congested while still leaving headroom unused in the other direction.
If you want to guarantee 100Mbps downstream, then you can't split the 2.4 Gbps GPON
downstream to any more than 24 ports.
If you were to have those 24 ports provisioned as 100 Mbps upstream as well, they
could collectively saturate the 1.2 Gbps uplink capacity - and the NBN fibre portion
is engineered to be contention-free.

OK, you could limit the number of port-splits to 12, so the GPON uplink channel never
becomes congested with a full complement of 100/100 symmetric services - but then you
would only ever use half or less of your collective downstream capacity, which is daft.

Why it is 100/40 at the top speed and not 100/50 is harder to answer.

P.