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Re: SONET/Ethernet clock tolerance


I think this is one of the biggest inter-culture gap between LAN and 
traditional (or Telecom) WAN.

At 8:35 PM -0600 00.3.30, Roy Bynum wrote:
> I am not sure why you are concerned about the regenerators budget timing
> specifications.  Loop timed or derived timed systems run on the clock
> specification of the incoming clock.

In datacom regenerators or media converters, they can easily re-clock 
with its local clock source by simply inserting/removing IDLE character.  
Hence they worry about the jitter specification only between each two 
regenerators.  This makes their PLL design very easily and inexpensive.

In Telecom world, on the other hand, we must be SYNCHRONOUS once we 
have packed the SONET frame.  This means SONET regenerators have to 
re-use the upstream incoming clock for the downstream signal to 
transmit.  This makes our PLL design very difficult, since we have to 
worry about jitter transfer at each regenerator and about its 
accumulation all over the regenerator chain.  I know that many 
traditional SONET regenerators even don't use PLL to meet this stringent 
jitter transfer specifications; they use more expensive SAW filter to 
extract the incoming clock.  Please consult with ITU-T Recommendation 
G.958 Clause 9.3: Jitter performance.

Once again, I agree that SONET-compliant PHY makes sense since it can be 
connected directly to install-base SONET regenerators.  Here I support 
Nortel's T1X1.5 proposal (Ether on SONET) since I have no other choice 
at present. (Or will anyone propose 64/66 on full-SONET?)

I don't believe SONET-framing make sense just for the OAM&P signaling 
to Ethernet Line Terminating Equipment (ELTE).  SONET framing is 
optimized for SYNCHRONOUS transport systems, and hence we should pay 
too heavy tax (read it pointer manipulation, SONET re-framing, or extra
buffers) when we want to change our clock for relaxing the jitter budget.

This is the background of my 10GENIE proposal (Layer-1 signaling in IPG) 
at Albuquerque.  I hope to see brand-new WAN for datacom, since I believe 
this is the most economical way to utilize our fiber networks.

Best Regards,
Osamu Ishida

NTT Network Innovation Laboratories
TEL +81-468-59-3263  FAX +81-468-55-1282