DWDM Metro Access Design

Michael K. Smith - Adhost mksmith at adhost.com
Mon Mar 21 20:01:50 CDT 2011

On 3/21/11 5:36 PM, "Livio Zanol Puppim" <livio.zanol.puppim at gmail.com>

>I don't know if this is the appropriate list for this kind of subject, so
>anyone knows another specific list, please tell me...
>I'm analysing several DWDM designs to implement at my city, but I'm still
>bit confusing about the Metro acess design. I'm supposed to build a
>ring topology with 6 pairs of fiber with an hub-and-spoke logical
>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
>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
>be worried with faults scenarios at the spokes, so that the optical
>does not stops. For example, if the OADM equipment at the spoke is down,
>lambda dropped at that site will be down too... Or at least, if we use a
>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
>I don't want the spokes sites interfering directly at the operation for
>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!  :-)

Michael K. Smith - CISSP, GSEC, GISP
Chief Technical Officer - Adhost Internet LLC mksmith at adhost.com
w: +1 (206) 404-9500 f: +1 (206) 404-9050
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