Subject: Re: minimising wavelength conversion in MPLambdaS systems

From: "J. Noel Chiappa" <[email-masked]>
Date: Mon, 31 Jan 2000 13:08:29 -0500

> From: John Hopkins <[email-masked]> 

    > wether lambda-assignment should be integrated with the process of
    > determing a route for the LmSP, or be a follow-on activity from
    > determing the route.

Well, given your goals, it would be greatly superior for it to be integrated.
If not, it could take a while for the algorithm to stumble across a path which
also happens to meet the additional wavelength allocation constraints (which
will not be available to the initial path selection algorithm). This will
especially true as the network size gets larger, and the number of potential
paths increases. Indeed, in many routing architectures, you may never get to
the path that meets your additional constraint - the routing algorithm won't
explore that far in the solution space.

    > it is very advantageous if the same wavelength can be used along an
    > entire lightpath traversing many nodes transparently (i.e.  with the
    > same lambda-in and lamba-out)

Well, this is certainly more painful to do in a Destination Vector routing
architecture (i.e. one that hands out routing tables). Their local
decision-making characteristic (in which the decisions about the path are made
by a number of independent nodes in parallel) are fundamentally at odds with
the global nature of the optimization you're trying to do. (Which doesn't mean
it's impossible, because classic Bellman-Ford algorithms minimize path length
- a global characteristic - with a local algorithm.)

But certain capabilities (e.g. making one pair's path slightly longer to allow
another pair to have a path without frequency conversion) can't really be done
in DV systems. I would recommend a Map Distribution based routing architecture
(i.e. one that distributes network maps as the basic data).

    > a distributed wavelength usage database for the optical domain ...
    > (One issue is how to keep this information up to date.)

As long as the network is small, this is not an issue. Existing Link-State
routing algorithms (a subset of MD algorithms) do this all the time.

	Noel