DWDM Metro Access Design


I don't know if this is the appropriate list for this kind of subject, so if
anyone knows another specific list, please tell me...

I'm analysing several DWDM designs to implement at my city, but I'm still a
bit confusing about the Metro acess design. I'm supposed to build a physical
ring topology with 6 pairs of fiber with an hub-and-spoke logical topology.
The ring will have about 40Km. At the HUB we'll install our
point-of-presence with a MPLS equipment, and at the spokes we'll use only IP
routers. We need an flexible design where we can add or remove spokes as
needed with the minimum effort possible. We are planning to have, at a
initial deployment, about 200 hundred spokes, and all these spokes are
talking only with the HUB site. Everything should work like in an FTTH or
FTTB design, no other type of transportation is allowed (wireless and

We can't use SONET/SDH. The solution must be only IPoDWDM or complemented
with TDMoIP at the access equipment.

The problem, is that all documents that I'm reading specifies that we should
be worried with faults scenarios at the spokes, so that the optical network
does not stops. For example, if the OADM equipment at the spoke is down, the
lambda dropped at that site will be down too... Or at least, if we use a lot
of lambdas, we need to keep and eye at the points where we have

We need bandwidth from 10Mbps to 1000Mbps at these spokes.

My question is:
Is it possible to make such a network in a way that we don't need to worry
about faults (electrical or others) at the spokes? If so, how can I do this?

I don't want the spokes sites interfering directly at the operation for the
whole network.

Thanks for your help.

Hello Livio:

At some point you will have a single point of failure, it's just a matter
of where. If you are running a single-threaded lambda or set of them into
a spoke site, that node will go down should your transport gear fail. If
you want your add-drop sites to be redundant through the network layer you
will have to feed each spoke site from the East and West side of your ring
on separate add-drop gear.

That will be expensive. If price is no object, you can do that and then
use your upper layer protocols to determine path availability. Or, you
can build your add drop site with a single device and built-in redundancy
(controller cards, power supplies, etc.) to keep the cost down.

Long story short, if you need those sites to stay up regardless of
anything else, you have to build two of everything at each site. It can
certainly be done and many a vendor would like to talk to you about
solutions I'm sure! :slight_smile:


I don't rally care about the uptime at the spokes. It's not my
responsability to maintain the spokes sites, we'll just give communication
to our network.

I know that I'll have single point of failure in my topology, like having
just one HUB, but I just don't want a spoke interfering in the opeartion of
my network. Ex.: I don't want a eletrical failure at one spoke interfering
in the operation of other spokes...

Thanks for your reply. :slight_smile:

What's the constraint that rules out using SONET or something similar, which is
designed to give you a robust ring topology? I think it's probably quite
important to know whether that's really, absolutely out of the question, or
whether it's a possibility to relax that in favour of a less painful higher
layer solution.