[LINK] DNS outage?
pbrooks-link at layer10.com.au
Wed Jul 29 11:59:31 AEST 2009
Glen Turner wrote:
> On 29/07/09 08:33, Paul Brooks wrote:
>> This does depend on there being a different path for traffic to be
>> re-routed onto. In practice if you happen to be very close to the cause
>> of the break (or your intended traffic source/sink is), there may not be
>> another path to steer traffic onto.
> That's still a matter of money and technology. Is the last mile to the
> customer a non-redundant link or is it part of a loop? As I posted
> before, I really hope the NBN adopts a loop topology for the last mile.
> That would go a long way to making the NBN more reliable than the current
> telephone network.
I wouldn't hold your breath, and I'm more ambivalent about the need.
I don't know of any serious contender that has a redundant loop all the
way out to the residence. The current contenders for FTTN architecture
- PONs of various flavours - are all non-redundant star networks, and
have an effective range of ~30-50km or more. Compared to the current
telephone network (a non-redundant star network from the exchange
building up to 5 - 7km long (average ~2.2km)) and simplistically the
non-redundant portion would appear to be worse, not better.
However, a PON OLT can generally service only up to 32 to 64 residences,
so the length of non-redundant star fibre is likely to be limited to the
radius that covers this number - in urban areas maybe only up to 500
metres or so - so in urban areas, there may be an improvement over the
current non-redundant portion.
Taking a step back though, the current last-mile network is very
reliable - in my own experience, I can count on one hand with fingers to
spare the number of times my connectivity has been down because of a
last-mile failure. The vast majority of service impairments I've
experienced have been due to problems further into the network, where
loops, mulitple paths, multiple power supplies and redundancy measures
should have kicked in transparently, but didn't. Of course, we also
have no knowledge of how many other failures deeper in the network occur
where the redundancy measures prevent anyone being impacted at all, as
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