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.
NTT Network Innovation Laboratories
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