scrambling vs block coding

["Michael M. Salzman" <msalzman@xxxxxxxxxx>]

Hi all,

 

The many cogent arguments about this subject highlight the requirements differences between long haul and LAN applications.  Although we have reason to expect that the packets sent along the LAN would emerge unscathed onto the long haul network, there is no fundamental requirement for those packets to be encoded and modulated in the identical fashion.  The LAN environment, whether SX or LX  will require Phy's that are cheaper, simpler and more robust.   Long haul applications on the other hand may derive an overwhelming cost advantage from the scrambling approach.   Thus the interconnection gateway between the LAN and the Long Haul would have the SX/LX phy's on the one side and some kind of a long haul port on the other.  That port could be a direct phy - for those cases where the signal simply enters a dark fiber, or it could be a port of a DWDM mux, that would insert the signal at a particular wavelength.

 

At the same time, a dual phy approach, leaves a few issues unresolved.   Among the benefits of the block coding approach is the ability to pass pseudo-data elements back and forth on the link.  These capabilities are most required for the long haul, where a quick recovery of the link is most desirable.   Furthermore, the ability to differentiate the level of the link problem in a long haul application is important.  It could be an amplifier failure, or a sync loss, or total transmitter failure.

 

Furthermore, would it not be desireable to enable loopbacks at a low level in the long haul link in order to idenitfy the location of the failed component?  Such schemes are often done outside the payload.  One answer is that 10 Gig links are likely to travel over a WDM infrastructure, providing its own physical fault isolation, hence a separate mechanism would not be required.  The flip side is that a block code with control codes would provide that extra measure of protection.  Frame based approaches for managing the link would require a full scale PHY and MAC at each device. 

 

Finally, retiming, regeneration muxing and switching devices along the way would benefit from the more rapid clock and error recovery of a block code.  It would reduce the demands on them. 

 

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Michael Salzman             Director of Business Development
                                                       Lucent Technologies 
Phone  408-871-4075                          Lan Systems Group
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Cell      408-829-4425                  msalzman@xxxxxxxxxx