Optical Networking

University of Southern Queensland

Where are Optical Networks Happening?

The Core networkMetropolitanNationalCan we combine the two?

Access NetworksAccess 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 TopicsWDM

Optical routingOptimal design of optical routing

Robustness & Reliability of Optical NetworksRobustness of the physical networkRobustness of logical networks

MPLSMPLS for VPNsLayered SwitchingLayered 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 networktopology!

Typical Topologies

Ring

Star

Grid

For each of these topologies itis appropriate to ask:

What is the optimal number of wavelengths for n (or nxm) nodes?

What is the maximum numberof 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? 30-40 nodes ~ 100 wavelengths should be enough

National? 20-30 nodes~ 100 wavelengths should be enough

National + Metropolitan?Probably still too big for pure optical networkCould have some overlap

Access networks: must be separate.

Reliability

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 reconfigurationcan 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.