Optical Networking University of Southern Queensland

Where are Optical Networks Happening?  The Core network  Metropolitan  National  Can we combine the two?  Access Networks  Access networks are switching to all optical at the moment  (Previous technologies: copper pair, Hybrid Fiber Coaxial (HFC))

What are the issues?  Cost: how to use the minumum of pits and ducts to deliver the maximum bandwidth.  Security  Reliability  Management of access, routing, reconfiguration.

List of Topics  WDM  Optical routing  Optimal design of optical routing  Robustness & Reliability of Optical Networks  Robustness of the physical network  Robustness of logical networks  MPLS  MPLS for VPNs  Layered Switching  Layered switching including optical

Optical Routing  Objective:  Allocate end-to-end paths along which transmission can occur at optical level.  Constraints:  The same colour (wavelength) must be used all along a path (otherwise its not purely optical);  Two routes using the same colour must be disjoint.

Logical vs Physical  The physical path traversed by a wavelength implements a logical path, connection, between two endpoints.  The collection of all these end-to-end “links” is the logical topology provided by the routing plan of the wavelengths.

Optimal Optical Routing  Objective: connect a certain number of nodes, including every possible O-D pair, with the minimum number of wavelenths, OR  Connect the maximum number of nodes, including every O-D pair, with a fixed number of wavelengths. Problem: it depends on network topology!

Typical Topologies  Ring  Star  Grid For each of these topologies it is appropriate to ask: What is the optimal number of wavelengths for n (or nxm) nodes? What is the maximum number of nodes for k wavelengths?

Answers  In the case of a star network, you should be able to answer these questions by yourselves.  In the case of a ring network, the answer to the first question is: Least integer above(1/2 x Greatest integer below((n^2)/4))

But, what does this answer mean?  We need to distinguish two types of routing:  duplex routing – each wavelength is used pairwise in both directions;  simplex routing – wavelengths are freely used in both directions.  The formula of Bermond assumes simplex routing.

Exercise Find the paths and colours for a ring network of four nodes which meets the Bermond bound, i.e. this must use just 2 colours. Hint, it is essential to use a different set of paths in the clockwise direction from in the anti-clockwise direction.

Solution

How do we know this works? Check that there is a path from A to B, and B to A, A to C, and C to A, etc, for each pair of nodes; Also check that the same colour is not used twice, in the same direction, on each link. If these conditions hold, we have a good solution.

Heuristic Methods  Shortest path routing  The “load” is defined to be the maximum number of paths passing on any link. The minimum required wavelengths is at least this large.  The required wavelengths might be larger than this due to the constraints on wavelength allocation.

Hybrid Networks  Allow optical electrical optical conversion at some nodes.  This allows networks of arbitrary size.  Allow splitting of optical signal (duplication of the optical signal) at nodes.  This expands the capacity so the number of nodes equals the number of wavelengths x the minimum number of outgoing links.

How Big?  Metropolitan? nodes  ~ 100 wavelengths should be enough  National? nodes  ~ 100 wavelengths should be enough  National + Metropolitan?  Probably still too big for pure optical network  Could have some overlap  Access networks: must be separate.

Reliability

 For reliability, networks must be made of rings  The size of each ring should not exceed a certain number, eg 10 nodes.  In addition, spare capacity is needed, for the backup paths.  But the backup paths might be provided in an upper layer, not in the optical layer  This reduces the capacity of optical networks

Upper Layers  A wavelength cannot be used by end-users  It would also be wasteful to allocate a whole wavelength to one user.  The next layer of choice in core networks is SDH, the Synchronous Digital Hierarchy.

Reliability of Upper Layer  To work out the reliability of an upper layer, we need to choose a mapping from upper layer links to optical paths.  The upper layer also has rings, and backup paths  The crucial property is:  Upper layer backup paths must be disjoint in the lower layer.

What are the layers?  IP / VOIP  SDH  WDM  Physical Reliability and reconfiguration can and should be managed in all of these layers. The lower the layer, the faster the recovery.

Example  See the study book

Solution Method  In the paper by Modiano et al, an integer-linear program problem which finds a “survivable” network design is identified.