Thursday, July 26, 2012

Next Generation Optical Transport Networks

When we say Optical Transport Networks (OTN), it could mean two things;
  1. OTN wrapper capability 
  2. OTN switching capability
To implement an OTN, there are many technological options available. From least cost (in general) to highest, the options are as follows;
Note: All options need to support OTN wrapper
  1. Fixed Optical Add Drop Multiplexers (FOADM)
  2. Re-configurable OADM (ROADM)
  3. Tunable ROADM (TROADM) (Wave-length Selective Switching (WSS)): supports Color-Less and Direction-less)
  4. FOADM with Automatically Switched Optical Networks (ASON)/ Generalized Multi-Protocol Label Switching (GMPLS) control plane
  5. ROADM with ASON/GMPLS control plane
  6. TROADM with ASON/GMPLS control plane
  7. FOADM with ASON/GMPLS control plane and OTN switching
  8. ROADM with ASON/GMPLS control plane and OTN switching
  9. TROADM with ASON/GMPLS control plane and OTN switching
Other than Color-Less and Direction-less, Contention-less is also a good feature to have on WSS systems. 

Cost of adding OTN switching capability vs. loosing sub-lambda grooming at intermediate sites need to be properly analyzed based on your current and future traffic matrix. 

To tell you the truth, OTN switching is more a hype than a reality. This is quite evident from the low number of OTN switching deployments currently in the world.

The prime advantage of OTN switching is the sub-lambda grooming at intermediate sites. The industry trend(both suppliers and operators) is to start without OTN switching and go for OTN switching in the future if all the lambdas run out/close to run out (aka
Wave-length blocking). This requires that you select a vendor who's capable of OTN switching but you need not purchase OTN switching components (cards) on day one.

You do not need OTN switching to achieve mesh protection. What is then required is ASON/GMPLS.

A good approach,adopted by many operators when publishing Request For Proposal (RFP)s for Optical Transport Networks is keeping the RFP open for all the options given above. It's required to give the fiber characteristics, locations and the traffic matrix (current and future). Based ion these inputs the vendors can come out with the least TCO option. The evaluation should also be based on lowest TCO (this covers both initial Capital Expenditure (CAPEX), future expansion CAPEX and the running Operational Expenditure (OPEX) such as site rentals, power etc.).  

When you want to do sub-lambda grooming at intermediate sites, you'll have to have OTN switching (CAPEX!).

When you have OTN switching, the earlier Point-to-Point lambda passed through several intermediate nodes at the optical domain (OOO) now need to go to electrical domain to do grooming (OEO) making it multi-segment. This requires several OTN ports (CAPEX!). However, you use only one lambda. Some call the latter as Layer 1-ASON and former as Layer 0-ASON.

If you do not do sub-lambda grooming at the intermediate site, you will have to have a separate lambda (CAPEX!) at the intermediate site, though the traffic goes to the same destination.

The above 3 CAPEX components need to be properly analyzed for the current and future traffic matrix. Then only the most optimized design and then the most optimized cost can be calculated.

For the comparison, following options are recommended to be used as ASON/GMPLS is better to have. This will also make sure that the comparison is more balanced.
  1. FOADM with ASON/GMPLS control plane
  2. ROADM with ASON/GMPLS control plane
  3. TROADM with ASON/GMPLS control plane
  4. FOADM with ASON/GMPLS control plane and OTN switching
  5. ROADM with ASON/GMPLS control plane and OTN switching
  6. TROADM with ASON/GMPLS control plane and OTN switching 
"Digital Optical Networking (DON)" technology claimed to be available with some vendors can address the sub-lambda grooming at optical level without using OTN switching (i.e using OOO as opposed to OEO) giving cost advantages.The component used is known as "Digital ROADM".


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