[c-nsp] Metro / NGN hardware/design

[Adam Armstrong]

Hi All,

This is quite a long query, but thanks in advance to anyone who reads it 
and has some suggestions for me! :)

I'm currently working at an Island telco which is moving away from the 
traditional centralised exchange + dslam + system-x model to a more 
distributed model using VoIP and MSANs. Our Island is approx 9 miles by 
5 miles and has ~35,000 telephone lines. The goal is to cut down loop 
length. Think of it like FTTC, but not quite.

We've built mini exchanges all over the island which will contain MSANs 
(a DSLAM which also connects the POTS to a voip soft switch at the 
exchange) which we intend to connect back to the old exchanges which 
will house the soft switches and PPPoE BRAS. I hope the image below 
explains roughly how we're planning to do it.

Image is : http://alpha.pimpmynetwork.org/~adama/sdr-model.jpg

My query is how would be best to link the mini exchanges back to the old 
exchanges. The plan devised before i joined was to use VPLS to create a 
large layer 2 network and run HSRP between a 7600 at each of the two 
main old exchanges. 6524s would be used at some of the mini exchanges, 
with others having their MSANs connected directly to each central 
exchange's 7600 with a fiber link.

My first issue is with VPLS, aside from requiring very expensive 
hardware, is it reliable enough for this? (we're the national telco, 
this will be carrying 999/911/112 calls)
The voice portion of the MSANs use the HSRP interface of the 7600s as 
their default gateway and talk to the soft switches across layer 3. 
There will be redundant soft switches at different sites, so an MSAN 
should be able to see the two soft switches at site 1, and its two 
backup soft switches at site 2. All with <100ms reconvergence!

The adsl portion of the MSANs needs to sit on a vlan with the relevant 
interface on the BRAS. It's likely that each msan will have many vlans. 
It's also likely that we'll be providing ethernet over copper and/or 
fiber services out of the mini-exchanges too.

Would it make sense to build a seperate highly-reliable layer 2 network 
just for the voice to focus on getting extreme uptime out it, and 
another higher-capacity, feature-rich network for the broadband/ethernet 
services?

Any suggestions on how to go about building such a layer 2 network with 
cisco kit? Any opinions of Cisco's REP? ME6524s in each main exchange, 
ME3400s in each mini-exchange, running REP?

A gige ring (or several) running around the island linking all of the 
mini-exchanges? A wdm ring (or several) delivering hub-and spoke 
connectivity from each mini-exchange to each main exchange?

Because of the voice nature, convergence should be as low as is 
realistically possible, preferably with the likelyhood of human error 
reduced as much as possible.

Thanks in advance,
adam.