šŖšµš šš¶š“šµš²šæ š¢šæš±š²šæ š¤šš šš®š¶š¹š š¶š» š„š²š®š¹ š”š²ššš¼šæšøš
š²š°š¤šš looks incredible on paper higher spectral efficiency more bits per symbol and huge theoretical capacity.
But in real networks especially long haul APAC routes it often collapses instantly.
The reason is simple
šš¶š“šµš²šæ š¼šæš±š²šæ š¤šš š¶šš»š š¹š¶šŗš¶šš²š± šÆš šŗš¼š±šš¹š®šš¶š¼š» ššµš²š¼šæš
šš š¶š š¹š¶šŗš¶šš²š± šÆš š¢š¦š”š„
As you push from š¤š£š¦š ā šš²š¤šš ā š²š°š¤šš the constellation points tighten the decision boundaries shrink and the signal becomes extremely sensitive to
ā¢ phase noise
ā¢ nonlinear interference
ā¢ filter narrowing
ā¢ ROADM insertion loss
ā¢ accumulated ASE from EDFAs
In short
ššµš² š°š®š½š®š°š¶šš šš¼š šš®š»š š±š²š½š²š»š±š š²š»šš¶šæš²š¹š š¼š» ššµš² š¢š¦š”š„ šš¼š šµš®šš²
This is why š°š¬š¬š QPSK or 16QAM is robust across most APAC fibers
and š²š°š¤šš is realistically a metro only modulation format.
šš¼šµš²šæš²š»š š¼š½šš¶š°š š¶šš»š š®šÆš¼šš š°šæš®šŗšŗš¶š»š“ šŗš¼šæš² šÆš¶šš š¶š»šš¼ š® šššŗšÆš¼š¹
šš š¶š š®šÆš¼šš ššµš²ššµš²šæ šš¼ššæ š³š¶šÆš²šæ š®š»š± š¢š¦š”š„ š°š®š» šøš²š²š½ ššµš¼šš² šÆš¶šš š®š¹š¶šš²
šŖšµš šš¶š“šµš²šæ š¢šæš±š²šæ š¤šš šš®š¶š¹š š¶š» š„š²š®š¹ š”š²ššš¼šæšøš
š²š°š¤šš š¹š¼š¼šøš š½š²šæš³š²š°š š¶š» š¹š®šÆš.
šš» šæš²š®š¹ š³š¶šÆš²šæ š»š²ššš¼šæšøš š¶š šÆšæš²š®šøš š³š®šš.
Where theory ends and physics begins.
šš¶š“šµš²šæ š¢šæš±š²šæ š¤šš š¶š š»š¼š š® š°š®š½š®š°š¶šš šš½š“šæš®š±š².
šš š¶š š®š» š¢š¦š”š„ š°šµš®š¹š¹š²š»š“š².
More bits per symbol only work if the signal quality can keep them separated.
šš šŗš¼š±šš¹š®šš¶š¼š» š¶š»š°šæš²š®šš²š šš²š»šš¶šš¶šš¶šš š²š š½š¹š¼š±š²š.
š¤š£š¦š ā šš²š¤šš ā š²š°š¤šš
Constellation points move closer.
Decision margins shrink.
Small noise becomes fatal.
šš š¶š š»š²šš²šæ š·ššš š¼š»š² š¶šŗš½š®š¶šæšŗš²š»š.
š”š¼š»š¹š¶š»š²š®šæ š±š¶ššš¼šæšš¶š¼š»
šššš šš¦š š»š¼š¶šš²
ššµšæš¼šŗš®šš¶š° š±š¶šš½š²šæšš¶š¼š»
š£š š
š„š¢ššš š³š¶š¹šš²šæš¶š»š“
They stack until the constellation collapses.
šš» š¹š®šÆš š²š°š¤šš š¹š¼š¼šøš šÆš²š®ššš¶š³šš¹.
šš» šæš²š®š¹ š³š¶šÆš²šæ š¶š š±š¼š²š š»š¼š.
Perfect fibers do not exist in live networks.
šš£šš š¹š¼š»š“ šµš®šš¹ š»š²ššš¼šæšøš š®šæš² šµš®šæššµ š²š»šš¶šæš¼š»šŗš²š»šš.
Mixed š.š²š±š® š®š»š± š.š²š±š± fiber
Variable splicing quality
EDFA only amplification
Long ROADM chains
High order QAM suffers first.
š²š°š¤šš š°š®š» šš¼šæšø š¶š» š°š¹š²š®š» šŗš²ššæš¼ š¹š¶š»šøš.
šš š³š®š¶š¹š š¼š» š¹š¼š»š“ šµš®šš¹.
That is why š°š¬š¬š š¤š£š¦š š®š»š± šš²š¤šš remain the backbone workhorses.
šš¼šµš²šæš²š»š š¼š½šš¶š°š š¶š š»š¼š š®šÆš¼šš š°šæš®šŗšŗš¶š»š“ šŗš¼šæš² šÆš¶šš.
šš š¶š š®šÆš¼šš šøš²š²š½š¶š»š“ ššµš¼šš² šÆš¶šš š®š¹š¶šš².
Capacity without margin is not engineering.
It is risk.
šŖš®š»š šš¼ šš»š±š²šæššš®š»š± š¢š¦š”š„ šŗš¼š±šš¹š®šš¶š¼š» šÆš®šš± šæš®šš² š®š»š± šæš²š®š¹ š»š²ššš¼šæšø š¹š¶šŗš¶šš
šš²šŗšššš¶š³šš¶š»š“ ššŖšš
Built for engineers who design real optical networks.
