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Re: SONET/Ethernet clock tolerance

I agree with Ishida-san.  

I think we need to understand that there are two separate issues here.

(1)  clock tolerance/accuracy specifications (i.e. +/-100ppm, +/-4.6 ppm, etc)
(2)  jitter budget specifications (jitter generation, transfer and tolerance)

The first one determines the lock range of any oscillators and phase look loops in the network. In SONET the worst case clock accuracy/tolerance is specified as +/- 20ppm and therefore every oscillator and clock recovery circuit in a SONET network should be able to lock on and hold to a +/- 20ppm signal.

The second one, as Ishida-san points out is, effectively determines the maximum number of  through-timed regenerators that can be traversed on an end-end SONET link to maintain a specified link bit error rate. 

These two specifications (i.e. clock tolerance/accuracy and jitter) are completely intendant. However from my experience in dealing with POS (packet over SONET) there is a tendency among some people to assume that they are related, in that a more accurate clock (say 4.6ppm rather than 20ppm) necessarily has better jitter performance. This is not the case. In fact SONET has a single set of jitter specifications (generation, transfer and tolerance) that are the same irrespective of whether an interface is clocked from a stratum 1 clock, a stratum 3 clock (4.6ppm) or a 'SONET minimum' clock (20ppm).

As a reference point POS interfaces use a 20ppm clock and are fully compliant with the SONET/SDH jitter specifications in Bellcore GR-253 and ITU G.958.

Gary ........................

At 02:04 AM 3/30/00 , Osamu ISHIDA wrote:
>I have discussed about jitter budget over the end-end SONET-framed link.  
>As I understand, in a chain of SONET(-lite) regenerators, the link jitter 
>budget should be specified for all the chain elements.  This means each 
>regenerator should meet very stringent jitter-transfer specifications.  
>I think this is not the issue of +/-100ppm or +/-4.6ppm.
>It would be appreciated if you let me know about jitter-transfer spec. 
>in "SONET-lite".
>Also please let me know if you have any further information about 
>"SONET-lite".  I have checked the T1.416 Standard Document approved 
>last December, but I can not find any statement about "+/-100ppm".  
>It seems to say just "be compliant to T1.105 (=SONET)".
>My understanding at present is that there is no "SONET-lite" standard 
>in ITU-T, T1, IETF, nor OIF.  Please correct me if I am wrong.
>Best Regards,
>Osamu Ishida
>At 8:37 AM -0600 00.3.29, Roy Bynum wrote:
>> The clock tolerances for regenerators (LRE) are very relaxed from those of
>> line terminating equipment (LTE).  It is the LTE that does the multiplexing
>> of the smaller TDM payloads into a full OC192 payload and puts the full
>> SONET overhead on it, scrambles it,  and sends it out.  The regenerator does
>> a line clock recovery, unscrambles the SONET frame, uses the section
>> overhead information to determine performance and fault issues, rescrambles
>> the SONET frame, and sends it out.  The regenerator re-times the signal
>> using the recovered clock, not a Stratum clock.  In the process the
>> regenerator removes any signal bit jitter that has been introduced by the
>> various forms of dispersion that occur in optical fiber over any distance.
>> Because it does not do the multiplexing and does not have the clock
>> tolerance issues that the LTE has, a regenerator is a lot less expensive
>> than an LTE.